MRI for Anterior Cruciate Ligament Injury 

MRI may alter the treatment of anterior cruciate ligament (ACL) injury (see the images below) by allowing confident diagnosis or exclusion of an ACL tear in patients with equivocal physical examination findings; however, the greatest contribution of MRI in ACL-injured patients is in the evaluation of coexisting internal derangements.^[1] Four specific examples are discussed below.

The first example is posterolateral-corner (lateral collateral ligament complex) injury. Presence of this comorbidity with an ACL tear largely mandates ACL reconstruction. The lateral collateral ligament (LCL) complex is also usually repaired, in part, because unoperated LCL injuries predispose ACL grafts to early failure.^[2] Furthermore, LCL injury hastens surgery since these injuries are optimally repaired within 3 weeks (whereas surgeons usually favor delaying surgery on patients with isolated ACL injuries until knee range of motion is optimized).^[3] Injury to the posterolateral corner can easily be missed upon clinical examination and arthroscopy,^{[4, 2]} yet it is often detected on MRIs.^[5]

Second are posterior cruciate ligament (PCL) tears (see the images below). If both PCL and ACL tears are present, instability is usually profound and reconstructive surgery of both ligaments is ordinarily necessary. PCL tears are readily diagnosed by using MRI, but they can be difficult to detect on physical examination, even those performed by experienced examiners.

The third example is meniscal tears. Meniscal repairs have a higher rate of failure in ACL-deficient knees than in ACL-reconstructed knees.^[6] Therefore, conservative ACL management is less likely to be recommended in patients who are undergoing meniscal repair. In addition, MRI diagnosis of a displaced bucket-handle tear may serve to hasten arthroscopic surgery.^[4]

Fourth are extensor-mechanism abnormalities. MRI-enabled diagnosis of clinically significant patellofemoral chondromalacia, patellofemoral osteochondral fractures, or other extensor derangements, may mitigate against patellar-autograft ACL reconstruction. Instead, patellar allograft or alternative autograft reconstructions may be elected.

These four examples notwithstanding, Vincken et al point out that it is actually the evaluation of the joint as a whole (the composite diagnosis) that is key to selecting patients for therapeutic arthroscopy. The composite sensitivity of MRI is reported to be 87-94%, specificity 88-93%.^[7] The high composite sensitivity indicates that utilization of MRI should function to decrease the number of unnecessary arthroscopies performed in patients with knee injuries.^[1] A study by Thomas et al concluded that MR may be overutilized in high-probability ACL-meniscal injury settings, but it confirmed that a negative MRI is reliable in excluding pathologies benefiting from therapeutic arthroscopy.^[8]

The anterior cruciate ligament (ACL) is the most commonly injured of the major knee ligaments. These injuries plague both athletes and nonathletes. The ACL is a vital ligamentous stabilizer of the knee that resists anterior translation and secondarily resists varus and valgus forces.^[9] The ACL also functions as a mechanoreceptor that relays information about knee tension to the central nervous system. Patients with ACL injury have variable knee instability that may limit even ordinary daily activities. They experience particular difficulty pivoting and ambulating on uneven surfaces. The torn ACL undergoes limited healing. Long-term morbidity is common with sequelae including injury to the articular cartilage, secondary meniscal tears, and osteoarthritis.

Diagnosis

ACL injury is usually diagnosed on the basis of the patient's history and physical findings or MRIs. Arthroscopy and arthrotomy are the criterion standards for diagnosis, but they are invasive and costly.^{[10, 11, 12, 13, 14]}

The image below shows an acute ACL tear.

The skilled clinician can diagnose as many as 90% of ACL tears based on history and physical examination findings.^{[15, 16]} Patients typically report an audible pop and "giving way" at the time of injury. A knee effusion usually develops over the next 24 hours. A tear is confirmed by physical examination, primarily by performing the Lachman test.^[9] The anterior drawer and pivot shift tests are often helpful, and arthrometric examination may be contributory.

