Lab 8 - Module 1 - Anatomy of the Knee: Page 10 of 10

Biomechanics of the Knee Joint

Mechanics of Motion: First one must consider the starting point of the knee when discussing the mechanics of motion, then the fixation must be defined. In most cases, knee biomechanics is described moving from flexion to extension, with the distal segment sabilized (closed kinetic chain).

1. During extension, the femoral condyles roll forward on the tibia (closed kinetic chain, or distal segment stabilized) until the posterior cruciate stops the forward roll and causes the femur to slide somewhat posteriorly. Due to the fact that the posterior cruciate attaches on the medial femoral condyle, a degree of spin occurs.

2. When, during this extension, the lateral surface of the joint has been "used up", there is still surface remaining for the medial surface to move. As the lateral surface comes to its stopping point, two things occur. First the anterior edge of the lateral femoral condyle locks up with its meniscus and, at almost the same time, the anterior cruciate fits into the intercondylar notch. Thus, the lateral surface of the joint is locked up but the medial surface still has room to move.

3. The final "screw home" occurs to allow the medial surface of the joint to come into full articulation. The lateral surface is locked up and the anterior cruciate acts as a pivot point around which rotation occurs. The femur internally rotates allowing the medial condyle to continue to roll and spin into its full locked position, while the locked-up lateral side merely slides (as a unit) forward slightly.