eMedicine Specialties > Radiology > Musculoskeletal

Knee, Meniscal Tears (MRI)

Author: Michael R Aiello, MD, Radiologist, St Elizabeth Medical Center, Utica, NY
Contributor Information and Disclosures

Updated: Jan 26, 2009

Introduction, Anatomy, Physiology, and Histology

Introduction

The knee menisci were once thought to be functionless remnants of a leg muscle and expendable components of the knee. Much has been learned through laboratory investigation, clinical experience, and radiologic imaging. The meniscus is now known to play an important role in the complex biomechanisms of the knee. For instance, it is involved in joint stability, load sharing and transmission, shock absorption,¹ and nutrition and lubrication of the articular cartilage.^2,3,4

Meniscal injuries are a common problem in sports; they are the most frequent injury to the knee joint. Such injuries are especially prevalent among competitive athletes, particularly those who play soccer, football, basketball, and (sometimes) baseball. In the past 25 years, the number of people participating in sports has greatly increased, resulting in a higher number of knee injuries.⁵

MRI accurately depicts the anatomy and pathology affecting almost every joint in the body. Because MRI is highly accurate, most leading orthopedic surgeons prefer MRI to arthroscopy.^6,7,8

Repair or resection of meniscal injuries is one of the most common orthopedic operative procedures. Meniscal injuries have a tremendous physical and financial impact on the population. It has been estimated that more than 1.7 million patients undergo meniscal surgery every year.

Various treatment options have been used for meniscal repair and reconstruction. The optimal treatment of meniscal lesions remains controversial. With the evolution of diagnostic MRI, it is no longer sufficient for the orthopedist to simply know that a meniscal tear is present. Treatment depend on knowledge of the exact type, location, and extent of the meniscal tear.

Anatomy

The menisci are C -shaped fibrocartilaginous structures attached to the condylar surface of the tibia. The limbs of the C face centrally. The superior meniscal surface is concave, which enhances contact with the curvilinear-shaped femoral condyle. Conversely, the undersurface of the meniscus is flat, which enhances contact with the flattened tibial plateau. The periphery (outer portion) of the meniscus is convex; it is thicker than the pointed central portion. The thick periphery allows for a firm attachment to the joint capsule (see Image 1).⁹

Medial meniscus

The medial meniscus (MM) is more crescentic in shape, whereas the lateral meniscus (LM) is more circular. The MM occupies 50% of the articular contact area of the medial joint compartment and is larger than the LM.^10,11

The posterior horn is thicker than that of the LM (see Image 2). The MM varies in width from about 6 mm in the anterior horn to 12 mm in the posterior horn. The transverse (intermeniscal) ligament connects the anterior horns of the MM and LM. The posterior fibers of the root of the anterior meniscus merge with the fibers of the transverse ligament (see Image 3).^9,11,12

The periphery of the MM is attached to the joint capsule throughout its length. The tibial portion of the capsular attachment of the MM is called the coronary ligament. At its mid portion, the body of the MM is more firmly attached to the tibia and femur through a thickening of the joint capsule, called the deep medial collateral ligament (MCL). The body is tapered when compared with the wider anterior and posterior horns. In 11% of patients, the anterior horn is not attached to the tibia. In this situation, it attaches directly to the transverse ligament or the anterior cruciate ligament (ACL).²

Lateral meniscus

The LM is more circular than the MM; it covers more of the tibial plateau (about 70%) than the MM. In addition, the LM is more uniform in width from anterior horn to posterior horn than the MM.^9,11

The LM contains the popliteus recess in its posterolateral margin. The recess allows passage of the popliteus tendon and sheath through the knee joint (see Image 6). The popliteus recess is located between the lateral recess and the joint capsule (see Images 5-7). The LM attaches to the lateral aspect of the medial femoral condyle through the meniscofemoral ligaments: the ligament of Wrisberg, which is posterior to the PCL and the ligament of Humphrey, which is anterior to the PCL. Both ligaments originate from the posterior horn of the LM (see Image 9).^5,10

The anterior horn of the LM attaches to the tibia between intercondylar eminence and the insertion of the ACL (see Image 14). The fibers of the anterior horn and ACL insertion may be contiguous (see Images 15 and 40). The posterior horn is secured to the tibia between the intercondylar eminence and the insertion of the MM. The LM measures about 10 mm in width.^5,10

At the posterolateral corner of the knee joint, the LM is attached to the joint capsule by the superior and inferior fascicles (see Images 7-8). The LM has a loose attachment to the joint capsule and is not anchored to the lateral collateral ligament (LCL), as the MM is to the deep portion of the MCL. The attachments of the LM to the femur and popliteus tendon enable it to couple its motion with that of the femur during rotation. Its greater mobility makes it less likely to become injured than the more firmly attached MM. It may normally move 1 cm in the anteroposterior direction.^9,10

Meniscomeniscal ligament

The meniscomeniscal ligament is an uncommon normal variant that runs from the anterior horn of 1 meniscus to the posterior horn of the other meniscus. It is named according to its anterior attachment.¹³

Discoid meniscus (see Image 41)

The discoid meniscus (DM) is a dysplastic meniscus that is frequently bilateral. This structure is enlarged and shaped like a half moon. It does not have the usual semilunar shape of the normal meniscus. The DM is more common in the LM by a ratio of 5:1 to 50:1. The incidence of occurrence in the LM is between 1.4 and 15.5%. The incidence of occurrence in the MM is about 0.3%.¹⁴

The DM may be described according to morphology as complete and incomplete. Both types are firmly attached to the tibia. A complete DM extends to the intercondylar notch of the tibia. The Wrisberg type of DM may have a normal appearance, but lacks any attachment to the posterior tibia and joint capsule. As a result, the meniscus retracts anteriorly from the pull of the meniscofemoral ligaments. This unrestricted motion puts it at risk for hypertrophy and trauma.¹⁰

An anterior megahorn DM is an overgrowth of the anterior horn and body with a normal posterior horn. The DM is more susceptible to tears and cyst formation than the normal meniscus. An incomplete DM is the most common type. Criteria for DM is that the horizontal diameter of the body of the meniscus is greater than 13-15 mm on coronal MRIs, as measured from the capsular margin to the free edge.

The DM is associated with other musculoskeletal anomalies, including the following: high fibular head, fibular muscular defects, hypoplasia of the lateral femoral condyle with lateral joint-space widening, hypoplasia of the lateral tibial spine, abnormally shaped lateral malleolus of the ankle, and enlarged inferior lateral geniculate artery.

Transverse, or intermeniscal, ligament

The transverse (intermeniscal) ligament connects the anterior horns of the MM and LM (see Images 3-4 , 43-44). It originates anterolateral to the central rhomboid attachment of the LM and inserts onto the anterior superior aspect of the anterior horn of the MM. The transverse (intermeniscal) ligament is absent in up to 40% of individuals.

Meniscofemoral ligaments

The meniscofemoral ligaments are oriented obliquely from the posterior horn of the LM to the medial femoral condyle (see Image 9). The more posterior ligament of Wrisberg is the larger of the 2. Its thickness may be half the thickness of the PCL. This ligament is present in 60% of cadavers.

The presence and size of these ligaments vary greatly. Their exact function is uncertain, but they may pull the posterior horn of the LM anterior to increase the coverage of the lateral femoral condyle and the LM.¹¹

Popliteus tendon

The popliteus tendon originates from the posterolateral surface of the femur, above the lateral femoral epicondyle in a groove (see Images 6 and 10-12). It retracts the posterior horn of the LM in flexion and internal rotation, reducing entrapment of the LM between the femur and tibia.¹⁰

Blood supply

The MM is supplied by the supreme, superior, and inferior branches of the medial geniculate artery. The LM is supplied by similar branches of the lateral geniculate artery.^2,11

The middle geniculate branch of the popliteal artery supplies the menisci through the vascular synovial covering of the anterior and posterior attachments to the joint capsule. These synovial vessels penetrate these attachments and give rise to the meniscocapsular vessels that branch into the menisci.¹¹

The microvascular structure of the meniscus consists of a perimeniscal capillary plexus originating in the capsular and synovial tissues of the joint. This plexus extends through 10-25% of the peripheral aspect of the meniscus. This limited blood supply has important prognostic implications in the treatment of meniscal tears.¹¹

On the basis of the degree of vascular penetration, the meniscus is divided into a red or vascular zone (outer one third) and a white or avascular zone (inner two thirds). The border between these areas is called the red-white zone or junction.

There is an absence of penetrating vessels in the posterolateral aspect of the LM adjacent to the popliteus tendon.

Nervous enervation

Nervous enervation is limited to the meniscal horns. The central one third of the menisci are devoid of enervation.¹¹

Physiology

The meniscus is a biphasic medium: it has a fluid phase and a solid phase. The fluid phase is the interstitial water, and the solid phase is the extracellular matrix, which is composed predominately of collagen. The interstitial water flows through the porous solid matrix at different rates under different circumstances to prevent damage and deformity of the meniscus as it fulfills its biomechanical function. The menisci are viscoelastic under load. That is, the tissue deforms and behaves differently, depending on the amount of load placed on it, as well as on the rate at which the load is applied.¹¹

The menisci provide added mechanical stability to the normal gliding of the femur on the tibia by deepening the surface of the tibial plateau to increase the congruity between the femoral condyles and the tibial plateau. It has been suggested that the posterior horn of the MM is the most important part of the meniscus in providing this function.

The meniscus protects the articular cartilage by acting as a buffer between the articular surfaces of the femur and tibia. The meniscus transmits axial and torsional forces across the knee joint. It prevents capsular interposition between the tibia and femur. It increases the surface area for motion of the femoral condyles. Vertically oriented collagens, located predominantly in the periphery of the meniscus, are converted into hoop stress in the menisci, decreasing the axial load on the articular cartilage. Loss of this function through a meniscal tear or as a result of meniscectomy may accelerate the breakdown of articular cartilage, with resultant DJD.

The menisci cushion mechanical loading and support 50% of the load about the knee in extension. In 90% flexion, 85% of the load is transmitted through the menisci. They act as a shock absorber.¹ As the knee joint compresses, the circumferential collagen fibers elongate and the meniscus extrudes peripherally, absorbing energy and reducing the shock to the adjacent cartilage and subchondral bone. They act as a spacer, separating the tibia and femur at low loads.

Nerves of the menisci may play an important role in proprioception of the knee. Evidence suggests that knees treated with meniscectomy have less proprioception than normal knees.

The menisci contribute significantly to joint lubrication. Because 74% of the total weight of the meniscus is water, compression of the menisci squeezes water out from them into the joint space, allowing smoother gliding of the joint surfaces. This pumping action also helps to distribute synovial fluid throughout the joint and aids in the nutrition of the articular cartilage.^10,11

Medial meniscus

The MM facilitates the transmission of 40-50% of the load of the medial compartment, whereas the LM may transmit as much as 65-75% of the load on the lateral compartment. The MM is subject to higher stresses than the LM. This has prognostic considerations when treating acute MM tears in the presence of ACL tears. In this situation, it is imperative to repair the meniscal tear while repairing the ACL tear. The medial meniscus also acts secondarily to provide anteroposterior stability; this stabilizing role of the medial meniscus becomes more important in the ACL-deficient knee.^15,16

Lateral meniscus

The LM may play a role in rotational control, especially in patients with posterolateral instability. The LM causes a greater proportion of the load on the lateral side of the knee than on the medial side, where the load is shared equally by the MM and the articular surface.⁵

Histology

The menisci are composed of dense fibrocartilage consisting of networks of collagen fibers and cells. The cells of the meniscus are called meniscal fibrochondrocytes because they are a mixture of both fibroblasts and chondrocytes. The cells in the superficial portion of the meniscus are similar to the chondrocytes found in the more superficial layer of the articular cartilage of the femur and tibia. The cells in the deeper portion of the meniscus are more oval in shape, but they also resemble the chondrocytes found in the tibia and femur. The function of the cells within the meniscus is to synthesize and maintain the extracellular collagen matrix, which is the scaffold of the meniscus.

The collagen is formed into bundles, which are organized into 2 zones: circumferential and transverse. The circumferential bundles are located primarily in the peripheral one third of the meniscus. The transverse bundles bridge the circumferential bundles and extend to the free (inner) edge of the meniscus, adding structural integrity to the meniscus and resisting compressive forces.

The middle perforating collagen bundle divides the transverse bundles into superior and inferior portions. Some of the transverse fibers act as tie fibers between the circumferential bundles that resist longitudinal splitting. The middle perforating collagen bundle may demarcate the sheer plane of the meniscus. In internal degeneration, the middle perforating collagen bundle corresponds to the predominately horizontal region of increased signal of grade 2 menisci, seen on proton density (PD)– and T2-weighted images. Elastic fibers bridge the collagen fibers throughout the meniscus.¹⁷

Ultrastructural studies demonstrate 3 layers of collagen fibers: superficial, surface and middle. Going from superficial inward, the fibers tend to become larger and more structurally significant.

Several types of collagen have been identified within the meniscus. Type I collagen predominates and makes up 95% of the meniscal collagen. It serves as the fibrous network for the dense and durable fibrocartilage of the meniscus. Types II, III, V, and VI collagen have also been identified within the meniscus, but in smaller amounts.²

For excellent patient education resources, visit eMedicine's Foot, Ankle, Knee, and Hip Center; Imaging Center; and Arthritis Center. Also, see eMedicine's patient education articles Knee Injury, Knee Pain, Knee Joint Replacement, Magnetic Resonance Imaging (MRI), and Osteoarthritis.

Pathology and Embryology

Pathology

Two pathways for healing in response to a tear in the periphery of the meniscus are described: extrinsic and intrinsic. Two types of tears are also discussed: traumatic and degenerative.

Extrinsic pathway

Once a meniscal tear occurs, a fibrin clot forms within its margins, creating a scaffold into which angiogenesis develops from the perimeniscal capillary plexus. The fibrin clot contains factors, such as platelet-derived growth factor and fibronectin, that act as chemotactic and mitogenic agents for the migration and development of reparative cells. Undifferentiated mesenchymal cells also migrate into the clot, and a fibrovascular scar develops, gradually sealing the lesion. Further inflammatory response and angiogenesis result in healing of the lesion in about 10 weeks in the dog.^3,5

It may take months or even years for the scar tissue to change into fibrocartilage, after which it resembles that of the meniscus. Differences between the newly formed fibrocartilage and mature fibrocartilage are recognizable and include increased cellularity and, at times, increased vascularity in the repair tissue.¹¹

Intrinsic pathway

The chondrocytes within the meniscus have an inherent capability of generating a healing response, even in the avascular region. Chondrocytes are assisted by the fibrin clot, which not only acts as a scaffold but also provides the chemotactic and mitogenic stimuli to promote healing. Meniscal tears may be classified into 2 types: traumatic and degenerative.

Traumatic tears

Traumatic tears are most commonly found in young, athletically active individuals. Traumatic tears are not necessarily associated with contact injuries. They are frequently associated with ACL tears and less commonly with PCL tears. Vertical longitudinal tears are the most common; transverse or radial tears are also common.^11,12

Degenerative tears

Degenerative tears tend to occur in patients older than 40 years. No history of a traumatic event is present. These tears have minimal or no healing capacity; horizontal cleavage tears, flap tears, and complex tears are most common.¹²

Embryology

The menisci develop from condensations of mesenchymal tissues within the limb bud; they assume an adult shape by the end of the eighth week of fetal development.

At birth, the menisci are completely vascularized; however, vascularization regresses, so that by 10 years of age, an avascular central third of the meniscus is present. The transition from vascular to avascular regions progresses so that by adulthood, only the peripheral 10-30% of the meniscus remains vascular. Weight bearing and knee motion may be responsible for this progressive change.^3,12

Clinical Details

Mechanisms of injury

Injuries to the healthy meniscus are usually produced by compressive forces coupled with rotation of the flexed knee as it starts to move into extension. The final type and location of the tear is determined by the direction and magnitude of the force acting on the knee and the position of the knee when injured. Most meniscal tears in sports are noncontact in nature and occur from deceleration, cutting, or landing from a jump. Other mechanisms of injury include twisting of the knee or squatting.

In knees with chronic ACL tears, recurrent episodes of anterior tibial subluxation on the femur occur, resulting in shearing forces on the menisci.

In skiers with acute LM tears and ACL tears, the mechanism of injury to the meniscus is an anterolateral rotatory translation. As the ACL disrupts, excessive anterolateral rotation of the tibia on the femur traps the LM between the posterolateral aspect of the tibial plateau and the central portion of the lateral femoral condyle. The LM is distorted when the tibia reduces, often resulting in a tear.

The orientation of the transverse tie fibers between the circumferential bundles is responsible for the tendency of the meniscus to tear with rotational forces. Rotation, especially when accompanied by axial compression, can squeeze the meniscus between the tibia and femur, generating tensile forces in the body of the meniscus high enough to damage either the transverse fibers, resulting in a longitudinal tear, or the circumferential fibers, resulting in a radial tear.

Findings on physical examination

General findings include the following:

• Knee giving way
• Clicking
• Locking of the knee in fixed flexion (This may occur immediately after displacement of a meniscal fragment between the tibia and the femur.)
• Pseudo-locking, which may be related to muscle spasm
• Limping (This is common, and patients may not be able to bear weight.)

