Mechanism of locking at the knee joint.

The articular contours of the femoral and tibial condyles show certain basic features that in themselves appear sufficient to effect locking at the knee joint during the terminal stages of extension. The essential feature on the femoral condyles is a convex surface adapted for rolling. On the tibia, the contours that chiefly influence rotation are the concave up slope on the anterior portion of the medial condyle that brings the rolling medial femoral condyle to a skidding halt, and the flattening or convex down slope on the anterior part of the lateral tibial condyle that allows the rolling of the lateral femoral condyle to continue further forwards, thus bringing about medial rotation of the shaft. Load on the joint must assist in the locking movement by enhancing the influence of the bony contours.