Correlation Analysis of Clinical Outcomes for Patients With Coronal Pelvic Obliquity After Total Hip Arthroplasty in Direct Anterior Approach

Objective: Abnormal pelvic coronal plane obliquity is a potential risk factor for cup instability during total hip arthroplasty. This study investigates the clinical function, acetabular cup position, leg length discrepancy, and improvement of obliquity in patients with infrapelvic obliquity after treatment with total hip arthroplasty in the direct anterior approach (DAA-THA). Methods: A total of 987 patients who underwent DAA-THA in the supine position from January 2017 to January 2021 were retrospectively analyzed, and 158 of them were included. The infrapelvic obliquity was classified into two types according to the direction of obliquity. Type I is when the pelvis tilts to the side of the affected lower limb, while type II is pelvic obliquity on the side of the healthy lower limb. Cases were further classified into two subtypes according to the angle of pelvic obliquity obtained: 0 degrees-3 degrees for type A; >= 3 degrees for type B. Clinical observation and follow-up were carried out at 1 day, 1 month, 3 months, 6 months, 1 year, and the last clinic visit (average 29 months). Standing hip radiographs were taken to measure the cup position, leg length discrepancy (LLD) and pelvic obliquity. The Harris score was used to evaluate hip function before and after surgery. Repeated measure ANOVAs were applied to compare multiple time points within groups, while the Fisher's LSD test was used for pairwise comparisons between the means of multiple samples across groups. Results: As the degree of pelvic obliquity increased for each subtype, the pre-operative Harris score decreased and pre-operative LLD increased. The parameters of cup position remained stable over time for each subtype. After DAA-THA, the Harris score improved significantly and the degree of pelvic obliquity and LLD improved for each subtype (p < 0.001). Although the last follow-up showed the lowest Harris score and the poorest recovery of pelvic tilt and LLD, type IB patients demonstrated the greater improvement compared to the other types. Conclusions: DAA-THA in supine position not only significantly improves the hip function of patients with infrapelvic obliquity, but also corrects pelvic obliquity and leg length discrepancy, while maintaining stable acetabular components. For patients with infrapelvic obliquity, in which the pelvis is oblique on the affected side and the angle is more than 3 degrees, the degree of functional improvement and correction is the greatest.