Effect of Changes in Iliocapsularis Cross-sectional Area on Hip Arthroscopy Outcomes: Clinical and Magnetic Resonance Imaging Follow-up

BACKGROUND: The function of the iliocapsularis (IC) muscle is still unclear. Previous studies have reported that the cross-sectional area of the IC may be useful in identifying borderline developmental dysplasia of the hip (BDDH). PURPOSE: To evaluate the pre- to postoperative changes in IC cross-sectional area in patients with femoroacetabular impingement (FAI) and to determine if there are any associations with clinical outcomes after hip arthroscopy. STUDY DESIGN: Cohort study; Level of evidence, 3. METHODS: The authors retrospectively evaluated patients who underwent arthroscopic surgery for FAI at a single institution between January 2019 and December 2020. Patients were divided into 3 groups according to lateral center-edge angle: BDDH group (20°-25°), control group (25°-40°), and pincer group (>40°). Supine anteroposterior hip radiographs, 45° Dunn view radiographs, computed tomography scans, and magnetic resonance imaging (MRI) scans were obtained for all patients preoperatively and postoperatively. The cross-sectional areas of the IC and the rectus femoris (RF) were measured on an axial MRI slice at the level of the femoral head center. Preoperative and final follow-up scores on the visual analog scale for pain and the modified Harris Hip Score (mHHS) were compared between groups with the independent-samples t test. RESULTS: A total of 141 patients (mean age, 38.5 years; 64 male, 77 female) were included. The preoperative IC-to-RF ratio of the BDDH group was significantly higher than that of the pincer group (P < .05). In the BDDH group, there was significant pre- to postoperative decrease in IC cross-sectional area and the IC-to-RF ratio (P < .05 for both) as well as a significant correlation between the preoperative IC cross-sectional area and the postoperative mHHS (r = 0.434; P = .027). CONCLUSION: Patients with BDDH had a significantly higher preoperative IC-to-RF ratio than patients with pincer morphology. A higher preoperative IC cross-sectional area was associated with better postoperative patient-reported outcomes after arthroscopy for the treatment of FAI combined with BDDH.