Anatomically, the MCL is composed of a superficial and a deep portion. The superficial MCL resides in the middle layer of the medial compartment of the knee and spans from the medial femoral condyle to its broad insertion at the metaphysis of the tibia, 4 to 5 cm below the joint line. Created by Sanjeev Bhatia, MDCreated by Sanjeev Bhatia, MD It is the principal restraint to valgus forces at the knee joint at all degrees of flexion. The deep MCL resides in the third layer of the medial compartment and is often separated from the superficial MCL by a bursa, which facilitates sliding of the two MCL components during flexion. The deep MCL attaches to the medial meniscus but does not aid in resisting valgus stress on the knee.
The severity of MCL injury is proportionally related to the number of ligament fibres torn in the superficial MCL. The most common location for MCL injuries is the femoral insertion.  Grade I injuries consist of a minimal number of torn fibres, grade II injuries have an increased degree of ligamentous disruption, and grade III injuries result in complete tearing of MCL fibres. Paradoxically, higher-grade MCL injuries are usually associated with less pain, perhaps because there is little or no tension on the injured ligament.  Patients with MCL injuries repeatedly experience a sense of valgus instability and altered biomechanics leading to post-traumatic arthrosis. Healing of grade I and II MCL injuries usually follows a relatively predictable sequence: haemorrhage, inflammation, proliferation, and re-modelling. 
Patients frequently have concomitant injury to the anterior cruciate ligament, posterior cruciate ligament, and meniscus. In knee injuries from cutting manoeuvres (occurring when an athlete forcefully changes direction on a weight-bearing foot), valgus forces placed on the knee usually tear the medial capsular ligament first, then the MCL, and finally the anterior cruciate ligament.  Because the meniscus is firmly attached to the deep MCL, meniscal tears commonly present alongside MCL tears.