Cup size and risk of dislocation after primary total hip arthroplasty.

ABSTRACT

We evaluated the relationship between cup diameter and dislocation risk in patients undergoing primary total hip arthroplasty (THA) with a 28-mm head. There were 50 dislocations, 28 of which occurred in 2221 (1.3%) THAs with a cup diameter smaller than 56 mm and 22 in 513 (4.3%) with a cup diameter of 56 mm or larger. Dislocation risk varied between 0.6% and 2.4% in the smaller cup group and between 4.1% and 5.2% in the larger cup group. The risk was substantially higher in the large cup group (unadjusted odds ratio, 3.5; 95% confidence interval, 2.0-6.2). Multivariable logistic regression revealed an adjusted odds ratio of 2.4 (95% confidence interval, 1.2-4.9). Patients with THA (28-mm head) had more than twice the risk of dislocation with cup size of 56 mm or higher compared to patients with smaller cups.