Abstract
OBJECTIVE: To determine the risks of primary total hip arthroplasty (THA) and total knee arthroplasty (TKA) in older patients with ankylosing spondylitis (AS). METHODS: We used administrative data from 1999 to 2013 on US Medicare beneficiaries to identify patients (< 75 yrs old) with AS and a comparison group without AS. Rates of primary THA and primary TKA were computed for each group, and standardized for age, sex, and race. We also examined risks of primary TKA among patients with AS by their THA status. RESULTS: We analyzed 52,568 patients with AS and 4,617,179 patients without AS. Between 1999 and 2013, the standardized incidence of primary THA in patients with AS doubled from 4.5 per 1000 patient-years (PY) to 9.6 per 1000 PY. Rates of primary TKA were higher in patients with AS than controls in all years. In 2013, the standardized incidence of primary TKA in AS was 12.3 per 1000 PY versus 5.7 per 1000 PY in the comparison group (RR 2.14, 95% CI 1.93-2.38). Rates of primary TKA were twice as high among patients with AS and THA than among those without THA (20.4 vs 10.2 per 1000 PY). CONCLUSION: Rates of THA in older patients with AS doubled over recent years, outpacing the increase in the general population. Rates of TKA were also substantially higher in older patients with AS. The increased risk of TKA in AS may be a consequence of damage from knee inflammation, or alterations in lower extremity biomechanics due to hip arthritis.