title: 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: OBJECTIVE
  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.
  
  
  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.
  
  
  CONCLUSIONS
  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.
authors.affiliation: Department of Orthopaedics, Institute of Surgical Sciences, Uppsala University Hospital, Uppsala, Sweden. nils.hailer@surgsci.uu.se
authors.forename: Nils P
authors.initials: NP
authors.lastname: Hailer
authors.forename: Rüdiger J
authors.initials: RJ
authors.lastname: Weiss
authors.forename: André
authors.initials: A
authors.lastname: Stark
authors.forename: Johan
authors.initials: J
authors.lastname: Kärrholm
mesh_terms: D000368:Aged; D019644:Arthroplasty, Replacement, Hip / Q000009:adverse effects / Q000379:methods; D005260:Female; D005265:Femoral Neck Fractures / Q000601:surgery*; D005270:Femur Head; D005271:Femur Head Necrosis / Q000601:surgery*; D006617:Hip Dislocation / Q000209:etiology*; D006801:Humans; D008297:Male; D008875:Middle Aged; D012086:Reoperation; D012737:Sex Factors