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Posteromedial Corner of the Knee: The Neglected Corner1 Ryan B. Lundquist, MD George R. Matcuk, Jr, MD Aaron J. Schein, MD Matthew R. Skalski, DC Eric A. White, MD Deborah M. Forrester, MD Christopher J. Gottsegen, MD Dakshesh B. Patel, MD Abbreviations: ACL = anterior cruciate ligament, AMRI = anteromedial rotational instability, MCL = medial collateral ligament, OPL = oblique popliteal ligament, PCL = posterior cruciate ligament, PMC = posteromedial corner, POL = posterior oblique ligament RadioGraphics 2015; 35:1123–1137 Published online 10.1148/rg.2015140166 Content Codes: From the Department of Radiology, Keck School of Medicine, University of Southern California, 1500 San Pablo St, Los Angeles, CA 90033 (R.B.L., G.R.M., A.J.S., E.A.W., D.M.F., D.B.P.); Department of Radiology, Southern California University of Health Sciences, Whittier, Calif (M.R.S.); and Department of Radiology, NYU Langone Medical Center, New York, NY (C.J.G.). From the 2013 RSNA Annual Meeting. Received April 10, 2014; revision requested August 11 and received September 4; accepted September 26. For this journal-based SA-CME activity, the authors, editor, and reviewers have disclosed no relevant relationships. Address correspondence to D.B.P. (e-mail: [email protected]). 1

SA-CME LEARNING OBJECTIVES After completing this journal-based SA-CME activity, participants will be able to: ■■List the important anatomic structures that make up the PMC of the knee. ■■Describe

the dynamic and static support that the structures of the PMC provide to the knee and discuss the consequences of overlooking or misidentifying injuries to these structures. ■■Correctly

identify injuries of the PMC.

See www.rsna.org/education/search/RG.

The posteromedial corner of the knee (PMC) is an important anatomic structure that is easily seen but often overlooked on magnetic resonance (MR) images. Whereas the posterolateral corner has been referred to as the “dark side of the knee” by some authors owing to widespread lack of knowledge of its complex anatomy, even less is written about the PMC; yet it is as important as the posterolateral corner in multiligament injuries of the knee. The PMC lies between the posterior margin of the longitudinal fibers of the superficial medial collateral ligament (MCL) and the medial border of the posterior cruciate ligament (PCL). The anatomy of the PMC can be complex and the literature describing it can be confusing, at times oversimplifying it and at other times adding unnecessary complexity. Its most important structures, however, can be described more simply as five major components, and can be better shown with illustrations that emphasize the anatomic distinctions. Injuries to the PMC are important to recognize, as disruption of the supporting structures can cause anteromedial rotational instability (AMRI). Isolated PMC injuries are rare; most occur in conjunction with injuries to other important stabilizing knee structures such as the anterior cruciate ligament (ACL) and PCL. Unrecognized and unaddressed injury of the PMC is one of the causes of ACL and PCL graft failures. Recognition of PMC injuries is critical, as the diagnosis will often change or require surgical management. ©

RSNA, 2015 • radiographics.rsna.org

Introduction

Injuries to the posteromedial corner (PMC) of the knee are often overlooked, yet the major anatomic structures found in the PMC are readily visible on modern high-field-strength magnetic resonance (MR) imaging systems. With proper understanding of these anatomic structures, their normal appearance at MR imaging, and their static and dynamic roles in supporting the knee, injuries may be easily recognized and understood. The PMC contains the structures lying between the posterior margin of the longitudinal fibers of the superficial medial collateral ligament (MCL) and the medial border of the posterior cruciate ligament (PCL). Within these borders are the five major components of the PMC. There is controversy regarding the existence of some of the structures, their nomenclature, and the number of their arms

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TEACHING POINTS ■■

Failure to address a PMC injury during surgical management of other injuries can lead to increased stress on reconstructed knee ligaments, eventually leading to their failure.

■■

The PMC has five major components: the semimembranosus tendon and its expansions, the oblique popliteal ligament (OPL), the posterior oblique ligament (POL), the posteromedial joint capsule, and the posterior horn of the medial meniscus.

