A study on the factors associated with the dimensions of the iliocapsularis muscle.

Purpose: To identify the correlation between the iliocapsularis muscle and other imaging parameters, with the aim of enhancing understanding of its function and primarily exploring the influence on hip stability and function.

Methods: We retrospectively evaluated patients who attended the sports medicine clinic of our department and who underwent arthroscopic surgery for femoroacetabular impingement (FAI) between January 2019 and December 2020. Preoperative supine anteroposterior hip radiography, 45° Dunn view radiographs, computed tomography (CT), and magnetic resonance imaging (MRI) were performed in all patients. The alpha angle and lateral center-edge angle, femoral and acetabular version, were measured on radiographs and CT. Cross-sectional area, thickness, width, and circumference of the iliocapsularis were measured on MRI. The thickness of the hip capsule was assessed in the midcoronal plane relative to the femoral head at three specific locations: at the level of the femoral head-neck junction, at a point midway between the midcapsule and the labrum), and at a point equidistant towards the greater trochanter. The Pearson correlation coefficient was used to test for the association between the imaging parameters and dimensions of the iliocapsularis muscle.

Results: A total of 120 patients (48 men and 72 women) were finally included in this study. There was a significant correlation between the dimensions of the iliocapsularis muscle and femoral version and Mckibbin index. Femoral version had a negative correlation between cross-sectional area (r = -0.28, P = 0.0045), thickness (r = -0.20, P = 0.043), width (r = -0.24, P = 0.012), and circumference (r = -0.27, P = 0.0052) of the iliocapsularis muscle. Mckibbin index also had negative correlation with cross-sectional area (r = -0.28, P = 0.0039), thickness (r = -0.27, P = 0.0054), width (r = -0.22, P = 0.025), and circumference (r = -0.26, P = 0.0075) of the iliocapsularis muscle. Besides, there was a significant correlation between the thickness of the iliocapsularis muscle and central acetabular version (r = -0.22, P = 0.025). No correlation was found between dimensions of the iliocapsularis muscle and alpha angle, lateral center-edge angle (LCEA), cranial acetabular version, and capsular thickness.

Conclusions: Our study revealed a negative correlation between femoral version, the Mckibbin index, and various morphological parameters of the iliocapsularis muscle, including cross-sectional area, thickness, width, and circumference. Higher levels of femoral version and the Mckibbin index were associated with a potential reduction in the dimensions of the iliocapsularis muscle. These findings suggest a biomechanical relationship between hip morphology and the structural characteristics of the iliocapsularis muscle, highlighting the importance of considering these factors in the assessment of hip stability and function.