Magnetic resonance imaging of the knee ligaments

Magnetic Resonance Imaging of the Knee Ligaments Alexander Stork, John F. Feller, Timothy G. Sanders, Phillip EJ. "firman, and Harry K. Genant HE KNEE IS a major weightbearing joint in the body. Beyond being a simple hinge joint, the knee is able to also perform anteroposterior, lateral, and rotational movements. The integrity of the knee joint depends on the muscles and tendons about the knee, the articular capsule, the intrinsic ligaments of the joint, and the bone architecture of the tibia and femur. 1 Ligamentous injuries of the knee are very common many of them resulting from trauma and sporting activities. 2 Before the introduction of magnetic resonance imaging (MRI), the diagnosis of internal derangement of the knee was mainly based on clinical examination, conventional radiography, conventional--and computed tomography (CT)--arthrography and arthroscopy. Since the early 1980s, MRI has become increasingly important in the imaging of knee pain. Today it is the primary imaging tool in the management of knee pain, particularly when internal derangement is suspected. MRI is free of risks and does not expose the patient to any radiation. Arthroscopy remains the standard for evaluation of internal derangement of the knee, but this technique is invasive and associated with a considerable risk for the patient. This article describes the normal anatomy and normal and pathologic MR appearance of the ligaments of the knee. It provides an overview of well-known ligamentous structures as well as ligaments that have recently received special attention in the literature, such as the oblique meniscomeniscal and medial patellofemoral ligament. The fibrous connection between the patella and tibial tubercle was considered a tendon (patellar tendon) and was therefore not included in this article.

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From the Osteoporosis & Arthritis Research Group, Department of Radiology, University of California, San Francisco, San Francisco, CA. Address reprint requests to Alexander Stork, MD, Osteoporosis & Arthritis Research Group, Department of Radiology, University of California, San Francisco, 350 Parnassus Ave, Suite 150, San Francisco, CA 94117. Copyright 9 2000 by W.B. Saunders Company 0037-198X/00/3503-0008510. 00/0 doi: 10.1053/sroe.2000. 7336

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TECHNIQUE

Patient Positioning and Coil For a routine MR examination the patient lies in the supine position with the feet entering the MRI machine first. Patient comfort is essential because even small pressure points can lead to patient movement and subsequent motion artifacts in the MR images. The knee is placed slightly bent in a circumferential extremity coil (with a send-receive, quadrature, or phased-array design), which provides a uniform signal-to-noise ratio across the knee. The joint space of the knee should be located at the midpoint of the coil. Markers can be attached to points of tenderness or palpable lesions. To align the anterior cruciate ligament parallel with the sagittal imaging plane the knee is routinely placed in a 10 to 15 degree external rotation. 3 This is less important as the slice thickness becomes thinner (<--3 ram). Too much external rotation may result in decreased accuracy in visualizing the menisci. An alternative to external rotation are oblique sagittal images parallel to the ACL as assessed on the axial localizer.

Imaging Protocols There are various combinations of sequences that can be used in routine MRI of the knee. Although there is no best combination, certain principles can help in designing a comprehensive protocol. 4 Routine MRI of the knee is performed in the axial, sagittal, and coronal planes. Some form of T2 weighting should be used in each of the three planes. Most authors recommend Tl-weighted sequences in the sagittal plane and optionally in the coronal plane. T2-weighted pulse sequences can contain proton-density-weighted images as doubleecho sequences. The availability of fast spin-echo (FSE) sequences has greatly improved the time efficiency of knee MRI. In T2 weighing with fat suppression, FSE sequences are sensitive to bone marrow edema and cartilage lesions and fluid collections in general. The echo train used should be short (-----4) to minimize image blurting especially for TE's in the intermediate-weighted range. However, the FSE technique is less sensitive to meniscal tears and should only be used for the Seminars in Roentgenology, Vol XXXV, No 3 (July), 2000: pp 256-276

MRI OF THE KNEE LIGAMENTS

evaluation of the meniscal cartilage in conjunction with T1 or PD SE or T2* GE images. The slice thickness of a routine imaging protocol should not exceed 4 mm in all planes. If needed for specific questions submillimeter resolution can be obtained with three-dimensional gradient-echo sequences. An alternative method of achieving a form of T2 weighting is the short tau inversion recovery (STIR) technique. Like the FSE technique this sequence is also valuable for the assessment of bone marrow. For the morphologic evaluation of hyaline cartilage 3D-GRE techniques yield the best contrast between cartilage and intraarticular fluid and fatty tissue while at the same time providing higher spatial resolution compared to the SE and FSE techniques. When evaluating the major ligaments of the knee joint, the cruciate ligaments should be evaluated primarily in the sagittal plane with the coronal and axial images giving important information as well. In the sagittal plane a double-echo (T2- and protondensity-weighted) spin-echo sequence is widely used. The collateral ligaments are best visualized in the coronal and axial planes with either fatsuppressed T2-weighted fast spin-echo, STIR, or T2*-weighted gradient-echo sequences. Frequently, one of these sequences is supplemented with a Tl-weighted sequence without fat suppression in the coronal plane.