Physical diagnosis may be difficult in large patients, in patients with strong secondary muscular restraints, and in patients with an acute injury and soft tissue swelling and guarding. Partial ACL tears are especially difficult to diagnose on physical examination.^[17] MRI may provide pivotal diagnostic information about the ACL in all of these settings.^{[18, 1]}

Treatment

Treatment of ACL tears ranges from conservative therapies to surgical ACL reconstruction. The patient's activity level (and expectation for activity in the future) is the most important factor guiding the choice of treatment.^[9] Associated meniscal and ligamentous injuries, the degree of laxity, and the patient's age and willingness to pursue vigorous postoperative physical therapy are other major determinants. ACL graft reconstruction stabilizes the ACL-deficient knee, thus increasing the range of activities tolerated and preventing reinjury from repeated subluxation. ACL reconstruction, however, has not yet been definitively proven to prevent long-term osteoarthritic deterioration.^{[9, 19]}

As noted above, it is generally believed that late ACL reconstruction decreases postprocedural stiffness and improves outcomes. Surgery is often delayed until swelling has subsided and range of motion is restored.^[9]

For patient education information, see the Foot, Ankle, Knee, and Hip Center and Sprains and Strains Center, as well as Knee Injury, Knee Pain, and Magnetic Resonance Imaging (MRI).

The anterior cruciate ligament (ACL) is a dense fibrous band composed of collagen fibrils. It is about 3.5 cm long and 1 cm in transverse diameter.^[20] It originates from the posteromedial aspect of the lateral femoral condyle and courses through the lateral intercondylar notch in an anterior, inferior, and medial direction. It inserts on the tibia approximately 23 mm posterior to the anterior edge of the tibia, just anterior and lateral to the medial intercondylar eminence (tibial spine).^{[20, 21]} The ACL is not as strong as the posterior cruciate ligament (PCL), and it is less strong at its femoral origin than at its tibial insertion.^[20]

The ACL is organized into an indeterminate number of linear fascicles that are often partially visible on MRI. The fascicles diverge (fan) distally into a larger foot-like insertion on the tibia that facilitates tucking of the ACL under the anterior femoral intercondylar roof.^[20] ACL fascicles are organized into anteromedial and posterolateral bundles that are named for their location relative to each other at tibial insertion.^{[22, 20]}

The biomechanics of the anteromedial and posterolateral bundles are complex and are under increasing investigation. It has generally been stated that the strong anteromedial bundle tightens with flexion of the knee and resists anterior translation of the tibia in flexion while the posterolateral bundle tightens with knee extension and resists hyperextension.^[20] The differing anatomic attachments and roles of the two bundles are the basis for the current intense investigational interest in double-bundle ACL reconstruction surgery.

Note that the anteromedial and posterolateral ACL bundles are not usually distinguishable on MR images. However, cadaver studies have raised the possibility that dedicated 3-Tesla imaging may ultimately allow useful evaluation of these bundles.^{[23, 24]}

The physiologic property in which part of the spiraled ACL is taut throughout the normal range of motion of the knee is termed isometry. Graft isometry is one stated goal of reconstructive surgery. In truth, this is probably rarely achieved by current surgical methods.

The ACL is an extrasynovial and intracapsular ligament. Bands of mesenterylike synovium, arising from the posterior intercondylar region of the tibia, surround the cruciate ligaments.^[20] This feature accounts for fluid often seen anterior to the normal ACL (and posterior to the PCL) on MRI. The extrasynovial location also helps to explain why hemarthrosis is often delayed in the setting of an acute ACL tear.

The primary blood supply to the ACL derives from arteries to the surrounding synovial membrane. These, in turn, derive from branches of the middle geniculate artery piercing the posterior capsule.^[20] The central core of the ACL is relatively avascular. This may partly account for the generally ineffective healing of ACL tears. Tibial nerve terminal branches innervate the ACL.^[20]

Mechanisms of anterior cruciate ligament (ACL) injury are numerous. Alpine-skiing ACL injury studies have served to demonstrate the complexity of this subject: a welter of characteristic mechanisms of injury have been identified in skiers including aggressive quadriceps contraction, boot-induced injuries, "phantom foot" injuries, hit-from-behind injuries, and various types of valgus, rotatory, and hyperextension injury. ACL tear mechanisms only superficially discussed in this article.

ACL tears occur with or without contact and with the knee in any position from flexed to fully extended. A common contact mechanism of injury is the valgus-abduction clip injury.^[21] These injuries are frequent in football players and occur with a lateral blow to the partially flexed knee. Coexisting medial and lateral meniscal tears are common, as are medial collateral ligament (MCL) injuries.

Hyperextension or varus-hyperextension from an anterior blow (eg, injury from a motor vehicle accident or contact sports) is the second most common contact mechanism of ACL injury. The posterior cruciate ligament (PCL) and/or posterolateral-corner structures are also frequently injured. With more severe hyperextension, the knee may dislocate; the popliteal neurovascular bundle or peroneal nerve may be injured in this setting.

