Elective total hip arthroplasty stratified by surgical approach: how does the risk of dislocation relate to bearing size, type, and implant fixation?

Dislocation after total hip arthroplasty (THA) is a serious concern that is rarely reported as a complication in register studies. The aim of this population-based register study was to evaluate the true cumulative incidence of dislocation by surgical approach following THA and to assess the impact of bearing size, bearing type, and stem fixation on dislocation risk. In this longitudinal cohort study based on data from 136,810 patients with a unilateral primary elective THA were included and followed up until 1st dislocation, revision, death, second THA of bilateral THA, or until the end of the observation period. The cumulative dislocation incidence was estimated using the Kaplan-Meier method, stratified by surgical approach. Multiple Cox regression models were fitted to estimate adjusted hazard ratios (HRs) for dislocation. The 1-year cumulative dislocation incidence was 1.1% (1.0-1.2) after use of the lateral approach and 2.3% (2.2-2.4) after the posterior approach. The use of femoral heads smaller than 32mm (reference) was associated with a higher risk of dislocation for both approaches (lateral HR 1.72, 95% CI 1.43-2.07, p<0.001; posterior HR 1.34, 95% CI 1.19- 1.51, p<0.001). For the posterior approach, head sizes bigger than 32mm or dual mobility cups (DMC) conferred a lower risk of dislocation (>32mm HR 0.64, 95% CI 0.9-0.83, p<0.001; DMC HR 0.21, 95%CI 0.1-0.41, p<0.001). The use of the posterior approach carried an increased risk of dislocation when compared with the lateral approach, however, this risk was mitigated when using head sizes of 36 mm and by DMC.