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spine itself (Fig 1 A ) . 7 Some fibers blend with the attachments of the lateral meniscus. Some posterior fibers attach to the intercondylar sulcus. The ACL is a fan-shaped structure--38 and 11 mm in average length and width, respectively7---creating a slight spiral as it crosses the joint. The ligament is divided into anteromedial and posterolateral bands that undergo differential tightening in flexion and extension. 7-9

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THE CRUCIATE LIGAMENTS

The anterior and posterior cruciate ligaments (ACL and PCL) act as major restraints to lateral rotation and anteroposterior motion of the tibia on the femur, 1 with the collateral ligaments and capsule acting as secondary restraints in these planes of motion. Furthermore, the cruciate ligaments are secondary restraints to varus and valgus stress. 5 Located intracapsularly and extrasynovially, they are enveloped by a fold of synovium that originates from the posterior intercondylar area of the knee. 6

Normal Anatomy of the A CL Proximally, the ACL originates from a fossa on the posteromedial aspect of the lateral femoral condyle. Distally, the ACL extends inferiorly and medially and attaches to the anterior intercondylar fossa anteriorly and laterally to the anterior tibial spine between the anterior attachments of the menisci. It does not attach to the anterior tibial

Fig 1. Normal ACL. (A) Anteromedial view of the ACL (Reprinted with permission. 21) (B) Sagittal Tl-weighted image in the plane of the ACL, The fibers of the ligament are usually taut and low in signal intensity (arrows),

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Normal MR Appearance of the ACL The A C L may be visualized on sagittal, coronal, and axial planes, ~~ although it is generally agreed that the ACL is best seen on sagittal images (Fig 1B). The anterior fibers of the ACL usually have low signal intensity on all pulse sequences. Predominantly the middle and posterior aspects of the ACL can demonstrate varying amounts of higher signal fat and loose connective tissue between the fibers. In general, the ACL tends to have a higher signal intensity than the PCL on Tl-weighted as well as T2-weighted images, but the signal intensity can be the same in both ligaments. The normal ACL appears straight even though a slightly lax A C L in not necessarily pathologic because tautness may be

affected by knee position. The should normally be at least as staat's line. The anteromedial bands of the A C L are difficult MR images. 4.9,11-13

slope of the ACL steep as Blumenand posterolateral to differentiate on

Traumatic Pathology of the ACL The A C L is one of the most frequently injured ligaments of the knee. ~-The function of the A C L as a primary knee stabilizer, the frequency with which it is injured, and the success of surgical reconstruction all make the diagnosis of an A C L tear important. The rationale for ACL reconstruction is the finding that without surgery, the ACL-deficient knee deteriorates and becomes more symptomatic

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C Fig 2. Complete ACL tears. Direct signs (A) Tl-weighted image of a mid-substance tear demonstrating discontinuous and indistinct fibers with abnormally high signal intensity (arrowheads). (B) Femoral attachment tear: Indistinct fibers seem to be ending abruptly near their femoral origin (arrow). In addition the slope of the ACL seems to be decreased in this region. This is confirmed on the T2-weighted coronal image (C) demonstrating edematous disrupted fibers near the femorai attachment of the ACL (arrows). (D) 11bial spine avulsion. A bony fragment (curved arrow) attached to fibers of ACL has been avulsed off the tibia. The fibers of the ACL are abnormal as well indicative of a partial tear or sprain (straight arrow).

MRI OF THE KNEE LIGAMENTS

with time. Clinically, this is manifested by instability, pain, and the early onset of degenerative changes. Consequently, reconstructive procedures most often using a patellar tendon graft are favored over conservative treatment especially in the young and active patient group. 14-16 On MRI, there are direct and indirect signs of a complete ACL tear. Direct signs are discontinuity of the ACL, abnormal ACL signal, and an abnormal contour/course of the ACL. ~~176 The slope of the completely torn ACL is usually not as steep as Blumenstaat's line. Mid-substance tears of the ACL are the most frequent rupture types (about 75%, Fig 2A), followed by proximal femoral attachment tears (20%, Fig 2B and C) and tibial avulsion fractures (5%, Fig 2D). 21 Impaction associated with twisting is the rather unique mechanism of injury to which most ACL tears can be attributed. The tibia rotates internally relative to the femur thereby causing the

Fig 3. Complete ACL tears. Indirect signs. (A) The PCL appears "buckled" (arrow). (B) The tibia is translated anterior to the femur (big, black arrow). The posterior horn of the lateral meniscus is subluxed posteriorly (white arrow). Also on this intermediate-weighted image the bone contusions (kissing edema) can be identified by lower signal intensity water replacing higher signal intensity fat (small arrows). (C) These contusions are better visualized on T2-weighted fat-suppressed images in this example in the coronal plane (arrowheads).