Noncontact mechanisms account for 70-80% of ACL tears.^{[21, 25]} The pivot-shift mechanism (see the images below) is most commonly implicated: the slightly flexed knee incurs a valgus load, with internal rotation of the tibia or external rotation of the femur. This twisting injury often occurs with rapid simultaneous deceleration and directional movements in skiers, football, basketball, or soccer players. Marked quadriceps loading at the time of injury has been implicated.^[25] Again, associated meniscal tears, collateral ligament injuries, or lateral patellar subluxation are common. Noncontact hyperextension, such as that occurring in a gymnast or cheerleader who misses a landing, is another mechanism of injury that often injures the ACL.^[21]

The incidence of ACL tears in females is increased over that of males for each hour of participation in activities at risk. Up to 10 times relative risk has been suggested; however, a review of the literature by Prodromos et al suggests a more modest 3.5 times greater risk (basketball being the sport with the greatest gender differential).^{[26, 27]} Explanations for this increased susceptibility are under debate.^[28] Lax joints, more common in females, appears to predispose patients to ACL injury.^{[29, 30]}

Relevant history and physical examination findings should be provided to the MR reader. Especially helpful is history regarding previous knee surgeries and dates of injuries. The authors have found it beneficial for technologists to place MRI markers at sites of pain and surgical scars.

Only basic guidance in performance of MRI of the knee is provided below.

Imaging protocols

Knee MRI protocols must be designed to yield diagnostic images of not only the ACL but also of the menisci, bones, articular cartilage, and other ligamentous structures of the knee. Furthermore, the requirements for good meniscus and cartilage imaging are more exacting than the requirements for diagnostic ACL imaging. As such, for the most part, a protocol that images the menisci and cartilage optimally also demonstrates the ACL adequately. This explains why most centers image patients in full knee extension, though the ACL is optimally evaluated with the knee in about 30° of flexion. Imaging in flexion complicates evaluation of the menisci and other knee structures.^[31]

The minimal protocol requirements of ACL imaging are sequences in all 3 planes (sagittal, coronal, axial), with T1-weighted (or proton density–weighted) and T2-weighted sequences in the sagittal plane.^{[32, 33]}

Any of the 3 imaging planes may prove pivotal in a given case (see the images below). The MR reader should routinely inspect the ACL in all planes and become familiar with the range of normal and abnormal in each plane. Axial images in particular provide a cross-sectional perspective free of partial volume artifact with the intercondylar roof and are invaluable in evaluation of the proximal ACL.^{[32, 34, 35, 36]}

Other technical considerations

The ACL is usually seen to greatest advantage on T2-weighted images, as opposed to T1-weighted or gradient-echo images obtained with short echo times.^{[37, 38]} This is due to confounding increased signal intensity seen in ligaments and tendons with short–echo time sequences owing to magic-angle effect and other factors. Fast spin-echo (termed turbo spin-echo by some vendors) T2-weighted sequences with fat saturation are performed faster than conventional T2-weighted sequences and have largely replaced these sequences.

Several methods for the prescription of sagittal images of the ACL by the MR technologist have been reported. Early recommendations were to allow patients to naturally externally rotate their legs and then to prescribe longitudinal images perpendicular to the table. However, this method leads to suboptimal, inconsistent results. The subsequent recommendation was to perform sagittal oblique slices 10-15° off a perpendicular to a bicondylar line along the posterior margins of the medial and lateral femoral condyles on an axial scout image. However, more recently, Garner et al have demonstrated that the true sagittal plane (perpendicular to the bicondylar line) is superior for evaluation of the ACL. (Unpublished data, Mayo Clinic, Jacksonville, FL. Presented at Society of Skeletal Radiology, March 2009) The authors have noted the same, and recommend this simpler approach.

Additional sequences

Katahira et al reported increased diagnostic accuracy prescribing oblique coronal images parallel to the long axis of the ACL off of an oblique sagittal image obtained as described above (double-oblique sequence).^[39] Several other investigators have reported similar findings.^{[40, 41]}

Investigators have also reported improved ACL visualization in the knee in mild (17-30°) flexion,^{[31, 42]} in part, because of decreased partial voluming of the proximal ACL with the intercondylar roof. Kinematic imaging for diagnosis of ACL tears has been proposed.^[43]

Ancillary methods of dedicated ACL imaging such as those described above have not found wide application. The authors suggest their use only when routine imaging is equivocal.

CT of the ACL

MRI cannot be safely performed in some clinical settings such as in patients with pacemakers. While reports are favorable regarding CT (or CT arthrogram) in the evaluation of the ACL, further studies are needed.^[44] Limited data indicate that CT may be reliable in confirmation of the negative ACL but less reliable in assessment of the torn ACL.^[45] Three-dimensional (3D) virtual CT has been described.^[46]

Normal MRI appearances

On sagittal images, the normal ACL (see the image below) appears as a solid band or as a striated band diverging slightly distally. As many as 4 striations may be present.^[33] The normal ACL is usually ruler-straight, though mild sagging convex inferiorly may be evident.

