Soft tissues contribution to hip joint kinematics and biomechanics

Purpose: To quantitatively describe the kinematic behaviour of the hip joint with particular interest in the contribution of the periarticular soft tissues to the stability. To quantitatively assess the sealing function of the acetabular labrum of an intact labrum, the effect of a labral-chondral separation and different surgical approaches. The biomechanics of the joint during specific clinical examinations was also assessed. Methods: All the kinematics tests, manually performed, have been acquired using a navigation systems. For the first part of the study, 4 hemi-corpse specimens were evaluated with the hip in 36 different positions with all the soft tissues intact, after removal of skin and muscles and after partial capsulectomy. During the second and the third part of the analysis 8 fresh frozen hips were analysed. They were tested during 3 different positions used for femeroacetabular impingement (FAI) diagnosis, during pivoting motion and distraction test. Contact area between acetabulum and femoral head, range of movement (RoM) and hip joint centre (HJC) displacement were evaluated. Results: Femoral head displacement occured in all 3 directions and increased as more tissue was removed. Considering both pivoting motion and distraction test there was an increase of the HJC displacement after labral tear. Analogously, the tested surgical approaches showed different results compared to intact condition. Our analysis confirmed that the hip position with Hyperextension-External Rotation is specific for the analysis of the posteriorsuperior region of the acetabulum. Conclusions: The study showed that hip joint does not act as a true ball-and-socket joint and the femoral head anatomical displacement is strongly affected by the removal of periarticular soft tissues, labral repair as well as labrectomy.