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.