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mid to anterior lateral femoral condyle to be compressed against the posterior lateral tibial plateau (kissing contusion). 22 Clinical tests to evaluate rupture of the ACL include the Lachman, flexionrotation-drawer, anterior drawer, jerk, pivot-shift, and Losee (in the order of sensitivity), and are basically reproductions of this abnormal motion that leads to an ACL tear. 21 As sequelae of the trauma mechanism and the resulting ACL deficiency, indirect signs at MR122-32 can be observed including an abnormal contour of the PCL, anterior tibial subluxation, uncovered posterior horn of the lateral meniscus, bone contusions--located as described above--and a deep lateral femoral notch (Fig 3). An association of ACL tears with bone contusions of the posterior lip of the medial tibial plateau has also been reported. 27 Concomitant TCL and medial meniscal injuries are common and should always be sought. 23 Depending on the

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with coexisting continuous fibers on the same or a different pulse sequence and a bowing or undulating contour of otherwise intact ACL fibers (Fig 4). A partial tear should only be diagnosed if the highly specific indirect signs of a complete ACL tear are absent. However, using these criteria MRI does not seem to be accurate in diagnosing partial ACL tears. 38

Normal Anatomy of the PCL

Fig 4. Partial tear of the ACL. The fibers of the ACL demonstrate an irregular undulating contour but are essentially continuous from the femoral to the tibial attachment (arrows).

magnitude of force on the bones, depression fractures of the posterior lateral tibial plateau are the most common. Several studies have reported the sensitivity and specificity of MRI for complete ACL tears to be above 90%. 10,17,19,23,24,26,28,30 Acute tears are easier to identify by the presence of edema, whereas chronic tears have a somewhat more variable appearance with bridging fibrous scars that can mimic an intact ligament. 2~ The most important criteria for diagnosing an ACL tear are the direct signs. 23 Among the indirect signs, bone contusions especially of the posterior lateral tibial plateau and an uncovered posterior horn of the lateral meniscus are highly specific for ACL tears but do not by any means preclude an ACL tear if n o t present. 22'23"26'28 In terms of imaging technique, it is advisable to evaluate the ACL in all imaging planes.l~ However, equivalent results for diagnosing complete ACL tears have been reported for sagittal images with proton-density and T2-weighted FSE images only. 33 According to some authors, the knee position inside the magnet has an influence on the diagnostic potential of MRI for the detection of ACL tears the partially flexed position being superior to the fully extended position, 34,35 The accurate diagnosis of a partial tear of the ACL is much more difficult than that of a complete tear especially when it comes to predicting the stability of the ligament and the likelihood of progression to a complete ACL tear, thereby potentially necessitating reconstructive procedures. 36'37 Criteria for a partial ACL tear are abnormal signal

The PCL originates in the concave lateral aspect of the medial femoral condyle, crosses the ACL, and attaches to the posterior intercondylar fossa and the posterior aspect of the tibial plateau (Fig 5A). The lateral meniscus extends some fibers to the PCL as it does with the ACL. The PCL's thickness decreases from its proximal to distal

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Fig 5. Normal PCL. (A) Posterolateral view of the PCL. (Reprinted with permission. 21) (B) Sagittal Tl-weighted image in the plane of the PCL. The PCL (arrows) is homogeneously dark in signal intensity, convex posteriorly, and has welldefined borders.

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Fig 6. Injuries to the PCL. (A) Tibial avulsion fracture. A bone fragment of the posterior tibial plateau only slightly displaced from the tibia is seen (white arrow). The PCL appears to be abnormal as demonstrated by the abnormal signal intensity (black arrows). (B) Partial tear of the PCL. The shape of the PCL is highly irregular (small arrows) and has focal areas of high signal (long arrow) intensity. However, most fibers are continuous. (C) Complete tear of the PCL. On this Tl-weighted image the course of the PCL is shown to be abnormal. At its midportion the ligament turns posteriorly and upward and ends abruptly (arrows). The femoral attachment is not visualized.

Fig 7. Anterior cruciate ligament cyst. This T2-weighted image shows circumscribed, multUoculated, high signal intensity cystic changes (closed arrows) posterior to the fibers of the ACL (open arrows).

Fig 8. Cystic changes near the tibial attachment of the ACL. The cyst contains high signal intensity fluid (T2weighted image) and is multiloculated (arrowheads).

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attachments. On average, the PCL is slightly thicker than the ACL but has about the same length. 7,8

Normal MR Appearance of the PCL The normal PCL is almost always visualized on more than one sagittal image. It usually has a diffuse low signal intensity regardless of pulse sequence (Fig 5B). In general, the signal intensity of the PCL is lower than that of the ACL on T1- and T2-weighted images but can also be the same. The PCL is curvilinear, convex posteriorly, and resembles a boomerang in appearance. It usually has well-defined borders. The ligament becomes increasingly taut in flexion and shows laxity in hyperextension. 1,8,9.39

Pathology of the PCL Tears of the PCL are far less common than tears of the ACL. In a report by Loos et al,4~ the percentage of PCL injuries among patients with knee surgery was only about 0.8%. The exact prevalence is difficult to estimate and likely to be higher because PCL injuries are difficult to diagnose clinically and might therefore be overlooked. 41,42 They occur most often during severe trauma like motor vehicle accidents or sportsrelated injuries, 4~ and coexisting injuries of the TCL, ACL, and menisci are common.4~ Mechanisms of injury include posterior translation of the tibia in a flexed knee (dashboard injury), forced hyperextension, continued valgus angulation after rupture of the ACL and TCL, and hyperflexion. The treatment of PCL injuries is very controversial. As there is evidence that PCL-deficient knees develop early degenerative changes, 43 there seems to be a trend towards surgical repair in patients with associated pathology and multidirectional instability. 41,44 Patients with avulsion fractures of the PCL should be treated surgically (Fig 6A). 45 The MRI direct signs of PCL tears are similar to those described for ACL tears: failure to identify the PCL, abnormal signal on all pulse sequences without continuous fibers, or depiction of PCL fibers with a focal discrete disruption of all visible fibers (Fig 6B). A partial tear or intrasubstance injury refers to PCLs that demonstrate signal abnormalities, disrupted fibers with continuous fibers, or abnormal contours (Fig 6C). 39'46'47 There are no specific indirect signs for PCL ruptures, even though only about 30% of PCL ruptures occur as the sole pathology. Usually there are other coexist-

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ing bony, ligamentous, and meniscal injuries. Similar to the ACL, the PCL can rupture at different sites, in this case the tibial avulsion fracture being the most frequent (70%, Fig 6A) followed by the mid-substance and femoral tear at about equal proportions. 21As with the ACL, injuries to the PCL must be confirmed on coronal and axial images as well.

