Minimal pre-operative leg length discrepancy as a risk factor of post-operative leg length discrepancy after total hip arthroplasty: a retrospective study of patients with non-traumatic osteonecrosis of the femoral head

Abstract Background Leg length discrepancy (LLD) is one of the troublesome complications of total hip arthroplasty (THA). Previously, several risk factors have been suggested, but they were subjected to their inherent limitations. By controlling confounding variables, we hypothesized that known risk factors be re-evaluated and novel ones be discovered. This study aimed to analyze the independent risk factors for LLD after primary THA in patients with non-traumatic osteonecrosis of the femoral head (ONFH). Methods We retrospectively reviewed patients with non-traumatic ONFH who underwent unilateral THA between 2014 and 2021. All patients were operated by one senior surgeon using a single implant. Demographic data, surgical parameters, and radiological findings (pre-operative LLD, Dorr classification, and femoral neck resection) were analyzed to identify the risk factors of ≥ 5 mm post-operative LLD based on radiological measurement and to calculate odds ratios by logistic regression analysis. Post hoc power analysis demonstrated that the number of analyzed patients was sufficient with 80% power. Results One hundred and eighty-six patients were analyzed, including 96 females, with a mean age of 58.8 years at the time of initial THA. The average post-operative LLD was 1.2 ± 2.9 mm in the control group and 9.7 ± 3.2 mm in the LLD group, respectively. The LLD group tended to have minimal pre-operative LLD than the control group (-3.2 ± 5.1 mm vs. -7.9 ± 5.8 mm p = 2.38 × 10 − 8 ). No significant difference was found between the groups in age, gender, body mass index, femoral cortical index, and implant size. Conclusion Mild pre-operative LLD is associated with an increased risk of post-operative LLD after primary THA in patients with ONFH. Thus, surgeons should recognize pre-operative LLD to achieve an optimal outcome and must inform patients about the risk of developing LLD.