Incidence of Heterotopic Ossification in Direct Anterior Approach to Total Hip Arthroplasty with use of Aspirin as Thromboembolic Prophylaxis

CONTEXT Heterotopic ossification (H.O.) is a common occurrence after total hip arthroplasty (THA) with significant potential clinical ramifications. Controversy still exists regarding the exact etiology of the disorder, including possible risk factors. Surgical technique, surgical approach, postoperative medication protocols and even thromboembolic prophylaxis have been implicated in the formation of H.O. Our study looked at one institution with a single surgeon performing direct anterior THA (DAA THA) in patients who received aspirin (ASA) as monotherapy for thromboembolic prophylaxis. METHODS Patients at a single institution who underwent DAA THA between 2015 and 2019 were identified by CPT code. 45 patients ultimately met inclusion criteria. Postoperative radiographs were analyzed retrospectively for H.O. according to the Brooker classification. Several patient characteristics and comorbidities were statistically analyzed using Chi-square tests, Fisher Exact tests, Wilcox rank sum tests, and Pearson correlation. RESULTS 12 patients (26.7%) were found to have heterotopic ossification (67% Class 1, 8% Class 2, 25% Class 3, and 0% Class 4); with a median follow up of 35 weeks (range: 12-96). 25% of these patients received ASA 325mg BID while 75% received ASA 81 BID. No statistical differences in development of H.O. were detected among age, gender, BMI, sex, race, diabetes, or NSAID use in the post-operative interval. There were significantly more smokers in the H.O. group (50% vs. 9%, p<0.006). CONCLUSIONS Our analysis aimed to quantify the incidence of H.O. with consistency in surgical approach and post-operative protocol. There have been few studies on this topic, and we believe it is very relevant with the increasing use of aspirin in the post-operative protocol for thromboembolic prophylaxis. Our retrospective analysis identified H.O. at rates similar to previous studies in DAA.