MRI for Posterior Cruciate Ligament Injuries

Updated: Nov 10, 2016
  • Author: Michael R Aiello, MD; Chief Editor: Felix S Chew, MD, MBA, MEd  more...
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Overview

Overview

MRI is the preferred examination for evaluating posterior cruciate ligament (PCL) injuries. [1, 2, 3, 4, 5, 6, 7, 8, 9, 10] It is the most sensitive and widely used modality for evaluating the PCL and the other cartilaginous and ligamentous structures of the knee. [11, 12, 13, 14, 15, 16]

MRI is superior to physical examination and has replaced CT and arthrography because it offers superior soft tissue resolution and is noninvasive. [17, 18] The sensitivity of MRI has obviated the use of arthroscopy as a diagnostic tool for evaluating PCL injuries in almost all patients. [19, 20, 21, 22, 23, 24]

MRI should be obtained in all patients with suspected PCL tears because of the high incidence of injury to other structures of the knee, such as the ACL, MCL, lateral collateral ligament (LCL), and menisci. [25, 26, 27, 28, 29, 30, 31]

PCL injuries are displayed in the magnetic resonance images below.

The normal femoral origin of the posterior cruciat The normal femoral origin of the posterior cruciate ligament is shown on this T1-weighted coronal image. Only a portion of the ligament is seen because of the normal oblique course of the ligament. The posterior cruciate ligament (black arrow) is of lower signal intensity than the anterior cruciate ligament (open arrow).
Combined anterior and posterior cruciate ligament Combined anterior and posterior cruciate ligament (PCL) tear. Proton-dense sagittal image demonstrates straightening of the orientation of the course of the PCL (straight black arrow in A) resulting from the loss of the normal restraining function of the anterior cruciate ligament secondary to a severe tear. The proton-dense coronal view shows almost all of the substance of the PCL within one image (C), reflecting the abnormal accentuated vertical orientation of the ligament.
Proton-dense sagittal image (A) and T2-weighted sa Proton-dense sagittal image (A) and T2-weighted sagittal image (B) show an extensive tear involving the proximal and distal portion of the PCL. The margins of the PCL are well delineated indicating the ligament is initial.

The roles of other radiologic modalities are as follows:

  • Plain radiography, including anteroposterior and lateral, is used an initial screening examination for evaluation of avulsion fracture (see the image below), dislocation, joint effusion, (lipo) hemarthrosis, and associated soft tissue injuries
    Anteroposterior radiograph of the right knee demon Anteroposterior radiograph of the right knee demonstrates interruption and discontinuity (black arrow) at the femoral origin of the posterior cruciate ligament, representing an avulsion fracture.
  • Stress radiography in the lateral projection using gravitational assistance or muscle contraction can be performed to evaluate posterior displacement of the tibia
  • Arthroscopy is no longer needed to make the diagnosis of PCL tears as a result of the development of MRI with its excellent sensitivity and specificity; arthroscopy is indicated only when conditions, such as patient size and motion, degrade the quality of MRI or when intraorbital metallic foreign bodies, intracranial metallic surgical clips, or pacemaker wires preclude the performance of an MRI examination
  • CT scan is excellent for identifying underlying fractures, including small fractures produced by avulsion of the PCL from either attachment site; however, it provides suboptimal contrast for determining ligamentous injuries; thus, for other uses, CT has been supplanted by MRI
  • Arthrography can visualize the PCL only indirectly; it is not indicated for evaluating the ligaments of the knee

The following are MRI technique guidelines:

  • To characterize PCL abnormalities with MRI, all 3 planes (axial, coronal, sagittal) should be used [32]
  • The use of a dedicated knee coil improves the signal-to-noise ratio
  • A small field of view (10-14 cm) helps improve spatial resolution but generally requires higher field strength magnets [33]
  • An acquisition matrix of 256 and 1-2 excitations (NEX) is routinely used
  • Slice thickness of 3-4 mm with a 10-20% interslice gap is adequate for good resolution of the PCL
  • Both high-field (1.5 tesla [T]) and low-field (0.2 T) MRI systems are accurate in making the diagnosis of PCL tears

The following are imaging protocols [34] :