Findings related to a DM include the following:

• Snapping knee syndrome: When the knee is flexed or extended, a snapping sound is heard because the DM may be hypermobile.¹⁸ The snapping is associated with knee joint pain or lateral joint line tenderness. This occurs only in a minority of cases.¹⁸
• Nonspecific physical findings are common.¹⁰
• The Wrisberg type of DM may present before adolescence with pain and with or without an audible palpable click.

Tests to evaluate the menisci

No one test provides predictive results for diagnosing meniscal tears. A combination of several positive results is highly predictive of meniscal tears.

Appley test

With the patient prone and the knee flexed at 90° the foot is rotated internally and externally, first with distraction and then compression. A positive result elicits pain with compression. This test is inaccurate, and its use is discouraged.

McMurray test

The patient is supine with the hip and knee flexed. The foot is alternatively internally and externally rotated, while the posteromedial and posterolateral joint line are palpating. To evaluate the MM, the knee is extended, and the foot is externally rotated. To evaluate the LM, the knee is extended, and the foot is internally rotated. Hearing or feeling a click during these maneuvers is indicative of a meniscal tear.

A true-positive result is rare, but the specificity is nearly 100%. More commonly, the tear elicits pain with a meniscal tear. This test is the most widely used clinical test to detect meniscal injuries, but its accuracy is inconsistent.¹⁹

Range-of-motion tests

Flexion pain or limitations in range of motion often result from posterior meniscal horn tears. Extension pain can be linked to anterior horn tears. Tenderness on palpation of the joint line is probably the most important finding.

Boehler test

This test is performed in the same way as testing for stability of the collateral ligaments. Valgus stress results in pain with LM tears. Varus stress results in pain with MM tears.

Apley grinding test

With the patient prone, the hip is extended and the knee is flexed more than 90°. Downward pressure is placed on the foot and the joint surfaces of the knee are rotated and compressed. Eliciting pain is indicative of a tear.

Payr test

The knee is flexed to 90°. The application of varus stress compresses the posterior horn of the MM. Eliciting pain indicates a meniscal tear.

Accuracy and limitations of tests

Accuracy

Large, blinded prospective studies with diverse patient populations suggest that for experienced clinicians, the sensitivity of physical examination for detecting meniscal tears is between 70-90%.^9,20,21,22

The negative predictive value (NPV) is no greater than 67%; therefore, about one third of meniscal tears are missed with clinical screening alone. In situations of multiple knee lesions, the accuracy of clinical examination in diagnosing meniscal tears decreases to 30%.^20,21

In the presence of acute ACL tears, the sensitivity for diagnosing MM tears is 45% and 58% for LM tears. The sensitivity of joint line tenderness for diagnosing meniscal tears is 75%. The sensitivity of the Apley grinding test for meniscal tears is about 45%. The sensitivity of the Payr test for diagnosing meniscal tears is about 40%.^9,20,21

Limitations

Unlike MRI, the clinical examination cannot demonstrate the location, shape, or length of a meniscal tear. These factors are important in treatment decisions.

The clinical diagnosis of meniscal tears becomes more difficult and unreliable in the presence of acute ligamentous injuries of the knee. The sensitivity for diagnosing MM tears decreases to 45% and the sensitivity for diagnosing LM tears decreases to 58% when the ACL is ruptured. Specificity also decreases, most likely due to the presence of tibial and femoral bone bruises that frequently accompany acute ACL tears. Pain from these bone injuries can cause joint-line tenderness, a finding that otherwise suggests the presence of a meniscal tear.^9,20,21,22

Distribution of meniscal tears

In knees with intact ligaments, MM tears are more common, due to higher biomechanical stresses in this location and a firmer attachment to the joint capsule. Only 18-20% of meniscal tears occur without associated ligament damage.

Meniscal tears associated with acute ACL tears

The incidence varies between 35% and 78%, and the majority are bucket handle tears.^15,22,23

These tears usually involve the vascularized outer one third of the posterior horn or meniscocapsular junction. This favorable location, within the richly vascularized portion of the meniscus makes these tears amenable to repair, or conservative treatment. In downhill skiers, the LM is more often involved. Tears tend to be longitudinal and are located at the periphery of the posterior horn, or the root of the meniscus at the joint capsule, medial to the popliteus tendon sheath hiatus.²³ Tears are either full thickness or partial thickness.^9,24

Meniscal tears associated with chronic ACL tears

After the initial ACL tear, the incidence of subsequent meniscal tears increases to between 53% and 97% over a 10-year period.

The incidence of MM tears increases with time. The incidence of LM tears with chronic ACL injuries was the same as with acute ACL injuries.²³ The MM is a significant restraint to anterior tibial translation after ACL disruption, suggesting the possibility of increased sheer forces on the meniscus after ACL tear. The relatively immobile MM may also be injured during abnormal knee motion during periods of instability.²²

Meniscal tears with chronic ACL injuries are often degenerative and complex, supporting the concept that chronic instability, seen with chronic ACL tears, leads to or accelerates degenerative meniscal tears and predisposes to early and accelerated DJD.

They are less amenable to surgery.²²

Meniscal tears with multiple ligament tears

With both ACL and MCL tears, LM tears are more common.²³

Isolated MM tears occurring without LM tears are uncommon. O'Donoghue's triad, the unhappy triad of combined ACL, MCL, and MM injury, is uncommon.

Many MM injuries in this setting are actually ruptures of the medial joint capsule and the deep fibers of the MCL in conjunction with peripheral meniscocapsular separation (MCS).

Other tears

Degenerative tears are more commonly occur in patients older than 40 years, and patients often are not aware of any injury, only the subsequent symptoms.

Meniscal tears in children are often associated with ACL tears.²⁵

Patients with DM tears may present with mechanical symptoms without a tear because the abnormally large DM may be hypermobile causing painful snapping with flexion and extension.¹⁸

Differential diagnosis

Differential diagnoses of meniscal tears include the following¹⁷ :

• Bone contusion
• Plica syndromes
• Popliteus tendinitis
• Osteochondritis desiccans
• Chondral damage from trauma
• Loose bodies
• Patellofemoral pain and instability
• Fat pad impingement syndrome
• Inflammatory arthritis
• Fracture
• Meniscotibial ligament sprain
• Discoid meniscus
• Synovial lesions

Differential diagnoses of a DM include the following¹⁷ :

• Any condition that presents as a snapping knee on physical examination (A snapping knee yields a snapping sound heard during flexion and extension of the knee.)
• Patellofemoral joint subluxation or dislocation
• Meniscal cysts
• Congenital subluxation of the tibiofemoral joint
• Subluxation and/or dislocation of the proximal tibial-fibular joint
• Snapping of the tendons about the knee due to the presence of osteophytes or a roughened bone surface

Preferred Examination

Magnetic resonance imaging

MRI is the most powerful, accurate, and noninvasive method for diagnosing meniscal tears. It is more accurate than physical examination and has influenced clinical practice and patient care by eliminating unnecessary diagnostic arthroscopies or by identifying alternative diagnosis that may mimic meniscal tears.^15,26,27,28

In many cases, MRI results lead to changes in the proposed management. One study determined that about one third of all diagnostic arthroscopies need not be performed if MRI is used. Another study showed that the use of MRI prevented 51% of diagnostic arthroscopic procedures; another study showed that with the use of MRI, the morbidity associated with arthroscopy was avoided.²¹

MRIs show many of the essential characteristics of meniscal tears critical to management, such as their location, shape, length, and depth. In this way, MRI helps allows an accurate assessment of stability and of the likelihood of tear propagation, and it enables one to determine whether the meniscal tear can be repaired. It is advantageous to know ahead of time if a given meniscal tear is repairable; the additional equipment, surgical assistants, and time needed for repair can be anticipated. Patients also benefit from knowing early on whether surgery is necessary. The recovery time for meniscal repair is longer than that for partial meniscectomy (PM). Patients may want to time surgery to fit with their other obligations.^15,16,29

When combined with clinical data, such as the patient's age, athletic requirements, and physical findings (eg, possible associated ligamentous injuries), a treatment plan may be developed by assessing the need for and timing of surgery and by determining the type of surgery (meniscal debridement, rasping, repair, partial or total resection, or meniscal transplantation). MRI may be used to identify other injuries, such as ligament tears, especially ACL tears, the presence of which may also influence the decision whether to perform surgery.^15,16

With MRI, physicians may obtain images in several planes, providing multiple perspectives on meniscal and ligamentous injuries. Other advantages include the following: (1) with MRI, the patient is not exposed to ionizing radiation; (2) MRI does not normally involve the intravenous administration of contrast material, the use of which is associated with a small but definite number of adverse effects; (3) MRI does not require joint manipulation; (4) MRI is painless and can be performed in less than 35 minutes; and (5) MRI does not require the intra-articular injection of iodinated radiographic contrast material, which is needed for arthrography. MRI results lead to alterations in therapy in about one third of cases.²¹

Arthrography

Arthrography has been supplanted by MRI except for patients who are too large to fit into the MRI unit or for patients who have contraindications to MRI (eg, intracranial aneurysm clips, orbital metallic foreign bodies are present).

Plain radiography

Plain radiography is extremely limited in the assessment of meniscal tears. Radiographs may be obtained to rule out unsuspected lesions, such as osteochondritis desiccans and loose bodies.

In the presence of a DM, radiographs may show widening of the medial or lateral joint compartments; hypoplasia of the lateral femoral condyle related to the increased size of the LM; a high fibular head; cupping of the lateral tibial plateau; or a squared-off lateral femoral condyle.

Magnetic resonance arthrography

Magnetic resonance (MR) arthrography is used to evaluate residual or recurrent meniscal tears after meniscal surgery. The detection of residual or recurrent meniscal tears following meniscectomy or meniscal repair is difficult with conventional MR images.³⁰

MRI Techniques, Normal Anatomy, Grading, and Criteria

Imaging systems, protocols, and sequences

Careful attention to detail must be made to achieve high accuracy in diagnosing meniscal tears.

MRI systems

MRI systems of low, medium, and high field strength can all produce accurate, diagnostic images for identifying meniscal abnormalities. In units with lower field strength, the number of signals averaged may need to be increased to obtain an adequate signal-to-noise ratio. This adjustment, however, increases the imaging time, which increases the risk of patient motion. Even a small amount of motion can degrade the images, jeopardizing the ability to diagnose meniscal tears.^{9,31,32,33,34,35,36,37,38,39,40,41}

An extremity coil is used to optimize the signal-to-noise ratio. A surface coil may be used for better detail in evaluating subtle lesions or suspicious areas.⁴²

Protocols and imaging planes

The knee is usually positioned in extension with slight external rotation to facilitate imaging the ACL.

High spatial resolution is required to show subtle tears. This requires a field of view of 16 cm or less, a section thickness of 5 mm or less (3-4 mm is preferred), and a matrix of at least 192 X 256 steps in the phase- and frequency-encoding directions. A skip of 0.5 to 1 mm is used between imaging sections. These parameters can be achieved by using a solenoid surface coil. An extremity coil is used to optimize the signal-to-noise ratio.^9,15,18

If subtle lesions or suspicious areas are identified by using the standard extremity coil, high-resolution images can be obtained by using a surface coil, provided that the area of interest is superficial enough to be encompassed by the surface coil with the small field of view. Scanning parameters in this situation include the following: field of view as small as 10 X 10 cm, matrix 256 X 512 (displayed at 512 X 512), section thickness of 3 mm with an 0.3-mm intersection gap, and 3 signals acquired.⁴²

Images must be obtained in both the sagittal and coronal planes. Sagittal images are obtained with the knee externally rotated to permit imaging in the plane of the ACL. Meniscal and ACL injuries frequently coexist. Axial images are also obtained to study the supporting ligaments around the knee. Changing coils during an MRI examination is not part of the standard examination, but is similar to changing transducers during ultrasonography to look at deeper or more superficial structures. In a small or remote radiology practice, the attending radiologist may not be available to supervise the MRI examination. In this situation, the patient can be called back for additional imaging with the surface coil.⁴²

Several factors should be considered in optimizing the imaging protocols. Imaging in all 3 planes is useful; however, not every sequence must be performed in every plane. Fluid-sensitive sequences are mandatory for detecting subtle areas of edema. Typically, some T2-weighted sequence is performed, usually in the sagittal and axial planes. Experience with a particular sequence may outweigh any theoretical improvements from a pulse sequence unfamiliar to the imager.¹⁸

A repetition time (TR) between 2200 and 2800 ms is needed to generate enough sections to image both menisci in the sagittal plane. Short echo times (TEs) are important when PD–weighted imaging is performed. With a TE of less than 26 ms, more than 90% of all meniscal tears can be detected. If the TE is increased to greater than 60 ms, less than 30% of tears are detected.¹⁸

For reviewing MRIs of the knee, the use of meniscal windows has become popular. This method consists of a region of interest, centered on the meniscus, zoomed to 1.5-2X magnification. A window width of 100 to 150 and a window level of approximately 1000 are used. Data indicate no significant differences in detecting meniscal tears by using these narrow compared with conventional window widths.¹⁸

T1- and PD-weighted sequences

T1-weighted images are not as sensitive as PD-weighted images for diagnosing meniscal tears.

Gradient-recalled echo sequences

Gradient-recalled echo (GRE) sequences are as accurate as conventional spin-echo images for diagnosing meniscal tears.²¹

However, GRE imaging is more limited in diagnosing ligament, muscle, tendon, bone marrow, and articular cartilage abnormalities. It is also less specific for meniscal tears as a consequence of spurious signal from artifacts or degeneration without a tear.¹⁸

Fat-suppressed sequences

Fat suppression can be applied to meniscal-sensitive sequences to rid the image of distracting high signal originating from the fatty marrow in the bones and the fat in the soft tissues. With fat suppression, the dynamic range signal of the menisci is increased, making meniscal tears more conspicuous (see Images 22-23).^6,7

Fast spin-echo sequences

Turbo or fast spin-echo (FSE) pulse sequences are not as effective as conventional spin-echo sequences for diagnosing meniscal tears because images with a short effective TE, as seen with FSE imaging, sacrifice high-spatial-frequency information for speed. Images of the menisci may appear blurred. Rubin and colleagues postulated that the presence of ghosting artifact (secondary to phase differences between even and odd echoes in the echo train) or the loss of meniscal signal intensity in meniscal tears from an increased magnetization transfer (as seen with FSE sequences) may be responsible for the lower sensitivity of this sequence.^9,17,43

A review of 6 studies, including the author's, showed a distinct discrepancy between the sensitivities of FSE and conventional spin-echo sequences. The sensitivities of fast spin-echo techniques for detecting a meniscal tear was approximately 80% whereas the sensitivities for conventional spin-echo sequences averaged approximately 90%. The authors postulated that abnormal meniscal signal may appear to extend to the meniscal surface secondary to blurring and may be incorrectly interpreted as a tear. Alternatively, the increased blurring and decreased resolution associated with FSE imaging can contribute to false negative results. Blurring is most evident with short TE sequences but short TE sequences are most proficient for detecting meniscal abnormalities. Blurring is also most conspicuous with long echo-train lengths, such as those incorporated with FSE imaging protocols. The authors urged abandoning FSE imaging because a loss of greater than 10% in sensitivity is unacceptable.⁴⁴

When FSE sequences are used, an echo train length of 4-6 should be used to reduce blurring. The sensitivity of FSE sequences for diagnosing meniscal tears is about 80% in all reports, whereas the sensitivity of conventional spin-echo techniques is at least 90%. If the sensitivity decreases from over 90% to 80% and if all that is gained is 2-3 minutes in imaging time, the use of FSE sequences for imaging the menisci is hardly justified.^6,7,45

The use of FSE sequences with high performance gradients is accurate as conventional spin-echo imaging in diagnosing meniscal tears. The following parameters are used: a TR of 1500 ms and an effective TE of 20 m, with K space centered on the second echo at 2X minimal interecho spacing and a length of 4.⁴⁵

Normal anatomy on MRIs

Structures in the sagittal plane

Centrally, the normal meniscus is composed of 2 separate triangular structures: the anterior horn and the posterior horn. The apices (free edges or inner margins) appear as sharp points of the triangle facing each other (see Image 34). On the lateral side of the knee, the triangular anterior and posterior horns of the LM are equal in size (see Image 16). On the medial side of the knee, the posterior horn of the MM is larger than the anterior horn (see Image 17).⁹

The popliteus tendon and its accompanying sheath course through the posterolateral portion of the posterior horn of the LM in an oblique anterosuperior to posteroinferior direction. It is seen on the more lateral images of the LM.

Two fascicles connect the posterior horn of the LM at the popliteus tendon sheath level to the joint capsule. The inferior fascicle is seen on the more lateral images through the tendon. Here, the superior fascicle is absent. More medially, both superior and inferior fascicles are present. The most medical images through the tendon show the superior fascicle and absence of the inferior fascicle. The thickness of the popliteus tendon sheath varies in size from a thin line to a thick band.

