In vivo MRI assessment of position dependent capsulolabral gap differences between neutral and flexion abduction...

The acetabular labrum plays an essential role in resisting distraction stress by closely coupling the femoral head. A previous cadaveric study suggested that specific hip postures allow the joint capsule to approximate the labrum, potentially enhancing labral adherence to the femoral head. However, whether such position-dependent changes occur in vivo remains unclear. This study aimed to investigate differences in the capsulolabral gap-the distance between the anterosuperior-joint capsule and labrum-between the hip neutral and flexion-abduction-external rotation (FABER) positions, and the location where these differences occur. Twenty-four hips of 24 healthy volunteers were examined using magnetic resonance imaging. A capsulolabral gap was evident around the anterior inferior iliac spine (AIIS) in the hip neutral but not in the FABER position. The largest capsulolabral gap (2.11 ± 0.93 mm) was significantly associated with the AIIS location (r = 0.65, p = 0.004). The maximal gap distance in the hip neutral position was positively significantly correlated with the articular-cavity area at the femoral neck in FABER (r = 0.85, p < 0.001). The capsulolabral gap around the AIIS in the hip neutral position was reduced in FABER owing to close capsule-labrum contact as the entire joint capsule changed, suggesting that the hip capsule contributes to a position-dependent labral-support mechanism.