Effect of femoral head diameter and operative approach on risk of dislocation after primary total hip arthroplasty

Background: It has been postulated that use of a larger femoral head could reduce the risk of dislocation after total hip arthroplasty, but only limited clinical data have been presented as proof of this hypothesis. Methods: From 1969 to 1999, 21,047 primary total hip arthroplasties with varying femoral head sizes were performed at one institution. Patients routinely were followed at defined intervals and were specifically queried about dislocation. The operative approach was anterolateral in 9155 arthroplasties, posterolateral in 3646, and transtrochanteric in 8246. The femoral head diameter was 22 mm in 8691 of the procedures, 28 mm in 8797, and 32 mm in 3559. Results: One or more dislocations occurred in 868 of the 21,047 hips. The cumulative risk of first-time dislocation was 2.2% at one year, 3.0% at five years, 3.8% at ten years, and 6.0% at twenty years. The cumulative ten-year rate of dislocation was 3.1% following anterolateral approaches, 3.4% following transtrochanteric approaches, and 6.9% following posterolateral approaches. The cumulative ten-year rate of dislocation was 3.8% for 22-mm-diameter femoral heads, 3.0% for 28-mm heads, and 2.4% for 32-mm heads in hips treated with an anterolateral approach; 3.5% for 22-mm heads, 3.5% for 28-mm heads, and 2.8% for 32-mm heads in hips treated with a transtrochanteric approach; and 12.1% for 22-mm heads, 6.9% for 28-mm heads, and 3.8% for 32-mm heads in hips treated with a posterolateral approach. Multivariate analysis showed the relative risk of dislocation to be 1.7 for 22-mm compared with 32-mm heads and 1.3 for 28-mm compared with 32-mm heads. Conclusions: in total hip arthroplasty, a larger femoral head diameter was associated with a lower long-term cumulative risk of dislocation. The femoral head diameter had an effect in association with all operative approaches, but the effect was greatest in association with the posterolateral approach.