  • The sagittal oblique plane (performed 10-14° off the triangular line) is most sensitive for evaluating the PCL
  • The standard protocol for assessing the PCL has been the spin-echo sequence, including T2 fast spin-echo imaging (T2FSE) with or without fat suppression in all 3 planes; T2-weighted images (T2WI), with and without fat suppression, accentuate edema and hemorrhage within and around the PCL [35]
  • Short T1 inversion recovery (STIR) protocols can be generated using a relaxation time (TR) of 4000 milliseconds, a TE of 18 milliseconds, a T1 of 140 milliseconds, and an echo-train length of 4
  • T1-weighted images (T1WI) can be generated using a TR of 500 milliseconds and a TE of 15 milliseconds
  • Conventional T2-weighted double echo images can be generated using a TR of 2000 milliseconds and dual-echo time (TE) sequences of 20 and 80 milliseconds [36]
  • T2 or T2 star images (T2*) can be obtained with refocused 20-dimensional Fourier transformation (2-DFT) gradient-echo images using a TR of 400 milliseconds, a TE of 20 milliseconds, and a flip angle (FA) of 20-30°
  • Gradient-echo volume imaging using a slice thickness of less than 1 mm can reduce imaging time
  • 3-Dimensional (3D) FT images can be acquired with a TR of 55 milliseconds, a TE of 15 milliseconds, and an FA of 10°
  • Axial 3D FT images can be acquired with a TR of 55 milliseconds, a TE of 15 milliseconds, and an FA of 10°; sagittal 3D FT T2WI can be generated using a TR of 33 milliseconds and a TE of 13 milliseconds

Normal MRI anatomy

On properly performed MRI examinations, the entire course of the PCL is visualized easily on a single or in the composite of 2 consecutive sagittal images but is not seen on a single coronal image. Such an appearance implies a more vertical course of the ligament related to buckling and foreshortening secondary to a PCL or ACL tear (see the images below).

Only a small portion of the posterior cruciate lig Only a small portion of the posterior cruciate ligament is seen on a coronal image. Increased signal intensity within the posterior cruciate ligament on this proton density image is the result of an interstitial tear.
Combined anterior and posterior cruciate ligament Combined anterior and posterior cruciate ligament (PCL) tear. Proton-dense sagittal image demonstrates straightening of the orientation of the course of the PCL (straight black arrow in A) resulting from the loss of the normal restraining function of the anterior cruciate ligament secondary to a severe tear. The proton-dense coronal view shows almost all of the substance of the PCL within one image (C), reflecting the abnormal accentuated vertical orientation of the ligament.

The normal PCL has a uniform low signal intensity and lacks the striations seen in the normal ACL except near the femoral insertion, where some signal may be seen, especially on T2 images. The normal PCL also has less signal intensity than the ACL (see the image below).

Proton-dense coronal image shows the relative inte Proton-dense coronal image shows the relative intensity of the anterior cruciate ligament (white arrow) and the posterior cruciate ligament (black arrow). The posterior cruciate ligament has less signal intensity because its fibers are more linearly organized than the helically arranged ACL.

Morphology of the normal PCL depends on the integrity of the ACL and depends on the degree of knee flexion. The PCL has a gentle convex posterior margin in extension or minimal flexion. When more flexion is applied, the ligament becomes taut and slightly thinner. Minimal buckling is normal.

When present, the ligament of Wrisberg is seen as an oblique fibrous band of low signal intensity extending from the posterior horn of the lateral meniscus to the medial femoral condyle. The ligament of Humphrey is a similar band of decreased signal intensity anterior to the PCL and is oriented in a plane similar to the ligament of Wrisberg (see the images below).

Proton-dense sagittal images of the knee. The cour Proton-dense sagittal images of the knee. The course of the normal ligament of Humphrey is indicated (black arrow) as it progresses from its origin at the posterior medial portion of the lateral meniscus (A), in front of the lower portion of the posterior cruciate ligament (B), the mid portion (C), and the proximal portion (D).
Proton-dense coronal images demonstrate the normal Proton-dense coronal images demonstrate the normal ligament of Wrisberg originating from the medial horn of the lateral meniscus and inserting at the lateral aspect of the medial femoral condyle near the femoral origin of the posterior cruciate ligament.

