Increased risk of dislocation after primary total hip arthroplasty in inflammatory arthritis: a prospective observational study of 410 hips.

BACKGROUND It is unclear whether patients with inflammatory arthritis have a higher risk of dislocation after hip replacement. PATIENTS AND MATERIAL We carried out a prospective study assessing the incidence of dislocation within 2 years after surgery for patients diagnosed with inflammatory arthritis (IA) and osteoarthrosis (OA). One single type of prosthesis was implanted using a lateral approach. Both diagnostic groups were compared by univariate analysis with respect to dislocation, sex, age, diagnosis, prior hip surgery, experience of the surgeon and malposition of the acetabular component. In a multivariate logistic regression approach, the difference in dislocation incidence was assessed after adjusting for the effect of the potential confounders given above. Between 1996 and 1999, 410 THA were performed: 70 in IA and 340 in OA. After 2 years no patients were lost to follow-up, but 12 patients had died, and 5 revisions were carried out for reasons other than dislocation. RESULTS The dislocation rate in patients with IA was higher than in patients with OA: 10% (7 hips) in the IA group and 3% (10 hips) in the OA group (p = 0.006). No significant differences were found among the risk factors for dislocation between the two groups. Multivariate logistic regression analysis showed that IA is an independent risk factor for dislocation (odds ratio (OR) 3.7, 95% CI 1.3-11), together with malposition of the cup in more than 55 degrees abduction (OR 7.7, CI 2.3-26) and increased anteversion (OR 7.6, CI 1.4-42.4). INTERPRETATION Our findings clearly suggest that inflammatory arthritis has to be considered as an independent risk factor for dislocation after primary THA.