Structures in the coronal plane

This is the best plane in which to image the meniscal bodies. Each meniscal body looks like a triangle with the pointed apex in the innermost part of the meniscus (see Image 13). The anterior and posterior horns appear as flat slabs. The root of the posterior horn of the LM is directed obliquely upward from a lateral to medial direction (see Images 2, 5-6, 34, and 54). The popliteus recess is located in the outer portion of the lateral joint compartment. It can be identified either by the presence of joint fluid within it or by the popliteus tendon originating from the distal lateral femur, above the joint, and passing through the sheath to insert on the back of the proximal tibia (see Images 5-6).^9,24

The insertion of the semimembranosus tendon is located posterior along the subarticular surface of the medial aspect of the proximal tibial metaphysis (see Images 37 and 43). This is not to be confused with a displaced meniscal fragment.

Meniscal flounce

A meniscal flounce is an uncommon meniscal variant characterized by a single symmetrical fold along the free edge of the meniscus. It appears as an S -shaped fold along the free edge on sagittal images and is associated with a truncated but normal meniscus on coronal images.

Normal meniscal signal intensity

The normal meniscus shows uniform, low signal intensity on T1- and T2-weighted images obtained with both conventional and FSE sequences (see Image 6). The low signal is related to a lack of mobile protons in the meniscal fibrocartilage. Subsequent dephasing of hydrogen nuclei results in T2 shortening, contributing to the low signal intensity on all pulse sequences.

Discoid meniscus

Differentiation between a true DM and a slightly larger but normal meniscus may be difficult.

On sagittal images, the DM has a thickened, bow-tie appearance on 3 consecutive sagittal images. The anterior and posterior horns of the normal meniscus are seen on several images near the intercondylar notch. With a complete DM, no distinct anterior or posterior horn is present. The normal meniscus rapidly tapers from the outer periphery to the center. The presence of equal or nearly equal meniscal height on 2 adjacent peripheral 5-mm-thick images indicates a DM. The anterior and posterior horns of the LM are normally equal in height. An asymmetric discoid LM may have an abnormally large anterior or posterior horn.¹⁷

On coronal views, the abnormal meniscal body extends more medially toward the intercondylar notch (see Image 41).

The posteromedial horn of the MM and the anterior horn of the MM near the roots may have a normal speckled appearance (see Images 34, 44, and 46).

Coronal images show the smallest width of the meniscal body, making this plane the most sensitive for showing meniscal enlargement. An asymmetric DM with an enlarged body may have a wide meniscal body on coronal images but normal anterior and posterior horns on sagittal images, emphasizing the need for coronal images. Incomplete DM may not extend into the intercondylar notch.

In children, grade 2 signal is frequently seen within the posterior meniscal horns. This is thought to represent normal vasculature, seen in the meniscus of a child. This disappears in adulthood.

Regarding the meniscofemoral ligaments, either the anterior or posterior ligament is present on 33% of MRIs. Both ligaments are present on 3% of examinations. One of the 2 ligaments predominates. The ligament of Humphry is best seen on sagittal images. It is occasionally seen on coronal images. The ligament of Wrisberg is best seen on posterior coronal images.¹⁷

Meniscal degeneration

Local increases in the degree of freedom of trapped water molecules within the substance of the meniscus occurs with age, resulting in increased T2 times. The appearance is that of increased signal intensity within the substance of the meniscus on short-TE images.¹⁴

MRI grading system for meniscal degeneration

An MRI grading system has been developed and correlated with a histologic model. Regions of degenerative show increased signal intensity in a spectrum of patterns or grades based on the distribution (morphology) of signal intensity relative to an articular surface of the meniscus. This basis is exclusive of the peripheral capsular margin of the meniscus, which is considered nonarticular.¹⁷

Grade 1

Grade 1 is a nonarticular, focal or diffuse region of increased signal intensity within the substance of the meniscus (see Image 16). This finding is correlated with early meniscal degeneration and chondrocyte-deficient or hypocellular region. The terms mucinous, myxoid, and hyaline degeneration are used interchangeably to describe the production and accumulation of an increased amount of mucopolysaccharide ground substance in stressed areas of the fibrocartilage of the meniscus. Such changes are a response to repetitive mechanical loading.

This appearance is found in healthy volunteers and asymptomatic athletes and not clinically significant.

Grade 2

Grade 2 is a horizontal, linear area of increased signal intensity within the substance of the meniscus that extends to but does not involve the (see Image 21). Such regions of abnormal signal are more extensive than in grade 1 degeneration, and no distinct cleavage plane or tear is present. Grade 2 is a continuation of progressive degeneration from grade 1 changes. Patients are usually asymptomatic.

Histologically, there is microscopic collagen fragmentation and clefting within the hypercellular region of the fibrocartilaginous matrix. The middle perforating collagen bundle, which divides the meniscus into superior and inferior halves, is the site of preferential accumulation of mucinous ground substance. It also represents the shear plane of the meniscus and is also the site of origin of horizontal degenerative meniscal tears.

The posterior horn of the MM is the most common location. It also is the most common site for grade 3 meniscal tears. The presence of grade 2 signal-intensity changes is not predictive of future progression to grade 3 meniscal tears. Grade 2 represents a point of potential structured weakening. Grade 3 tears, when they develop, are adjacent to or are in continuity with areas of grade 2 changes.

Grade 2C is a subcategory in which linear signal intensity extends to the articular surface on a single image. When found in symptomatic patients, about 50% have a tear. There are no additional features that can discriminate a torn meniscus from an intact meniscus with grade 2C signal intensity. This is not a very common occurrence, appearing in only 3% of patients in one study. Most patients with grade 2C signal are not treated with arthroscopy because they do not have symptoms referable to the site of abnormality, but about 50% of patients with grade 2C signal and knee symptoms have meniscal tears. Grade 2C signal might represent more extensive degeneration than seen with Grade 2 signal and can progress to degenerative tears.⁴⁷

Grade 3

Grade 3 is a region of abnormal signal intensity within the meniscus extending to and communicating with at least 1 articular surface of the meniscus (see Image 21). Multiple foci of grade 3 signal-intensity changes may be present in 1 meniscus.

About 5% of grade 3 tears are actually intrasubstance cleavage tears. These are closed meniscal tears and diagnosed only with surgical probing of the meniscus. They may be missed on routine arthroscopy if surface extension is not identified.¹⁴

False-negative correlations with arthroscopy have been described. This commonly occurs in the LM, either peripheral or posterior when an associated ACL tear is present. The improved spatial resolution and signal-to-noise ratio achieved with a surface coil may improve diagnostic accuracy in this situation and be related to spurious interpretation of areas of fraying as meniscal tears. Such lesions may present with pain related to edema and ingrowth of the synovium within the tear because the meniscus is an enervated structure.^24,42

MRI criteria for meniscal tears

Two MRI criteria have been established for diagnosing meniscal tears. If prior surgery has not been performed on the meniscus, the accuracy in diagnosing tears is 90%.¹⁵

Criterion 1

Criterion 1 is increased internal signal intensity in the meniscus (see Images 10, 22-25, 32-33, 38-39, 41-42). An article discussed the concept of the "Two-Slice-Touch Rule." The authors describe a positive predictive value of 94% for meniscal tears for the medial meniscus and 96% for the lateral meniscus when the tear is present on two consecutive images. The positive predictive value was 55% and 36% for medial and lateral meniscal tears when seen only on one slice.^15,24

The abnormal signal intensity must be in contact with one articular surface, either the superior or interior surface or at the tip (free edge) of the meniscus (see Images 10, 22-25, 32-33, 38-39, 41-42). If the contact with the articular surface appears on 2 or more consecutive images, the accuracy of the diagnosis of meniscal tear increases (see Images 50-51).^14,15

The need for short TEs is important. Most other tissue disorders are characterized by an increase in free water and unbound protons. In meniscal tears, hydrogen nuclei are bound to macromolecules. The bound protons have a shorter T2 relaxation than do protons in free water. Meniscal tears also result in the absorption of synovial fluid in the margins of the tear. This may be related to the loss of the normal tight collagen spiral, resulting in an increased mobility of water molecules. Water molecules are trapped, increasing the local spin density. The increased meniscal signal within the tear probably results from an increase in the local spin density and not from an increase in the T2 signal.¹⁵

The rate of proton rotation is shortened by the interaction of synovial fluid and large macromolecules, resulting in shortening of T1 and T2 values, increasing the sensitivity of PD-weighted images in revealing meniscal pathology. Such changes cause a local increase in the degree of freedom of trapped water molecules, resulting in increased T2 times, allowing the detection of increased signal with short-TE sequences. Increased signal within the meniscus is best seen on short-TE images obtained by using PD-weighted or GRE sequences.¹⁴

Although most meniscal tears are well seen on PD-weighted images, they are not visualized as well on T2-weighted images, unless there is a wide cleft at the site of the meniscal tear that freely communicates with joint fluid. If such a situation is present, confidence in diagnosing a meniscal tear is high. However, this finding is not common.

Criterion 2

Criterion 2 is an abnormal meniscal shape in the (see Images 22-33, 38-39, 42, 48-51, 60-62). Comprehensive knowledge of the normal MRI anatomy of the menisci is required.

Meniscal tears are more confidently diagnosed when they are seen on both sagittal and coronal images. The presence of a meniscal tear on both these views decreases the rate of false-positive diagnoses. However, some tears at the meniscocapsular junction can be seen only on 1 of these views.

In writing the MRI report about a meniscal tear, the radiologist should understand the use of standard nomenclature for meniscal tears and describe the location, plane, shape, completeness, length, and number of tears.¹⁸

Planes, shapes, lengths, and thicknesses of meniscal tears

Multiple cross-sectional representations of a meniscal tear have to be translated into a 3-dimensional (3D) description for the benefit of the arthroscopist.^15,16

Meniscal tears occur in 2 primary planes: vertical (see Images 22-23, 38-39, and 42) and horizontal (see Images 24 and 33).

The 3 basic meniscal tear shapes are longitudinal (see Image 38-39, 42, and 67), radial (see Images 22-23, and 68), and horizontal (see Images 24, 33, 43, 45, 63, and 69).

Vertical tears are aligned perpendicular to the coronal plane of the meniscus and can be subdivided into longitudinal (see Images 38-39 and 42) and radial tears (see Images 22-23). They occur as traumatic lesions in younger patients.⁴⁸

Longitudinal tears separate the meniscus into inner and outer fragments and occur parallel to the outer margin of the meniscus (are perpendicular to the tibial plateau and propagate parallel to the circumferential axis of the meniscus) (see Image 67). These tears are equidistant from the outer (peripheral) meniscal margin throughout their entire course. Longitudinal tears are more commonly traumatic in etiology and occur in younger and more physically active patients. They are also commonly found in patients who have also acutely torn their ACL. Such meniscal tears are located in the posterior horn of the LM, central to the popliteus tendon.⁹

In addition, these tears do not involve the free-edge (the inner part) of the meniscus on any image. They are often located in the middle or outer third of the meniscus (see Image 38) and usually begin in the posterior horn.¹⁵

Short tears, or those confined to the posterior horn, may be visible only on sagittal images. Longer tears propagate into the body of the meniscus. These are seen on both sagittal and coronal images.

Meniscal tears are either partial thickness (see Image 25, 29, 31, and 38) or full thickness (see Images 24, 38, 39, and 44). A partial thickness longitudinal tear contacts only the superior or inferior articular meniscal surface, but not both (see Images 21-22, 30, 38-39).

A full thickness vertical tear contacts both the superior and inferior articular meniscal surfaces, completely dividing the torn part of the meniscus into an inner and outer portion (see Images 19-20, 29, 38-39). Such tears can lead to the development of bucket-handle tears.⁹

Bucket-handle tears

These tears are displaced vertical longitudinal tears and usually involve the MM (see Images 38-39, 42, and 52). The separated central (inner) fragment, when viewed axially, resembles the handle of a bucket. The remaining larger peripheral portion of the meniscus resembles the bucket. These tears account for about 10% of all meniscal tears.^6,7,18

Central or anterior movement of the inner portion of a longitudinal tear results in a bucket-handle tear. The central fragment in a displaced bucket-handle tear may be partially displaced into the intercondylar notch, inferior and anterior to the PCL. Partial displacement is seen with shorter tears. The central fragment is often well visualized on coronal images and poorly visualized on sagittal images. Central fragments of bucket-handle tears of the posterior horn of the LM are often displaced anteriorly so that the torn and displaced fragment lies on top of the anterior horn of the LM. This occurs because the ACL prevents the meniscal fragment from completely migrating into the intercondylar notch. In this situation, the "height" of the anterior horn of the LM is almost twice its normal height. This is best seen on sagittal views.¹⁴

An absent bow-tie sign is helpful for diagnosis of bucket handle tears of the meniscal body (see Image 52). The normal body of the meniscus is 9-12 mm in width and should be seen on 2 consecutive sagittal images and, as described in normal MRI anatomy, has the shape of a bow tie. When a bucket handle tear is present, part of the free edge of the meniscus is missing. The inner portion of the meniscal body will be absent. Confirmation almost always occurs in the form of a displaced meniscal fragment that is visualized elsewhere in the knee joint.

When no displaced fragments are found, an absent bow-tie sign may be related to a normal but small meniscus. In this situation, both the medial and LM are small; bucket handle tears of both the medial and LM, occurring at the same time, are rare. Another cause of an absent bow-tie sign is a normal postoperative meniscus in patients who have undergone PM.

These injuries may be further classified as single, vertical longitudinal tears, displaced bucket-handle tears, broken bucket-handle tears, and double and triple vertical longitudinal bucket-handle tears. They are 3 times more frequent in the MM when compared with the LM and may be associated with ACL tears. Bucket-handle tears are commonly seen in young adults with a history of locking, extension block, or slipping of the joint due to displacement of the central fragment toward the intercondylar notch.

Radial tears (transverse tears)

These are vertical tears and propagate perpendicular to the main axis of the meniscus (see Images 10, 22-23, 32, and 68). These injuries are devastating because a full thickness tear destroys meniscal integrity, ie, the ability of the meniscus to distribute hoop stress. Hoop stress is the normal outward force generated in the meniscus in all directions as a result of weight bearing.

The force is distributed in the meniscus by collagen fibers, located around the circumference of the meniscus from apex to periphery. The collagen preserves the normal shape and integrity of the meniscus in weight bearing. Radial tears transect these fibers. The meniscus is normally attached to the tibia at the anterior and posterior ends. During weight bearing, the meniscocapsular attachments pull the meniscus outward. A radial tear occurring between the tibial attachment points causes the free, unattached edges of the meniscus at the point of the tear to temporarily pull outward, expanding the width of the tear, exposing a bare spot on the adjacent tibia and femur, allowing abnormal stresses on the unprotected articular cartilage and bony surfaces, and resulting in articular cartilage destruction and subsequent bone alteration, leading to accelerated degenerative disease.

A complete radial tear extends all the way through the meniscus from the apex to the periphery (see Image 23). When it involves the meniscal body, the meniscus is split into an anterior and posterior fragment (see Image 29). The middle third of the LM is a common location. This injury begins at the free edge (inner margin) and extends a variable distance toward the periphery.⁹

Small tears may be difficult to recognize on MRIs. Missed radial tears constitute a large proportion of errors made in image interpretation of meniscal pathology. The key feature of recognition is that they involve the free edge of the meniscus. Thus, the inner point of the meniscal triangle is absent or blunted on 1 or more images. Radial tears of the meniscal body are best seen on sagittal images (see Image 32). They disrupt the normal bow-tie configuration of the meniscus on 1 or more images. Radial tears of the anterior and posterior horns are best seen on coronal images (see Images 10, 22-23).

Like longitudinal tears, radial tears are commonly traumatic and occur in younger, more physically active patients . Tears in the MM usually occur in the posterior horn and are more common in older patients (see Images 10, 22-23). Small tears of 3 mm or shorter may be asymptomatic.

Tears located near the posterior horn of the LM are associated with ACL tears. They may be associated with more complex meniscal tears, such as vertical longitudinal tears or peripheral horizontal cleavage tears. They are devastating because the circumferential fibers are disrupted. The meniscus is prevented from developing the necessary hoop stress that normally helps dissipate forces across the knee.

It is common to see a radial tear as one component of a complex tear.

Oblique (parrot beak) tears or flap tears are a form of radial tears. They begin at the free (inner) edge, like other radial tears, but then curve into a longitudinal orientation, similar to longitudinal meniscal tears, as the tear extends toward the meniscal periphery (see Image 70). As the tear is traced on sequential images, it moves closer to the outer portion of the meniscus and then remains equidistant from the outer meniscal margin on subsequent images, as seen on longitudinal tears. Oblique tears are the most common meniscal tears.^15,16,17

A high prevalence of radial tears is present in postoperative patients who have had partial meniscectomy. Magee et al indicated a 32% prevalence when viewing 100 postoperative MRIs. This may be related to altered biomechanics of knee function after partial meniscus removal. Stresses may be redistributed that predispose to radial tears.

Horizontal tears

These tears are also called cleavage or fish-mouth tears. They divide the meniscal tear into a top (superior) portion and a bottom (inferior) portion (see Images 44-45, 49, 63, and 69-70). They usually begin on the undersurface of the meniscus (see Images 43 and 63).⁹

Although horizontal tears may appear to extend deep into the meniscal substance on MRIs, the tears, as seen at arthroscopy, may extend only a few mm into the meniscus from the point where the abnormal signal contacts the meniscal surface. When it extends to the periphery of the meniscus, the cleft between the fragments can allow the egress of joint fluid to the meniscosynovial border, where it may become trapped, forming a meniscal cyst (see Images 47 and 59). Most are degenerative, occurring in older patients with osteoarthritis.