The ACL shows low-to-intermediate signal intensity, higher than that of the PCL. The distal ACL demonstrates relatively increased signal intensity, presumably due, in part, to distal divergence of fascicles. Data from one study confirmed that increased internal signal intensity is the result of macroscopic (rather than histologic) features of the ACL and that, in elderly patients, internal degeneration accounts for some of the observed increased signal intensity.^[47]

On coronal images, normal ACL (see the image below) is usually well seen, though fascicles often appear attenuated and single or few in number. The lateral position of the ACL in the intercondylar notch of the femur is apparent in the coronal plane; the PCL is seen medially.

In the axial plane, the proximal ACL is well seen (see the image below) and appears as an elliptical low-signal intensity band adjacent to the lateral wall of the upper intercondylar notch. It gradually moves away from the wall and splits into a horseshoe (fan-shaped) array of fascicles as it approaches its tibial insertion.^[34] The distal ACL is difficult to critically evaluate on axial images.

Pitfalls in interpreting normal findings

The ACL is poorly visualized in 5-19% of healthy knees in the sagittal plane.^[33] T1-weighted or gradient-echo sagittal images are especially likely to demonstrate the normal ACL poorly. However, the absence of hemorrhage or edema in the expected location of the ACL, a normal appearance of the ACL in other planes, and the absence of secondary signs of ACL injury is almost always sufficient to confirm that the ACL is normal.^[33] Smith et al noted that the ACL is highly likely intact when poorly visualized on either the T1- or T2-weighted sagittal sequence and normal in appearance on other sequences.^[48]

Partial-volume superimposition of the inner aspect of the lateral femoral condyle on the proximal ACL may produce a pseudomass that mimics an acute ACL tear on sagittal images.

If section thicknesses of 4 mm or less are routinely used and if other imaging planes are correlated, this is not a diagnostic problem.^[33]

The proximal origin of the ACL is often less well seen than the remainder of the ACL on sagittal images, partly because of its proximity to the adjacent intercondylar roof; however, this part of the ACL is usually well demonstrated on axial images. A repeat sagittal sequence with approximately 30° flexion of the knee may also be helpful.^[31]

Studies report variable 78-100% sensitivity and 68-100% specificity of MRI for the diagnosis of ACL tears.^{[32, 37, 38, 49, 50, 51, 52, 53, 8, 54]} Difficulties in diagnosis of proximal, partial, or chronic tears account for many of the errors in evaluation of the ACL. Sensitivity is also significantly decreased if other major ligamentous injuries are present in the knee.^[55] Data regarding children are less than that for adults. Decreased accuracy of MRI has been reported in preadolescents,^[56] but a study of patients aged 5-16 years demonstrated a sensitivity of 95% and a specificity of 88%.^[16] Accuracy is not affected significantly by magnet field strength.^[57]

Most ACL tears (about 70%) occur in the middle aspect of the ligament;^[20] 7-20% occur proximally near its origin. Only 3-10% occur distally at the tibial attachment.^[33]

Primary signs of ACL tear

Primary signs of acute ACL tear (ie, MRI abnormalities of the ACL proper) allow for high accuracy in the diagnosis of ACL injury, even in the absence of secondary signs.^{[16, 38, 49, 52, 58]}

The primary signs of an ACL tear include nonvisualization, disruption of the substance of the ACL by abnormal increased signal intensity, abrupt angulation or a wavy appearance, and an abnormal ACL axis. The axis of the ACL is abnormal if it is clearly more horizontal than a line projected along the intercondylar roof (Blumensaat line) on sagittal images. The ACL axis can be quantitated (although the authors have not found this to be necessary); a less than 45° angle of the long axis of the ACL relative to a line parallel to the tibial plateau (the ACL angle) is reportedly sensitive and specific for ACL tear.^[59]

A common presentation of an acute ACL tear is nonvisualization or near-nonvisualization, with replacement by a cloud of focal edema and hemorrhage. An acute tear manifesting as enlargement of the ACL and increased internal signal intensity but with visible intact fascicles has been termed an interstitial tear. These appearances must be differentiated from mucoid degeneration of the intact ACL (discussed later in this article).