Plain radiograph findings

Plain film radiography demonstrates avulsion fractures to the tibial or femoral insertion site (see the image below).

Anteroposterior radiograph of the right knee demon Anteroposterior radiograph of the right knee demonstrates interruption and discontinuity (black arrow) at the femoral origin of the posterior cruciate ligament, representing an avulsion fracture.

Lateral radiographs may demonstrate posterior subluxation of the tibia on the femur. When dramatic or obvious, this is pathognomonic for a PCL tear. Soft tissue swelling may be seen anterior to the proximal tibial plateau on lateral radiographs. Knee effusions/hemarthroses may be seen on lateral radiographs.

Injury to the posterior cruciate ligament (PCL) has been overlooked as a cause of internal derangement of the knee. Improved basic science knowledge of the anatomy and biomechanics of the PCL has provided the orthopedic surgeon with new information on which to base treatment decisions. [37, 38]

MRI has revolutionized the evaluation of the knee for acute and chronic injuries. Its advent has revealed that the PCL is subject to injuries more often than previously believed. Although the significance and presence of this injury may not be recognized immediately, the late onset of instability and arthritis resulting from injury may herald irreversible limitations in activity, leading to debilitation. Correct diagnosis of PCL tears can be challenging but rewarding for both physician and patient. [39, 40]

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Anatomy

The posterior cruciate ligament (PCL) is the major stabilizer of the knee. It provides most of the restraint against posterior tibial displacement on the femur during flexion. The posterior fibers of the PCL prevent hyperextension. During flexion, the anterior fibers tighten and help prevent hyperflexion. The PCL resists internal tibial rotation during flexion by winding around the anterior cruciate ligament (ACL). In terminal knee extension, the PCL and ACL, with the guidance of the menisci, help externally rotate the tibia to its correct position in relation to the femur (screw home mechanism). The PCL also stabilizes the knee against excess varus or valgus angulation. Biomechanical studies have revealed a strong functional interaction between the PCL and the posterolateral structures of the knee. Posterior stability is impaired significantly when both of these complexes are damaged. [41]

The meniscofemoral ligaments (MFL) are associated intimately with the PCL. Their function is to pull the posterior horn of the lateral meniscus anterior and medial during flexion, balancing the action of the popliteus muscle, which, in addition to the MFL, attaches to the posterior horn of the lateral meniscus. The MFL also may function as a secondary restraint to posterior tibial translation after complete rupture of the PCL. Meniscotibial ligaments are involved. [42, 43]

The PCL originates from the lateral surface of the medial femoral condyle. According to Covey et al, the average length of the ligament is 38 mm and the average width of the ligament is 13 mm at the mid portion. [44] The femoral origin is in 2 parts, including a flat upper portion and a convex lower border, which conforms to the shape of the articular surface of the medial femoral condyle. The tibial attachment site is located on an inclined recessed shelf, posterior and inferior to the articular surface of the tibial plateau. The tibial attachment site is smaller than the femoral origin site (see the images below).

The normal femoral origin of the posterior cruciat The normal femoral origin of the posterior cruciate ligament is shown on this T1-weighted coronal image. Only a portion of the ligament is seen because of the normal oblique course of the ligament. The posterior cruciate ligament (black arrow) is of lower signal intensity than the anterior cruciate ligament (open arrow).
Proton-dense sagittal image shows the gentle conve Proton-dense sagittal image shows the gentle convex sloping course of the normal posterior cruciate ligament. Courtesy of Javier Beltran, MD, Maimonides Medical Center.
Proton-dense sagittal image demonstrates the norma Proton-dense sagittal image demonstrates the normal tibial insertion of the posterior cruciate ligament. The insertion site is a vertically inclined structure posterior to the articular surface.

The PCL, like the ACL, is enclosed by a synovial envelope originating from the posterior aspect of the joint capsule. Thus, the PCL and ACL are intra-articular but extrasynovial. [45] The PCL, like the ACL, is composed of individual fasciculi that unite into 2 major bundles, the larger anterolateral and the smaller posterolateral bundles. Both are named for their respective attachment sites on the femur, which are anterior and posterior, and on the tibia, which are lateral and medial. A smaller posterior oblique bundle also has been described.

