L Andrew Ashton, Warwick Bruce, Jerome Goldberg and Willliam Walsh
Concord Repatriation & General Hospital, Concord, NSW, Australia

A retrospective study was carried out to evaluate prophylaxis for heterotopic ossification (HO) about the hip joint post total hip arthroplasty (THA). Between 1990 and 1996, 20 patients with known risk for developing HO were treated prophylactically to prevent this complication. Patients at risk were divided into 3 groups based on risk factors for HO formation (previous ipsilateral hip HO formation, previous contralateral hip HO formation and bilateral hypertrophic osteoarthritis) Single fraction radiotherapy of 600, 700 or 800 cGy was administered postoperatively to all patients. The aim was to irradiate all patients within 72 hours of THA. 12 (60%) patients received in addition a short course of postoperative indomethacin for less than 13 days.
Patients in this study were investigated for the following treatment variables: relative risk for forming HO, radiotherapy doses administered, time delays between surgery and irradiation, combined radiotherapy and indomethacin treatment versus radiotherapy alone, and surgical approach used for THA. Heterotopic ossification in patients was measured radiographically by use of the Brooker grading sytem, and was assessed clinically by use of the Harris Hip Score (HHS).
A significant difference was found between relative risk groups (p = 0.02). Patients with previous HO formation in the ipsilateral hip joint were at greater risk of developing HO than those with previous contralateral HO formation. Moreover both of these groups were at greater risk than those with advanced bilateral hypertrophic osteoarthritis. Other variables studied showed differences that were not significant due to small sample numbers.
This study, though limited by sample number, addresses questions regarding effective radiotherapy dosage, time delays acceptable before irradiation postoperatively, usefulness of short course postoperative indomethacin, and preferred operative approaches to minimise HO.

Key words: heterotopic bone; hip arthroplasty; radiation; indomethacin