Miscellaneous tears

Other tears shapes may be thought of as combinations of these 3 basic patterns (see Image 22). Multiple tears can present simultaneously in a meniscus, involving different portions of or the same region. A single meniscal tear, containing a combination of longitudinal, radial or horizontal cleavage planes, is called a complex tear (see Images 24 and 33). A common type of complex tear is composed of horizontal and radial components (see Image 22). Nearly all of these tears are degenerative in origin.^15,16

Fraying or fibrillation of the free edge of the meniscus is seen as an area of increased signal intensity at the apex of a normally shaped meniscus. If abnormal morphology (truncation and foreshortening) is present, a tear is likely. Fraying can occur anywhere along the meniscal surface (see Image 41).¹⁷

Indirect signs of a LM tear include an abnormal or absent superior popliteal-meniscal fascicle and the presence of pericapsular edema, posterolaterally, is associated with a tear to the posterior horn of the LM. However, caution is needed in this setting. Johnson and De Smet reported that the superior popliteal-meniscal fascicle was not seen in 2 of 66 patients with intact menisci. De Smet and Asinger showed that 2 of 13 patients with posterior pericapsular edema did not have LM tears.^24,49,50

Displaced meniscal tears

A fragment of a torn meniscus, partially attached to the meniscus proper and migrating to any position within the joint; usually the intercondylar notch anterior and parallel to the PCL (see Images 46-47), anteriorly toward the infrapatellar fat pad, on top of the anterior horn (see Images 48-49), backward to lie above, below or behind the posterior meniscal horn, or superomedial to the medial femoral epicondyle or inferomedial to the tibial plateau. Displaced meniscal fragments occur in 9-24% of meniscal tears. Any shape of a meniscal tear can result in a displaced fragment.⁵¹

MRI diagnosis depends on visualizing the torn, deficient meniscus or the displaced inner meniscal component.⁶

Displaced meniscal fragments are often clinically significant lesions requiring surgery because of pain and knee locking.

Types of displaced meniscal tears

Bucket-handle tears are the most common pattern, occurring in 10% of meniscal tears. They result from vertical, longitudinal, or oblique tears. These tears often involve the entire meniscus but isolated tears of the anterior horn, posterior horn, or (more commonly) the posterior horn and body. The most reliable sign is visualization of the displaced fragment. Typical locations of the displaced fragment include the intercondylar notch anterior and parallel to the PCL (double PCL sign) (see Images 46-47); and ventrally or horizontally juxtaposed to the anterior horn (see Images 48-49). An absent bow-tie sign indicates the meniscal body is absent. Any meniscal segment, however, can be involved.⁷

A flap tear is a short-segment horizontal meniscal tear with either superior or inferior displacement of the meniscal fragment (see Images 50-51). This type is less frequent; superior displacement is more common.

Horizontal tears become displaced either by the top or bottom portion flipping over to lie above or below the remainder of the meniscus or by sliding toward the inner part of the knee. These tears usually involve the MM.⁵¹

Inferomedial displaced tears from the MM are uncommon. When the displaced fragment extends inferior and medial to the tibial plateau, deep to the MCL, it may go unnoticed by the arthroscopist because the meniscus surface may appear intact (see Image 58).

Inward displacement of the free edge of an oblique tear results in a displaced parrot-beak tear and can precipitate mechanical symptoms, such as locking, catching and giving way. Displaced fragments can prevent closed reductions of knee dislocations. The fragment can completely separate from the rest of the meniscus to become a free fragment.⁴⁸

A meniscal body appearing unusually small should prompt a careful search for a displaced fragment.⁶

Signs and terminology used to describe the displaced meniscal fragment

Several signs may be present in the same patient.⁵²

An absent bow-tie sign may be identified on sagittal images and represents a torn meniscal body segment (see Image 52). When present, it is indicative of a free meniscal fragment in 97% of patients.^6,52

Bucket-handle tears with centrally displaced fragments into the intercondylar notch may also be present. A double-PCL sign is seen in 39% of patients, and the notch-fragment sign is seen in 51% of patients (see Image 63). In this setting, the fragment is adjacent to but not at the same level as the PCL on sagittal images. The fragment is slightly more medial in location. It originates from the MM. A disproportionate posterior horn sign is seen in 21% of patients. A larger central portion of the posterior meniscus, when compared with the smaller periphery of the same posterior meniscus, indicates a displaced meniscal fragment. The meniscal tear usually originates from the anterior horn.⁵²

Bucket-handle tears with an anteriorly displaced meniscal fragment can be observed (see Images 48-49). The flipped-meniscus sign is seen in 63% of patients (see Images 50-51). The criterion is a tear or nonvisualization of the posterior meniscus with the maximum height of the anterior horn of the involved meniscus greater than 8 mm (see Images 48-49 and 53).^45,52

Pseudohypertrophy of the anterior horn of a meniscus occurs when an anterior meniscal horn appears abnormally large (see Images 48-49 and 53). The meniscal body or posterior horn is unusually small. This indicates a portion of the torn body or posterior horn has flipped anteriorly and lies behind the anterior horn. The abnormal meniscal fragment and the adjacent normal meniscus are separated by joint fluid, which has increased signal intensity on T2-weighted images.

Differential diagnosis of displaced meniscal tears

The differential diagnosis includes the following: ligament of Humphry, loose bodies, osteophytes, and fracture fragments.

Stable versus unstable tears

The stability of tears is determined by a number of factors, including the length, location, and completeness of the tear. Probing the meniscal tear during arthroscopy is critical for determining stability.

A stable vertical longitudinal tear occurs when the central (inner) fragment of a meniscal tear cannot be displaced more than 3 mm from the intact meniscal periphery. Any meniscal tear with a displaced fragment is unstable.

Longitudinal tears that are relatively long are unstable; their length is assessed on multiple 3- to 4-mm sections in either plane and they extend through the full thickness of the meniscus or contain fluid on T2-weighted images.

Some meniscal tears do not show a free fragment at the time of MRI. Whenever, however, the inner margin of the tear can be displaced to a position where it can be entrapped between the rotating femur and tibia when probed at arthroscopy, it is considered an unstable tear.

Miscellaneous lesions

Meniscal contusion

Meniscal contusion occurs when the meniscus gets trapped between the tibia and femur, usually as a result of trauma. A contused meniscus demonstrates increased signal within its substance that might resemble a tear. The signal is indistinct and amorphous, rather than the sharp and discreet signal seen in a tear. Often, there is an adjacent bone contusion.⁶

Meniscocapsular separation

MCS is a tear of the periphery of the meniscus at the meniscosynovial junction (see Images 47, 57-59, and 66). The attachment of the MM is more commonly involved due to its firmer contact with the joint capsule. The site more frequently involved is the capsular attachment of the posterior horn of the MM at the tibia, at the meniscotibial (coronary) ligament. MCS is frequently associated with ligamentous injuries about the knee. They rarely propagate into the meniscal periphery, though they can be an extension of a full-thickness tear.

An abnormal superior meniscal fascicle is highly associated with a lateral meniscal tear, but the LM can rarely be normal in this setting. Such a finding should intensify the evaluation of the LM for possible tears. The presence of posterior pericapsular edema is another indirect, but nonspecific sign, that when coupled with an abnormal superior popliteal meniscal fascicle should alert the imager to the possibility of a lateral meniscal tear.^24,53

MCS of the LM can occur at the superior or inferior popliteal-meniscal fascicle adjacent to the popliteus tendon (see Images 51 and 60). Disruption of the fascicular attachments between the popliteus tendon and the LM can result in gross instability and is associated with a tear of the posterior horn of the LM.²⁴

Spontaneous healing is common because of the rich blood supply in the meniscal periphery.

MRI findings include an increased distance between the periphery of the meniscus and the tibia and fluid between the MCL and the medical meniscus.⁵⁴

Meniscal cysts

Meniscal cysts occur more frequently in the medial compartment than elsewhere because meniscal tears are more common in the MM. This observation contrasts with previous reports, indicating that they are more common in the lateral compartment. There are 3 reasons for this discrepancy⁵⁵ :

First, reports indicating a lateral compartment preference were written before the use of MRI. MRI is more sensitive than PE in the detection of small meniscal cysts. There is a relatively scant amount of fatty soft tissue on the lateral aspect of the knee compared with the medial aspect. This may explain why more lateral cysts were reported because, in that region, cysts are more apt to appear as a palpable mass.

Second, other diagnostic methods, such as knee arthrography, lead to underestimations of the true incidence of meniscal cysts. Diagnosing meniscal cysts with arthrography requires the extravasation of contrast material through the meniscal tear and cyst, but this does not always happen.

Third, meniscal cysts are occasionally overlooked at arthroscopy because visualization of the posterior horn of the MM can be difficult. Thus, cysts in the posterior medial compartment were underdiagnosed.

Medial parameniscal cysts are more symptomatic that other conditions because of their location adjacent to the MCL (see Images 47 and 59). The incidence is between 1-2% and 7-8% and these cysts most commonly occur in men aged 20-40 years. Medial meniscal cysts are most commonly adjacent to the posterior horn (see Images 54-56).⁹

LM cysts are most commonly located adjacent to the anterior horn or body. MM cysts are twice as common as LM cysts, and MM tears are twice as common as LM tears. The tears are most commonly formed when horizontal tears extend to the meniscal periphery, allowing joint fluid to escape into the parameniscal soft tissue. The fluid subsequently encapsulates and becomes symptomatic due to mass effect. Occasionally, the cyst can be confined to the meniscus. This is referred to as an intrameniscal cyst.

It is important to recognize the association between meniscal cysts and meniscal tears. If the cyst is resected without addressing the tear, the cyst may recur.

The MRI appearance is a fluid-filled region adjacent to a horizontal meniscal tear.

Meniscal ossicles

Meniscal ossicles are rare and often mistaken for intra-articular loose bodies.

These appear as circumscribed ossification with fatty bone marrow in the center. The fatty marrow helps distinguish this entity from intraarticular loose bodies. Meniscal ossicles are usually located in the posterior horn of the MM.⁵⁶

Chondrocalcinosis (see Image 64)

Chondrocalcinosis is calcification of the menisci, synovium, and/or articular cartilage due to the deposition of calcium pyrophosphate dihydrate crystals, dicalcium phosphate dihydrate, calcium hydroxyapatite, or any combination of these crystals. The condition is seen in conditions such as gout, degenerative disease, hemochromatosis, crystal deposition disease, and hypercalcemia. These intra-articular crystals are weakly positive fringent monourate crystals under polarized light.

The prevalence is 5-14% and increases with age. A multitude of conditions and mechanisms are implicated in the pathogenesis.

Intrameniscal calcium can demonstrate increased signal intensity, which can mimic a meniscal tear on T1-weighted, intermediate PD-weighted, or T2-weighted images. Correlation with plain radiographs, especially in patients with the associated conditions, reduces the incidence of error.⁹

Diagnostic Accuracy

Diagnostic accuracy

Isolated meniscal tears

When only prospective, double-blind studies of at least 200 menisci are considered, the sensitivity for detecting MM tears is 86-96% with a specificity of 84-94%. For LM tears, the sensitivity decreases to 68-86%, and the specificity is 92-98%. The NPV is 91%, indicating that if a properly obtained MRI of the knee does not show a meniscal tear and if the patient's symptoms necessitate surgery, the likelihood of finding a meniscal tear at arthroscopy is less than 10%.^14,21,24,57

If the abnormal meniscal signal contacts the articular surface on a single section in a single imaging plane, the accuracy of diagnosing tears in the MM is 55%, and the accuracy for tears in the LM is 30%. The accuracy rate approaches 100% if the meniscal tear is seen on at least 2 consecutive images (see Images 24 and 33).²⁴

FSE images are less sensitive than conventional spin-echo images for diagnosing meniscal tears, even when the imaging parameters are optimized. Two prospective studies in a large number of menisci examined with FSE imaging showed a sensitivity of 83% for meniscal tears; this is lower than most studies using conventional spin-echo techniques.

With 3D fast MRI, there is 95% concurrence between MRI and arthroscopy in detecting meniscal tears and 100% concurrence for detecting meniscal degeneration.

MRI is accurate for predicting repairable meniscal tears and sensitive for determining nonrepairable tears. One study showed 89% (103 of 116) of meniscal tears were categorized the same at MRI and arthroscopy with respect to repairable vs nonrepairable groups. The same study, however, showed that MRI had a variable accuracy for predicting meniscal tear configuration found at arthroscopy.

Errors in diagnosing meniscal tears can be classified as unavoidable (discordance between MRI and arthroscopy), equivocal (interobserver differences in interpretation), or interpretive (normal MRI variants mistaken for meniscal tears)

Accuracy for diagnosing meniscal tears in adolescents is similar to that in adults. One group found a 92% sensitivity and 87% specificity for the medial meniscus and a 93% sensitivity and 95% specificity for the LM.⁵⁸

False-negative errors in diagnosing LM tears

False-negative errors accounted for the largest number of errors in several studies and usually involve the body or posterior horn. The tears are usually small and stable and do not require PM or repair. They often go undiagnosed at arthroscopy unless the meniscus is probed or compressed.^14,24,57

False-negative diagnoses outnumber false-positive diagnoses by about 3 to 1. False-negative tears of the posterior horn of the LM are associated with ACL tears. The reason may be related to the anatomy of the posterior horn of the LM. It has a shorter radius of curvature than the MM. The posterior horn is oriented obliquely upward. Such obliquity could result in volume averaging that could blur the connection of intrameniscal signal to the articular surface. Of these results, 40% were unavoidable, even when they were retrospectively evaluated.^14,24,57

False-positive tears of the LM

Most of the findings involve vertical or horizontal degenerative tears in the posterior horn. There has been a gradual decrease in the number of false-positive diagnoses as a result of better recognition of normal anatomy and postsurgical variations and improved arthroscopic techniques.

The presence of either meniscal distortion or abnormal signal contacting the joint surface on only one view is not specific for a LM tear as confirmed by arthroscopy because healed tears can have an MR appearance similar to that of an acute tear.⁵³

False-negative tears of the MM

Most of these results are in the outer half of the posterior horn or the inner third of the meniscal body. Two thirds of these tears are small, stable, and treated conservatively. Like the LM, most are not diagnosed directly at arthroscopy; rather, the diagnosis is made on the basis of the behavior of the meniscus to probing or compression.

Closed or confined intrasubstance tears may be misdiagnosed on MRI as grade 2 signal-intensity changes. These tears may also be missed at arthroscopy if the meniscal surface is not probed. This may contribute to a higher rate of false-negative diagnoses.^14,24,57

False-positive tears of the MM

These are more common than false-negative diagnoses of MM tears and almost always involve the posterior horn.^14,24,57

Causes include the following: (1) chondrocalcinosis in the meniscus mimicking a tear and (2) healed meniscal tears that often continue to show increased signal intensity that contacts a meniscal surface.

Regarding limitations in the criterion standard, ie, arthroscopy, most MM tears present on MRIs but not identified at arthroscopy. These tears occur on the undersurface of the posterior horn (see Image 34). This location is an arthroscopic blind spot. Although additional viewing portals and angled arthroscopies may be used, there are still some areas of the MM that cannot be visualized. False-positive findings include vertical, degenerative oblique, horizontal, and meniscocapsular tears. Justice and Quinn reported that 20% of false-positive tears were described as frayed on arthroscopy and not frankly torn, as seen on MRIs.

Causes of false-positive MRIs

Causes of false-positive MRI results include the following:

• Truncation artifacts
• Vacuum-joint phenomenon
• Magic-angle phenomenon
• Increased conspicuity of intrameniscal signal intensity (grade 2 signal intensity) on GRE images
• Misinterpretation of normal anatomic structures and anatomic variants
• Errors related to previous meniscectomy
• Errors related to misinterpreting changes after meniscal repair as evidence of a new tear
• Tears missed on arthroscopy
• Intra-articular loose bodies, which can obscure the meniscal margins and mimic a tear.
• Intrameniscal signal intensity in patients with osteoarthritis who later went on to develop grade 3 signal-intensity abnormalities in the same region (The initial MRI changes are postulated to represent early separation of the meniscus and may be an early and symptomatic precursor of a horizontal cleavage tear.

Causes of variation between MRI and arthroscopy

Causes of variation between the accuracy rate of MRI and arthroscopy in diagnosing meniscal tears are varied.

Radiologists have different learning curves for interpreting knee MRIs, and arthroscopists have different levels of experience. Cassells has noted that "some surgeons learn arthroscopy slowly or not at all, and some are not very capable in this (diagnostic arthroscopy) respect." The accuracy of arthroscopy in diagnosing meniscal tears has been reported to be between 68-98%, depending on the experience of the examiner and the location of the tear. Nonetheless, surgeons generally regard arthroscopy as a standard of reference.

Differences in descriptive terminology concerning meniscal damage are another factor. To one arthroscopist, a free-edge abnormality may be described as fraying and, to a radiologist, the same abnormality may be diagnosed as a small tear. Different types of MRI equipment, surface coils, and field strengths cause variability.

