Fracture originated at the tip of heterotopic ossification of femur by minor external force: report of two cases.

Fracture of femur without evidence of severe osteoporosis rarely occurs as a result of minor external force such as a gentle passive or an active range of motion exercises. We report two cases of femoral shaft fracture occurred at the tip of heterotopic ossification of femur by a minor external force, in which the involved femur shows no evidence of remarkable osteoporosis. The possible mechanism inferred by authors is as follows: 1) heterotopic ossification limits range of motion of the hip joint; 2) a new lever arm is formed at the tip of heterotopic ossification where energy can be concentrated; 3) therefore, fractures develop even by a minor external force. Search for similar cases and further discussions on possible mechanisms and prevention of femoral fractures in patients with heterotopic ossification will be necessary.