Shielding of the hip prosthesis during radiation therapy for heterotopic ossification is associated with increased failure of prophylaxis.

Purpose: Radiation therapy (RT) is frequently administered to prevent heterotopic ossification (HO) after total hip arthroplasty (THA). The purpose of this study was to determine if there is an increased risk of HO after RT prophylaxis with shielding of the THA components. Methods and Materials: This is a retrospective analysis of THA patients undergoing RT prophylaxis of HO at Brigham and Women’s Hospital between June 1994 and February 2004. Univariate and multivariate logistic regressions were used to assess the relationships of all variables to failure of RT prophylaxis. Results: A total of 137 patients were identified and 84 were eligible for analysis (61%). The median RT dose was 750 cGy in one fraction, and the median follow-up was 24 months. Eight of 40 unshielded patients (20%) developed any progression of HO compared with 21 of 44 shielded patients (48%) ( p = 0.009). Brooker Grade III-IV HO developed in 5% of unshielded and 18% of shielded patients ( p = 0.08). Multivariate analysis revealed shielding ( p = 0.02) and THA for prosthesis infection ( p = 0.03) to be significant predictors of RT failure, with a trend toward an increasing risk of HO progression with age ( p = 0.07). There was no significant difference in the prosthesis failure rates between shielded and unshielded patients. Conclusions: A significantly increased risk of failure of RT prophylaxis for HO was noted in those receiving shielding of the hip prosthesis. Shielding did not appear to reduce the risk of prosthesis failure.