Arthroscopy may not detect intrasubstance degenerative cleavage tears. The medial femoral condyle might interfere with arthroscopic visualization of the posterior horn of the MM (see Image 34). The periphery of the meniscus at the meniscocapsular junction is difficult to image on MRIs. Most false-positive MRI findings occur in the posterior horns of both menisci. These regions are acknowledged to be the most difficult for arthroscopists to examine. The validity of the assumption that arthroscopy is the most reliable method for assessing the outer third of the posterior horns and the inner third of the body of both menisci has been questioned. The inferior surface of the MM and, to a lesser extent, of the LM, cannot be routinely seen arthroscopically.

NPV of MRI for diagnosing meniscal tears

The NPV provides an estimate of the patient's likelihood of having no meniscal tear at arthroscopy when MRI indicates no tear. NPV is 100% for the MM and 91% for the LM. Because of the high NPV, a negative MRI can be useful to exclude patients from unnecessary arthroscopy. Screening MRI can be performed in patients with clinically suspected knee injuries before a diagnostic arthroscopy is considered. If MRI results are normal, conservative treatment can be instituted. Arthroscopy can be reserved for patients with persistent symptoms in spite of treatment.²²

The prevalence of meniscal tears in asymptomatic patients increases with age. One study showed an incidence of 13% in patients younger than 35 years and 36% in patients older than 45 years.

The sensitivity of MRI in detecting displaced meniscal tears depends on the size of the displaced meniscal fragment. When less than one third of the meniscus is torn, the displaced fragment will most likely not be seen on MRIs. Conversely, when the displaced fragment involves more than two thirds of the meniscus, it will be identified.

Regarding the accuracy of MRI of the menisci in adolescents, Major and Beard found a sensitivity and specificity of 92% for MM tears and a 93% sensitivity and 95% specificity for LM tears.⁵⁸

Isointensity in the meniscus relative to fluid on T2-weighted MRIs, has a sensitivity of 60% and specificity of 92% for diagnosing new or recurrent meniscal tears. The accuracy of MR arthrography in diagnosing recurrent meniscal tears is 88%; that of routine MRI is 66%. The greater the amount of meniscus removed, the higher the accuracy of MR arthrography and the lower the accuracy of routine MRI. In patients with minimal meniscal resection, the accuracy of both techniques was 89%. When only a small meniscal remnant is present, the accuracy of MR arthrography improved further to 100%, while that of routine MRI decreased to 50%.⁵⁹

Regarding sensitivity, specificity, and accuracy for diagnosing meniscal tears in the presence of chondrocalcinosis, for the LM, sensitivity is 78%, specificity is 71%, and accuracy is 78%. For the MM, sensitivity is 89%, specificity is 72%, and accuracy is 81%.⁶⁰

Using 4 criteria, VandeBerg reported sensitivities of 18-54% and specificities of 92-100% in diagnosing unstable meniscal tears. The criteria were the following: (1) presence of a displaced meniscal fragment, (2) visibility on more than 3 coronal sections of 3-mm thickness and 2 sagittal sections of 4-mm thickness, (3) more than 1 orientation plane or more than 1 pattern (contour irregularity, peripheral separation, or tear), and (4) intrameniscal high signal intensity on T2-weighted spin-echo images.⁶¹

Positive predictive values (PPVs) were 92-100%. NPVs ranged between 39% and 52%. Regarding bucket-handle tears, sensitivity has been reported as low as 44-64%. Sensitivity of the absent bow-tie sign has been reported to be as high as 97% and as low as 77%.⁶²

For meniscal tears occurring with ACL tears, the accuracy of diagnosing meniscal tears with MRI decreases, but there is a greater decrease with clinical diagnosis. The sensitivity of MRI for diagnosing MM tears is between 84% and 88%. For the LM, it is between 68% and 83%.^24,62

Regarding reasons for decreased sensitivity with MRI, there is a different distribution of meniscal tears between knees with intact and torn ACLs. With ACL tears, meniscal tears are often in the periphery of the posterior horn of the LM. This is a difficult region to evaluate arthroscopically. Fortunately, meniscal tears in this region do not often require surgical treatment. Finding a severe injury like an ACL tear may distract the radiologist from identifying more subtle, but equally important meniscal injuries. Subluxation of the knee from combined ligament tears may result in unusual anatomic appearance of the menisci, distorting their appearance, making recognition of meniscal tears more difficult. Patients with more severe injuries, such as concomitant ACL tears and meniscal tears, are less likely to remain motionless during MRI examinations because of pain. Patient motion results in poorer diagnostic images.

For meniscal tears with more severe ligamentous injuries, the accuracy is further decreased. One study showed that the sensitivity for diagnosing MM tears is 57% when 2 or more knee ligaments are torn. The sensitivity for diagnosing LM tears is 57%. With MCS, Rubin et al and Pfirrmann indicated that none of the MRI findings correlated with the arthroscopic results. The PPV for medial MCS was only 9%, and that for the lateral MCS was 13%. One reason is that many MCS injuries heal by the time arthroscopy is performed as a result of the rich vascularity of this region. Another reason is that such injuries may not be visible arthroscopically. Saw et al, however, found excellent correlation between MRI and arthroscopy.¹⁵

For DM tears, MRI does not have a high PPV for diagnosing meniscal tears (57%) when the standard criterion of abnormal meniscal shape is applied.

Regarding the predictability of MRI in determining meniscal tear reparability: MRI is only moderately reliable in predicting the reparability of a meniscal tear. Routine use of MRI for determining this is not indicated. Accuracy rates vary between 69% and 74%. The type of meniscal tear is an important parameter, affecting tear reparability. The accuracy of MRI in determining reparability according to tear type varies between 14% and 64% in the study by Mataya et al. The specificity in predicting reparability is between 74% and 97%. The sensitivity is 29%. The PPV is 50%, and the NPV is 80%.⁶³

Concerning the predictability of MRI in determining meniscectomy, values are as follows: sensitivity, 68%; specificity, 75%; PPV, 90%; and NPV, 43%. The total false-positive rate for diagnosing meniscal tears is about 1.5%. The total false-negative rate for diagnosing meniscal tears is about 4.8%, which compares with a false-negative rate of 2-13% for arthrography and 5-10% for arthroscopy. The accuracy for MRI diagnosis of meniscal tears is nearly 90%.^64,65

Pitfalls

Effect of TE and the magic-angle phenomenon

Meniscal tears become less visible when TE values are longer than 16 to 20 ms. Using TEs of shorter than 16 ms may result in increased signal within the normal meniscus. This signal intensity is not in contact with the meniscal surface.^{9,17,20,30,56,57,66}

Short TEs needed for meniscal imaging may result in artificially increased signal intensity in the meniscus because of the magic-angle phenomenon. This is most common in the posterior medial horn of the LM where the main magnetic field (b ₀) is orientated along the head-to-foot axis of the patient (typical for most superconducting, high field-strength systems).

The normal posterior horn of the LM slopes obliquely upward from lateral to medial, as it ascends from the lateral tibial plateau to its insertion on the posterior tibial eminence. On coronal images, it often achieves an angle of 55° relative to the long axis of the leg. This is the angle where the magic-angle phenomenon occurs when short TEs are used. This orientation leads to shortening of the apparent T1 relaxation time, resulting in an increase in signal intensity within the posterior horn, which may simulate a tear or grade 2 signal-intensity change.

This finding can be differentiated from true meniscal tears by increasing the TE; magic-angle findings disappear. True meniscal tears remain visualized. This finding can also be eliminated by imaging the knee in slight abduction, which alters the orientation of the posterior horn of the LM in relation to Bo.

Change in the normal shape of the meniscus

Any change in the normal meniscal shape, in the absence of prior surgery, indicates a tear, with 2 exceptions: (1) the DM, and (2) buckling of the MM producing a wavy appearance. This is similar to a phenomenon called meniscal flounce seen occasionally during arthroscopy. This last finding is normal. (See Images 22-33 , 38-39 , 42 , 48-51 , 53 , and 60-62.)

Effects on GRE images

On GRE images, high signal intensity is present within the normal meniscus. Caution is needed to avoid using a window for which the images are too bright because this can lead to overdiagnosing the presence of meniscal tears. GRE images are vulnerable to susceptibility artifacts occurring in the vicinity of any ferromagnetic substance (microscopic metal shavings from prior surgery) or gas occurring during a normal vacuum phenomenon in the knee.^{9,17,20,30,56,57}

Mimics

Entities that may mimic meniscal tears include the origin of the meniscofemoral ligaments (see Image 9) from the posterior horn of the LM, the popliteus tendon passing next to the posterior-lateral corner of the LM (see Images 5-6 and 34), and the attachment points of the intermeniscal ligament to the anterior meniscal horns (see Image 43). These errors can be avoided by following the anatomic structures on subsequent images. The obliquely orientated meniscomeniscal ligament may be confused with a meniscal fragment in the intercondylar notch region. Careful attention to axial and coronal images will avoid this pitfall.

Small radial tears may be difficult to diagnose; they may be seen in only 1 plane. Radial tears of the posterior meniscal roots can be overlooked. On sagittal images, distortion of the usual triangular shape of the posterior meniscus may be visible only on the image adjacent to the PCL. A torn ACL, loose bodies, an intercondylar osteophyte, or prominent meniscofemoral ligament may mimic a displaced meniscal fragment.⁴⁶

Regarding grade 2 versus grade 3 signal-intensity change in the meniscus, determining whether the signal intensity in the meniscus extends to the articular surface can be difficult. Evaluating for any change in the morphology of the meniscus at this site may be helpful. If the meniscus is distorted or abnormally shaped, the signal intensity is likely grade 3.

The transverse intermeniscal ligament and surrounding fat is another mimic (see Images 43-44). In 30% of cases, the fat surrounding the transverse intermeniscal ligament may mimic a meniscal tear. On sagittal images, the ligament crosses between the tibial attachment of the ACL and the Hoffa infrapatellar fat pad. Axial 3D Fourier-transformed sections of 1 mm or thinner may demonstrate the normal course of the ligament if a meniscal tear is still uncertain after standard MRI. Tears of the anterior horn of the LM are rare. The central anterior ligamentous attachment of the anterior horn of the LM is rhomboid and normally directed upward on sagittal images. It frequently contains increased internal signal intensity. On serial sagittal images, the ligament may be traced from the anterior horn of the LM to the anterior horn of the MM.

A prominent vessel from the lateral geniculate artery rarely produces a pseudotear adjacent to the anterior horn of the LM. The lateral inferior geniculate artery, arising from the popliteal artery at the level of the knee joint, courses anteriorly around the lateral aspect of the knee and participates in the rich collateral circulation in this area. It is closely applied to the LM through its course. It lies within a region of periarticular fat between the meniscus and the LCL. When the artery lies adjacent to the anterior horn of the LM, it can appear as a tear on sagittal images. The artery can be traced on adjacent sagittal images so as not to be confused with a tear.

The junctional region between the posterior horn of the MM and the joint capsule contains peripheral vessels. The signal intensity here can mimic the appearance of a meniscocapsular detachment. Edema from MCS is more diffuse and ill defined.⁶⁷

It may be difficult to differentiate between meniscal fraying and meniscal tearing.

The semimembranosus tendon can be mistaken for a displaced meniscus (see Image 37).

The popliteus tendon sheath may be mistaken for a tear in the posterior horn of the LM (see Images 5-6 and 34). True tears of the periphery of the LM in this region usually have a different orientation than that of the tendon sheath (see Image 65). When in doubt, this structure can be traced on subsequent sagittal images and normalcy can be confirmed by using PD- and T2-weighted axial images.

After lateral meniscectomy, the low-signal popliteus tendon may be mistaken for a retained posterior horn meniscal remnant.

The normal concave edge of the periphery of the meniscus may demonstrate grade 2 signal on peripheral sagittal images through the body of the meniscus. This is more commonly seen in the MM and is caused by partial volume averaging of fat and the neurovascular structures in the soft tissues adjacent to the concavity of the meniscus. This is seen in up to 20% of MMs and 6% of LMs. Thin-section coronal images show a normal meniscus and its normal concave margin.

A speckled pattern of increased signal within the anterior horn of the LM near its central attachment on sagittal images is a normal finding. It represents the intimate relationship between the meniscus and the fibers of the ACL at its insertion. The medial most portion anterior horn of the MM may also exhibit this appearance (see Image 35).⁴⁶

The meniscal insertion of the meniscofemoral ligament may mimic a vertical tear through the posterior horn of the LM. This appearance is caused by the interposition of fat between the ligament and its meniscal attachment. The fat signal intensity disappears on fat-suppressed images. This pseudotear extends obliquely from the superior meniscal surface and is directed posteroinferiorly toward the inferior meniscal surface. The pseudotear less commonly appears as a vertical line parallel with the meniscal periphery.

Distention of a bursa between the deep and superficial fibers of the MCL  or distention of the deep portion of the semimembranosus MCL bursa can mimic fluid between the joint capsule and the meniscus.^{9,17,20,30,56,57}

Vacuum phenomena

Vacuum phenomena cause magnetic susceptibility artifacts. Intra-articular gas can normally occur when traction is applied to the joint. Gas, normally present in tissues, comes out of solution as a result of reduced pressure. This leads to magnetic susceptibility artifacts, which appear as low signal intensity on T1-weighted MRIs or as a blooming artifact on GRE images. This gas may be mistaken for a meniscal tear. Gas collecting in the medial joint space between the articular cartilage of the femur and tibia can appear as a triangular signal void that simulates meniscal tears or an abnormal volume of meniscal tissue resembling a DM, or a displaced torn meniscal fragment.

Magnetic susceptibility is the ratio of the intensity of magnetization of the applied magnetic field. Bone, air, gas, and ferromagnetic materials have a susceptibility markedly different from that of soft tissue. Ferromagnetic materials have the greatest difference; bone, air, and gas have a lower difference. Artifacts occur when there is a great and abrupt difference between the magnetic susceptibility of adjacent tissues. Such differences are caused by inhomogeneities in the magnetic field resulting from intrinsic magnetic field gradients generated in such a region. These gradients result in the misregistration of spatial information in the frequency encoding direction because pixel information is associated with the wrong frequency. The result is areas of artifactually high and low signal intensity.

These artifacts are also seen with GRE imaging because the spin dephasing is not fully refocused by means of gradient reversal techniques. They are worse with low readout gradients, as seen in a large field of view. The artifact is maximal when the region of varying susceptibility is perpendicular to the main magnetic field.^{9,17,20,30,56,57}

Pseudo–bucket handle tears

Separate portions of the posterior horn of the LM may be mistaken for a bucket handle tear on coronal images. Normal meniscal signal is present on the sagittal images.

MCL bursae

The bursa of the MCL is located between the periphery of the body of the MM and the MCL. Fluid within the bursa may be mistaken for a peripheral meniscocapsular tear on T2W1.

Aliasing

Aliasing occurs when all the anatomical structures of a desired body segment are not included in the field of view of the imaging section. This can be eliminated by increasing the field of view, oversampling by applying saturation pulses outside the region, or using surface coils.

Chemical-shift artifacts

Chemical-shift artifacts are found at the interface between fat and other structures. These increase with field strength and are more pronounced in images acquired with a narrow bandwidth.

Motion artifacts

Motion artifacts arise from flow in pulsating blood vessels, and reduced levels of patient cooperation, which is sometimes related to pain and apprehension.

Inhomogeneous fat saturation

Inhomogeneous fat saturation is caused by a nonuniform magnetic field. It is more pronounced on frequency-selective, T2-weighted, spin-echo images. It is also found in the presence of metallic artifacts.

Truncation artifacts

Truncation artifacts are lines that traverse the menisci that can be confused with tears and are problematic when a 128 X 128 matrix is used with a 128 phase-encoded axis orientated in the superior to inferior direction.

Truncation artifacts result from the use of Fourier transformation methods used to construct MRIs of high-contrast boundaries, such as the interface between the articular cartilage and the meniscus. When an attempt to recreate the boundary is made, an inaccurate representation is produced. The image exhibits a series of overshoots and undershoots (a sine-integral function), occurring in alternate pixels as one moves away from the boundary. This appears as lines of increased or decreased signal intensity. The size of the artifact is the difference between the degree of over- and undershooting. Such a line of high signal within the low signal of the meniscus may mimic a tear. One manifestation is the ringing artifact (Gibbs phenomenon), which occurs near highly contrasting interfaces.

These artifacts are subtle, uniform in thickness, and run parallel to the meniscal surface, 2 pixels away from the articular cartilage. The lines extend beyond the boundary of the meniscus and can be attenuated if the phase-encoded axis in a 128 X 256 matrix is orientated anterior to posterior instead of superior to inferior. Alternatively, they can be affected by data extrapolation algorithms or image filtering. The most effective way of diminishing the artifact is to use an acquisition matrix of 192 X 256 or 256 X 256.^{9,17,20,30,56,57}

Chondrocalcinosis

Calcification of the menisci and/or articular cartilage due to the deposition of calcium pyrophosphate dihydrate crystal, dicalcium phosphate dihydrate, calcium hydroxyapatite, or any combination of these substances (see Image 64). Calcium pyrophosphate dihydrate crystal is the most common crystalline arthropathy. These intraarticular crystals are weakly positive birefringent monourate crystals under polarized light microscopy.^{9,17,20,30,56,57}

The prevalence is 5-14% and increases with age. A multitude of conditions and mechanisms are implicated in the pathogenesis. Intrameniscal calcium can demonstrate increased signal intensity that can mimic a meniscal tear on T1-, PD-, and T2-weighted MRIs. No single theory has satisfactorily explained the cause of the high signal intensity of calcium on MRIs.^{9,17,20,30,56,57}

Correlation with plain radiographic results reduces the incidence of this error.