The anterolateral bundle tightens during flexion and relaxes during extension. The posteromedial bundle acts in a reverse manner; it relaxes during flexion and tightens during extension.

The PCL is twice as strong as the ACL. It contains a larger cross-sectional area and possesses a higher tensile strength, explaining its lower rate of injury. The PCL fibers are oriented more vertically than the more oblique fibers of the ACL.

The smaller MFL are an important component of the PCL complex. They connect the posterior horn of the lateral meniscus to the lateral aspect of the medial femoral condyle adjacent to the origin of the PCL. They consist of the ligament of Humphrey, which is smaller and courses anterior to the PCL, and the ligament of Wrisberg, which is larger and courses posterior to the PCL.

In the ligament of Wrisberg, 3 types of proximal insertions have been described as follows:

  • Type I is seen with a frequency of 45% and inserts on the medial femoral condyle, above the femoral attachment of the PCL (see the image below)
    Sagittal T2-weighted image shows the normal femora Sagittal T2-weighted image shows the normal femoral insertion of the ligament of Wrisberg above the insertion of the posterior cruciate ligament.
  • Type II, seen in 31%, inserts into the proximal portion of the PCL near the femoral insertion site
  • Type III, seen in 21%, inserts into the distal half of the PCL

Variations in the insertion site may cause diagnostic pitfalls when evaluating PCL tears. Either ligament is found in more than 80% of knee specimens. Stoller reports that both are present simultaneously in 6-8% of knees. [46] Their visualization may improve in the presence of edema and hemorrhage following PCL tear.

The major vascular supply to the PCL is the middle genicular artery (MGA), a branch of the popliteal artery. The MGA also supplies the synovial sheath, which is a major contributor to the blood supply of the PCL. The MGA originates behind the popliteal surface of the distal femur and passes anteriorly to enter the posterior capsule of the knee joint at the level of the intercondylar notch. The base of the PCL is supplied by some of the capsular vessels arising from the popliteal and inferior genicular arteries. The various vessels enter the PCL at various levels and run within the ligament in a superior and inferior direction. The main innervation of the PCL is from the posterior articular nerve, a branch of the posterior tibial nerve. [47, 48]

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Pathophysiology

Classification

Posterior cruciate ligament (PCL) ruptures can be classified into isolated or combined injuries. [49, 50] Isolated related injuries can be classified further into partial (grade I and II) or complete (grade III) tears, according to the amount of posterior tibial subluxation as determined by the posterior drawer test. [51]

Grading is as follows:

  • Grade I - Posterior tibial subluxation of 1-5 mm on the posterior drawer test
  • Grade II - Posterior subluxation of 5-10 mm
  • Grade III - Posterior subluxation greater than 10 mm

Grade III or complete PCL tears must be distinguished from combined injuries because the prognoses are different. [52] Isolated PCL injuries are treated conservatively and have an excellent prognosis. Combined injuries involving the PCL have a more guarded prognosis. They are treated with surgical repair or reconstruction within 3 weeks of injury. Surgical results are better than results seen with conservative management; however, it is difficult to distinguish clinically between the two injuries. [53]

Mechanism of injury

Hyperflexion of the knee by high-velocity forces acting on the anterior tibia is the most common cause of PCL tears. This results in posterior displacement of the tibia on the femur. It is seen in motor vehicle accidents in which the knee strikes the dashboard and in soccer sliding injuries in which an athlete receives a blow to the anterior tibia from a slide tackle.

Hyperextension of the knee occurs in football players. The posterior capsule initially is torn, after which tearing of the PCL occurs. The ACL is often torn. Rotational injuries with associated varus or valgus stress are a common cause of PCL tears. The medial collateral ligament (MCL) is torn and the ACL may be torn.

Knee hyperflexion in internal rotation with the foot in dorsiflexion or plantar flexion also causes PCL tears. The anterolateral bundle of the PCL comes under increased pressure and tears while the posterior bundle remains intact. Hyperflexion without an associated force on the anterior tibia occurs in freestyle wrestlers.