■■

The semimembranosus is the main dynamic stabilizer of the PMC.

■■

The PMC is a dynamic system, and injury to any one structure is capable of disabling the functional cascade of the posteromedial capsule.

■■

A hallmark of PMC injuries is AMRI.

and the arms’ respective attachments. However, the major elements seem to be defined in similar ways by the majority of the literature we surveyed. This confusion about the structures of the PMC has led to confusion in biomechanics studies, as some authors have considered the posterior oblique ligament (POL) to be part of the MCL, and others have designated all structures posterior to the superficial MCL as the posteromedial joint capsule (or simply the posteromedial capsule). Despite the variability in interpretation of the structures, we will try to summarize and to simplify the information contained in our references. Keep this in mind while reading, as some articles may list the names or attachments of the structures in the PMC slightly differently from each other. Even with this variability, the primary functionality of the PMC can be understood, and this understanding can help guide treatment, leading to better patient care. Anteromedial rotational instability (AMRI) is defined as excessive anterior translation and external rotation of the medial tibial plateau with respect to the femur, accompanied by opening of the medial joint space. The many static and dynamic structures of the PMC help to prevent AMRI, and, likewise, AMRI can be an indication of an underlying PMC injury. Often comorbid with other injuries, PMC injures can be an important cause of patient disability. Failure to address a PMC injury during surgical management of other injuries can lead to increased stress on reconstructed knee ligaments, eventually leading to their failure (1,2). In this article, we will discuss the anatomy of the PMC and the biomechanics of its structures as it relates to knee support; illustrate common injuries that we have seen in clinical practice; and discuss the clinical manifestation of such injuries and the treatment options that are available. Knowledge of the anatomy and common pathologic conditions of the PMC can help to prevent misdiagnosis.

Figure 1.  Axial proton-density–weighted fat-saturated MR image of the knee illustrates the borders of the PMC (white outline). Arrow = main semimembranosus tendon.

Anatomy of the PMC

The anatomy of the medial aspect of the knee has been described by using two different concepts. Initially, the medial-side supporting structures were described by Warren and Marshall (3) as consisting of three layers, where layer I consisted of the deep or crural fascia, layer II included the superficial MCL, and layer III was made up of the joint capsule and deep medial collateral ligament. Later, Robinson et al. (4) divided the medial aspect of the knee from the medial edge of the patellar tendon to the medial edge of the medial head of the gastrocnemius into thirds, with the longitudinal fibers of the superficial MCL in the middle third, and structures anterior and posterior to it in the anterior and posterior thirds, respectively. The superficial MCL is part of Warren and Marshall’s layer II and forms the middle third of the medial side of the knee. The structures between the posterior margin of the superficial MCL and the medial margin of the PCL form the posterior third, designated as the PMC (Fig 1).

Components of the PMC The PMC has five major components: the semimembranosus tendon and its expansions, the oblique popliteal ligament (OPL), the posterior oblique ligament (POL), the posteromedial joint capsule (or simply the posteromedial capsule), and the posterior horn of the medial meniscus. Although the superficial and deep portions of the MCL, including its meniscotibial ligament, function in close association with the structures of the PMC and form the medial supporting structures, they are not typically considered to be part of the PMC.

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Figures 2, 3.  (2) Illustration of the semimembranosus tendon and its five major expansions. (3) Illustration of the POL with its three major arms, and of the semimembranosus tendon with four of its expansions.

Semimembranosus Tendon and Expansions The PMC has been referred to as the semimembranosus corner due to the important role the semimembranosus tendon has in providing dynamic stability to the knee (2). The semimembranosus tendon (Figs 2, 3) has five major arms or expansions: the direct arm (principal attachment), the capsular arm, the extension to the OPL (OPL insertion), the anterior arm (tibial or reflected arm, pars reflexa), and the inferior (popliteal) arm (1,5–7). Smaller minor semimembranosus tendon extensions also exist (4,5,8,9), seen on some studies but not others; for the sake of simplicity we will discuss only the five major extensions. Some authors describe eight distinct attachments (9). The main tendon bifurcates into the direct and anterior arms just below the joint line, with the extension to the OPL