Nontraumatic Pathology of the Cruciate Ligaments The most important nontraumatic pathology of the cruciate ligaments are cruciate ligament cysts, also called intraarticular ganglia, intraligamentous cysts, intercondylar cysts, or cruciate ganglionic cysts. The reported prevalence on MRI examinations of the knee ranges from 0.2% to 1.3%. 48-5~ Cruciate ligament cysts may be symptomatic, the typically painful symptoms resembling other internal derangement of the knee. Attempts to treat symptomatic intraarticular ganglions have been made with good results including arthroscopic resection and CT-guided biopsy. 48,49 The etiology of intraarticular ganglia is still unclear. Theories include hemiation of the synovium into surrounding tissue, displacement of synovial tissue during embryogenesis, degeneration of connective tissue after trauma, and proliferation of pluripotent mesenchymal cells. On MRI intraarticular ganglions are ovoid and well-circumscribed with signal intensity isointense to fluid including low Tl-weighted and high T2-

Fig 9. Mucoid degeneration of the ACE The fibers of the ACL are indistinct and abnormally high in signal intensity but remain continuous and taut (arrows), In the absence of trauma this represents mucoid degeneration of the ACL.

MRI OF THE KNEE LIGAMENTS

weighted signal intensity. Most intraarticular ganglia are multiloculated (Fig 7A). 5~ Care must be taken not to misdiagnose an intraarticular ganglion as a meniscal cyst. 52,53 Meniscal cysts are usually associated with a meniscal tear, occur laterally more frequently than medially, and are situated at the base of the menisci in most cases. Nevertheless, they can also expand into the region of the intercondylar notch. Cystic bone changes around the attachment sites of the cruciate ligaments (Fig 8) are observed in about 1% of MR examinations. 54 These probably represent resorptive processes or intraosseous ganglions that might develop secondarily to stress exerted on the bone by the cruciate ligaments. Another nontraumatic entity that has not been well recognized in the literature yet is mucoid

Superficial fibl of the M Deep fib~ of the MC Posterior oblique lig

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Fig 10. Normal MCL. (A) Medial view of the t w o layers of the MCL. (Reprinted with permission. 2~) (B) Coronal Tl-weighted image in the plane of the MCL. The TCL (superficial MCL) can be identified as a smooth low signal intensity band (black arrows). In the absence of joint effusion the meniscotibial and meniscofemoral ligaments (white arrows) abut the femur/tibia and can be difficult to identify. (C) Sagittal proton-density-weighted image depicting the meniscofemoral (white arrow) and meniscotibial (black arrow) ligaments posterior to the medial meniscus (see also Fig 15A).

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degeneration of the cruciate ligaments. In the absence of trauma the cmciate ligament appears thickened and abnormal in signal. The poorly defined fibers remain recognizable, are continuous and taut, sometimes compared with a celery stalk (Fig 9).

THE MEDIAL AND LATERAL COLLATERAL LIGAMENT The medial and lateral collateral ligaments (MCL and LCL) are integral parts of the medial and lateral supporting structures and should therefore be viewed in conjunction with important adjacent structures. The LCL and surrounding supporting structures being more posterior than the MCL are also referred to as the posterolateral complex.

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The Medial Supporting Structures The medial supporting structures of the knee are divided into three layers. 55 The most outer layer (I) consists of the deep fascia surrounding the sartorius muscle and overlies the gastrocnemius. The middle layer (II) is the superficial MCL, the strongest and also palpable part of the MCL. To better distinguish the superficial MCL from the deep MCL (see next section), it has also been termed the tibial collateral ligament (TCL). This terminology is also used in this article. The deep layer (III) consists of the medial capsular ligament. This ligament consists of two parts referred to as the meniscotibial (coronary) and meniscofemoral ligament connecting the medial meniscus to the outer margins of the tibial plateau and the femur. The capsular ligament is sometimes referred to as the deep MCL. The semitendinosus and gracilis muscles pass between layer I and II. Posteriorly the semimembranosus tendon merges with the joint capsule at its tibial insertion in layer III. The anterior extension of the three layers on the medial side of the knee is described below in the section on the medial patellofemoral ligament. The TCL is about 8 to 11 cm long and is proximally attached to the medial femoral epicondylus just distal to the adductor tubercle. Distally it inserts about 5 cm inferior to the tibial plateau posterior to the pes anserinus insertion. Posteriorly the TCL and the capsular ligament (layer III) merge to form the posterior oblique ligament, and in this way establish a close connection between the TCL and body/posterior horn of the medial meniscus (Fig 10A). 55-57The TCL and capsular ligaments are separated by a bursa, which reduces friction during knee flexion. 58,59 The function of the TCL varies with knee position. In extension, it is taut and it limits hyperextension. When the knee is flexed the TCL provides primary valgus stability. The importance of the TCL function increases as flexion increases, and the posterior capsular structures become more lax. 5

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in its entire length from its femoral to tibial attachment (Fig 10B). 37'57'6~The separation of the TCL and capsular ligament by the intraligamentous bursa can sometimes be seen on T2-weighted images. 58 The capsular ligaments are generally best seen on coronal (Fig 10B), sagittal (Fig 10C), or radial images, 4 especially when the joint capsule is distended and the layers of the MCL are separated by fluid (see Fig 15A).