In a study of 48 MRI examinations of the knee with isolated PCL tears, 69% of the tears occurred in the midsubstance, 27% proximally. Meniscal tears were detected in 25% of the knees, involving all segments of both menisci except for the anterior horn of the medial meniscus. Focal cartilage lesions were seen in 23% and usually affected the central third medial femoral condyle and medial trochlea. Knee fractures were present in 12.5% of the knees, and 48% had bone bruises that usually involved the central to anterior tibiofemoral joint. The presence of a fracture and the proximal location of the PCL tear were both associated with hyperextension injury. [54]

Associated injuries

Posterior tibial dislocations resulting from PCL disruption can damage the tibial and peroneal nerves. Peroneal nerve injury is more common in combined injuries involving the arcuate complex of the posterolateral corner of the knee. Most neurapraxia resolves with conservative therapy within 18 months. The incidence of vascular injury, such as thrombosis and transection of the popliteal artery, can be as high as 14% regardless of whether the dislocation was reduced spontaneously.

Bone contusions along the inferior aspect of the femoral condyle and the anterior aspect of the tibial plateau can be seen in hyperextension injuries.

Frequency of PCL injuries

PCL injuries constitute 3-20% of knee injuries. The rate may be higher because acute tears often go undiagnosed. More than one half of PCL injuries occur through traffic and industrial accidents; less than one half occur through sports-related injuries. PCL injuries are rare in children.

According to Mink et al, combined injuries to the PCL and other structures of the knee are much more common (97%) than isolated PCL injuries (3%). [55] The ACL is injured most commonly (65%), followed by the MCL (50%), the medial meniscus (30%), the posterior capsule, and fibular collateral ligament.

Complete tears occur in approximately 40% of cases; partial tears in approximately 55%; and avulsion tears in 7%.

Site of injury

The anatomic site of injury depends on the mechanism of injury and the strain velocity of the ligament. High-velocity injuries produce more midsubstance tears. Low-velocity nonimpact injuries may produce more avulsion fractures.

The mid portion of the PCL is the most frequent site of injury, followed by the proximal portion near the femoral insertion.

The tibial insertion site is strong and difficult to tear. Avulsion fractures are more common at this site and are more frequent in children. [56]

Rotational injuries with associated varus or valgus stress most commonly produce a PCL tear at the femoral attachment site.

Sequelae of PCL tears

Chronic tears of the PCL result in increased stress on the patellar ligaments and quadriceps tendon, resulting in chronic tendinitis. Increased stress at the patellofemoral joint may result in grinding related to bone-on-bone contact and chondromalacia. PCL insufficiency may result in a lateral shift in the center of rotation of the knee joint, resulting in articular cartilage degeneration of the medial compartment. An increased risk exists of medial meniscus tears and posterolateral instability. These changes may occur within 5 years of injury. There is an increased risk of medial meniscus tears and posterolateral instability after a complete PCL tear has been sustained. These changes may occur as early as 5 years from the time of injury.

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Clinical Presentation

History

Posterior cruciate ligament (PCL) evaluation begins with an attempt to determine the mechanism of injury. This may provide important information regarding the potential severity of the injury and possible damage to associated structures within the knee.

Physical examination

Pain is present diffusely or may be located posteriorly at the site of an avulsion fracture to the tibial or femoral insertion. Pain also may be present over the anterior tibia as a result of the flexed knee striking the dashboard. The painful knee can make the physical examination challenging even to the experienced examiner.

Swelling and hemarthrosis may be present. Abrasions may be present over the anterior tibial region from dashboard injuries.

An inability to bear weight on the affected extremity is a sign of severe injury. Some individuals with isolated PCL tears may be able to continue with activity. In chronic PCL injuries, the patient may walk with high-heeled shoes, flexing the knee 10-15° to prevent full knee extension because of instability.

Clinical testing to evaluate PCL integrity

The basic function of these tests is to demonstrate posterior proximal tibial subluxation relative to the distal femur with the knee in flexion.

