INTRODUCTION: Pelvic obliquity (PO) in dysplastic hip osteoarthritis (DHOA) can present as either upward or downward tilting of the affected side. This study investigated the influence of preoperative PO direction on postoperative clinical outcomes and hip-spine morphology in patients undergoing total hip arthroplasty (THA). MATERIALS AND METHODS: Data from 116 (21 men, 95 women) patients with unilateral DHOA, who underwent THA at a single institution between June 2018 and September 2023 and exhibited ≥ 2° of PO, were analyzed. Patients were categorized into two groups: upward PO (U-PO [≥ 2° upward tilt, n = 35])
and downward PO (D-PO [≥ 2° downward tilt, n = 81]). Patient demographic information, surgery-related factors, hip function scores, and radiographic parameters of the hip, lower limbs, and spine were compared between the groups. RESULTS: Except for the duration of hip disorders, no significant differences were observed in patient background and surgical data between the groups. Preoperatively, the U-PO group exhibited a larger acetabular offset, greater hip adduction angle, longer functional leg length on the affected side, and greater ipsilateral convex lumbar scoliosis than the D-PO group (P = 0.034, P <
0.001, P <
0.001, and P <
0.001, respectively). Postoperatively, a greater hip adduction angle and longer functional leg length discrepancy persisted in the U-PO group compared to those in the D-PO group (P <
0.001 and P = 0.002, respectively). The median (interquartile range) residual PO was greater in the U-PO group (3° [0-4°]) than that in the D-PO group (1° [0-3°]) (P = 0.009). Compared with the D-PO group, the mean postoperative hip Japanese Orthopaedic Association scores were significantly lower in the U-PO group (85 [81-92] vs. 92 [85-96], P = 0.016). CONCLUSION: The U-PO group exhibited greater residual hip adduction angles, longer functional leg lengths on the affected side, and less improvement in PO after THA than the D-PO group, resulting in poorer postoperative hip function.