Medial Supporting Structures: Medial Collateral Ligament and Posteromedial Corner

Semimembranosus Tendon Complex

The semimembranosus tendon is the most important dynamic stabilizer of the PMC. It originates at the posterolateral aspect of the ischial tuberosity. Distally, it divides into five anatomically distinct tendinous branches, each inserting into the posteromedial region of the knee: (1) the direct arm (principal attachment), (2) the capsular arm, (3) the extension to the OPL (OPL insertion), (4) the anterior arm (tibial or reflected arm, pars reflexa), and the (5) inferior (popliteal) arm (see Figs. 11.3 to 11.9 ). The insertion sites include the infraglenoid tubercle of the posteromedial tibial plateau, the posterior capsule, the posterior horn of the medial meniscus, and a portion of the capsule deep to the MCL. Axial MR images may often demonstrate a small amount of fat accumulation between the different arms of the semimembranosus tendons at the level of the joint line, which is considered a normal variant. Axial sections obtained at the level of the upper portion of the femoral condyles may demonstrate the semimembranosus tendon before it branches into the five different arms as a “ C ”-shaped structure or as a “ 3 ”-shaped structure adjacent to the distal semimembranosus muscle.

The direct arm has an anterior course deep to the anterior arm and inserts at the tubercle on the posteromedial tibial condyle, also known as the tuberculum tendinis . Immediately prior to its tibial insertion, it attaches to the posterior aspect of the coronary ligament of the posterior horn of the medial meniscus. The insertion site of the direct arm is best visualized on both sagittal and axial images. The anterior and direct arms together may form a low-signal-intensity band along the posteromedial aspect of the tibia, distal to the articular margin and inseparable from each other (see Figs. 11.3 to 11.9 ).

The capsular arm of the semimembranosus tendon is best visualized on axial images. This arm is seen as a deep-lying, flat, striated structure arising from the posterior medial aspect of the tibial plateau, which then combines with the posteromedial capsule to join the capsular portions of both the OPL and POL. This arm is better appreciated as a separate structure when the joint is distended by fluid, although this is not very common. Hence, the capsular arm is seen as a flat striated structure located in the posterior medial aspect of the tibial plateau, inseparable from the adjacent direct and anterior arms arm while continuous to the OPL (see Figs. 11.3 to 11.9 ).

The anterior or tibial arm, also known as the reflected arm , extends anteriorly, passing under the POL, and inserts on the medial aspect of the proximal tibia under the tibial collateral ligament. Most commonly, this arm is seen on medial peripheral sagittal images as a low-signal-intensity structure curving anteriorly and taking an almost horizontal course. When the knee is fully extended, there is angulation of the anterior arm, resulting in a focal area of high signal intensity within the tendon with a echo time (TE) of 20 ms consistent with a magic angle artifact ( Fig. 11.10 ). On sequential coronal images, the horizontal portion of the anterior arm is seen as a round or oval hypointense or as a thin structure adjacent to the medial aspect of the tibia, under the tibial collateral ligament and POL ( Fig. 11.11 ). The thickness of the anterior arm varies significantly between normal individuals. On sequential axial images, the anterior arm is demonstrated as a hypointense structure adjacent to the posterior medial aspect of the tibia, extending anteriorly in consecutive sections (see Figs. 11.4 to 11.9 ).

The inferior or popliteal arm extends more distally than the anterior and the direct arms. It passes under the distal tibial segments of the POL and the tibial collateral ligament, inserting just above the tibial attachment of the tibial collateral ligament. On sequential axial MR images, the inferior arm is seen as a low-signal-intensity structure extending inferiorly and anteriorly from the posterior margin of the joint line, deep to the MCL. The anterior arm is sometimes further divided into medial and lateral divisions, with the attachment to the medial tibia forming the medial division and the lateral division extending over the popliteal fascia (see Figs. 11.3 to 11.9 ). *

* References .

Posterior Oblique Ligament (Ligament of Winslow)

The POL or ligament of Winslow originates in the adductor tubercle of the femur. Its anterior margin combines with the posterior margin of the superficial MCL and its posterior margin joins the joint capsule, which is part of layer III. The POL has three bundles of attachment that are inseparable: (1) the superficial arm, or fascial tissue of the pes anserinus, (2) the tibial arm, or the posterior medial aspect of the tibia and the posterior horn of the medial meniscus, and (3) the capsular arm, which merges with the capsular arm of the semimembranosus tendon (SMT) and OPL (see Fig. 11.3 ). The tibial arm is considered the thickest and most important of these arms. This arm extends posteriorly and obliquely and inserts into the posteromedial aspect of the medial meniscus and the adjacent posteromedial aspect of the tibia, migrates deep to the anterior arm of the semimembranosus tendon, and finally merges with the main part of the tendon (see Fig. 11.3 ). The tibial arm is seen best on the sequential coronal and axial images, running immediately posterior to the superficial MCL as it joins with the capsule while inserting into the medial meniscus and the adjacent tibia. The capsular arm of the POL lies superior to the joint line of the knee. This arm is continuous with the posterior joint capsule and combines with the capsular arm of the semimembranosus tendon to form the proximal aspect of the OPL (see Fig. 11.3 ). Using the axial images, the capsular arm is commonly seen at the level of the femoral condyle, while on coronal images it presents as a thin hypointense band that extends posteriorly from the superficial MCL to combine with the medial aspect of the posterior capsular structures. Finally, the superficial arm inserts into the sheath and tibial insertion points of the semimembranosus tendon, combines with the posterior margin of the superficial MCL, and passes superficially to the anterior arm of the semimembranosus tendon (see Fig. 11.3 ). The superficial arm is best visualized on coronal images as a thin hypointense band superficial to the direct insertion of the semimembranosus tendon and extends from its origin to insert into the medial aspect of the tibial plateau.

Oblique Popliteal Ligament

The OPL is a thin, broad, lateral extension of the semimembranosus tendon. It both covers and blends with the posterior medial capsule to extend beyond the midline of the joint and unites with the arcuate ligament from the posterolateral aspect of the knee. The OPL originates from both the capsular arm of the POL and the lateral expansion of the semimembranosus. It transcends laterally to insert into the osseous or cartilaginous fabella, the meniscofemoral portion of the posterolateral joint capsule, and the plantaris muscle (see Fig. 11.3 ). The fibrous attachment of the OPL attaches to the lateral aspect of the PCL facet. Coronal and sagittal images may demonstrate this thin structure as a low-signal-intensity line indistinguishable in most cases from the posterior capsule but in continuity with the main tendon of the semimembranosus ( Figs. 11.12 and 11.13 ).