The capsular ligaments provide a passive stabilising force to protect the hip against edge loading

Aims: In the native hip, the hip capsular ligaments tighten at the limits of range of hip motion and may provide a passive stabilising force to protect the hip against edge loading. In this study we quantified the stabilising force vectors generated by capsular ligaments at extreme range of movement (ROM) and examined their ability to prevent edge loading. Methods: Torque-rotation curves of the hip joint were obtained from nine human cadaveric specimens to define the rotational restraint contributions of the capsular ligaments in 36 positions. A ligament model was developed to determine the line of action and effective moment arms of the medial/lateral iliofemoral, ischiofemoral and pubofemoral ligaments in all positions. The net force vector generated by the capsule was evaluated in each position and functioning ligament forces and stiffness were determined. Results: The medial and lateral arms of the iliofemoral ligament generated the highest net inbound force vector in positions combining extension and adduction providing anterior stability. The ischiofemoral ligament generated the highest net inbound force in flexion, adduction and internal rotation (FADIR), reducing the risk of posterior dislocation. In this position the hip joint reaction force moved 0.7° inbound for each Nm of internal capsular 17restraint, preventing edge loading. Conclusion: The hip ligaments contribute to keep the joint force vector inbound from the edge of the acetabulum at extreme ROM. Preservation and appropriate tensioning of these structures following any type of hip surgery may be crucial to minimising complications related to joint instability