EP4.4 Dynamic Capsular Morphology: Ultrasonographic Assessment of Normal Hip Capsular Motion

Abstract Introduction Hip capsule anatomy with the joint in a static position is well understood, however structural changes the capsule undergoes throughout motion have not been well described. The purpose of the current study was to investigate the dynamic properties of the anterior hip capsule throughout normal hip range of motion in young adults without hip pain, and to determine patient demographic and radiographic characteristics that correlate with changes in hip capsule thickness throughout range of motion. Methods Patients without hip pain or prior hip surgery were evaluated in this study. All patients underwent radiographic evaluation of bilateral hips including anteroposterior and 45-degree modified Dunn lateral views. Dynamic hip ultrasound evaluation was performed with patients in a supine position, and anterior hip capsule thickness was recorded with the ipsilateral hip in 6 different positions. Hip capsule thickness was compared, and the relationship between change in hip capsule thickness and preoperative patient and radiographic characteristics was evaluated. Results 82 hips from 41 participants were evaluated, with a mean age of 24.6 ± 2.7 years, and a mean BMI of 24.8 ± 3.8 kg/m2. Mean anterior hip capsule thickness was found to be 4.8 ± 1.1 mm with the hip in 0° extension, neutral rotation, which increased to a mean thickness of 10.7 ± 2.5 mm in 90° flexion and maximum internal rotation. The anterior capsule became significantly thicker in all positions of hip flexion compared to 0° extension, neutral rotation (p < 0.001). The percent increase in hip capsule thickness as the hip moved from the position 0° extension, neutral rotation to 90° flexion, internal rotation was significantly higher in female patients (249% vs 202%, p=0.003), those with Tonnis 0 degenerative changes (229% vs 177%, p=0.009), and those without cam morphology (241% vs 198%, p=0.003). Conclusions The anterior hip capsule becomes significantly thicker as the hip ranges from extension to flexion, and the degree of thickening may be impacted by patient sex, the degree of degenerative changes in the joint and the presence of femoral head cam morphology. These findings provide normative values for dynamic hip capsule thickness that can be used for future studies investigating capsular dynamics in the symptomatic hip.