Which ligament would you expect to be torn in a knee joint that has received a direct blow from the lateral side?
a. medial collateral
b. lateral collateral
c. posterior cruciate
d. medial patellar reticulum
The answer is: a. medial collateral ligament
The answer to this problem seems a bit counter-intuitive at first until one considers the mechanism of injury. Because the blow to the knee is delivered from the lateral side one might be tempted to conclude that the lesion affected lateral structures rather than medial ones. This ignores, however, the severe gapping or disarticulating forces that would be applied to the knee. In this classic knee injury a blow delivered to the lateral knee creates a valgus stress at the knee which, especially when the foot is planted and does not release, causes the medial compartment articular surfaces (medial femoral condyle and medial tibial plateau) to move away from one another. In a conventional stress x-ray one would see a gapping of the medial compartment in excess of the joint space seen in an unstressed film. The medial collateral ligament (MCL) articulates the medial compartment of the knee and therefore a severe valgus stress may lead to excessive tensile forces being applied to the MCL causing it to stretch and/or tear.