arising approximately 2 cm proximal to the bifurcation. The direct arm or principal attachment inserts below the joint line at the tubercle on the posteromedial medial tibial condyle called the tuberculum tendinis. Some authors have found the insertion in a groove just proximal to the tubercle (10). It passes deep to the anterior arm to arrive at this location. Just before its tibial attachment, the direct arm attaches to the posterior aspect of the coronary ligament of the posterior horn of the medial meniscus. The capsular arm blends with the posteromedial capsule and coalesces with the capsular portions of the OPL and POL. The extension to the OPL blends into the OPL, as the name suggests. The anterior arm inserts into the medial proximal tibia, superior to the insertion of the superficial MCL and passing under the POL (1,5,9). The inferior or popliteal arm passes beneath the POL and attaches

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Figure 4.  Axial proton-density–weighted fat-saturated MR images of a normal knee at the level of the femoral condyles (a), the joint space (b), just below the joint line (c), the tibial plateau (d), and the tibial metaphysis (e). The images demonstrate the superficial MCL (straight white arrow), POL (curved arrow in a and b), main semimembranosus tendon (arrowhead in a), OPL expansion of the semimembranosus (arrowhead in b), inferior arm of the semimembranosus (arrowhead in c–e), tibial arm of the semimembranosus (curved arrow in d), and direct arm of the semimembranosus (black arrow in d). G = gracilis, MG = medial head of the gastrocnemius, S = sartorius muscle and tendon, ST = semitendinosus.

to the tibia just above the superficial MCL attachment. Some authors have divided this arm into medial and lateral divisions, with the attachment to the medial tibia forming the medial division. The lateral division extends over the popliteal fascia (9,11). The insertion of the direct arm is best seen on sagittal and axial images (Fig 4). It is difficult to distinguish this arm from the anterior arm, and together the two form a low-signal-intensity band along the posteromedial aspect of the tibia distal to the articular margin. Best seen on coronal MR images, the anterior arm appears as an oval hypointense structure along the medial tibia, deep to the MCL (Fig 5). The anterior arm can frequently be seen on sagittal images along the medial tibial plateau as a low-signal-intensity structure with a nearly horizontal distal aspect (Fig 6). It can also be identified on axial images as it courses anteriorly and distally to its insertion. Increased signal intensity related to magic angle phenomena may be seen along the posterior aspect of this arm. The extension to the OPL is best visualized on axial images. The capsular arm as it blends with the capsular arm of the

POL is difficult to separate and independently identify from the direct arm, proximal portions of the anterior arm, and the extension to the OPL. When seen, it has a flat striated appearance and is best seen on axial images at the level of the joint space. It is best seen when the joint is distended by fluid. On axial MR images, the inferior arm appears as a hypointense structure extending inferiorly and anteriorly from the posterior margin of the joint line, deep to the MCL. The lateral division of this arm is best seen on sagittal and axial images as a bandlike structure (1,6,11,12).

POL (Ligament of Winslow) Hughston and Eilers (13) initially described the POL (Figs 2, 3) and assigned significance to it, although recently, Robinson et al (4), in their dissection study, did not find a discrete ligament, and referred to all structures posterior to the superficial MCL simply as the posteromedial joint capsule. The ligament is part of layer II, and its anterior margin blends with the posterior margin of the superficial MCL. Posteriorly, it blends with

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Figure 5.  Coronal T2-weighted fat-saturated MR images of the knee show normal appearances of the MCL (arrows in a), POL (curved arrow in b–d), anterior arm of the semimembranosus tendon at the posterior horn of the medial meniscus (white arrowhead in c) and in the posterior joint capsule (white arrowhead in d), inferior arm of the semimembranosus tendon (black arrowhead in c), and OPL (arrow in d).

Figure 6.  Sagittal proton-density–weighted fat-saturated MR images show the normal anatomy of the knee at the level of the body of the medial meniscus (a), in a plane lateral to a (b), and in the mid medial joint space (c). The images demonstrate the tibial arm of the semimembranosus (white arrowhead in a), inferior arm of the semimembranosus (black arrowhead in a), POL (arrow in b) in close relation to the medial meniscus, and OPL (arrow in c).

