The risk of revision due to dislocation after total hip arthroplasty depends on surgical approach, femoral head size, sex, and primary diagnosis. An analysis of 78,098 operations in the Swedish Hip Arthroplasty Register

Abstract

Background and purpose: The effects of patient-related and technical factors on the risk of revision due to dislocation after primary total hip arthroplasty (THA) are only partly understood. We hypothesized that increasing the femoral head size can reduce this risk, that the lateral surgical approach is associated with a lower risk than the posterior and minimally invasive approaches, and that gender and diagnosis influence the risk of revision due to dislocation.

Patients and methods: Data on 78,098 THAs in 61,743 patients performed between 2005 and 2010 were extracted from the Swedish Hip Arthroplasty Register. Inclusion criteria were a head size of 22, 28, 32, or 36 mm, or the use of a dual-mobility cup. The covariates age, sex, primary diagnosis, type of surgical approach, and head size were entered into Cox proportional hazards models in order to calculate the adjusted relative risk (RR) of revision due to dislocation, with 95% confidence intervals (CI).

Results: After a mean follow-up of 2.7 (0-6) years, 399 hips (0.5%) had been revised due to dislocation. The use of 22-mm femoral heads resulted in a higher risk of revision than the use of 28-mm heads (RR = 2.0, CI: 1.2-3.3). Only 1 of 287 dual-mobility cups had been revised due to dislocation. Compared with the direct lateral approach, minimally invasive approaches were associated with a higher risk of revision due to dislocation (RR = 4.2, CI: 2.3-7.7), as were posterior approaches (RR = 1.3, CI: 1.1-1.7). An increased risk of revision due to dislocation was found for the diagnoses femoral neck fracture (RR = 3.9, CI: 3.1-5.0) and osteonecrosis of the femoral head (RR = 3.7, CI: 2.5-5.5), whereas women were at lower risk than men (RR = 0.8, CI: 0.7-1.0). Restriction of the analysis to the first 6 months after the index procedure gave similar risk estimates.

Interpretation: Patients with femoral neck fracture or osteonecrosis of the femoral head are at higher risk of dislocation. Use of the minimally invasive and posterior approaches also increases this risk, and we raise the question of whether patients belonging to risk groups should be operated using lateral approaches. The use of femoral head diameters above 28 mm or of dual-mobility cups reduced this risk in a clinically relevant manner, but this observation was not statistically significant.

Figure 1.

Frequency of revision surgery due to dislocation plotted against time elapsed after the index procedure.

Figure 2.

Survival functions with the endpoint “revision due to dislocation” by surgical approach. A Cox proportional hazards model was used in order to calculate adjusted survival functions with 95% CI (dotted lines) for the 3 surgical approaches lateral (n = 35,460), posterior (n = 41,904), and minimally invasive (n = 734). The endpoint was revision due to dislocation. Survival functions were adjusted for the mean of the covariates gender, age, primary diagnosis, and femoral head size. The lateral approach was associated with the lowest risk of revision due to dislocation.

Figure 3.

Survival functions for patients with a femoral neck fracture with the endpoint “revision due to dislocation” by femoral head size. A Cox proportional hazards model was used to calculate adjusted survival functions with 95% CI (dotted lines) for patients with a femoral head fracture, for the femoral head sizes 22 mm (n = 1,394) and 28 mm (n = 57,194), and dual-mobility cups (n = 287). The endpoint was revision due to dislocation. Survival functions were adjusted for the mean of the covariates gender, age, and surgical approach but fixed for the diagnosis femoral neck fracture. Dual-mobility cups were associated with the lowest risk of revision due to dislocation.