Primary signs of tear involving the proximal ACL should be sought on axial images. The linear hypointense band representing the proximal ACL may be attenuated, fragmented, completely or partially replaced by hemorrhage, or displaced away from the lateral wall of the intercondylar notch.^[34]

Secondary signs of ACL tear

MRI findings of an ACL tear apart from abnormalities of the ACL proper are termed secondary signs. The sensitivity of these signs is limited;^[49] therefore, the absence of secondary signs in no way excludes ACL disruption. Certain signs, however (discussed below), have greater than 80% specificity for ACL injury. As a consequence, they may allow for a fairly confident diagnosis of tear when primary signs are equivocal.^{[49, 51, 52, 60, 61, 62, 63, 64, 65, 66, 67, 68]}

Secondary signs with high specificity for ACL injury include pivot-shift bone bruises/osteochondral fractures, anterior translocation of the tibia, and Segond fractures (see the images below).

Pivot-shift bone bruises and fractures

With a pivot-shift rotatory injury of the ACL, there is external rotation of the lateral femoral condyle relative to the fixed tibia. This shift allows the lateral femoral condyle to impact the posterolateral tibial plateau, frequently causing characteristic bone bruises of one or both bones.^{[38, 64, 66]} The lateral femoral condyle bone bruise is usually near the anterior horn lateral meniscus but may be more posteriorly located if the injury occurs during flexion. The tibial bone bruise subtends the posterolateral corner of the tibia.

With more severe injury, osteochondral fractures may accompany these bone bruises. MRI demonstrates linear subchondral fracture lines or cortical contour flattening. Sagittal MR images (and lateral radiographs) may reveal a "deep lateral femoral-notch sign" that manifests as an exaggerated (>1.5 mm-deep) condylopatellar notch of the lateral femoral condyle.^{[61, 69]} Pivot-shift fractures of the posterolateral tibial plateau are easily seen on MR but often occult on radiographs. This manifests as a subtle impaction fracture or as a posterior capsular bony avulsion fragment.^[70]

Characteristic pivot-shift bone bruises (and osteochondral fractures) of the tibia or femur indicate a greater than 90% likelihood of ACL injury.^[21] However, pivot-shift bone bruises without an associated ACL tear can occur, usually in the pediatric or adolescent population.^[71]

Contrecoup bone bruises occur frequently with ACL tears and involve the posteromedial tibial plateau at, or near, the semimembranosus tendon insertion.

Bone bruises were originally reported to persist on MR images for about 6 weeks.^[62] It is now clear however that bone bruises commonly remain visible on MRI studies 12-14 weeks after injury.

MRI-enabled diagnosis of pivot-shift bone bruises has prognostic implications. Articular cartilage injury is a significant determinant in the long-term development of osteoarthritis and subchondral bone bruises are a marker for underlying articular cartilage at risk. Further long-term studies are needed to assess this issue.

Anterior translocation of the tibia

Anterior translocation of the tibia is the MRI correlate to anterior drawer instability elicited on physical examination, and as such, indirectly suggests ACL incompetency. The radiologist should seek this finding on a sagittal image through the middle of the lateral femoral condyle. If the tibia translocates anteriorly to the extent that the distance between vertical tangent lines through the posterior margins of the femur and tibia exceeds 5 mm, acute or chronic ACL tear is highly likely.^{[60, 68]}

Tibial translocation is also present if a vertical line tangent to the posterior cortex of the tibial plateau courses through, or anterior to, the posterior horn meniscus (the "uncovered meniscus sign"). This occurs because the meniscus moves with the femur as the tibia translates anteriorly.

Segond fracture

A Segond fracture (see the images below) is a stereotypical fracture of the tibia that has a 75-100% association with ACL tear.^[20] The Segond fracture is an elliptical, vertical, 3 X 10-mm bone fragment paralleling the lateral tibial cortex about 4 mm distal to the plateau. Segond fractures have historically been attributed to traction avulsion of the middle third of the meniscotibial capsular ligament; more recently, slips of the iliotibial band and lateral collateral ligament complex have been implicated.^[72]

On MR images, the Segond fracture fragment is often inconspicuous and easily overlooked. In fact, the MR reader is often tipped off instead by associated focal bone bruise of the adjacent lateral tibial plateau. The Segond fragment demonstrates a marrow-edema pattern in the acute setting; in the long term, it usually shows isointensity relative to marrow (or low signal related to osseous sclerosis) and may fuse to the underlying bone.^{[20, 73]}

On plain films, a Segond fracture must be distinguished from a Gerdy tubercle bony avulsion anterolaterally (with iliotibial band traction); this fracture is optimally seen on a radiograph with external rotation. In contrast, a Segond fracture is best seen on a true anteroposterior radiograph (and is also readily shown on a tunnel view).^{[74, 20, 20]}

