The hip joint capsule : mechanical properties and contribution to joint stability in total hip replacement
The hip joint capsule is a complex soft tissue structure that comprises a number of ligaments with different thickness and strength respectively, as well as a thinner membrane-like part that provides the structural integrity for the capsule. The role of the capsule is twofold, forming a closed environment around the joint surfaces, in which the synovial fluids are retained, but also providing a passive joint resistance that acts in addition to the surrounding muscles and mainly in extreme limb positions and in unexpected limb loading. Despite its important role, the capsule is commonly removed in Total Hip Replacement (THR), either fully or partially, to clear the view to the joint. The little existing knowledge on the mechanical properties of the capsule and its performance in situ during various walking activities makes it difficult to argue for its preservation and repair following THR, especially due to the additional efforts that this brings along for the surgical team. This project is an attempt to provide this required information and to highlight any changes in joint resistance that occur as a result of a complete or partial removal of the hip joint capsule. A cadaveric approach was used at first, in which complete human hip joints with intact capsules were tested. A sequential removal of the capsular ligaments was carried out, in order to reveal the total contribution to joint resistive moments of the individual ligaments. Anterior ligaments were distinguished from posterior ones and the tests were carried out in those limb positions in which the joint would be least stable under absence of the respective ligaments. Partial damage of the posterior aspects of the capsule showed increased risk for posterior joint dislocation when the limb was in flexion and internal rotation compared to the effects of an anteriorly damaged capsule. A 3-D computer model of the hip joint with its capsular ligaments was created, in order to confirm the findings from the cadaveric study, but also for the purpose of simulating effects in post-operative anatomies. Different methods of creating the geometric model were attempted and it was achieved to create a model that would be used to simulate the resistive moments produced by the individual ligaments. For simplicity, the ligament action was simulated by linear spring elements. The findings of the cadaveric experiments could not be reproduced, because of the way the ligament forces were modelled, which does not consider the viscoelastic properties of soft biological tissues. However, it was still possible to draw valuable conclusions on the effect that various prosthetic component attributes have on the total joint moments. Recommendations were made for the optimal approach to the capsule in THR.