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Figure 7.  (a, b) Axial (a) and sagittal (b) protondensity–weighted images show the semimembranosus tendon (arrow) and OPL (arrowhead) in a normal knee. (c) Coronal T2-weighted fat-saturated MR image shows the normal OPL (arrowhead). G = gracilis, MG = medial head of the gastrocnemius, S = sartorius muscle and tendon, ST = semitendinosus.

the joint capsule (layer III) and is inseparable from it (1,10,14). Most authors have described the origin of the ligament to be the adductor tubercle, but LaPrade et al (10) in their anatomic study found the origin to be distal and posterior to it. Wijdicks et al (15) also found the insertion point to be distal and posterior to the adductor tubercle. The distal end comprises three arms that may not be separately discernible. The central or tibial arm is the thickest and most important of these. It extends posteriorly and obliquely to attach to the posteromedial aspect of the medial meniscus and adjacent posteromedial aspect of the tibia, passing deep to and beneath the anterior arm of the semimembranosus tendon, and merging with the main part of the tendon. The superior or capsular arm is mainly superior to the joint line and is continuous with the posterior joint capsule, merging with the capsular arm of the semimembranosus tendon to form the proximal OPL. The distal (or superficial) arm attaches to the sheath and tibial insertion point of the semimembranosus tendon, blends with the posterior margin of the superficial MCL, and passes superficially to the anterior arm of the semimembranosus tendon (1,5,7,10,14). The POL is best visualized on coronal and axial images (Figs 4–6). The three arms run continuously with each other, are difficult to distinguish from each other, and may not be seen as separate structures. The capsular arm is identifiable on axial images at the level of the femoral condyle, and on sequential coronal images as a thin hypointense band extending posteriorly from the superficial MCL to merge with the medial aspect of the poste-

rior capsular structures. The superficial arm is better seen on coronal images extending from its origin to the medial aspect of the tibial plateau as a thin hypointense band, superficial to the direct insertion of the semimembranosus tendon. The central arm is best visualized on sequential coronal and axial images just posterior to the superficial MCL as it blends with the capsule and inserts into the medial meniscus and the adjacent tibia (1,5,11).

Oblique Popliteal Ligament The OPL (Fig 7) arises from the capsular arm of the POL and lateral expansion of the semimembranosus. Laterally, the OPL attaches to the osseous or cartilaginous fabella, the meniscofemoral portion of the posterolateral joint capsule, and the plantaris muscle. The OPL also has a fibrous attachment to the lateral aspect of the PCL facet (9). The OPL is considered a component of both the PMC and posterolateral corner. Controversy exists on the nature of the OPL. Benninger and Delamarter (16) challenged the status of this structure as a ligament and

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Figure 8.  Sagittal proton-density–weighted (a) and coronal T2-weighted fat-saturated (b) MR images of a normal knee demonstrate the posterior horn of the medial meniscus (arrow) and meniscotibial ligament (arrowhead).

proposed, based on their analysis, that the oblique popliteal expansions be termed the oblique popliteal tendon. The OPL is a thin structure and is difficult to distinguish from the posterior joint capsule. When it is somewhat thicker, it can be identified on sequential sagittal and axial images as a band of tissue extending obliquely from the main tendon of the semimembranosus laterally and proximally toward the lateral femoral condyle (5,12).

Posteromedial Joint Capsule and Posterior Horn of the Medial Meniscus The knee joint capsule, part of layer III, forms the deep MCL, with its meniscotibial and meniscofemoral components, along the medial aspect of the knee. It then continues posteriorly, where it is reinforced externally by the POL, a component of layer II, forming the posteromedial joint capsule (Fig 8). More posteriorly, the posteromedial joint capsule passes deep to the medial head of the gastrocnemius, and then extends laterally to become the posterior joint capsule. Along the posteromedial side of the knee, the capsule is strengthened by the POL and expansions from the semimembranosus. The medial meniscus attaches to the capsule posteromedially and the meniscotibial ligament anchors the meniscus to the tibia. Some authors have mentioned a separate arm from the semimembranosus to the meniscotibial ligament, while others consider it to be part of the direct arm (9,11). The medial meniscus is thought to be linked to the semimembranosus through a neurophysiologic pathway. Stimulation of the posterior horn of the medial meniscus has resulted in somatosensory evoked potentials in the semimembranosus muscle (17). The attachment of the peripheral surface of the meniscus to the capsule and the tibia is best evaluated on sequential coronal and sagittal images posterior to the superficial MCL (Figs 5, 6).