Fracture of the tibial spine

The ACL does not actually insert on the anterior tibial spine; it inserts immediately lateral to it. Thus, tibial spine fractures can be seen in patients with a normal ACL; nevertheless, the possibility of an ACL tear (or ACL insufficiency) should be borne in mind when these fractures are detected. Tibial spine avulsion with ACL insufficiency or injury indicates a hyperextension mechanism in most cases. While relatively more common in the pediatric population, the majority of ACL tears in children are not associated with a tibial osseous avulsion. Only 5% of adults with traumatic ACL insufficiency have an associated tibial avulsion. These injuries are often isolated in children, but they imply a high-force injury in adults and other internal derangements are usually present.^{[21, 75, 76, 77]}

On MRI images, tibial spine fracture fragments may be small and difficult to appreciate. The MRI reader must be alert for their presence, especially in MRI exams in children.

Several tibial spine fracture classification systems have been proposed.^[78] Treatment is somewhat controversial; however, surgery is often performed in the setting of displaced larger fractures.

Summary of fractures commonly associated with ACL injury

When interpreting knee radiographs, one must be alert for 4 principal fractures that have a high association with ACL injury or insufficiency: the Segond fracture, the lateral femoral condyle osteochondral fracture (corresponding to "deep sulcus" sign seen on MR images), the pivot-shift fracture of the posterolateral tibia, and the tibial spine avulsion fracture.

As noted above, the MR reader should be especially careful to search for subtle Segond fracture fragments (or adjacent associated focal bone bruises of the tibia) and tibial spine avulsions. These findings may be helpful in the setting of equivocal primary MR signs of ACL injury.^[69] CT correlation may be helpful in difficult cases.

Less useful secondary signs of ACL tear

Several secondary signs of ACL injury have relatively low specificity for ACL tear and are less useful than the signs discussed above.

Buckling or redundancy of the PCL^[79] occurs frequently with ACL tears, but it also occurs with hyperextension of the normal knee^[62] and with quadriceps dysfunction (see the image below).^[21]

Kissing anterior femoral and tibial bone bruises indicate a hyperextension injury and were found to be associated with ACL tears in about 50% of patients in one study.^[80] Similarly, avulsion fractures of the proximal fibula (termed the "arcuate sign") indicate a hyperextension/varus injury. The LCL complex is usually torn, and the ACL was torn in 13 of 18 patients in one study.^[81] PCL tears are also often present in hyperextension injuries. In severe cases, the knee frankly dislocates with extensive ligamentous injuries: the ACL is torn in most knee dislocations.^[82]

Cadaveric studies have shown no normal synovial recesses in the triangular space inferior to the intersecting ACL and PCL as observed on sagittal images. Therefore, Lee et al hypothesized that fluid in this location may indirectly indicate abnormality of the cruciate ligaments, but this has not been confirmed in a clinical setting.^[83] As noted previously, fluid recesses anterior to the ACL are a common finding in normal ACLs. Edema in the region of the ACL is an abnormal but nonspecific finding;^[67] other evidence is needed to make a definitive diagnosis of tear.

Partial ACL tear

Partial tears of the ACL are not uncommon, accounting for 10-43% of all ACL tears^{[16, 17, 34, 84]} and account for a higher percentage of ACL tears in the pediatric population.^[85] The natural history and optimal treatment of these injuries is still being worked out.^{[17, 86]} A tear involving less than 25% of the ACL has a favorable prognosis; a tear involving 0.5-0.75 of the ACL has a 50-86% probability of progressing to a complete tear.^[17] Prognosis is guarded overall, with 38-62% of stable conservatively treated patients progressing to instability.^{[84, 86, 87]}

On physical examination, partial tears are often difficult to diagnose. Factors including soft tissue swelling and guarding may compromise the examiner's ability to evaluate laxity.^[17] In cadavers, laxity is absent on physical examination and arthrometric testing when only the anteromedial band of the ACL is transected. Thus, it is not surprising that some patients with only mild detectable laxity (suggestive of partial tear) actually have a complete tear.^[88]

Although MRI is accurate in differentiating the normal from abnormal ACL, it is unreliable in the diagnosis of partial tears.^{[62, 86, 89]} Differentiating high-grade partial ACL tears from complete tears is generally not possible. For example, even patients with a completely nonvisualized ACL with hemorrhage, or patients with an abnormally horizontal ACL axis on MRI, occasionally have partial rather than complete tears.^[86] Further, secondary signs of ACL injury, such as bone bruises, while correlating generally with severity of injury, do not necessarily indicate a complete tear.^[65] In the converse, patients with normal knee MRIs are found occasionally to have partial ACL tears. False-negative MRIs in patients with a partial ACL tear are especially likely if the tear is partial or chronic.

