Summary

This study defined the Iliocapsularis muscle morphology in patients with hip microinstability.

Abstract

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 under-coverage remains unclear. This study compares IC muscle dimensions between patients with MI, BDDH, and mixed-type femoroacetabular impingement (mFAI), and assesses IC muscle morphology in relation to hip pathology.

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 (18º- <25º), MI (25º-39º), and mFAI (>39º) groups. Pre-operative radiographs and MRI scans were reviewed, and IC muscle dimensions (width, depth, cross-sectional area [CSA]) were measured by three independent observers. A priori power analysis was performed. Statistical analyses included comparative tests, inter-observer correlation coefficients (ICC) and receiver operating characteristic (ROC) analysis.

Results

Ninety-five patients were included in this study. IC depth, width, CSA, and combined IC and iliopsoas (IC+IP) CSA were significantly smaller in the MI group compared to BDDH and mFAI groups (p<0.05 for all). Inter-observer agreement was good to excellent for IC width (ICC 0.87), CSA (ICC 0.87), and combined CSA (ICC 0.96), but poor for depth (ICC 0.26). The MI group had a higher proportion of female patients (p=0.003). No significant differences were noted in radiographic markers (Alpha angle (p=0.093), Ischial Spine sign (p=0.083), Cross-over sign (p=0.11), and Posterior Wall sign (p=0.41)). ROC curves showed moderate discriminative ability for diagnosing MI based on MRI anatomical measurements (AUC = 0.67).

Conclusion

No detectable IC hypertrophy was observed in the MI group compared to BDDH and mFAI patients. The study offers a reproducible measurement technique with good inter-observer agreement. However, this study highlights the limited prognostic value of IC muscle mass in predicting hip microinstability as well as the limited utility of 2-dimensional radiographic markers for evaluating 3-dimensional conditions.