THE MEDIAL COMPARTMENT
The Medial Collateral Ligament
The medial collateral ligament originates from the adductor tubercle and proceeds distally 10-12 cm, fanning out and inserting over a broad area of the tibia under the pes anserinus, blending with the periosteum. It has a superficial and a deep component. The superficial ligament has no attachments to the underlying structures. The deep portion inserts into the superior aspect of the medial meniscus. The ligament can also be thought to have anterior and posterior fibers. The anterior fibers are thick and tighten during knee flexion, while the posterior fibers are relaxed.
The Medial Capsuloligamentous Complex
The medial capsuloligamentous complex is a complicated structure. Anatomically, it has three layers, which are numbered with Roman numerals I, II, and III from superficial to deep.
Deep Layer (III)
This is the capsule of the knee. It can also be conceptualized as anterior, midportion, and posterior components. It is thin anteriorly. The midportion is the deep layer of the medial collateral ligament, which has attachments to the medial meniscus. The posterior component fuses with the middle layer (II) to become the posterior oblique ligament. Further posteriorly, the layer thins, and is reinforced by the semimembranosus tendon, which is an important stabilizer of this posteromedial corner of the knee.
There are five reflections of the semimembranosus tendon: 1. oblique popliteal ligament. 2. posterior oblique ligament and medial meniscus rim. 3. posteromedial tibial metaphysis. 4. anteromedial tibial metaphysis, running deep to the superficial MCL. 5. investing fascia around the popliteus muscle
Intermediate Layer (II)
This layer contains the superficial medial collateral ligament. Posteriorly, this layer and the deep layer form the posterior oblique ligament which has an attachment to the posterior portion of the medial meniscus. This area is an important insertion for the semimembranosus tendon, as a secondary stabilizer in the ACL deficient knee. Some controversy exists about whether the posterior oblique ligament is truly a separate ligament or simply the posterior portion of the superficial MCL
Superficial Layer (I)
This layer consists of the deep fascia of the thigh and leg, and sartorius muscle. Anterior to the superficial MCL, the fibers of the intermediate and superficial layers are inseparable.
The “Posteromedial Capsule”
The “posteromedial capsule” refers to the confluence of the capsuloligamentous layers II and III posterior to the medial collateral ligament but anterior to the posterior cruciate ligament. The posteromedial capsule plays a crucial role in the medial and rotatory stability of the knee; the integrity of the capsular complex directly affects the prognosis of injuries to the MCL because it adds a component of rotation to the tears.