Factors Associated with Residual Pelvic Obliquity after Total Hip Arthroplasty for Dysplastic Hip Osteoarthritis: A Retrospective Observational Study

Introduction: In patients with dysplastic hip osteoarthritis (DHOA), pelvic obliquity (PO) may present as either an upward tilt or a downward tilt on the affected side. Upward PO (U-PO) tends to persist more frequently after total hip arthroplasty (THA) than downward PO (D-PO), but the contributing factors remain unclear. This study aimed to identify factors associated with residual PO after THA, based on the preoperative direction of PO. Materials and Methods: This retrospective study included 116 patients (21 men, 95 women) who underwent unilateral THA for DHOA at a single institution between June 2018 and September 2023, and showed PO ≥2° on preoperative standing whole-spine radiographs. Based on the tilt direction, patients were categorized into the U-PO group (≥2° upward PO; n=35) or the D-PO group (≥2° downward PO; n=81). Pre-and postoperative radiographic parameters of the spine and lower limbs were analyzed to determine factors associated with residual PO. Results: In the U-PO group, PO improved to <2° in 14 patients and persisted in 21. In the D-PO group, PO improved in 50 and persisted in 31. Multivariate logistic regression identified limb lengthening (odds ratio [OR], 1.390; 95% confidence interval [CI], 1.010–1.910; P =0.045) as an independent factor associated with residual PO in the U-PO group. In the D-PO group, preoperative lumbar scoliosis angle (OR, 1.140; 95% CI, 1.010–1.290; P =0.039) and lumbar lateral bending mobility (OR, 0.680; 95% CI, 0.541–0.855; P <0.001) were independent predictors of residual PO. Conclusion: In the U-PO group, greater limb lengthening was associated with residual PO following THA. In contrast, in the D-PO group, a larger preoperative lumbar scoliosis angle and reduced lateral spinal flexibility contributed to residual PO.