Clinical cutoff value of lumbar bending range for predicting residual pelvic obliquity after total hip arthroplasty in dysplastic hip osteoarthritis

INTRODUCTION: Residual pelvic obliquity (PO) after total hip arthroplasty (THA) for dysplastic hip osteoarthritis (DHOA) may adversely affect coronal balance and postoperative outcomes. Although lumbar bending range (LBR) is considered critical for PO improvement, the clinical cutoff value for predicting residual PO remains unclear. This study aimed to determine the cutoff value of LBR associated with residual PO after THA. MATERIALS AND METHODS: Among 382 patients who underwent primary THA for unilateral DHOA between July 2019 and June 2024, 98 patients with preoperative downward PO of ≥ 2° on the affected side were included. Patients were classified into a residual group (postoperative PO ≥ 2° at 1 year) and an improvement group (postoperative PO < 2°). Demographic data and radiographic parameters of the hip, lower limbs, and spine were compared. Multivariate logistic regression analysis was performed to identify factors associated with residual PO, and receiver operating characteristic (ROC) analysis was used to determine the optimal LBR cutoff value. RESULTS: Residual PO was observed in 28 patients (29%). Compared with the improvement group, the residual group demonstrated significantly greater Crowe index values, preoperative radiographic leg length discrepancy, and preoperative PO. Affected-side LBR was significantly smaller in the residual group (4.3° ± 3.0° vs. 7.7° ± 4.0°, P < 0.001), whereas the unaffected-side LBR exhibited no significant difference. Multivariate analysis identified affected-side LBR as the only independent factor associated with residual PO (odds ratio, 0.698; 95% confidence interval, 0.580-0.840; P < 0.001). ROC analysis demonstrated an optimal affected-side LBR cutoff value of 4.5° (area under the curve, 0.783). CONCLUSIONS: Reduced affected-side LBR was strongly associated with residual PO following THA in patients with DHOA presenting with downward PO. An affected-side LBR cutoff value of 4.5° is helpful in predicting residual PO and assist in coronal compensatory capacity before THA.