OP5.7 Using Dynamic Joint Space During Physiological Loading to Objectively Measure Hip Stability

Background: Hip instability has been generally defined as extraphysiologic hip motion that causes pain with or without the symptom of hip joint unsteadiness. There are currently no objective measures of dynamic hip stability that can reliably classify patients according to their functional instability.

Purpose: To provide a reference for stable hip motion by quantifying dynamic hip joint space in healthy controls.

Methods: Twenty-four healthy adults (13F; age 21.9±2.2 years) were enrolled in this study. Synchronized biplane radiographs were collected at 50 images per second during treadmill walking and squatting. Subject-specific bone models were created by segmenting computed tomography (CT) scans of each participant’s femur and pelvis, and bone motion was determined by a validated volumetric model-based tracking technique. The acetabulum was split into five radial regions and three sagittal segments. For each motion frame, the minimum subchondral bone distance in each of the radial and sagittal acetabular regions was identified and normalized to gait cycle (walking trials) or hip flexion (squat trials). The change in minimum gap within each acetabular region was determined for each trial as the difference between the smallest and largest minimum gaps. Differences between acetabular regions, side-to-side differences (SSD), and sex differences were identified through a repeated measures linear model and Benjamini-Hochberg correction for comparisons across multiple acetabular regions.

Results: The change in minimum gap during gait was larger in the anterior-inferior and posterior-superior radial regions than in the anterior-superior and superior radial regions (mean difference 0.4±0.1mm, p<0.001). No differences in regional minimum gap were identified during squat. During both gait and squat, the average change in minimum gap varied no more than 0.1mm between sagittal regions. No significant SSD or sex differences were identified during gait or squat (all differences under 0.2±0.2mm).

Conclusion: This data indicates that, for healthy hips, there is variability in dynamic stability between radial regions of the acetabulum during gait, with higher stability in the superior and anterior-superior regions but less stability in the anterior-inferior and posterior-superior regions. In contrast, dynamic stability in healthy hips is consistent during squat across radial regions of the acetabulum.