The posterior drawer test is the most accurate clinical test to determine posterior tibial subluxation. PCL injuries have been graded I, II, and III, depending on the amount of subluxation determined by the test. Posterior tibial subluxation of 1-5 mm is considered a grade I injury. The tibia remains anterior to the femoral condyle despite the subluxation. Subluxation of 5-10 mm is a grade II injury. The tibia is flush with the femoral condyle. Further posterior tibial subluxation is considered a grade III injury.

In the posterior sag test, the hip and knee are flexed to 90°. With complete PCL tears, the tibia sags and becomes subluxed posteriorly relative to the femur.

In the quadriceps active test and in the presence of a PCL tear, active contraction of the quadriceps muscle with the knee from 60-90° of flexion causes the tibia to move anteriorly, and normal posterior tibial sag is eliminated.

The reverse pivot test evaluates posterolateral instability related to associated injuries of the posterolateral compartment of the knee (arcuate ligament complex, consisting of the arcuate ligament, lateral collateral ligament, popliteus muscle, and lateral head of the gastrocnemius muscle). The patient is supine, with the examiner standing on the side of the injured leg. One hand grasps the foot and externally rotates the tibia. The other hand is placed on the lateral aspect of the knee. Posterolateral instability results in posterior subluxation of the lateral tibial plateau.

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Findings and Limitations of MRI and Arthroscopy

MRI findings

Posterior cruciate ligament (PCL) tears are intrasubstance, partial, or complete avulsions. In over 90% of patients with PCL tears, the PCL measured more than 7 mm in anteroposterior diameter on sagittal T2-weighted images. [57] One feature of a torn PCL is a striated appearance, similar to the normal ACL with longitudinally oriented lines of increased signal intensity. [57] An acutely torn PCL usually maintains its continuity as a single structure. [57]

Intrasubstance tears are interstitial (confined within the ligament). They contain hemorrhage and edema that increases in signal intensity on long TR/TE sequences. Abnormal signal within the ligament may be increased inhomogeneously on long TR/TE sequences. They can involve much of the course of the ligament causing diffuse enlargement but with well-preserved outer margins (see the image below).

Proton-dense sagittal image (A) and T2-weighted sa Proton-dense sagittal image (A) and T2-weighted sagittal image (B) show an extensive tear involving the proximal and distal portion of the PCL. The margins of the PCL are well delineated indicating the ligament is initial.

Partial tears contain eccentric regions of increased signal intensity within the ligament, extending to interrupt a portion of one of the margins of the ligament on long TR/TE sequences with preservation of some normal ligament fibers. They may contain a circumferential ring of hemorrhage and edema around the margins of the PCL with preservation of most of its internal architecture (halo sign), which is dark on short TR/TE sequences and bright on long TR/TE sequences (see the images below).

Partial tear of the proximal PCL. The proton densi Partial tear of the proximal PCL. The proton density sagittal image (A) shows increased signal intensity in the proximal portion of the PCL. This is clearly delineated on the T2-weighted sagittal image (B).
Partial tear of the proximal femoral portion of th Partial tear of the proximal femoral portion of the PCL. The proton density sagittal image (A) demonstrates increased signal intensity in the proximal femoral portion of the PCL, confirmed on the T2-weighted sagittal image (B). This is corroborated on the proton density coronal image (C). Tears to the PCL must be demonstrated on two different orientations.
Interstitial tear of the mid and distal PCL. Proto Interstitial tear of the mid and distal PCL. Proton density sagittal image (A), T2-weighted sagittal image (B).
Proton-dense–weighted sagittal image shows an exte Proton-dense–weighted sagittal image shows an extensive partial tear of the mid substance of the posterior cruciate ligament (large black arrow). The uppermost part of the ligament is intact on the proton-dense images. Low signal tendon is absent in the region of injury, and replacement by edema is seen. The normal ligament of Humphrey (small arrow) is visualized better because it is adjacent to the high signal intensity edema around the torn posterior cruciate ligament.
A sagittal fat-saturated proton density (sag FS PD A sagittal fat-saturated proton density (sag FS PD) image showing increased signal, abnormal thickness, and abnormal contour to the tibial insertion site of the PCL, indicating a partial thickness tear. This is an unusual site for tears because the ligament is strongest here. Avulsion injuries at this site are more common.

