Magnetic Resonance Imaging Characteristics of the Iliocapsularis in Hip Microinstability: A Comparative Analysis

Background: Iliocapsularis (IC) muscle hypertrophy in borderline developmental dysplasia of the hip (BDDH) and developmental dysplasia of the hip (DDH) suggests a possible role as a dynamic hip stabilizer. However, its significance in hip microinstability (MI) without acetabular undercoverage remains unclear. Purpose: To compare IC muscle dimensions and fatty infiltration between patients with MI, BDDH, and mixed-type femoroacetabular impingement (mFAI), and assess the association between IC muscle morphology and hip pathology. Study Design: Case-control study; Level of evidence, 4. Methods: A retrospective analysis was conducted on patients undergoing arthroscopic surgery for hip pathology between January 2014 and December 2022. Patients were categorized based on lateral center-edge angle (LCEA) into BDDH (18° to <25°), MI (25° to 39°), and mFAI (>39°) groups. Preoperative radiographs and magnetic resonance imaging (MRI) scans were reviewed, and IC muscle dimensions, including width, depth, cross-sectional area (CSA), IC fatty infiltration, and combined IC and iliopsoas (IP) (IC+IP) CSA, were measured by 3 independent observers. A priori power analysis was performed. Statistical analyses included parametric and nonparametric comparative tests, interobserver correlation coefficients, and receiver operating characteristic analysis. Results: A total of 95 hips were included in this study. IC depth, width, and CSA and IC+IP CSA were significantly smaller in the MI group ( P < .05 for all). Interobserver agreement was good to excellent for IC width (IRC, 0.87), IC CSA (IRC, 0.87), and IC+IP CSA (IRC, 0.96), but poor for IC depth (IRC, 0.26). The alpha angle did not significantly differ among groups ( P = .093). The MI group had a higher proportion of female patients ( P = .003). No significant differences were noted in radiographic markers, including the ischial spine sign ( P = .083), crossover sign ( P = .130), and posterior wall sign ( P = .41). Conclusion: No detectable IC hypertrophy was observed in the MI group compared with the BDDH and mFAI groups, with patients with MI showing smaller IC width, depth, and CSA. The study offers a reproducible measurement technique with good interobserver agreement. IC muscle mass has limited prognostic value in predicting hip MI.