Pathology of the TCL and Capsular Ligament Injuries to the TCL in patients with knee trauma are common. They can occur alone or in conjunction with other ligamentous, meniscal, or bony pathologies. 61-65The common mechanism of injury is a valgus force applied to the flexed knee. Bony

Normal MR Appearance of the TCL and Capsular Ligaments MR evaluation of the TCL is best accomplished on coronal and axial images. Some sort of T2 weighting in conjunction with fat suppression is important. The TCL normally appears as a smooth, low-signal-intensity structure that can be followed

Fig 11. Grade I injury. (A) This T2*-weighted image shows edema superficial and deep to the TCL (curved arrows). The TCL itself appears normal (straight arrows), (B) On axial images the ligament displays a normal cross section (arrows).

MRI OF THE KNEE LIGAMENTS

contusions associated with TCL tears can occur on the lateral side from direct trauma or a impaction fracture as well as on the medial side possibly from avulsive forces during the ligament tear. 4,61 The above-mentioned fibers of the posterior oblique ligament cause many TCL tears to occur together with tears of the body or posterior horn of the medial meniscus. Once the TCL gives way, the ACL becomes the major restraint against valgus force and becomes susceptible to tearing. Isolated TCL tears are usually treated conservatively. 66,6v Therefore, the indication for an MR examination primarily lies in the exclusion of other pathology potentially necessitating surgical repair. Certain criteria have been developed to grade the severity of TCL injuries on MRI. A grade I sprain/tear consists of edema and possibly hemorrhage, which extends into subcutaneous fat. The ligament is continuous, thin, and dark in signal intensity (Fig 11). A grade II tear is considered if there is morphologic disruption or internal high signal or

Fig 12. Grade III injuries. (A) The TCL is completely disrupted at its mid-portion (arrow). (B) In this case, the tear occurred proximally at the femoral attachment site (arrow). (C) This image shows an example of a distal TCL tear (arrow).

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fluid in the TCL bursa. A grade III is diagnosed if there is complete disruption of the ligament (Fig 12). Other sensitive signs for TCL injuries are fascial edema and loss of demarcation from adjacent fat. 37,61 Tears can occur on the femoral side (65%, Fig 12B), the tibial side (25%, Fig 12C), or at the level of the joint (10%, Fig 12A). 2~ Another entity that is considered to be related to a history of trauma is Pellegrini-Stieda disease. Calcification of the proximal attachment of the TCL and a thickened TCL itself are criteria of this condition and are low on T1- and T2-weighted pulse sequences. However, care must be taken because TCL tears with an avulsed epicondylar fragment can have the same appearance. 4 Both can also demonstrate marrow signal intensity in the proximal TCL (Fig 13). The capsular ligaments are most conspicuous in the presence of joint effusion and when the layers of the MCL are separated by fluid (Fig 14A). Tears of the meniscofemoral or meniscotibial ligaments lead to meniscocapsular separation. 55 In fact, menis-

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cific though. If both the meniscofemoral and meniscotibial attachments tear a free-floating meniscus can be observed (Fig 14C).

The Posterolateral Complex (Posterolateral Supporting Structures)

Fig 13. Pellegrini-Stieda disease. (A) Frontal radiograph and (B) a coronal Tl-weighted MR image demonstrate ossification (arrow) and thickening (arrowheads) of the proximal TCL.

cocapsular separation is more frequent than a tear of the TCL because the capsular ligaments are weaker and tear first. Therefore, there is an association of TCL tears with meniscocapsular separation. 58 Even in the absence of any other pathology, a meniscofemoral separation can be of clinical significance if symptomatic. The posterior horn of the medial meniscus is most susceptible to meniscocapsular separation because it is the least mobile part. On MRI, this phenomenon is best evaluated on T2- or T2*-weighted images in the coronal plane. Fluid is seen extending along the base of the meniscus to the space just deep to the superficial part of the TCL (Fig 14B). Another sign in favor of the diagnosis of meniscocapsular separation is an uncovered posterior tibial plateau on sagittal images meaning that the meniscus is displaced forward leaving more than 5 mm of the tibial cartilage uncovered? However, this sign is rather nonspe-