Recently, the ultrasonographic appearance of the main posteromedial structures, including the POL, the direct and the anterior arms of the semimembranosus, and the OPL, has been described (18).

Biomechanics and Function of the PMC

The natural tendency of the POL is to be relaxed when the knee is flexed and tight when the knee is extended. When the knee is completely extended, the tibia rotates externally to achieve a “screw in” position, tightening the PMC, forming a pouch around the medial femoral condyle. With the onset of flexion, as the tibia rotates internally, the PMC becomes relaxed, but the semimembranosus, due to its contraction, tenses the posteromedial capsule and its ligaments. With increasing flexion, the posteromedial capsule becomes relaxed and folds upon itself (1,4,14,19). The PMC is a synergistic structure with myotendinous, ligamentous, and meniscal components. These components provide static and dynamic support to the knee, which constrains anteromedial rotation of the tibia (7). Dynamic support of the PMC is formed by the myotendinous units and their aponeuroses; static support is provided by the surrounding ligaments and joint capsule (1,5,9). Passive support comes from the “brake stop” function of the posterior horn of the medial meniscus (Fig 9), described later. Rotational support is also from the deep meniscotibial attachments. Active support of the PMC comes from the pes anserinus muscles and tendons when the knee is flexed; the vastus medialis muscle when the knee is extended; and the semimembranosus with its support of the superficial MCL and POL during active flexion (1,19).

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Figure 9.  Illustration demonstrates the function of the medial meniscus and meniscotibial ligament in the brake stop mechanism of support of the knee (arrow). See text for explanation.

The semimembranosus is the main dynamic stabilizer of the PMC. During contraction, the semimembranosus flexes and internally rotates the tibia, and acts as an active restraint to valgus motion when the knee is extended, and to external rotation when the knee is flexed. It produces traction on the posterior horn of the medial meniscus. By placing traction on the posterior horn of the medial meniscus, the semimembranosus tendon prevents injury to the posterior horn from compression between the femur and tibia (12). The semimembranosus is oriented parallel to the femur; therefore, this protective effect on the medial meniscus is maximal at 90° of flexion and decreases as the knee moves away from 90° flexion. It causes tension of the OPL and posterior joint capsule, stabilizing them; works synergistically with the popliteus, thus participating in the stability of the posterolateral corner; and works synergistically with the biceps femoris muscle for dynamic knee stability for internal and external rotation (1,7,13,19,20). The POL provides restraint against valgus stress, anterior and posterior tibial translation, and internal and external rotation. Substantial increases in load to the POL were seen with valgus and external rotation after sectioning of the MCL. The POL is an important stabilizer against internal rotation at all knee flexion angles. The OPL is the primary restraint to knee hyperextension and is important to prevent genu recurvatum (21–24). When the meniscus is stable on its tibial platform and the meniscotibial ligament is intact, the posteromedial meniscus provides a brake stop function to anterior translation of the tibia (Fig 9). If the posteromedial meniscus is not stable on its tibial platform, for instance owing to meniscotibial ligament insufficiency, the brake stop function is lost. With the brake stop function lost, the meniscus and femoral condyle