These limitations notwithstanding, MRI clearly does reveal abnormalities in some patients with partial tears and normal physical examinations; positive MRI findings should not be ignored outright because of a negative Lachman test. MRI has at least a theoretical advantage over the criterion standards of arthroscopy or arthrotomy with regard to partial intrasubstance tears. As such, MRI probably maintains some utility in deciding between arthroscopic and conservative treatment.

In view of the ambiguities discussed above, what should be the approach of the MRI reader in interpreting the ACL? In general, the possibility of a partial tear should be raised when there is high probability of a tear clinically, and the MRI is equivocal. As examples, a partial tear can be suggested in the following settings: (1) unequivocal direct signs of ACL tear are present but at least one section shows a straight, taut-appearing ACL;^[90] (2) a partial-thickness increased signal intensity is observed in the substance of the ACL^[90] (care taken to avoid mistaking this finding for intra-ligamentous degenerative changes or partial-volume imaging of normal synovial recesses that closely invest the ACL);^[86] and (3) secondary signs of ACL tear (eg, pivot shift bone bruises) are present but the ACL proper appears normal.

Such problem cases obviously require diligent clinical and Lachman-test correlation, and the MRI reader could consider additional problem-solving MRI sequences. Ultimately, the MRI reader should, in most cases, be able to place findings into either a complete/high-grade tear (high risk) category or a normal/ low-grade tear category (low risk).^{[34, 35, 55, 90, 91]}

Vincken et al reported that the decreased accuracy of MRI for the diagnosis of partial ACL tear is to some degree irrelevant to patient care.^[7] This is because high-grade tears usually coexist with other detectable major knee injuries, allowing patients to be appropriately selected for arthroscopy. The converse is true for relatively low-grade tears. In other words, the high composite (whole-joint) accuracy of MRI partially overcomes limitations in ACL evaluation with regard to management of patients. There is also hope that advances in technology, such as the advent of 3-Tesla imaging, will improve partial tear diagnosis. Studies by Steckel et al have indicated the potential of MRI to resolve the anteromedial and posterolateral ACL bundles.^{[23, 24]}

Treatment recommendations for patients with partial ACL tears are evolving. Factors favoring conservative treatment include advanced age, a normal or near-normal Lachman result, low athletic demands, and less than 50% involvement of the ACL fibers on arthroscopy. Most young and highly active patients, patients with a clearly abnormal Lachman result, and patients with greater than 50% or posterolateral band involvement on arthroscopy are best treated with ACL reconstruction.^[92]

Chronic ACL tear

The MRI reader will not uncommonly encounter nonacute ACL tears. These injuries are often associated with meniscal tears and secondary osteoarthritis. (See the images below.)

MRI signs of chronic ACL tear are largely the same as those of acute ACL injury except that bone bruises and edema about the knee are absent.^{[67, 93]} The most common MR finding with chronic tear is a fragmented ACL.^[67] Complete nonvisualization of the ACL may also occur with only fat signal intensity evident in the lateral intercondylar notch, the "empty notch" sign.^[33]

The chronically torn ACL tear may attach to the PCL.^[21] The authors, however, have noted that this is most often an endoscopic observation and is less frequently appreciated on MRI, even in retrospect. These patients may have a clinical endpoint of anterior translation of the tibia with Lachman testing, resulting in a false-negative clinical examination.

The diagnosis of chronic ACL tear by MRI is usually straightforward; however, false-negative results occur. The chronic nondisplaced ACL tear may appear entirely normal,^[67] presumably because hypointense mature scarring is difficult to distinguish from the normal hypointense ligament. In other instances, the only sign of a chronic ACL tear may be a very subtle angulated contour or flattened axis of the ACL.^[94] The MRI reader must be especially diligent in the nonacute injury setting to avoid underdiagnosis;^[67] in the setting of a positive Lachman test result, a negative MRI should be viewed as a possible false negative.

Mucoid degeneration and ganglion cysts of the ACL

Mucoid degeneration of the ACL can mimic an interstitial ACL tear.^{[95, 96]} The etiology of this uncommon entity is uncertain, but it may lie along a continuum of senescent degeneration of the ligament.^[96] It is increasingly evident to this author and others that mucoid degeneration of the ACL is a fairly common entity.^[97]

Reported patients are usually older than 30 years. Patients may be asymptomatic, but they frequently have pain and limited flexion of the knee. The knee is stable, with a negative Lachman test result.