Treatment

Background

The intact meniscus performs the critical function of load distribution across the knee joint. Load distribution is radically altered after total meniscectomy (TM). Much of the load-bearing role of the meniscus is concentrated in the circumferentially orientated collagen fibers in the meniscal periphery. As long as they are intact, the meniscus can develop hoop stress to ease the load of the femur on the tibia, even if a portion of the central inner margin of the meniscus is absent, torn, or nonfunctional.^2,12,15,68

Before arthroscopy, TM was the only treatment for a torn meniscus. It was soon recognized that this led to remodeling of the distal femur and proximal tibial surfaces, leading to premature or accelerated osteoarthritis and consisting of ridge formation from the margin of the femoral condyle over the site of the removed meniscus, joint space narrowing, and flattening of the femoral condyle. These findings have been referred to as the 3 classic Fairbanks changes and are indicative of the high level of unsatisfactory long-term outcomes in patients undergoing this procedure.^2,12,15,68

Roos et al showed a relative risk of 14 for developing radiographic arthritis in patients who undergo TM, compared with control subjects. They proposed that such patients may develop premature osteoarthritis 10-20 years earlier than patients with primary osteoarthritis.⁶⁸

The degree of degenerative change is directly proportional to the amount of meniscus removed.

Sequelae

Total meniscectomy

The first critical function lost is that of load bearing. Without the meniscus, the size of the contact area between the tibia and femur is reduced by one third to one half. This loss results in a greater load transmitted to a smaller area and in higher tensile stresses in the collagenous compounds of the articular cartilage and abnormal stress on the adjacent subcortical bone trabeculae; this appears on radiographs as juxta-articular bone sclerosis (see Image 65). Stiffness increases in the subchondral bone and plays a role in subsequent articular cartilage damage. The shock absorbing function decreases by about 20%. The contact stress on the articular cartilage increases in direct proportion to the amount of meniscus removed.

Joint instability develops, even with intact ligaments. The problem is accelerated in ACL-deficient knees. Meniscectomy converts the normal gliding motion of the knee into a sliding motion causing friction and wear in the articular surface. Since gliding motion is provided mainly by the periphery of the meniscus, preservation of this zone is important.¹⁵

After TM, trabecular bone density increases in the proximal tibia. This is more marked than after PM. The risk of developing radiographic signs of osteoarthritis after TM is high. One study showed joint space narrowing in 71% of patients and joint-space narrowing accompanied by osteophyte formation in 48% at 21 years after TM. The relative risk for the presence of the more advanced radiographic changes was 14.⁶⁸

The rate of symptomatic complaints related to TM is high and increases with time. At 4.5 years after TM, the rate of symptomatic complaints is 53%; after 14.5 years, it had increased to 67%. The incidence of knee instability was 10% after 4.5 years following TM and increased to 36% at 14.5 years. About 46% of patients give up or reduce their sports activity at 14.5 years after TM, and 6.5% change occupations at 4.5 years after TM. The causes are knee symptoms that follow TM.

There is a high rate of symptomatic complaints. Jorgenson et al found that patients have a high incidence of knee pain while climbing stairs shortly after TM. The occurrence of pain on climbing stairs, knee swelling, or objective instability early after TM is an indicator of later symptomatic worsening.

Lateral meniscectomy leads to a higher incidence of osteoarthritis than with medial meniscectomy. This may be related to a higher portion of the lateral tibial plateau normally covered and protected by the lateral meniscus, than on the medial side.

In conclusion, the risk of premature osteoarthritis and joint instability has altered the treatment of meniscal tears from TM to PM, when repair cannot be accomplished.

Partial meniscectomy

Recognition of the importance of the meniscus in maintaining proper knee function and in preventing accelerated osteoarthritis has led to a preference of PM over TM. Lateral meniscectomy leads to a higher incidence of osteoarthritis than medial meniscectomy although no compartment is spared of this complication (see Image 62). This outcome may be related to a larger proportion of the lateral tibial plateau normally covered and protected by the LM.

By removing only a portion of the damaged meniscus, the residual meniscus may help to retain part of the critical weight-bearing function. Symptomatic radial tears can be treated by trimming the margins of the tear.⁶⁹

After PM for bucket-handle tears, Tapper reported good-to-excellent results, as defined by minimal to no symptoms or limitations of activity at 10-13 years after surgery. In another study, there was a high degree of patient satisfaction after PM. About 98% indicated their symptoms improved, and 82% returned to athletic participation.

The incidence of postoperative morbidity, hospital stays, and time on crutches is decreased with PM when compared to TM. Also, the incidence is decreased in patients older than 45 years, as compared with PM.⁶⁹

PM is used to treat complex tears, degenerative tears and large radial flap tears. Avascular tears (those occurring in the inner two thirds of the meniscus) and tears associated with unstable ACL-deficient knees in patients older than 40 years are frequently treated by PM. Parrot-beak or flap tears are irreparable because they involve the avascular inner edge of the meniscus. Treatment is PM with transection of the meniscus through the base and contouring the remaining margins to form a stable rim. PM is used for longitudinal, vertical, or bucket-handle tears for which repair is contraindicated.

Any abrupt changes in the contour of the remaining meniscus must be removed so the meniscus does not catch between the femoral condyle and tibial plateau during weight bearing. Such actions propagate the tear.

Although it may be impossible to make the remaining meniscus perfectly smooth, normal weight-bearing forces tend to smooth out the remaining meniscus.⁷⁰

Meniscal repair

All attempts are made to preserve the shock-absorbing and local distribution functions of the meniscus. Complications such as accelerated osteoarthritis still result from PM. Because of the importance of the meniscus, therapy has focused on saving the injured meniscus, especially the LM because of the increased load transmission through it. Removal of 16-34% of the meniscus increases joint surface contact of the tibia and femur by 350%. Although PM has decreased the risk of accelerated osteoarthritis, significant risk still remains.⁶⁹

Factors affecting treatment decisions

When the possibility of meniscal repair is considered, the following considerations must be evaluated: (1) chronicity of the tear; (2) associated injuries, especially ACL and PCL tears; (3) extent and type of tear; and (4) location of the tear.^{3,5,12,22,71,72,73}

Meniscal repairs are reported to heal better when performed closer to the time of injury. Better meniscal healing has been reported when the repair is performed 8-19 weeks after injury.

If repair to an ACL tear is done at the same time, the outcome of the meniscal repair improves, possibly related to the presence of serum-derived growth factors, present in the hemarthrosis. Cooper, Arnoczky, and Warren, in one article, showed on a 5% failure rate with simultaneous ACL repair, compared with 40% failure rate in isolated meniscal tears.⁵

The peripheral blood supply of the meniscus is an important factor in healing. If an incision is made in a meniscus and if no communication is present between the incision and the synovial membrane or meniscal periphery, no healing occurs at 2-week to 3-month follow-up. If the incision communicates with these regions, there is evidence of tissue ingrowth and healing. Tears in the avascular portion of the meniscus can heal if they communicate with the meniscal periphery, synovium or perimeniscal capillary plexus.

Longitudinal tears of the MM in the setting of acute ACL tears need to be repaired. The MM is subject to higher stresses than the LM. Such tears have a higher propensity to propagate over time. Many meniscal tears can be ignored unless the surgeon is convinced that they are causing the patient's symptoms or that a risk of tear propagation exists.

Meniscal repair is associated with a longer recovery period and a higher frequency of repeat surgery than PM. Patient satisfaction is also lower than with PM.

Partial-thickness horizontal tears, shallow radial tears of 5 mm or less, and short vertical or oblique longitudinal tears, can be treated conservatively, especially when the knee ligaments are intact or have been successfully repaired. These vertical or oblique longitudinal tears are usually shorter than 8 mm and involve 50% of the thickness of the meniscus or less.^{3,5,12,22,71,72,73}

MCS can be treated conservatively with casting.

Laboratory studies have shown that torn menisci can function biomechanically if the peripheral circumferential fibers remain intact. The tear itself does not predispose to accelerated osteoarthritis. Sometimes, these tears may be simply rasped to promote healing. Rasping provokes a neovascular response creating a blood supply.

Incidental meniscal tears have been found in between 5.6% and 16% of asymptomatic patients. LM tears are more common in this group, as 30% of asymptomatic volunteers in one study had meniscal abnormalities consisting of a linear area of Grade 2 signal. There is an increased incidence with age.⁷⁴

Many types of meniscal lesions, especially degenerative tears, do not result in secondary damage to the underlying articular cartilage, so "when in doubt, leave it in."

The relatively high incidence of abnormal findings in asymptomatic patients underscores the danger of relying on a diagnostic test without careful consideration of clinical signs and symptoms.⁵⁶ Complete radial tears disrupting all fibers in the periphery of the meniscus are debilitating and render the meniscus nonfunctional because the circumferential collagen bundles are destroyed. Removing the torn meniscal fragment does not change prognosis, but this is done only if there is a risk of the torn segment becoming entrapped within the joint, resulting in knee-locking. It is doubtful that such a meniscus will ever regain the functions of providing hoop stress to the meniscus, even if such a tear heals after repair. Fortunately, most radial tears are shallow, partial-thickness tears that extend only through one third to one half of the width of the meniscus.⁵⁶

Crushing injuries of the vascular anterior and posterior horns tend to heal spontaneously or become asymptomatic with conservative treatment.^75,76,77

Meniscal tears occurring within 4 mm of the meniscocapsular junction in the vascular zone heal better after repair (see Image 38). The meniscal horns (anterior and posterior) have a better supply than the meniscal body, which is more conducive for healing. Menisci containing large horizontal cleavage tears and tears in degenerated menisci do not do as well with repair than acute peripheral tears in nondegenerated menisci. Only about 20% of meniscal tears are suitable for repair.^3,12,14

Tears of the avascular portion of the meniscus that communicate with the synovium peripherally and perimeniscal capillary plexus can be repaired.

Midsubstance meniscal tears can heal if the perimeniscal synovium is abraded; rasping of both meniscal tear surfaces; and implementation of an exogenous fibrin clot onto the repaired surface.

One study showed that isolated meniscal repairs showed a 41% failure rate when only synovial rasping was used, but only an 8% failure when both rasping and fibrin clot implantation was used. A healing rate of approximately 90% was reported.^75,76

Most types of tears with rim widths of up to 5 mm can be repaired if they can be stabilized and coapted.⁷⁸

The amount of time elapsing between injury and treatment may not be a major factor, as Weiss and Morehouse indicated that both acute and chronic lesions up to 8 years after injury could be repaired.

The age of the patient has less of a correlation with outcome than previously thought. In a population with an average age of 44.2 years and no one younger than 40 years, 86% were symptom-free 26.5 months after surgery.

Anterior central meniscal lesions (disruption of the osseous attachment of the anterior meniscal horn to the tibia) are repairable.

Treatments for specific tears

Meniscal tears with ACL injuries

ACL tears are present in about 80% of knees with repairable meniscal tears. Repaired menisci have a greater likelihood of becoming injured if ACL tears are not repaired.

The percentage of repairable meniscal tears declines over time; thus, early arthroscopic evaluation and repair are important.

Some LM tears can be left untreated during ACL reconstruction. These include (1) posterior horn avulsion tears (attempting to remove this fragment may damage the lateral femoral condyle); (2) vertical tears totally posterior to the popliteus tendon (repair is difficult and may damage the microvasculature about the meniscus); and (3) stable vertical, longitudinal, or radial tears, if stable.^15,16

Meniscal tears not seen on MRI

Many do not require repair. These include short, partial-thickness, stable tears; far peripheral tears in the vascularized red zone; and peripheral, posterior horn tears of the LM in ACL-deficient knees.^32,57,79

Unstable meniscal tears

These do poorly without surgery because they tend to propagate and lead to accelerated osteoarthritis.

Meniscal tears in older patients

These tears are more likely to be degenerative, precluding successful repair.

Isolated meniscal tears

Healing may be impaired because tears tend to be more central, occurring more frequently in knees with yet unidentified biomechanical derangement; these are more degenerative in nature.

Discoid menisci

Many DMs are asymptomatic and do not require operative treatment. Treatment of the unstable inner segment of the DM requires saucerization (partial resection) so it appears like a stable crescent, similar to that of the normal meniscus. The Wrisberg ligament-type of DM has a tendency to displace medially into the intercondylar notch. It is best treated by TM.

Some patients with symptoms in the presence of intrasubstance degeneration without a surface tear have undergone saucerization. Some patients with symptomatic discoid menisci have undergone TM. These menisci showed prominent grade 2 signal intensity, which is thought to represent an intrameniscal cleavage tear. Children and adolescents with symptomatic DMs may require TM.

Indications for immediate treatment of meniscal tears

Professional, competitive, or highly motivated recreational athletes may not want to invest the recovery time required for observation to see if symptoms resolve. Laborers who rely on their lower extremities for their occupation are also candidates for immediate intervention.^15,16

Meniscal tears associated with ACL tears should be treated sooner than later because the inherent potential of a meniscal lesion to heal spontaneously in an unstable knee diminishes over time.

Delay in treating locked knees related to displaced meniscal fragments can cause cartilage damage by virtue of local increased pressure, especially with weight bearing.

Contraindications to meniscal repair

Contraindications to meniscal repair include the following: (1) tears 10 mm or thinner, as these tend to be stable; (2) partial-thickness (<50%) tears; (3) short, longitudinal tears that can be displaced less than 3 mm from the peripheral rim; (4) shallow radial tears shorter than 3 mm; (5) a degenerated and macerated meniscus; and (6) complex tears.^75,76,77

Factors influencing healing

Factors influencing healing are several. The length of the meniscal tear does not affect healing rate. The patient's age does not alter healing rate. The rim width of the meniscal tear was not a significant factor until it exceeds 4 mm.^75,76,77

Operative technique

Partial meniscectomy

Two types exist: segmental and circumferential. Segmental PM involves the entire width of the meniscus out to the capsular rim. This destroys the ability of the meniscus to transmit load. With circumferential PM, some length of the central portion of the meniscus is resected. The meniscus retains local transmission properties proportional to the width of the peripheral meniscus that is preserved. This is used to treat radial tears and is needed because right angle edges of each segment create stress concentrations such that each segment is prone to increased sheer stresses in rotational motion between the tibia and femur. The result is further peripheral propagation or anterior or posterior turning resulting in a flap tear.

Primary meniscal repair

Three approaches for primary meniscal repair are available. These are classified by the duration of suture passage through the menisci: inside-out, outside-in, and all-inside. The inside-out technique is the most common.⁷⁰

Although nonabsorbable sutures are the most commonly used material, absorbable and nonabsorbable arrows and sutures have been used.⁷⁰

Open meniscal repair may be indicated for tears of the periphery of the posterior horns, occurring within 1-2mm of the meniscosynovial junction. The reason may be better exposure.²⁹

With techniques to treat central, avascular meniscal tears, the underlying challenge is to bring a blood supply to the tear. Adjuvants such as fibrin clot, trephination, creation of vascular channels, and use of synovial flaps have all been attempted. These methods have been tried because standard technical repair of complex tears in this region is more demanding and has a higher incidence of failure.

Healing of midsubstance tears can be stimulated through abrasion of the perimeniscal synovium and rasping of both meniscal tear surfaces. Vascular channels have been created through the body of the outer rim of the meniscus to bring an avascular lesion into contact with a blood supply. The large avascular channels, however, disrupt the normal meniscal architecture, bringing synovial flaps to the tear, an attempt to facilitate neovascularization from the perimeniscal capillary plexus. Trephination has also been used to stimulate blood supply. The use of exogenous fibrin clot to promote healing is the most common technique for treating meniscal tears.

Henning and Lynch found the failure rate for treating isolated meniscal tears to be 41% if there was no concurrent use of fibrin clot and an 8% failure rate with the use of fibrin clot.⁷⁷

Allograft menisci have been shown to attach and heal to recipient menisci in experimental models of allograft menisci transplants. If used after meniscectomy, there is a delay in the appearance of osteoarthritis. This procedure is indicated for young patients who have undergone TM and who are at risk for developing accelerated osteoarthritis by age 40 years. Persistent pain after meniscectomy is the most common indication for allograft transplantation. Allografts contribute to knee stability in ACL-deficient knees. The anterior and posterior horns of the meniscus and the meniscotibial attachments are preserved so they can function as firm anchors for the generation of hoop stresses.^17,72

Regarding meniscal tears in children, caution is needed when MRI is used to guide treatment. One study showed grade 2 or grade 3 lesions in 51% of asymptomatic children aged 9-15 years. Correlation with physical findings is mandatory. Arthroscopy is indicated in any child with significant symptoms and a corroborating physical examination. A treatment classification scheme called SAKS is used for meniscal injuries: S = size, shape and stability of a tear; A = acuity; and KS = knee stability.^25,33

Meniscal tears can be described by the zone of injury:

• Zone 0 - Injury to the synovial-perimeniscal vascular plexus
• Zone 1 - Injury to the red or vascularized zone of the meniscus
• Zone 2 - Injury to the red-white junction of the meniscus
• Zone 3 - Injury to the white avascular zone

In the stable knee, meniscal tears in the periphery less than 1 cm in length and displacement of 3 mm or less are treated conservatively. If the injury is chronic, the interface between the meniscal edge is rasped. A longitudinal, noncomminuted, reducible meniscal tear in zones 0-2 is repaired. In chronic tears, the inner fragment edges are rasped and a fibrin clot is used. Repair is not performed if the central tear is macerated with multiplanar tears or tears that cannot be reduced anatomically. The fragments should be debrided and the remaining stable meniscal rim contoured. Unstable tears in the central white zone are excised. The rare radical tear or horizontal tear is debrided, leaving a stable, intact remaining meniscus. The smaller leaf of the horizontal tear is resected and contoured. If both leaves are equal in size, the inferior leaf is excised.²⁵

In adolescents with an unstable knee from a grade 3 MCL injury and a peripheral meniscal tear, repair of both is performed. If the meniscal tear is in the avascular zone, the fragment is debrided and the collateral ligament injury is treated conservatively. In patients with meniscal injuries and ACL tears, repair of both is done, if possible.²⁵

The postoperative meniscus

Problems with routine MRI

Standard MRI protocols are less reliable than other methods in diagnosing meniscal tears in patients who have undergone previous meniscal surgery or in those receiving conservative treatment for meniscal tears.