move as a unit, sliding over the tibial articular surface. In this situation, no stabilizing function remains, causing increased stress on the other structures and increased risk of injury to the tibial articular surface and meniscus. The PMC injury decreases dynamic function of the medial meniscus and the brake stop mechanism, therefore decreasing support of the knee (1,2,5,7,19,25). The superficial MCL is the primary restraint to valgus stress in all angles of flexion. The deep MCL is a secondary static stabilizer providing restraint to valgus stress. The PMC is an additional stabilizer, providing one-third of the restraint to valgus stress when the knee is in extension. After isolated MCL injury, the POL acts as an important secondary stabilizer for rotation and valgus stress (14,19–21,24,26–28). The PMC is an important secondary stabilizer of pure anterior tibial translation. Many patients with anterior cruciate ligament (ACL) ruptures therefore continue to function at a high level in athletics. On the other hand, if PMC injury is not recognized or addressed at ACL reconstruction, there is an increased risk of failure of the graft (1,7,28). The POL is a secondary stabilizer of posterior tibial translation, particularly in extension. This stabilizing effect increases with tibial internal rotation. The POL support for posterior tibial translation is especially important in PCLdeficient knees (14,23,27,28).

PMC Injuries

The PMC is a dynamic system, and injury to any one structure is capable of disabling the functional cascade of the posteromedial capsule. Without dynamic support from the semimem­ branosus, the stresses are transferred to the remaining components of the PMC, leading to their failure over time and ultimately leading to instability (1,12).

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Figure 10.  External oblique radiograph (a), axial T1-weighted MR image (b), and sagittal T1-weighted MR image (c) demonstrate a semimembranosus avulsion fracture of the posterior tibial plateau and tear of the posterior horn of the medial meniscus (arrow) in a 19-year-old man with a basketball injury.

A hallmark of PMC injuries is AMRI. AMRI is defined as external rotation with anterior subluxation of the medial tibial plateau relative to the distal femur. The PCL acts as the central axis during this translation. Simultaneous MCL and PMC injuries create the potential for AMRI. In AMRI, there is an abnormal excess opening of the medial joint space in abduction at 30° of knee flexion, with a simultaneous anteromedial rotatory subluxation of the medial tibial condyle on the central axis of the intact posterior cruciate ligament. This subtle additional rotatory instability differentiates it from isolated open-book instability seen with isolated medial collateral ligament injury (1,12,29,30). Deficiency of the PMC can lead to persistent valgus instability and failure of ACL and PCL reconstructions (14,23). Similarly, treatment of medial compartment ligaments without repair of the POL often leads to persistent instability (31).

Of patients with isolated and combined medial-side injuries with symptomatic AMRI who were treated with surgery, POL injury occurred in 99%, semimembranosus injury occurred in 70%, and peripheral meniscal detachment occurred in 30%. Simultaneous injury of all three was seen in 19% (7). In another study, nearly all patients in surgically treated ACL and MCL injuries had POL tears (32). Findings of semimembranosus injury can include avulsion fracture of the tibial attachment, partial or complete tendon tear, and tendinosis (1,5). Avulsion fractures at the insertion of the direct arm (Fig 10) are more frequently seen than a complete tear of the tendon, although both are not common. Many cases of avulsion fractures of the semimembranosus insertion along the posteromedial tibia have been described. These are postulated to be secondary to valgus stress with external rotation and are associated with injury of the ACL, MCL, and medial meniscus (33–36). There are few reports in the literature on partial tears, although they have been commonly noted at surgery (7,37). Partial tears or strains have increased signal intensity in the tendon with surrounding edema in the context of an acute injury. Complete tears are seen as disruption of the tendon with retraction (Fig 11) and hematoma filling the gap. Strain of the tendon is seen with edema and fluid around an otherwise intact tendon. With tendinosis, thickening and increased signal intensity are seen in the tendon, but accumulation of fat in the normal

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Figure 11.  MR images of knee injury in a 21-year-old baseball player owing to a collision on the baseball field. (a) Axial proton-density–weighted MR image at the middle of the level of the femoral condyles demonstrates a torn and retracted semimembranosus tendon with the gap occupied by the semitendinosus tendon (ST) adjacent to the medial head of the gastrocnemius (MG). The sartorius muscle and tendon (S) and gracilis tendon (G) are also seen. The superficial MCL and POL (black arrowhead) are torn proximally, and the retracted torn end of the superficial MCL is visualized (arrow). The medial retinaculum (white arrowhead) is also torn, and a bone contusion is seen in the lateral femoral condyle (*). (b, c) Midcoronal (b) and posterior coronal (c) T2weighted fat-saturated images demonstrate complete tears of the proximal MCL (arrow in b) and POL (arrow in c) with meniscocapsular separation (arrowhead in c). Note the bone contusions in the proximal tibia (* in b). (d, e) Sagittal proton-density–weighted MR images of the intercondylar region (d) and medial femoral condyle (e) show a complete proximal PCL tear (white arrow in d), tear of the posterior capsule and OPL (black arrow in d), and meniscocapsular separation (arrowhead in e).

