Abstract
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.