On arthroscopy, the ACL is enlarged and often impinges on the intercondylar notch sidewalls or roof. Histologic examination of the ligament demonstrates extensive, patchy, yellow internal deposits, which represent a mixture of fibrous elements and mucoid degeneration.

Treatment has consisted of mainly meticulous piece-by-piece debulking of the yellowish material. Some fascicles of the ACL may be sacrificed in this procedure. Notchplasty may also be performed to reduce ACL-notch impingement.

MRI appearances are characteristic. The ACL is enlarged with diffusely increased non–fluid-like increased signal internally that splays apart intact ACL fascicles. The splayed linear ACL fascicles have a "celery-stalk" appearance. Appearances could be mistaken for an interstitial ACL tear; however, discordant history, a negative Lachman result, and a lack of secondary signs of ACL tear should suggest the correct diagnosis.^{[21, 95, 96, 98, 99, 100]}

Intraligamentous and extraligamentous ganglion cysts of the ACL are a related but distinct entity. The extraligamentous cysts are extremely common but do not present a diagnostic problem. These appear as well-defined lobular, often septated, cysts immediately adjacent to the ACL. These cysts are usually asymptomatic incidental findings, though a variety of symptoms has been reported. The smaller cysts may be difficult to differentiate from normal synovial recesses.^[98]

Ganglion cysts in the substance of the ACL are less common but have been reported in all age groups.^{[101, 102, 103, 104, 105]}

On MRI, ganglion cysts appear as fusiform well-defined cysts oriented along the long axis of the substance of an otherwise normal-appearing ACL. When the cysts are small, this appearance may be mistaken for that of a partial ACL tear,^[102] but differentiation is usually not difficult.

Several authors have described ACL ganglion cysts that developed in young patients after trauma.^{[101, 106]} As in diffuse mucoid degeneration of the ACL described above, the knee is stable with a negative Lachman result. However, symptoms may be clinically significant, and patients often benefit from arthroscopic probing with release of the mucinous material, with or without partial debridement.^{[20, 98]}

Ganglion cysts of the ACL can be difficult to appreciate on standard arthroscopy. Therefore, diagnosis may depend on MRI; the MRI reader who recognizes the abnormality may alert the arthroscopist to probe the ACL or add a posterior portal approach.^{[98, 99, 96]}

Deltoid-shaped ACL

Calpur et al reported 2 patients with a markedly widened deltoid (triangular) distal ACL at the tibial insertion.^[107] Symptomatic impingement of the intercondylar-notch structures was reported, and successful trimming of the distal ACL (ligamentoplasty) was performed in both patients.

Recommendations

The goal of this article is to educate the MR reader in interpretation of the normal and abnormal ACL. To this end, several points should be reemphasized. Regarding generation of images, one should obtain spin-echo or fat-saturated fast spin-echo images in all 3 planes, including both T1- and T2-weighted sagittal images. Sagittal images should be obtained in the true orthogonal plane: oblique-sagittal imaging of the ACL is no longer recommended for routine imaging.

One must become familiar with normal and abnormal appearances of the ACL in all planes. On sagittal images, one should critically evaluate the ACL axis relative to the intercondylar roof. The proximal and distal aspect of the ACL should also be carefully evaluated; tears or osseous avulsions may be readily overlooked in these locations. Axial sequences are especially useful in evaluating the proximal ACL. One should also look for secondary signs of ACL tear, including the often subtle MRI findings in Segond fractures.

One should be wary of a normal MRI in a patient with a positive Lachman test result and a history of remote injury; look for subtle flattening of the ACL axis or subtle angulation of the ligament as clues of a chronic tear. In the converse, be wary of overdiagnosing an interstitial tear in patients with mucoid degeneration of the ACL. Lachman test results should help in differentiating these entities.

In difficult cases, obtain additional history. One may also consider performing an additional flexed-knee imaging or a double-oblique oblique coronal thin-section T2-weighted sequence aligned along the long axis of the ACL.

Finally, one should avoid satisfaction-of-search error. When the ACL is torn, look especially diligently for other internal derangements, especially peripheral lateral meniscal tears.

Future of ACL imaging

Much remains to be learned about MRI of the anterior cruciate ligament. Studies to determine the association between MRI findings and long-term functional patient outcomes are lacking. There is also much room for improvement in MRI diagnosis of partial and chronic tears of the ACL. Fortunately, faster and better MRI images can be anticipated as a result of continued technological advances in instrumentation, software, and contrast agents.