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Figure 12.  Knee injury in a 22-year-old male skier, incurred after a jump. Coronal T2-weighted (a) and axial proton-density–weighted (b) fat-saturated MR images of the knee demonstrate a sprain of the POL (arrow).

Figure 13.  Knee injury in a 22-year-old woman, incurred during a soccer game. (a) Axial proton-density– weighted fat-saturated MR image obtained just above the joint line shows a sprain (arrow) affecting the POL and joint capsule. (b) Sagittal proton-density–weighted fat-saturated MR image of the medial compartment shows meniscocapsular separation (arrowhead) and joint effusion (arrow).

tendon has been recognized as a normal variant and should not be mistaken for tendinosis (1,5). Commonly seen findings of POL injury include avulsion fracture of the tibial attachment, partial or complete tear of the ligament (Figs 12–14), and chronic injury (5). The grading system for POL tears is the same as that for MCL tears (1). A sprain, or grade I injury, is seen when the ligament is normal in thickness with continuity, but surrounded by edema. A partial tear, or grade II injury, is seen when there is ligament thickening and partial fiber disruption with increased surrounding edema and hemorrhage. A complete tear, or grade III injury, is seen when no fibers remain intact. These injuries are best seen on axial and coronal MR images (Figs 11, 14).

Posteromedial meniscocapsular injury or meniscocapsular separation (Fig 15) is best seen in the sagittal plane, with low-grade sprain of the capsule seen as thickening and increased signal intensity of the capsule with surrounding edema. However, the presence of edema superficial to the posteromedial joint capsule in itself does not signify injury, as such edema may be secondary to other causes such as a ruptured or leaking popliteal cyst; the finding has to be interpreted in the context of the history, mechanism of injury, and other imaging features. Injury to the OPL is usually best seen in the axial and sagittal planes. Although the OPL is difficult to see, its injury is inferred from injury of the posterior joint capsule. Irregularity, thickening, or disruption of the posterior joint

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Figure 14.  MR mages in a 23-year-old male football player injured during a game. (a, b) Sagittal (a) and axial (b) proton-density–weighted fat-saturated MR images show tears of the PCL (black arrow), proximal superficial MCL (straight white arrow in b), and proximal POL (curved white arrow in b). (c, d) Midcoronal (c) and posterior coronal (d) T2-weighted fat-saturated images show a complete tear of the proximal superficial MCL (white arrow in c), proximal attachment of the meniscofemoral ligament (arrowhead in c), and proximal POL (arrow in d). Note the bone contusion in the lateral tibial condyle (* in c) and tear of the PCL (black arrow in c).

capsule or OPL with surrounding edema may be seen (Figs 11, 16) (12).

Clinical Manifestation of PMC Injuries

In one series, the most common mechanism of injury, accounting for 72% of injuries, was sports, particularly football, basketball, and skiing. The remaining injuries were secondary to fall or motor vehicle accident (7). In another series, with concomitant MCL and POL reconstruction, sports and pedestrian accidents were the most common, accounting for 75% of the cases (31). PMC injuries rarely occur in isolation. They are more often associated with injuries that dominate the clinical picture and may mask injuries to the PMC. Common associated injuries include

ACL injuries including the O’Donoghue triad of ACL, MCL, and medial meniscal injuries; PCL injury; combined ACL and PCL injury; and MCL injury. Patients may present with medial knee pain and tenderness. At physical examination, the clinician may perform an abduction stress test, anterior drawer test, or posterior drawer test, which are tests used for evaluating the integrity of the MCL, ACL, and PCL, respectively. The abduction stress test is performed at 0° and 30° of flexion. Increased medial joint space at 30° of flexion but not at 0° of flexion indicates that the PMC is still functional, but any increase in opening of the joint space at 0° of flexion indicates concurrent injury of the posteromedial corner. Increased laxity at 30° of flexion, when combined with anterior rotatory

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Figure 15.  Knee injury in a 35-year-old male soccer player. Sagittal proton-density–weighted fat-saturated MR images obtained in the intercondylar notch (a) and medial compartment (b) show a complete ACL tear (arrow) and meniscocapsular separation (arrowhead).

