Development of heterotopic ossification around the hip. A long-term follow-up of patients who underwent surgery with two different types of endoprostheses.

Heterotopic ossification has been reported in many pathological situations, most important clinically as a sequel to hip arthroplasty and spinal trauma. The etiology of heterotopic ossification is yet not clear, but the disease is supposed to be connected with trauma. Heterotopic bone was found in 53% (1.2% with the severe form) of 623 patients operated on at the Orthopaedic Hospital of the Invalid Foundation, Helsinki, Finland; the operations included 849 arthroplasties. The rate of heterotopic ossification was higher after revision arthroplasty, following operation of the contralateral side, in men, and in primary coxarthrosis, and the incidence was higher with the Brunswik (metal-on-plastic) endoprosthesis than in the McKee-Farrar type (metal-on-metal). Heterotopic bone formation generally seemed to increase and to be more manifest during long-term observation.