Purpose: The goal of this study was to determine the role of soft tissue and osseous constraints in hip biomechanics using a unique robotic testing apparatus.
Methods: Four fresh-frozen human cadaveric hemi-pelvises without degenerative changes or dysplasia were stripped of all soft tissue except the ligamentous capsule and the intra-articular structures. All hips were tested using a robotic manipulator/universal force-moment sensor testing system to measure and compare end-range of motion (ROM) and kinematic translations in "capsule vented" (a small hole in the capsule) and "capsule separated" (capsular ligaments separated from each other) states. Then, the "capsule vented" state was compared to the condition in which the capsule and labrum were removed to calculate bone and soft tissue forces with 40 N of load applied in six different directions along three axes.
Results: There were no significant differences in end-ROM or kinematic translations between the "capsule vented" and "capsule separated" states. Bone forces significantly increased with loads applied in the anterior, posterior and superior directions. Soft tissue forces increased significantly with loads applied in the medial, lateral and inferior directions.
Conclusion: The individual hip capsular ligaments act independently of each other to resist end-ROM. Both osseous and soft tissue constraints are important to hip biomechanics depending upon the direction of applied force. The clinical relevance is that surgical management for hip disorders should preserve the soft tissue constraints in the hip when possible to maintain normal hip biomechanics.