Chondral fracture of the lateral trochlea of the femur occurring in an adolescent: mechanism of injury.

The trochlea of the femur is a very unusual site for chondral fracture. Little is known of the mechanism of injuries confined to the articular cartilage of the trochlea of the femur. A very unusual case of chondral fracture of the lateral trochlea of the femur occurring in an adolescent is reported here. The mechanism by which this injury occurred could be evaluated in this patient. The cartilage on the convex surface of the lateral trochlea was likely avulsed proximally by shear force of the patella during rapid extension of the weight-bearing knee from a flexed position. From a viewpoint of mechanism, this injury differs from the more usual osteochondral or chondral fractures of the weight bearing area of the femoral condyle, which are usually accompanied by twisting forces.