Femoral neck fracture following avascular necrosis of the femoral head.

Four cases of femoral neck fracture following avascular necrosis of the femoral head were studied histologically. All four patients were women who had received steroid therapy, three of them for systemic lupus erythematosus and the other for idiopathic thrombocytopenic purpura. Two types of fracture were found according to the site and the mechanism of fracture. One was at the junction between the necrotic bone and the repairing bone, and it can thus be regarded as a stress fracture. The other type of fracture commenced at the superior portion of the junction between the femoral head and neck, which was weak due to the repair reaction. The fracture line extended to the inferior cortex of the femoral neck, as often occurs in the elderly. In one patient, the femoral neck fracture was the first sign of avascular necrosis of the femoral head.