Adverse loading of the hip joint after joint preserving and joint replacement surgery

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The proposed research will determine how the natural healthy hip protects itself against potentially damaging loads. These can be generated when the direction of the force vector between the femoral head and acetabulum passes close to the edge of the acetabular surface (edge loading). The reduced acetabular surface area available to support the load can increase contact pressure on the cartilage surface and increase strain in the labrum. These types of load are generated when the hip is close to its range of motion limit, for example pivoting, standing up from a low chair or tying a shoelace. When the hip is rotated into these positions, the hip ligaments pull taut to constrain rotation and protect against impingement. However, our pilot data indicates that when the ligaments pull taut they also make a contribution to the direction of the hip force vector, rotating it into the acetabulum, away from the edge and thus help protect against edge loading. The proposed research will measure this function of the ligaments in the natural hip, using cadaver specimens to best represent the properties of the hip ligaments. Once we have established how the natural hip protects itself against edge loading, we will simulate different types of (a) hip joint preserving surgery and (b) hip replacement surgery to investigate whether this function is maintained after surgery. We suspect it is not maintained because the ligaments are cut during surgery to access the hip (they have to be - they completely surround the joint), and the extent of repair currently practiced, if any, is variable. If we find that ligament function is disabled after surgery, we will investigate how different repair methods can restore their function, and propose new repair methods if required. This research is important for hip joint preserving surgery, which aims to correct joint pathology and prevent osteoarthritis developing, because the adverse loading described above would increase the risk of cartilage degeneration and labral tears. Joint preserving surgery will only be a success if it does not accelerate what it seeks to prevent, hence the importance of the research. The research is also important for hip replacement surgery. Edge loading is an established mechanism that is reported to occur in 70% of hips and has been related to squeaking and revision. Reducing the risk of edge loading will reduce the occurrence of these complications and may enable new treatment concepts that do not have to withstand this loading mechanism.