Association between cup fixation screw and iliopsoas impingement after total hip arthroplasty

Background: Iliopsoas impingement after total hip arthroplasty can result not only from acetabular cup but also from cup fixation screw. However, research addressing this screw impingement is scarce, leaving the details undetermined. This study aimed to elucidate the incidence and threshold of symptomatic iliopsoas impingement attributable to protrusion of the cup fixation screw into the iliopsoas muscle and to evaluate its impact on postoperative radiographic imaging findings and patient-reported outcome measures. Methods: A total of 152 hips were included in this study. The symptomatic threshold of screw protrusion was determined using a receiver operating characteristic curve, and patients were divided into low-protrusion and high-protrusion groups using this threshold. The area and Hounsfield Unit values of the iliopsoas muscle on CT and the Japanese Orthopaedic Association Hip Disease Evaluation Questionnaire postoperatively were investigated. Findings: 10 hips (6.5%) were exhibited symptoms of IPI in this series. The threshold for screw protrusion length was identified as 6.4 mm. Patients in the high protrusion group exhibited significantly larger area and lower Hounsfield Unit values of the iliopsoas muscle. In addition, the high protrusion group revealed significantly lower scores (total, pain, movement, mental). Furthermore, subscales scores (pain, movement) in the high protrusion group didn't improve from 3 months to 12 months postoperatively with significance. Interpretation: This study underscores the imperative for surgeons to consider the length of the cup fixation screw. This careful consideration is crucial for mitigating the incidence of postoperative iliopsoas impingement and enhancing total hip arthroplasty outcomes.