Complete absence of a portion of the ligament with interposition of hemorrhage and edema blurring the margins is present (see the image below). Focal areas of edema and hemorrhage can replace the ligament at the site of bone attachment. Disruption can involve the tibial insertion, the femoral origin, or the intersubstance. The PCL, with its accompanying bony fragment, is retracted away from the insertion site. Often, bone marrow edema exists at the fracture site. Focal areas of edema and hemorrhage can replace the ligament at the insertion site.

Proton-dense sagittal (A) and coronal (B) images d Proton-dense sagittal (A) and coronal (B) images demonstrate complete interruption of the proximal portion of the posterior cruciate ligament.

Associated bone findings include hyperextension injuries, which may demonstrate contusion of the tibial plateau and adjacent femoral condyle, and hyperflexion (dashboard) injuries, which may show contusion of the proximal anterior tibia.

The chronically torn PCL can demonstrate a mild diffuse increase in signal intensity on long TR/TE sequences. The ligament assumes a serpiginous course, becomes irregular in outline, and is no longer taut in flexion

Spontaneous repair may occur after an acute injury. The PCL may parasitize the blood supply of the ACL to propitiate this repair.

The ligament may retain its continuity but be replaced by a fibrous scar. Since scar and the normal ligament both are of low signal intensity, the anatomically intact but functionally torn ligament may be a source of interpretive error.

Arthroscopic findings

Arthroscopic findings in PCL injuries are both direct and indirect. Direct findings include damage to the PCL, similar to those seen on MRI, including midsubstance tears, interstitial tears, and avulsion fractures.

According to Fanelli et al, indirect findings occur secondary to PCL insufficiency and include the sloppy ACL sign and degenerative changes of the patellofemoral joint and medial compartment [58] ; the sloppy ACL sign demonstrates increased laxity of the ACL related to gravity-assisted posterior tibial drop back; the ACL tension returns to normal when the tibia is reduced.

Limitations of techniques

The sensitivity and specificity of MRI in making the diagnosis of PCL tears is high. Sensitivity has been reported to be as high as 100%; reports of specificity have ranged from 84-100%.

Unlike injuries to the ACL, PCL injuries may not compel an athlete to stop activity, providing a false sense of security. Tense hemarthrosis may not develop immediately, and frequently an initial lack of soft tissue swelling exists, which may result in a delay in diagnosis. [59]

Physical examination in the acute setting may be falsely negative because of hamstring spasm or hemarthrosis. The posterior sag sign in this setting may be normal. Examinations under anesthesia have been falsely negative.

The presence of other associated injuries, such as ACL, MCL, and meniscus injuries, may divert attention away from the PCL.

The anterior drawer test may be misinterpreted as positive because the examiner may not be aware that he or she may be merely reducing the gravity-assisted posterior sag.

Soft tissue swelling and ecchymoses over the anterior tibia are only indirect signs of PCL injuries. Further investigation with MRI is needed.

Plain radiograph limitations include the following:

  • Joint effusions, although commonly present, are not specific for PCL injuries; they are seen in myriad traumatic, degenerative, and neoplastic conditions
  • Avulsive fractures can be confused with a loose body
  • The poor soft tissue resolution offers no information concerning the status of the PCL or other ligamentous and cartilaginous structures of the knee

Arthroscopy limitations include the following:

  • The PCL is difficult to view arthroscopically; the caudal two thirds are seen poorly from an anterior approach because of synovium and subsynovial fat
  • The ligament is obscured by the ACL
  • The tibial attachment of the PCL is below the articular surface of the tibial plateau
  • An intact ligament of Humphrey may be mistaken for an intact PCL in patients with a PCL tear
  • The PCL can be obscured by a chronic torn ACL adhering to the PCL
  • Capsular tears may prevent joint distention, which is needed to visualize the PCL
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Differential Diagnosis

Regions of increased signal intensity within the ligament with normal morphology on T1WI require conventional T2, T2, or FS T2 FSE images to differentiate between eosinophilic degeneration (EG) or the magic angle phenomenon (MAP) and PCL tears. [60]

Eosinophilic degeneration may simulate an intrasubstance tear. It consists of focal regions of increased signal intensity within the ligament on short TR/TE sequences that lose signal on long TR/TE sequences. The contour and margins of the ligament are intact. EG is seen more frequently in older persons.