Several factors are responsible for problems encountered when evaluating the postoperative meniscus. After injury, the signal intensity of the meniscus may never return to normal. It has been postulated the persistent signal intensity represents granulation tissue or an asymptomatic meniscal tear. Sites of meniscal healing may appear as lines of hyperintensity reaching the articular surface of the meniscus; these mimic new tears unless preoperative images are available for comparison.^3,14

Meniscal morphology is often distorted after PM. Physicians interpreting MRIs of the knee in this setting may expect the partially resected meniscal surface to be smooth, and they may not be aware that normal postoperative distortion of the meniscal surface may be present. Blunting of the meniscus tip may be related to operative shaving and is a normal, acceptable appearance of the postoperative meniscus.

Since the goal of surgery is to preserve as much meniscal material as possible, only a portion of the abnormal meniscus may be removed. This is especially true with horizontal cleavage tears in which only the central and most fragmented portion of the meniscus is removed. The peripheral portion of the affected meniscus, often containing abnormal signal intensity, is left. Sometimes, a portion of the cleaved meniscus is preserved. In this situation, a meniscus with preoperative grade 2 signal intensity now has that same finding in contact with the new meniscal surface; this can falsely appear as grade 3 signal intensity. This phenomenon is called signal conversion or intrameniscal signal conversion. Thus, basic criteria, which are helpful in diagnosing meniscal tears in preoperative patients, are unreliable in the postoperative knee.

There is a high incidence of radial tears in patients who have undergone partial meniscectomy. Magee et al describe a 32% rate of occurrence.

Role of MRI in patients with persistent or recurrent symptoms after meniscal surgery

Assessment of the stability of the remaining meniscal tissue and to differentiate meniscal from extrameniscal tissue causes of pain. The assessment includes evaluation of the meniscal remnant and overlying articular cartilage, the detection of any loose osteocartilaginous loose bodies, the evaluation of ligaments, the detection of synovitis, and the identification of osteonecrosis.

Evaluation of menisci based on the amount of meniscus resected

For PM with less than 25% meniscal resection, conventionally accepted criteria for meniscal tears can be used with confidence. The presence of grade 3 signal intensity can be used to diagnose meniscal tears with an accuracy comparable to that of a virgin meniscus. Blunting of the meniscal tip is an accepted appearance for the stable meniscus. This definition requires a history of a PM because meniscal blunting, in the absence of prior surgery, is evidence for a meniscal tear.

For PM with 25-75% meniscal resection, meniscal evaluation using traditional criteria for meniscal tears is unreliable. Part of a meniscal tear may not be resected, leaving grade 3 signal intensity at the meniscal surface. Signal conversion may be present.

In this situation, 3 criteria are used for distinguishing a retorn meniscus from a stable postoperative meniscus: (1) grade 3 signal intensity may be present on T2-weighted images.⁹ Better correlation with arthroscopy occurs if the signal extended to 2 articular surfaces. Using only PD-weighted images is fraught with error. This is the most common of the positive findings. The abnormal signal intensity represents fluid extending into the cleavage plane of a meniscal tear, within the meniscal remnant. This is highly specific but not sensitive. Applegate et al showed a sensitivity of 56% and a specificity of 90%. (2) A displaced meniscal fragment may also be present. This is highly specific but not sensitive. (3) Grade 3 signal may be contacting the meniscal surface at a location other than the repair site.³⁴

For PM with more than 75% of the meniscal removed, the presence of a displaced fragment is the most specific criteria for meniscal tear.

Regarding the appearance of menisci treated conservatively or with surgical repair in patients having healed by clinical evaluation, most menisci show persistent abnormal intrameniscal signal intensity, unchanged from the preoperative MRIs.²⁰

MRI findings in conservatively treated peripheral tears include persistent grade 3 signal-intensity changes in the meniscus. With increasing time (usually a period of months), the signal intensity within some tears may diminish or disappear. This represents conversion of scar tissue to fibrocartilage, but this is seen only in a minority of cases.³⁴

MRI findings after TM include joint-space narrowing, articular cartilage loss, and subchondral low signal intensity within the bone. This appears before the appearance of sclerosis on plain radiography.

MR arthrography

Fluid within the meniscal remnant or at the meniscal repair site is indicative of a meniscal tear. This observation, in light of the difficulties evaluating the postoperative meniscus, have led to the use of MR arthrography using gadolinium-based contrast agents. This technique is used when more than 25% of the meniscus has been previously removed and a meniscal tear is suspected.

Gadolinium-based contrast agents (gadopentetate dimeglumine [Magnevist], gadobenate dimeglumine [MultiHance], gadodiamide [Omniscan], gadoversetamide [OptiMARK], gadoteridol [ProHance]) have been linked to the development of nephrogenic systemic fibrosis (NSF) or nephrogenic fibrosing dermopathy (NFD). For more information, see the eMedicine topic, Nephrogenic Fibrosing Dermopathy. The disease has occurred in patients with moderate to end-stage renal disease after being given a gadolinium-based contrast agent to enhance MRI or MRA scans. 

Worldwide, over 200 cases have been reported, according to the FDA. NSF/NFD is a debilitating and sometimes fatal disease. Characteristics include red or dark patches on the skin; burning, itching, swelling, hardening, and tightening of the skin; yellow spots on the whites of the eyes; joint stiffness with trouble moving or straightening the arms, hands, legs, or feet; pain deep in the hip bones or ribs; and muscle weakness. For more information, see the FDA Public Health Advisory or Medscape.

Although the use of Gd-based agents for MR arthrography is considered an off-label use by the US Food and Drug Administration, its use is widespread.^9,70 The low viscosity of these agents may allow better penetration of contrast into meniscal tears than joint fluid. Two milliliters of a Gd-based contrast material is diluted in 250 mL of normal sodium chloride solution, and 10 mL of this solution is mixed with 5 mL of iodinated contrast material and 5 mL of 1% lidocaine. A total of 20-30 mL of this solution is injected into the joint. T1-weighted fat-saturated images are obtained in the axial, oblique-coronal, and oblique-axial projections. With this method, the sensitivity of diagnosing meniscal tears in the postoperative knee increased from 56% to 89%, with a specificity of 86%.^34,59

Meniscal tears treated by suture repair are considered healed if no contrast agent enters the repair site. Contrast material extending partially into the repair indicates either a partially healed tear or a recurrent partial-thickness tear. Full-thickness tears that have not healed or have healed and then return show contrast material extending through the entire tear.

Sharp edges, abrupt changes in contour, and free meniscal fragments are not normally left by the arthroscopist and should always indicate a new tear.

Accuracy of MR arthrography

MR arthrography is more accurate than conventional MRI to diagnose meniscectomy. One study showed a sensitivity of MRI of 66% in this setting while that for MR arthrography was 86%.^34,59

Operative technique for arthroscopy and arthroscopic surgery

These procedures facilitate the identification of meniscal pathology and expand treatment options.

They can be used to differentiate intersubstance cleavage tears from grade 2 signal-intensity changes in the meniscus by probing the articular meniscal surface.

These techniques can also be used to determine whether a tear is in the vascular or avascular zone of the meniscus. Bleeding at the site of the tear indicates the tear is in the vascular zone. Lack of bleeding is not a conclusive finding because the distending pressure of the irrigating joint fluid can occlude the capillary circulation and prevent bleeding. Clinical judgment is needed. If the tear is within 3 mm of the periphery of the meniscus, it is considered vascular. If it is 5 mm or more from the periphery, it is considered avascular. If it is 3-5 mm it is indeterminate; the patient's age is important. Vascular penetration of the meniscus is greater in skeletally immature individuals.

Arthroscopy and arthroscopic surgery promote meniscal healing with a decrease in postoperative morbidity. Meniscal tears that are not peripheral are treated with arthroscopic surgery. Peripheral tears can be treated by either an arthroscopic or open approach. Meniscal tears not suitable for repair but requiring surgery are treated by PM. This can be done arthroscopically.

These procedures are easier to perform than open surgery and are often necessary to determine optimal treatment. They are also used to diagnose tibial surface tears of the meniscus by probing. Open surgery is only appropriate for peripheral meniscal attachments.

Factors favorably influencing repair

Factors favorably influencing repair include the following: a rim width of less than 3 mm, a tear shorter than 2.5 mm, and a tear located in the LM.⁷²

Failure of meniscal repair

Factors leading to failure include a rim wider than 3 mm and repair performed with resorbable sutures. These sutures tend to disappear before meniscal healing is complete, predisposing the meniscus to recurrent tearing.⁷²

About 60% of failures occur within the first 6 months, and 86% occur within the first 18 months.⁷²

Pitfalls

The posteromedial corner of the knee is the single greatest source of diagnostic errors, especially in tight knees (see Image 63). The insertion site of the anterior horn of the MM is under the patellar fat pad at the junction with the tibial plateau. This is difficult to visualize arthroscopically. Accuracy for detecting undersurface tears of the posterior horn of the MM is only 45-65%.^66,80

Treatment outcomes

Nonsurgical management

Successful recovery from a meniscal tear is helped by a gradual resolution of symptoms over 6 weeks with a return to normal activity by 3 months. Many meniscal tears heal spontaneously.^15,16,81

With time, symptoms abate and reparative arthroscopy may be canceled. A study of patients on the waiting list for arthroscopic surgery showed that 25% had complete relief and 47% had partial relief of symptoms. Many, however, changed their level of activity. Ultimately, 39% canceled their upcoming surgery.⁸¹

Total meniscectomy

Patient dissatisfaction can be as high as 36-40%. Removal of the MM results in problems with joint stability. Results of a 10-year follow-up study showed that 30% of patients had marked disability, 32% had fair or poor results. In addition, 55% of men and 90% of women complained of pain.^2,75,76

Results in relative overloading of the articular surface resulting in cartilage degeneration and accelerated osteoarthritis. Degenerative changes are more frequent with lateral meniscectomy.^75,76

Partial meniscectomy

Patients' functional status after PM deteriorates over time. One study showed that 62% had an excellent or good result after PM. Only 48% were able to maintain their preinjury activity level due to increasing symptoms. A 54-month follow-up study showed a 38% progression of degenerative changes in the medial compartment and 24% progression in the lateral compartment.^3,73

Degenerative changes in the knees appeared with the same frequency after flap tear resection as often as the more radical bucket handle resection.^3,73

PM of the MM in a varus-angulated knee or of the LM in a valgus-angulated knee had an increased risk for subsequent degenerative change when compared with PM in knees with normal alignment.

Factors yielding unfavorable results to PM include the following: age older than 40 years; degenerative, horizontal cleavage and complex tears; and chondromalacia worse than grade 2 in the articular cartilage of the knee.

PM in the presence of underlying articular cartilage degeneration increases the degree of osteoarthritis, as seen on radiographs 12-15 years after surgery.

Arthroscopic meniscal repair

The success of meniscal repairs in patients with 2 or more years follow-up varies from 62-92%. Eggli et al reported a 27.5% overall failure rate after 7.5 years. Most failures occurred after the first 6 months. Overall, 11% of LM tears and 26% of MM tears failed after surgery. Repairs performed 8-19 weeks after injury. With simultaneous ACL repair, healing rates of 90% have been reported.^71,72,82

Factors influencing favorable meniscal healing include the following: time from injury to surgery less than 8 weeks, peripheral tears, patients younger than 30 years, and tears shorter than 2.5 cm.^71,72

Isolated meniscal tears and open surgical repair

Clinically successful repairs have a protective effect on the meniscus. DeHaven et al showed that 85% of joint compartments were normal on radiographs, and only 15% had mild juxta-articular bone sclerosis or joint-space narrowing 10.5 years after surgery.^12,71

For isolated meniscal tears healing rates vary from 50% to 91%. The higher healing rates reported above may be due to the introduction of healing-inducing techniques, such as fibrin clogs and parameniscal abrasion.^12,71

With open surgical repair, healing of traumatic meniscal tears, located within 4 mm of the meniscocapsular junction, occur in 90%. Long-term follow-up healing rates at 5-10 years after repair vary between 70% and 90%. Failure is more likely to occur in unstable ACL-deficient knees. The re-tearing rate is higher than in ACL stable knees.^12,71,83

Meniscal tears from repaired radial tears are significantly lower than in controls, in terms of yield stress, maximum stress, and elastic modules. No significant improvement occurs after 13 weeks after surgery.

Re-tearing rates of about 15% have been reported in early follow-up. Long-term follow-up shows a re-tearing rate of 21%.^12,71,83

Multimedia

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Media file 1: Sagittal fat-saturated proton density–weighted image demonstrates the concave superior meniscal surface (arrows), which improves contact with the femoral epicondyles, and a flat undersurface, which improves contact with the tibial plateau. The periphery (outer edges) is thicker than the central portion (arrowhead), allowing for firm attachment to the joint capsule. Note the normal bow-tie appearance of the meniscal body.

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Media file 2: Coronal fat-saturated proton density–weighted image shows the relative size of the posterior horns of the medial and lateral menisci. The posterior horn of the medial meniscus (left arrow) is thicker than the posterior horn of the lateral meniscus (right arrow). Note the normal dark appearance (relative lack of signal intensity) in the menisci. The medial portion of the posterior horn of the lateral meniscus (ie, the meniscus on top of the fibula) is directed upward obliquely, from a lateral to medial direction. This is its normal course.

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Media file 3: Axial fat-saturated proton density–weighted image demonstrates the transverse (intermeniscal) ligament (arrows) connecting the anterior portions of the medial and lateral menisci.

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Media file 4: Coronal proton density–weighted image shows the intermeniscal ligament (arrow) connecting the anterior horns of the medial and lateral menisci.

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Media file 5: Coronal fat-saturated proton density–weighted image shows the popliteus recess containing joint fluid and located between the lateral aspect of the posterior horn of the lateral meniscus and the joint capsule. An extensive tear is present in the posterior horn of the medial meniscus (arrow). Note the normal oblique upward orientation of the posterior medial horn of the lateral meniscus.

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Media file 6: Coronal fat-saturated proton density–weighted image shows the dark appearing popliteus tendon (arrows) passing through the popliteus recess. The posterior medial horn of the lateral meniscus is directed obliquely upward.

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Media file 7: Sagittal proton density–weighted image of the lateral knee compartment. It shows the superior fascicle (arrow) attaching the posterior horn of the lateral meniscus with the joint capsule. Hyperintense (bright) fluid is present in the popliteus recess. The inferior fascicle is not yet visualized.

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Media file 8: Sagittal fat-saturated proton density–weighted image shows the inferior fascicle. In this location, the superior fascicle is not present. Note the normal bow-tie appearance of the meniscal body.

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Media file 9: Coronal proton density–weighted image shows the ligament of Wrisberg originating from the posterior medial horn of the medial meniscus and passing obliquely upwards (arrow) to attach to the posterolateral aspect of the medial femoral epicondyle.

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Media file 10: Coronal fat-saturated proton density–weighted image shows the popliteus tendon originating from an undulation of the lateral femoral condyle. From there, it passes through the popliteus recess to insert on the proximal posterior tibial metaphysis. A radial tear (arrow) is present in the posterior horn of the medical meniscus.

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Media file 11: Coronal proton density–weighted image posterior to the knee joint shows the normal junction of the popliteus tendon and muscle passing obliquely downwards to insert on the posterior tibial metaphysis. There is a tear to the conjoint tendon of the biceps muscle and lateral collateral ligament (LCL, arrow).

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Media file 12: Coronal proton density–weighted image of the popliteus tendon (outer arrows), which passes obliquely caudal, left to right, becoming the popliteus muscle (lower central arrow) and inserting on the proximal posterior metaphysis.

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Media file 13: Coronal fat-saturated proton density–weighted image of the mid knee shows the normal appearance of the body of the medial and lateral menisci. The apices (inner portions) are the thinnest part of the meniscus and are more central in the knee joint. The periphery, meniscal bases, outer portion (arrow and arrowhead) is the thickest part and contains the blood vessels supplying the meniscus.

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Media file 14: Coronal fat-saturated proton density–weighted image of the anterior knee shows the horizontal attachment of the anterior horn of the lateral meniscus (arrow) attaching near the intercondylar eminence.