The structures of the posterolateral complex are the fabellofibular ligament (short lateral ligament if no fabella is present), arcuate ligament, the popliteal muscle and tendon, the coronary (meniscotibial) ligament, the oblique popliteal ligament (of Winslow), and the LCL (Fig 15A). 68 The LCL extends from the lateral epicondyle to the proximal lateral surface of the fibular head. It is 5 to 7 cm long, extracapsular, and free from meniscal attachment. The deepest and most posterior fibers form the arcuate ligament, which extends posteromedially to the posterior portion of the joint capsule and the lateral femoral condyle. The superficial posterior capsular fibers end at the variably sized fabellofibular ligament. Both the arcuate and fabellofibular ligament are rather variable and not identifiable in all knees. 68,69 Just deep to the lateral collateral ligament runs the popliteus tendon of the popliteus muscle. This muscle originates from the posterior surface of the proximal tibia, attaches to the posterior horn of the lateral meniscus, and terminates in the lateral femoral condyle. Apart from the LCL, the fibular origin or the popliteus muscle is observed in almost all knees in cadaveric studies with a considerable width. It completes the span of the popliteus tendon between the lateral femoral condyle and the fibular styloid and is thought to have a major stabilizing effect. 69 The oblique popliteal ligament of Winslow is a posterior capsular fibrous band running from proximal lateral to distal medial. The LCL is separated from the lateral capsular ligament by a variable amount of soft tissue that is predominantly fatty. The function of the LCL is to resist varus forces. Together with the arcuate and fabellofibular ligament as well as the fibular attachment of the popliteus muscle, it also protects against excess external rotation.

Normal MR Appearance of the Posterolateral Complex The LCL is best seen on coronal and axial images and appears as an obliquely oriented cordlike band of low signal intensity on all pulse sequences (Fig 15B and C). Thin-section peripheral

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Fig 14. Capsular ligaments. (A) In the presence of surrounding edema and joint effusion, the meniscofemoral and meniscotibial (arrows) ligaments become more conspicuous. (B) Meniscocapsular separation. The meniscofemoral as well as the meniscotibial ligaments are disrupted (arrows) with fluid extending from the joint cavity to the space deep to the TCL. (C) Free-floating meniscus: The TCL and the capsular ligaments are completely disrupted and a large joint effusion is present. The medial meniscus (arrow) is entirely surrounded by joint fluid.

sagittal images can depict well the attachment of the LCL to the fibular head. 4,57,7~ In regards to the posterior capsule and its ligamentous structures like the arcuate, fabellofibular and oblique posterior ligament axial images are beneficial (Fig 15C). In addition, oblique coronal images can be obtained. 7~

Pathology of the Lateral Supporting Structures Injuries to the posterolateral complex of the knee are relatively uncommon and usually occur with other pathologies especially ruptures of the ACL. The mechanism of injury is usually hyperextension resulting from an anteromedial and varus force to the proximal tibia. 72The identification of injuries to the posterolateral complex is important because the treatment results for early surgical repair are better than operations performed in a chronic stage. 43,72 On MRI, the criteria for ruptures of the LCL are basically the same as for the medial collateral

ligament including signal, shape, and contour abnormalities, and incomplete or complete discontinuity (Fig 16). 71 Injuries to the other stabilizing structures of the posterolateral complex, namely the fabeltofibular and arcuate ligament and popliteal tendon (Fig 16), frequently coexist. Because the fabellofibular and arcuate ligament can only be identified in about half of normal knees with an optimized imaging sequence, 7~ it is also difficult to accurately diagnose injuries to those structures on MRI. TRANSVERSE GENICULATE (MENISCOMENISCAL) LIGAMENT

The transverse geniculate ligament runs in the coronal plane connecting the anterior horns of the medial and lateral menisci. It is located in the posterior part of Hoffa's fat pad anterior to the joint cavity (Fig 17). The ligament is of variable thickness and may be absent. The overall frequency of

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popliteal lig.~-~_~L~/lllF~iiiD/F abella

E.'~~ ~ , ~ M . popliteus TCL~ a n d tendon Ug. ]~r/~':~Arcuate Posterior/~l~li~i~F..j/~-,~.,~ "- L - L obliquel i g . ~ L; ~ ~Fabellofibular liig.

Fig 15. Normal anatomy of the posterolaterat complex. (A) Posterior view of the knee capsule. (Reprinted with permission.2q (B) Coronal Tl-weighted image depicting the normal LCL as a low signal intensity cordlike structure (arrows). (C) Axial Tl-weighted image demonstrating the LCL (long arrow), posterior capsule with its fabellofibular/arcuate ligament (curved arrow), popliteus tendon (open arrow), and biceps femoris muscle and tendon (arrowheads). (D) Coronal image in the plane of the posterior capsule showing the oblique popliteal ligament (short arrows). Semimembranosus tendon at its tibial attachment (curved arrow). Lateral femoral condyle (open arrow).

visualization of the ligament on MRI is about is important to recognize the MRI appearance of this ligament because, if present, it can be confused with a lateral meniscal at its anterior horn (Fig 17B). 74 These so-called pseudotears of the anterior horn of the lateral meniscus are seen in about one third of the knee MR examinations. 50%. 73 It

MENISCOFEMORAL L I G A M E N T

The meniscofemoral ligament is a variable structure extending from the lateral aspect of the medial femoral condyle to the posterior horn of the lateral meniscus (Fig 18A). Positioned anteriorly to the PCL, it is called the Ligament of Humphrey, posteriorly to the PCL it is referred to as the ligament of Wrisberg (Fig 18B). 75 It is important not to confuse the MR appearance of this ligament

with a meniscal tear of the posterior horn of the lateral meniscus or an osteochondral fragment (Fig 18C). 74'76-78 In anatomic studies, a meniscofemoral ligament was present in 70% to 100% of cases. 7,75,78-8~Both ligaments (Humphry and Wrisberg) can coexist in the same knee, but in some cases no meniscofemoral ligament is identified. In terms of function, the meniscofemoral ligament is thought to pull back the posterior horn of the lateral meniscus during flexion thus preventing entrapment of the meniscus between the femur and tibia.56,75