In the magic angle phenomenon, increased signal intensity may be present on the upward sloping portion of the PCL on short TE images, mimicking a tear. It is present in anatomic components of the ligament oriented 55° to the main static magnetic field, along the long axis of the magnetic bore. The phenomenon can be distinguished from a true PCL tear using long TE-weighted imaging sequences. When using proton-density imaging, the artifact may persist if the TE is 20 milliseconds or less. If the abnormal signal focus persists, a true PCL abnormality is present. Artifacts from MAP disappear when long TE sequences are used (see the image below).

Magic angle phenomenon. The T1-sagittal image (A) Magic angle phenomenon. The T1-sagittal image (A) demonstrates increased signal intensity at the apex of the posterior cruciate ligament, which might indicate a tear. The proton-dense–weighted image (B) demonstrates that region of the ligament to be normal. Anatomic components oriented approximately 55° to the main static magnetic field demonstrate this phenomenon.

A ganglion cyst may attach to the PCL. It can be differentiated from a true PCL tear by its well-marginated bright echogenicity on T2WI and an intact underlying PCL.

Giant cell tumors (GCT) of the PCL tendon usually are attached to the periphery of the ligament. The tumor usually is lobulated and multinodular and contains foci of decreased signal related to hemosiderin deposition. Sheppard et al reported that the structural integrity and configuration of the ligament are preserved in GCT. [61]

Bucket handle tears from the medial meniscus can be displaced laterally to lie beneath the PCL, giving the appearance of a double PCL. [62] The "double PCL sign" can be distinguished from a PCL tear by following the course of the normal PCL and by appreciating the meniscal tear (see the image below).

Proton-dense coronal images demonstrate a bucket h Proton-dense coronal images demonstrate a bucket handle tear of the medial meniscus within the free fragment displaced medially. This is termed the double posterior cruciate ligament sign and can be confused with a posterior cruciate ligament tear. Courtesy of Javier Beltran, MD, Maimonides Medical Center.
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Treatment and Prognosis

Treatment

An isolated partial midsubstance tear should be treated nonoperatively with aggressive physical therapy. [63, 64, 65] Avulsion fractures of the PCL require open repair. The prognosis for patients with these injuries is excellent.

Controversy surrounds the treatment of isolated tears. Although primary repair was advocated initially, results of conservative treatment have been promising. [66] Most patients have functionally stable and almost completely asymptomatic knees. [67]

The dysfunctional PCL may be compensated by quadriceps activity. Contraction of the quadriceps produces an anterior drawer bone that can correct the posterior sag caused by a torn PCL. Complete interstitial tears of the PCL should be treated if other ligamentous injuries of the knee are present, such as ACL, MCL, LCL, or arcuate ligament complex injuries. Chronic PCL tears that have failed physical rehabilitation should be treated surgically.

A critical length of the distal component of the torn PCL on MRI may predict the ability to perform early proximal femoral repair of the ligament versus reconstruction. Goiney et al studied 27 knees with complete disruption of the PCL that had been evaluated using preoperative MRI and underwent either early reattachment to the femoral insertion or reconstruction. Knees with a distal fragment PCL length of 41 mm or greater underwent early proximal femoral repair, with the distal stump being attached to the distal femur. Knees with a distal PCL length of 32 mm or less were not able to undergo repair because of insufficient length, and as a result, reconstruction was performed. [68]

Prognosis

Neurapraxia resulting from injuries to the tibial or peroneal nerves usually heals with conservative therapy within 18 months. Avascular necrosis of the medial femoral condyle can be seen following PCL reconstruction. The cause may be related to the size and location of the surgically created tunnel. [69, 70, 71]

Future ligament stability is predicted imprecisely solely on MRI findings. Patients with isolated PCL tears have a good prognosis because of a high degree of normal revascularization from the inferior genicular artery. Patients with intersubstance tears have a good prognosis. Tearing of the underlying synovium, as in partial or complete tears, exposes the torn PCL to synovial fluid. According to Andrish et al, synovial fluid has a deleterious effect on healing. [72, 73]

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