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Media file 15: Coronal fat-saturated proton density–weighted image shows the attachment of the anterior horn of the lateral meniscus. In this patient, the anterior horn attachment is near the tibial attachment of the anterior cruciate ligament (ACL, arrow).

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Media file 16: Sagittal fat-saturated proton density–weighted image of the lateral compartment shows the relative equal size of the anterior and posterior horns of the lateral meniscus. The meniscal body has the normal configuration of a bow tie.

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Media file 17: Sagittal fat-saturated proton density–weighted image of the medial compartment shows the larger posterior horn (arrowhead) and the smaller anterior horn.

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Media file 18: Sagittal proton density–weighted image of the medial compartment. The thin apex of the meniscal body connects the large anterior and posterior horns at this level, giving the appearance of a bow-tie configuration (arrow).

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Media file 19: Sagittal proton density–weighted shows a small amount of abnormal signal intensity in the anterior horn of the medial meniscus (arrow). This represents a grade 1 change in signal intensity.

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Media file 20: Coronal proton density–weighted image shows extensive grade 2 signal intensity in the anterior and posterior horns of the medial meniscus. However, the signal intensity does not extend to a joint surface.

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Media file 21: Sagittal fat-saturated proton density–weighted image shows abnormal signal intensity in the posterior horn of the medial meniscus, which appears to extend close to the inferior surface. This represents grade 2C changes in signal intensity. It can be difficult to differentiate grade 2 and grade 3 changes. Injuries causing grade 2C signal intensity can progress to degenerative tears.

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Media file 22: Coronal fat-saturated proton density–weighted image of the knee shows a bucket tear of the posterior horn of the medial meniscus. It is a full-thickness tear involving both the superior and inferior articular surfaces. The wide separation of the margins of the tear usually results in poor outcomes with surgical repair. Also present is a horizontal tear of the meniscal body extending from the margin of the bucket handle tear to the meniscal base. Such tears usually occur in older patients and are not usually amenable to surgical repair.

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Media file 23: Coronal fat-saturated proton density–weighted image of the knee shows a full-thickness radial tear in a location similar to that in Image above. The ability of fat saturation to remove the high signal intensity of fat from the signal intensity bone marrow and subcutaneous tissue makes it an excellent way to highlight meniscal tears.

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Media file 24: Sagittal proton density–weighted image of the mid portion of the medial compartment shows a full-thickness horizontal tear of the posterior horn of the medial meniscus extending from the base to the superior surface. In addition, image shows amputation of the inferior apex of the posterior horn. The combination of these 2 tears involving the same part of the meniscus makes this injury a complex tear.

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Media file 25: Sagittal proton density–weighted image shows an amputated inferior base of the posterior horn of the lateral meniscus; this represents a partial tear.

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Media file 26: Sagittal proton density–weighted image shows almost complete disappearance of the posterior horn of the medial meniscus. The loss of the normal shock-absorbing function of the meniscus predisposes the person to the loss of cartilage and subsequent bone abnormalities typical of degenerative arthritis.

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Media file 27: Sagittal fat-saturated proton density–weighted image shows amputation of the apex of the posterior horn of the lateral meniscus. The anterior horn is deformed, and the image shows irregularity and deformity of the adjacent portion of the femur, which may have occurred with the injury to the anterior horn.

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Media file 28: Sagittal proton density–weighted image shows an enlarged, abnormal, globular posterior horn of the medial meniscus with abnormal signal intensity. The abnormal appearance is related to edema from the meniscal tear.

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Media file 29: Image shows a uniform decrease in the size of the body of the medial meniscus (arrow). The superior apex of the body of the lateral meniscus (arrowhead) has been amputated.

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Media file 30: Image shows amputation of much of the inferior portion of the apex and part of the base of the body of the medial meniscus.

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Media file 31: Coronal proton density–weighted image shows that most of the posterior horn of the lateral meniscus has been amputated (vertical arrow). An extensive partial-thickness tear is present on the undersurface of the body of the medial meniscus and extends to the meniscal base (oblique arrow).

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Media file 32: Sagittal fat-saturated proton density–weighted image of the medial compartment showing a large area of increased signal intensity (arrow) representing joint fluid which, occupies the site where the normal meniscus should be located. This cleft within the meniscus represents a large,, full-thickness radial tear of the meniscal body. It is full thickness because the tear involves the superior and inferior articular surfaces of the meniscus. The radial tear divides the meniscal body into anterior and posterior portions.

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Media file 33: Sagittal fat-saturated proton density–weighted image of the same patient as in Image 24, slightly more medial in the knee joint. Image shows the horizontal tear in the posterior horn of the medial meniscus (arrow). The amputation of the apex of the inferior horn is more evident on this image than on the other one. When a meniscal tear is visualized on at least 2 consecutive images, the accuracy that the tear is real and not an artifact approaches 100%. Its presence is almost always verified on arthroscopy.

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Media file 34: Coronal fat-saturated proton density–weighted image shows abnormal signal intensity in the posterior horn of the medial meniscus (MM) extending to the undersurface near the junction with the joint capsule. Such tears may be missed on arthroscopy because that part of the knee joint is difficult to access. Also present is a tear to the posterior medial horn of the lateral meniscus (LM) as it slopes obliquely inward. A false-positive diagnosis of meniscal tear can be made when one evaluates this region because of the magic angle effect. Tears persist when the echo time (TE) is varied and when T2-weighted images are obtained. True tears can also be confirmed by visualizing them on sagittal or axial projections.

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Media file 35: Sagittal proton density–weighted image shows the tibial insertion site of the posterior horn of the medial meniscus (MM).

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Media file 36: Sagittal fat-saturated proton density–weighted image shows the tibial insertion of the posterior horn of the medial meniscus (MM). This portion of the meniscus has a normal, speckled appearance.

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Media file 37: Sagittal fat-saturated proton density–weighted image of the posterior knee compartment shows the normal insertion (arrow) of the semimembranosus tendon. The insertion site is near the posterior horn of the medial meniscus (MM), and it is not to be mistaken for a displaced meniscal fragment.

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Media file 38: Sagittal fat-saturated proton density–weighted image shows a full-thickness tear to the periphery of the anterior horn of the medial meniscus (MM). Tears in this location have a good likelihood of healing without surgical repair because they occur in the zone with a good blood supply to the meniscus. Also present is a partial thickness tear to the undersurface of the posterior horn of the MM.

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Media file 39: Coronal proton density–weighted image shows a full-thickness,, vertical bucket handle tear (arrow) through the base of the medial meniscus (MM).

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Media file 40: Coronal fat-saturated proton density–weighted image shows the tibial insertion of the anterior horn of the lateral meniscus (LM, arrows) and the intimate association sometimes seen between the insertion of the meniscus and the tibial insertion of the anterior cruciate ligament (ACL). Normal interdigitations of the ACL are present at this site. Fat is also present and appears hyperintense on T1- and proton density–weighted images. This is not to be confused with a meniscal or ACL tear.

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Media file 41: Coronal fat-saturated proton density–weighted image shows irregularity to the upper (femoral) surface of the body of the lateral meniscus (LM, outer arrow), indicating fraying. Fraying usually occurs at the meniscal apex. Soft tissue densities (inner arrow) are present under the apex of the meniscus, indicating debris or a free meniscal fragment at this level. The body of the LM is unusually thick and longer than usual, indicating a discoid meniscus. The normal-sized medial meniscal body is present for comparison. Discoid menisci occur about 5 times more often here than in the LM, and they are more prone to injury.

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Media file 42: Sagittal proton density–weighted image shows a vertical bucket handle tear (arrow) through the periphery of the posterior horn of the medial meniscus (MM). Tears in this location do not usually heal spontaneously because this portion of the meniscus lacks blood supply.

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Media file 43: Sagittal fat-saturated proton density–weighted image of the paramedian portion of the lateral joint compartment. The transverse intermeniscal ligament (arrowhead) is about to unite with the anterior horn of the medial meniscus (MM, arrow). Fat is normally present in this region and can mimic a ligament or meniscal tear. By carefully following the course of the ligament on sequential images and by observing a uniformly well-defined, hypointense structure on every image, this pitfall can be avoided. A small, ill-defined, linear soft tissue density is present under the anterior horn. It is separated from the anterior horn by bright fluid. This is a rare tear in this region. The brightness is joint fluid in the tear.

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Media file 44: Sagittal fat-saturated proton density–weighted image of the paramedian portion of the medial knee. The transverse intermeniscal ligament is about to insert on the anterior horn of the medial meniscus (MM). The anterior horn is normally speckled. The anterior horn is partially displaced off the anterior surface of the tibia by a radial tear more laterally (picture is not shown). A tear involves the posterior horn of the MM (arrow).

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Media file 45: Sagittal proton density–weighted image shows a full-thickness horizontal tear of the posterior horn of the lateral meniscus (LM, arrow) involving the superior articular surface.

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Media file 46: Sagittal fat-saturated proton density–weighted image shows a well-defined,, soft tissue density in front of the posterior cruciate ligament (PCL). It is speckled and looks like the normal posterior medial horn of the medial meniscus (MM), but it is in the wrong place. This finding represents a displaced meniscal tear involving the posterior medial horn. The position of the meniscus is referred to as a double PCL because it looks like 2 of these ligaments are present.

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Media file 47: Coronal fat-saturated proton density–weighted image of the same patient as in Image 46 shows the same speckled displaced meniscal fragment (inner arrows). Also present is an extensive horizontal tear to the body of the medial meniscus (MM) with a lot of hyperintensity at the periphery (outer arrows). This represents a meniscal cyst. Repair of the cyst without repair of the underlying meniscal tear results in recurrence of the cyst.

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Media file 48: Sagittal fat-saturated proton density–weighted image shows soft tissue (arrows) lying on top of the anterior horn of the medial meniscus (MM), separated from it by high signal intensity (bright joint fluid). This finding represents a displaced meniscal fragment. The posterior horn of the MM is abnormally shaped and has abnormal signal intensity. This is the origin of the displaced fragment. The height of the combination of the displaced fragment and the anterior meniscus is greater than 8 mm. The height of the meniscal tissue equal to or greater than this number is a good sign of a displaced meniscal fragment.

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Media file 49: Coronal fat-saturated proton density–weighted image of the same patient as in Image 48 shows a different view of the meniscal fragment and the anterior horn. As in Image 48, an appearance of a double anterior meniscus is shown.

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Media file 50: Coronal fat-saturated proton density–weighted image of the posterior knee Shows an extensive horizontal tear involving the undersurface of the posterior meniscus. Also present is a flap on the undersurface of the meniscus that has been displaced medially and that is now directed vertically and inferiorly. Note the normal insertion site of the semimembranosus tendon (arrow) that might be confused for a displaced meniscal fragment.

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Media file 51: More anterior image of the same patient as in Image 50 shows the tear to the undersurface of the posterior horn of the medial meniscus (MM, large solid arrow). Image also shows a displaced meniscal fragment lying on top of the body of the lateral meniscus (LM, small solid arrow). The lateral meniscal tear was located more anterior in the body (picture not shown). Open arrow indicates abnormal signal at the origin of the popliteus tendon, representing a partial tear.

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Media file 52: Sagittal proton density–weighted image of the outer portion of the lateral side of the knee shows an absence of the body of the lateral meniscus (LM, arrow). The body of the meniscus should be visualized peripherally (see Images 1, 8, 16, and 20). The normal appearance of the meniscus has the appearance of a bow tie; the bow tie is absent here.

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Media file 53: Sagittal fat-saturated proton density–weighted image shows an unusually tall anterior meniscal horn. On closer inspection, a separation is present between the upper (arrowhead) and lower (arrow) portions. The upper portion is a displaced meniscal fragment from the posterior meniscus lying on top of the anterior horn. Most of the posterior meniscus is absent from its usual location. When the height of a meniscus is greater than 8 mm, it likely represents the combination of a displaced meniscal fragment and a normal meniscus.

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Media file 54: Coronal fat-saturated proton density–weighted image of the posterior portion of the knee joint. A circular, fluid-filled structure (arrow) is present in the upper portion of the most medial portion of the posterior horn of the medial meniscus; it represents a meniscal cyst.

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Media file 55: Axial fat-saturated proton density–weighted image shows the location of the cyst in the most medial portion of the medial meniscus.

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Media file 56: Sagittal fat-saturated proton density–weighted image shows the cyst (arrow) in a different projection.

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Media file 57: Coronal fat-saturated proton density–weighted image obtained more posteriorly in the same patient as in Image 47. Abnormal signal intensity (small arrows) is present medial to the base of the posterior horn of the medial meniscus (MM). No medial collateral ligament (MCL) is discernible at this level; this appearance indicates a tear. This is fluid of the meniscus from its normal attachment with the MCL, representing meniscocapsular separation. The insertion of the semimembranosus tendon (large arrow) has abnormal signal intensity, ie, it is whiter and less well defined than it is in Image 37; this finding indicates a tear.

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Media file 58: Coronal fat-saturated proton density–weighted image obtained more posteriorly than Image 57. The separation of the base of the meniscus from the medial collateral ligament (MCL, large solid arrow) is evident. Image shows interruption and abnormal signal intensity in the MCL (small arrows), indicating a tear. Large open arrow indicates a soft tissue density, either blood products or a free meniscal fragment below the joint space between the tibia and MCL. It is easy to see how inferomedial displaced tears from the medial meniscus (MM) can go unnoticed on arthroscopy. The tear is hidden,, and it is difficult to bend the arthroscope around the tibia to see below the joint. A fracture is present in the central portion of the articular surface of the tibia (oblique central arrow in the tibia),, and another tear is present in the posterior medial horn of the medial meniscus near the tibial insertion site.

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Media file 59: Image in a different patient shows a cyst in the body of the medial meniscus (MM) with meniscocapsular separation. At this level, the medial collateral ligament (MCL, arrow) is intact.

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Media file 60: Sagittal proton density–weighted image obtained from the outer portion of the lateral aspect of the knee. Image shows interruption of the inferior fascicle (arrow) of the lateral meniscus (LM). Compare this with the normal inferior fascicle in Image 8. No pericapsular edema is present.

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Media file 61: Sagittal proton density–weighted image of the same patient as in Image 60 but slightly more medial. Image shows abnormal signal intensity with interruption of the superior fascicle of the lateral meniscus (LM). Compare this finding with the normal superior fascicle in Image 7. No pericapsular edema is present.

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Media file 62: Sagittal proton density–weighted image of a patient who underwent partial meniscectomy of the body and posterior horn of the medial meniscus (MM). Image shows slight hypertrophy and increased signal intensity to the posterior femur, indicating secondary degenerative arthritis related to removal of the meniscus.

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Media file 63: Coronal fat-saturated proton density–weighted image of the posterior aspect of the knee. A soft tissue density (arrow) lies between the lateral portion of the medial femoral epicondyle and the posterior cruciate ligament (PCL). This represents the notch fragment. A horizontal tear is present on undersurface of the body of the medial meniscus (MM). The difference between the notch fragment sign and the double PCL sign is merely one of position of the meniscal fragment. The fragment is more medial with the notch fragment sign than in the double PCL sign.

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Media file 64: Oblique radiograph of the right knee demonstrates bright triangular lines on either side of the joint. This finding represents calcium in the triangular menisci, which represents chondrocalcinosis. A globular calcium opacity (arrow) is above the periphery of the body of the lateral meniscus; this might represent a calcified loose body, either in the joint proper or in the popliteus recess. The presence of chondrocalcinosis can result in a false-positive diagnosis of a meniscal tear.

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Media file 65: Coronal fat-saturated proton density–weighted image of the posterior aspect of the knee shows the normal vertically oblique course of the popliteus recess. Immediately medial to the recess is a longitudinally oblique area of high signal intensity (arrow) located in the posterior horn of the lateral meniscus (LM), communicating with the recess and representing an oblique tear.

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Media file 66: Coronal proton density–weighted image shows a displaced medial meniscal body (arrow) bowing the medial collateral ligament (MCL). The thin meniscus apex is pointing upward, and the thicker base is more caudal in position, indicating meniscocapsular separation with reorientation of the position of the meniscus. In addition, the meniscus has twisted 180° because the concave femoral surface is directed toward the femur. If it had simply flipped upward, it would face the MCL.

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Media file 67: Axial illustration of a full-thickness longitudinal tear of the posterior horn. The meniscus is viewed from above in (a), sagittal in (b), and coronal in (c). For Image 67-70, A = anterior, L = lateral, M = medial, and P = posterior.

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Media file 68: Axial illustration of a full-thickness radial tear of the posterior horn. The meniscus is viewed from above. The orientation is the same as in Image 67.

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Media file 69: Axial illustration of a full-thickness horizontal tear of the posterior horn. The meniscus is viewed from above. The orientation is the same as in Image 67.

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Media file 70: Axial illustration of an oblique (parrot beak) tear of the posterior horn. The meniscus is viewed from above. The orientation is the same as in Image 67. In B, image 1 is most lateral, image 2 is middle, and image 3 is most medial. In C, image 1 is most anterior, image 2 is middle, and image 3 is most posterior.

Keywords

meniscal tears, knee menisci, meniscal injury, medial meniscus, lateral meniscus, meniscomeniscal ligament, discoid meniscus, transverse ligament, intermeniscal ligament, meniscofemoral ligament, Appley test, McMurray test, Boehler test, Apley grinding test, Payr test

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