Timing of first-time dislocation varied by head size after total hip arthroplasty for nontraumatic osteonecrosis

The cumulative probability of a first-time dislocation (CPD) after total hip arthroplasties (THAs) with a 22-mm head was reported in 2004 to rise steadily to 7% at 25 years. Later reports employing larger heads indicated that dislocations were concentrated in shorter periods after THA. Therefore, dislocations may occur differently over time depending on head sizes, which has not been clearly demonstrated. The purpose was to examine this hypothesis.

With first-time dislocation as the endpoint, the Cox proportional-hazards regression model and the Kaplan-Meier analyses were applied to 6,339 THAs performed for nontraumatic osteonecrosis of the femoral head, considering all possible influencing variables. Follow-up was 6.5 years on average (range, 0 to 27).

First-time dislocations occurred in 289 THAs (4.6%) at a mean of 2.9 years (range, 0 to 19) following THA, with head sizes of 22, 26, 28, 32, and ≧ 36 mm in 33, 76, 86, 58, and 36 THAs, respectively (dislocation rate: 13.5, 8.2, 5.7, 2.5, and 2.7%, respectively; p < 0.001 in χ2 test). CPD increased linearly after THA in the 22-mm group, while larger-head groups showed a steep early rise followed by a decline.

First-time dislocations occurred differently over time with different head sizes.