Upward versus downward pelvic obliquity in dysplastic hip osteoarthritis: direction-specific factors associated with residual obliquity after total hip arthroplasty.

INTRODUCTION: Pelvic obliquity (PO) in dysplastic hip osteoarthritis may tilt upward (U-PO) or downward (D-PO) on the affected side. U-PO tends to persist more frequently after total hip arthroplasty (THA) than D-PO; however, the contributing factors remain unclear. This study investigated factors associated with residual PO after THA according to preoperative PO direction. METHODS: This retrospective study included 116 patients (21 men, 95 women) who underwent unilateral THA for dysplastic hip osteoarthritis with preoperative PO ⩾2°. Patients were classified into U-PO (n = 35) or D-PO (n = 81). Pre- and postoperative radiographic parameters of the spine and legs were analysed to identify factors associated with residual PO. RESULTS: In the U-PO group, leg lengthening was independently associated with residual PO (odds ratio [OR], 1.390; 95% confidence interval [CI], 1.010-1.910; p = 0.045). In the D-PO group, greater preoperative lumbar scoliosis angle (OR, 1.180; 95% CI, 1.030-1.360; p = 0.022) and reduced lumbar lateral bending mobility (OR, 0.681; 95% CI, 0.540-0.858; p = 0.001) were independently associated with residual PO. CONCLUSIONS: Direction-specific assessment of PO may help anticipate residual coronal malalignment after THA, with greater leg lengthening associated with residual U-PO and spinal deformity and reduced lumbar flexibility associated with residual D-PO.