THE MEDIAL PATELLOFEMORAL L I G A M E N T

The medial patellofemoral ligament (MPFL) has recently received extensive attention in the surgical

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Fig 16. Injuries to the posterolateral complex. (A) Complete tear of the LCL, biceps femoris tendon, and popliteus tendon. The LCL and biceps femoris tendon are highly irregular in shape and abnormally high in signal intensity (straight arrows). The popliteus tendon cannot be seen at its usual location (curved arrow). (B) Tear of the LCL close to its femoral attachment (arrow) as well as popliteus tendon rupture. (C) Axial image shows the biceps femoris tendon to be intact (arrow), whereas a normal LCL cannot be found.

and radiological literature because of its clinical importance in preventing recurrent lateral patellar dislocation. 81-s5Among the various medial ligamentous structures, the MPFL has been shown to be the

major medial soft-tissue constraint preventing lateral patellar dislocation.82,85 Warren and Marshall 55 first introduced the concept of a three-layered system describing ligamen-

Fig 17. Normal transverse geniculate ligament (A) in the axial plane (arrows) and (B) in the sagittal plane (short arrow). Note that in the sagittal plane the lateral attachment site of the ligament can mimic a tear (long arrows) of the anterior horn of the lateral meniscus (curved arrow).

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A~/J! i: ~-

Humphrey's lig.

Wrisberg's lig

A

Fig 18. Normal meniscofemoral ligament. (A) Posterior view of the meniscofemoral ligament. (Reprinted with permission.21) (B) Sagittal MR image displaying both the ligament of Humphrey anterior to the PCL (open arrow) and the ligament of Wrisberg posterior (closed arrow) to the PCL (curved arrow). (C) At its attachment site to the posterior horn of the lateral meniscus, the meniscofemoral ligament can be confused with a meniscal tear (arrow).

tous structures of the medial aspect of the knee in which each ligament was reproducibly found within a given layer. Posteriorly, the layers are easily separable, but as one moves toward the patella, the layers fuse into a single fascial plane. Layer one is the most superficial layer and is located just deep to the subcutaneous fat. The medial retinaculum is described as the anterior confluence of the first and second layers. The second layer is located just deep to the first layer and contains the MPFL, the superficial component of the medial collateral ligament, and the patellotibiat ligament, which extends from the tibia at the insertion site of the gracilis and semitendinosus muscles to the lower pole of the patella. Layer three is the deepest of the layers and is composed of the joint capsule and the patellomeniscal ligament, located just deep to the patellotibial ligament. The MPFL is located in the second layer and is variable in size and thickness, extending from the

adductor tubercle of the medial femoral condyle to the medial aspect of the upper third of the patella (Fig 19). It is located just deep to the distal belly of the vastus medialis obliquus (VMO) muscle. 55,82 The MPFL is responsible for between 53% and 60% of the restraining force preventing lateral patellar dislocation. The patellomeniscal ligament contributes approximately 22% of the total restraining force, whereas the medial retinaculum and patellotibial ligament provide no significant restraining force. 86 Surgical repair of the MPFL following lateral patellar subluxation results in restored stability in most cases. 82 MRI clearly depicts the MPFL in the axial plane as a dark bandlike structure extending from the medial aspect of the upper pole of the patella to the adductor tubercle (Fig 20). 83 MRI can play a significant role in determining the location and extent of MPFL injury following transient patellar dislocation. Most MPFL injuries begin at the

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271

Medial patellofemoral ligament

Vastus medialis

dductor tubercle Vastus medialis

Fig 19. The medial patellofemoral ligament (MPFL). (A) A line drawing in the axial plane through the knee demonstrates the normal location of the MPFL as it extends from the upper pole of the patella to the adductor tubercle. It is located deep to the vastus medialis muscle. (B) A line drawing of the medial aspect of the knee demonstrates the normal course of the MPFL. Note that the MPFL inserts on the adductor tubercle adjacent to the attachment site of the adductor magnus tendon and the medial collateral ligament.

Adductor magnus

Medial patellofemor ligament Adductor tubercle

Medial reti~

Medial collateral ligament

B

proximal aspect of the ligament adjacent to its femoral attachment site. Disruption of the MPFL is seen on MRI as a discontinuous wavy band with extensive surrounding edema (Fig 21). Avulsion of the MPFL off of the adductor tubercle is seen as fluid extending between the ligament and the adductor tubercle (Fig 22). Stretching or sprain of the MPFL may also occur and appears as a wavy but intact ligament with surrounding edema. 87

As previously stated, the MPFL is located just deep to the VMO, and fibers of the MPFL actually blend with the deep fascia of the VMO at the level of their attachment to the adductor tubercle of the medial femoral condyle. 88 On sagittal MR images, the VMO sits directly on top of the adductor tubercle, but following injury to the MPFL, edema is commonly seen deep to the VMO resulting in displacement of the VMO away from the medial

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0

Fig 20. MR appearance of the normal MPFL. T2-weighted axial image with fat saturation of the knee reveals the normal MPFL (straight arrow). It extends from the superior pole of the patella to the adductor tubercle (curved arrow) of the medial femoral condyle. It is located just deep t o the vastus medialis obliquus muscle (arrowheads).

femoral condyle. Elevation of the VMO is best demonstrated on T1- and T2-weighted sagittal images and is an indirect sign of MPFL injury (Fig 23). When clinical history or MR findings, such as the typical bone marrow edema pattern, suggest prior transient dislocation of the patella, the MR images should be studied closely for evidence of MPFL injury. Disruption or avulsion of the MPFL is now considered by many orthopedic surgeons to

Fig 21. Disrupted MPFL. T2-weighted axial image with fat saturation of the knee reveals complete disruption of the MPFL (arrow) with extensive surrounding edema following acute dislocation of the patella.

