Increased hip flexion angle and protrusion of the anterior acetabular component can predict symptomatic iliopsoas impingement after total hip arthroplasty: a retrospective study.

Abstract

Iliopsoas impingement (IPI) is a known complication of total hip arthroplasty (THA). The aim of this study was to identify risk factors and the cut-off cup protrusion length for symptomatic IPI. We retrospectively reviewed 218 hips (46 men, 172 women) of 190 consecutive Japanese patients who underwent primary THA at our institution. Hips with IPI (<i>n</i> = 24) and control hips (<i>n</i> = 194) were compared according to age, body mass index, and clinical scores. We evaluated leg-length discrepancy, global offset, implant alignment angles, pelvic inclination, and anterior cup protrusion on plain radiographs and computed tomography scans. Symptomatic IPI had an incidence of 11.0% and a predilection for hips with osteonecrosis. Pre- and postoperative hip flexion angles were significantly greater in hips with symptomatic IPI (<i>p</i> = 0.013 and <i>p</i> = 0.006, respectively). Cup protrusion length was significantly greater in hips with IPI (4.7 mm vs. 1.4 mm; <i>p</i> = 0.001). Receiver-operating characteristic curve analysis identified a threshold cup protrusion length of 3.9 mm (sensitivity 0.89, specificity 0.63) for symptomatic IPI. The incidence of symptomatic IPI tended to be lower in patients with greater posterior pelvic sagittal inclination despite the cup protrusion length being greater. An increased hip flexion angle and protrusion of the anterior acetabular component predicted symptomatic IPI. The threshold cup protrusion length suggesting mild IPI might be about 3.9 mm and could be useful for identifying candidates for conservative treatment.