Morphological risk factors associated with dislocation after bipolar hemiarthroplasty of the hip in patients with femoral neck fractures—a nested case-control study

Background The relationship between preoperative hip measurements and dislocation after bipolar hemiarthroplasty is presently unclear. In the current study, we investigated the morphological risk factors associated with dislocation after bipolar hemiarthroplasty of the hip in patients with femoral neck fractures. Methods Between January 2011 and June 2017, a nested case-control design study was used to analyze the risk factors for dislocation in 348 patients who had undergone bipolar hemiarthroplasty because of femoral neck fractures. Twelve patients underwent at least one dislocation postoperatively. Sixty patients without dislocation were selected as controls matched in terms of time of surgery, age, and sex, at a ratio of 1:5. Patient acetabular measurements were compared between the dislocation group and the control group, including the center-edge angle, abduction angle, acetabular width and depth, depth-to-width ratio, femoral neck offset, leg length discrepancy, and femoral head coverage ratio. A multivariate logistic regression model was used to evaluate the morphological risk factors of dislocation. Results Postoperatively, the incidence of dislocation was 3.4%. A smaller center-edge angle was found to be a risk factor associated with dislocation after bipolar hemiarthroplasty of the hip. Patients with small acetabular depth and a small acetabular depth–width ratio were prone to dislocation. Patients with a center-edge angle of ≤ 45.4° or an acetabular depth of ≤ 19.12 mm were more likely to suffer dislocation. Conclusions Careful preoperative measurements before bipolar hemiarthroplasty of the hip are important. Surgical intervention for femoral neck fracture patients with a shallow acetabulum should be carefully planned and total hip arthroplasty should be considered when necessary.