KNEE | POSTEROMEDIAL CORNER (semimembranosus corner)

The posteromedial corner lies between the posterior margin of the medial (tibial) collateral ligament and the medial border of the posterior cruciate ligament.

Many of the normal individual components of the posteromedial corner are difficult to see on MRI because of their complex anatomy, thinness and close association with other structures.

Many injuries of the posteromedial corner can be identified by recognizing edema and soft-tissue thickening at the posteromedial knee, posterior to the medial collateral ligament in the expected location of the posterior oblique ligament.

Posteromedial corner structures (knee stabilizers)

1. Posterior oblique ligament (POL, Ligament of Winslow)

2. Semimembranosus tendon (dynamic stabilizer)

3. Oblique popliteus ligament (OPL)

4. Posterior horn of medial meniscus

5. Medial head of gastrocnemius tendon

6. Posteromedian joint capsule

7. Pes anserinus

8. TCL

Posterior oblique ligament

The posterior oblique ligament can be identified by location, as it forms the capsular layer posterior to the superficial medial collateral ligament.

Posterior oblique ligament originates from medial femur (adductor tubercle) and has 3 distal arms:

a) central or tibial component - is the most prominent and attaches strongly to the PHMM

b) superior or capsular arm - is continuous with the posterior joint capsule and the oblique posterior ligament

c) distal or superficial arm

Following trauma, injuries to the posterior oblique ligament may be accompanied by marrow edema due to avulsive forces related to the strong proximal attachment of the posterior oblique ligament at the adductor tubercle.

Semimembranosus tendon

The semimembranosus tendon has five major arms or expansions attaching to bone, capsule, ligaments and the medial meniscus:

a) the direct arm (principal attachment), - 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.

b) the capsular arm, - The capsular arm blends with the posteromedial capsule and coalesces with the capsular portions of the OPL and POL.

c) the extension to the OPL (OPL insertion),

d) the anterior arm (tibial or reflected arm, pars reflexa), - The anterior arm inserts into the medial proximal tibia, superior to the insertion of the superficial MCL and passing under the POL

e) the inferior (popliteal) arm

Oblique popliteal ligament

Connects the posterior medial and lateral knee.

Attaches medially to the capsular arm of the posterior oblique ligament.

Medial gastrocnemius tendon and semimembrabosus tendon

As the medial gastrocnemius tendon and SM tendon can be directly visualized, injuries to these structures can be specifically diagnosed by recognizing the intrinsic abnormalities of the affected regions

Other causes of posteromedial corner edema or fluid

Ruptured popliteal cyst

parameniscal cyst

pes anserine bursitis

Posteromedial corner injury can result in increased stress on the cruciate ligaments and can result in anteromedial rotatory instability (AMRI) of the knee. Up to 88% of injuries can be associated with a cruciate ligament injury, overwhelmingly the ACL.

Reference

1. https://radsource.us/posteromedial-corner-injury-of-the-knee/

2. https://pubs.rsna.org/doi/pdf/10.1148/rg.2015140166