Figure 16.  Sagittal proton-density–weighted fat-saturated MR images in a 22-year-old male football player with a knee injury demonstrate a tear of the OPL and posterior capsule (white arrow) and PCL sprain (black arrow in b).

subluxation of the medial tibial plateau, is indicative of injury of the PMC. With combined injuries of the MCL and POL, there are noticeable increases in internal and external tibial rotation. The anterior drawer test is performed with the foot in neutral rotation and external rotation. Increased anterior translation with the tibia in external rotation is indicative of AMRI. The posterior drawer test is performed with the knee in neutral and internal rotation. Increased tibial translation with the tibia internally rotated suggests a combined injury of the PCL and the PMC (30,38,39). Radiographs are usually obtained after knee injury as a preliminary study. A limited series may be obtained in an acute setting, but a complete series, including weight-bearing radiographs, is obtained in chronic instability. In acute injury, this

may show soft-tissue swelling, fractures including avulsion fractures, or residual laxity (in cases of knee dislocations). Stress views are usually painful in the acute setting and are more commonly obtained in cases of chronic injury. Radiographs with valgus and posterior stress are obtained to assess medial-sided and combined PCL and capsular/ collateral ligament injury, respectively (40,41). Computed tomography (CT) may be performed to evaluate fractures, but the soft-tissue structures are seldom evaluated. For knee dislocations, consideration should also be given to conventional, CT, and MR angiography and other noninvasive methods to evaluate the popliteal vasculature. The mainstay of diagnosis is clinical examination, but MR imaging is important in uncovering hidden injuries of the PMC, particularly when

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the physical examination is limited due to pain, swelling, and/or spasm. MR imaging is therefore an important part of presurgical planning (7,42).

Treatment of PMC Injuries

Isolated PMC injuries are uncommon and, as with isolated MCL injuries, can be treated conservatively (14). When PMC and MCL injuries are both present, stronger consideration of the patient for surgery may be warranted (1,7,14). Symptomatic, functional AMRI in patients with isolated or concurrent ACL and PMC injuries is an indication for surgical treatment of the PMC (7,29). When both the PCL and PMC are injured, surgical repair of both ligaments is indicated (14). Surgical repair of the PMC is also considered when multiligament (ACL, PCL, and MCL) injuries are present. Various techniques of repairing or reconstructing the POL have been described along with reconstruction of the MCL, including plication, advancement, augmentation, and reconstruction with autografts and allografts. However, in a setting of multiligament injury, considerations of whether to treat medial-side injuries conservatively or surgically, and of the timing, technique, and order of repair or reconstruction, are evolving and dependent on the treating surgeon (14,31). The long-term outcome of ACL reconstruction is worse when PMC injury is not addressed at surgery. Patients with isolated PCL injuries not managed with surgery have a change in the kinematics of the medial compartment, which can result in a characteristic pattern of medial compartment arthrosis. Combined PCL and PMC injury can exacerbate this change in kinematics with increased posterior tibial translation. Patients with concurrent PCL and PMC injuries who undergo PCL reconstruction without addressing the medial knee injury may remain disabled or experience failed PCL reconstruction (1,14).

Conclusion

Injuries to the PMC can be an important source of morbidity for patients with knee injuries and can easily be overshadowed by concurrent injuries that may dominate the clinical picture. By paying close attention to the PMC in patients with acute knee injuries, serious patient morbidity can be avoided.

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Posteromedial Corner of the Knee: The Neglected Corner.

The posteromedial corner of the knee (PMC) is an important anatomic structure that is easily seen but often overlooked on magnetic resonance (MR) imag...
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