Heterotopic ossification.

Heterotopic ossification is becoming increasingly recognized as a phenomenon that can complicate trauma to the head and spinal cord. It can be a disabling accompaniment of thermal injury, and it may seriously compromise results in hip arthroplasty and the treatment of acetabular fractures. Etiologic factors, which are imprecise and incompletely understood, vary with the clinical situation. The five cases reported here illustrate the radiologic appearances, complications, and diagnostic problems, including the difficulty in determining the timing of surgical resection. Reported for the first time are cases of neurogenic heterotopic ossification associated with bilateral shoulder involvement and bilateral ulnar nerve entrapment at the elbow. Heterotopic ossification may mimic acute arthritis.