Fig 22. Avulsion injury of the MPFL T2-weighted axial image with fat saturation of the knee reveals fluid signal intensity (black arrow) between the MPFL (straight white arrow) and the adductor tubercle (curved arrow) in this patient with a surgically proven avulsion of the MPFL off of the adductor tubercle.

Fig 23. Elevation of the vastus medialis obliquus muscle. T2-weighted sagittal image with fat saturation of the knee reveals edema (arrow) between the medial femoral condyle and the vastus medialis obliquus muscle. Normally, the vastus medialis obliquus muscle sits directly on top of the medial femoral condyle. Following acute patellar dislocation, as in this case, the vastus medialis obliquus muscle is elevated off of the medial femoral condyle secondary to edema at the femoral attachment site of the MPFL.

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273

ANTERIOR Anterior cruciate ligament

Transverse meniscal ligament

Menisco-n

leniscus

Medial m(

Fig 24. The oblique meniscomeniscal ligament. The normal anatomy of the medial oblique menisco-meniscal ligament is indicated in this line drawing. It runs from the anterior horn of the medial meniscus to the posterior horn of the lateral meniscus, passing obliquely through the intercondylar notch between the tibial attachments of the anterior and posterior cruciate ligaments.

Posterior cruciate ligament POSTERIOR

Fig 25. MR appearance of the oblique meniscomeniscal ligament. (A) T2-weighted fast spin-echo axial image of the knee w i t h fat saturation demonstrates the normal course of the oblique meniscal ligament (arrow) as it passes from the anterior horn of the medial meniscus to the posterior horn of the lateral meniscus. (B) Tl-weighted coronal image of the knee demonstrates the oblique menisco-meniscal ligament as a thick cordlike structure (arrow) extending through the intercondylar notch. On contiguous coronal images, it can be traced from the anterior horn of the medial meniscus to the posterior horn of the lateral meniscus. (C) Proton density sagittal image of the knee through the intercondylar notch at the level of the posterior cruciate ligament (PCL) demonstrates the oblique meniscomeniscal ligament (straight arrow) as a low signal intensity structure anterior to the PCL. This can easily mimic a displaced meniscal fragment in the sagittal plane. A curved arrow notes the PCL.

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be an indication for surgery f o l l o w i n g patellar dislocation. OBLIQUE M E N I S C O M E N I S C A L L I G A M E N T

The oblique m e n i s c o m e n i s c a l l i g a m e n t is a relatively u n c o m m o n intermeniscal l i g a m e n t that runs obliquely from the anterior horn o f one m e n i s c u s to the posterior horn o f the opposite m e n i s c u s (Fig 24). 89,90 It has b e e n previously reported in both the anatomic and arthroscopic literature, but has only recently r e c e i v e d attention in the radiology literature because o f its propensity to m i m i c a displaced meniscal f r a g m e n t on M R I . 9t The ligament has a reported p r e v a l e n c e ranging f r o m 1% to 4 % and is n a m e d for its anterior attachment site. 89.9~ Hence, the medial oblique m e n i s c o m e n i s c a l ligament originates f r o m the anterior horn of the medial meniscus and traverses obliquely through the intercondylar notch, passing b e t w e e n the anterior and posterior cruciate ligaments to insert on the posterior horn o f the lateral meniscus. The lateral oblique m e n i s c o m e n i s c a l ligament originates f r o m the anterior horn o f the lateral m e n i s c u s and inserts on the posterior horn o f

the medial meniscus. This ligament has no k n o w n function. The oblique m e n i s c o m e n i s c a l ligament d e m o n strates decreased signal intensity on all M R pulse sequences, similar to the adjacent meniscal tissue. As a result, the oblique m e n i s c o m e n i s c a l ligament, like other anatomic structures that lie in close p r o x i m i t y to the m e n i s c u s (ie, m e n i s c o f e m o r a l ligament, anterior transverse meniscal ligament, or the popliteal ligament) m a y r e s e m b l e a displaced meniscal fragment, m i m i c k i n g either a flap tear or a bucket-handle tear. The l i g a m e n t is best visualized on axial or coronal M R images, where it can be traced f r o m its origin to its insertion (Fig 25). On sagittal images, it typically lies just beneath the posterior cruciate ligament, and it is in this plane that the oblique m e n i s c o m e n i s c a l l i g a m e n t is most likely to m i m i c a displaced meniscal fragment. 9~ Familiarity with the normal anatomic course o f this ligament, as well as correlation o f findings f r o m all three i m a g i n g planes, will aid in correctly identifying this l i g a m e n t as a normal anatomic variation rather than misinterpreting it as a displaced meniscal fragment.

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