Pelvic Tilt and the Pubic Symphysis to Sacrococcygeal Junction Distance: Risk Factors for Hip Dislocation Observed on Anteroposterior Pelvis Radiographs

Background: Dislocation after total hip arthroplasty (THA) is among the most common causes of early revision in contemporary practice. Abnormal spinopelvic alignment increases risk for dislocation, but methods to identify such are limited and can be complex. We sought to determine the effect of pelvic tilt, using a novel radiographic measurement, on dislocation risk by evaluating those with and without a history of dislocation. Methods: Using our institutional total joint registry, we identified 10,082 primary THAs performed between 2006 and 2015. Postoperatively, 177 dislocated (1.7%). Dislocators were matched 1:1 to control patients who did not dislocate. Pelvic tilt was calculated using the pubic symphysis to sacrococcygeal junction distance (PSCD) from a supine anteroposterior pelvis radiograph both preoperatively and postoperatively. The association between dislocation and both pelvic tilt and PSCD was then evaluated by logistic regression. Mean follow-up was 3 years. Results: Patients who dislocated had more posterior pelvic tilt (mean pelvic tilt of 57° vs 60°; P = .02) and smaller PSCDs (mean 41 mm vs 46 mm; P = .04) than controls. Patients with a PSCD <0 mm (symphysis above sacrococcygeal junction) had 9-fold odds of dislocation compared to those with a PSCD >50 (odds ratio 9; P = .006). Conclusion: Patients who dislocated following primary THA had more posterior pelvic tilt. Additionally, those with a PSCD <0 had 9-fold odds of dislocation. Assessing the PSCD can alert a surgeon of increased risk for dislocation and identification of a negative PSCD should encourage further investigation or optimization of the preoperative plan to minimize dislocation risk.