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

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.

The proposed research will quantify the contribution of the capsular ligaments to the overall hip joint contact force for the native hip. Our hypothesis is that these ligaments contribute to the hip's natural defence against adverse loading of the acetabular cartilage and labrum. If we demonstrate this and surgeons implement our repair methods in their surgical practice, the impact will be:

1. Improved patient function/proprioception after joint preserving surgery
2. Increased survival rate of joint preserving surgery. This surgery is not yet proven to prevent osteoarthritis progression. Our work will ensure that the procedures are soundly based and greatly decrease the probability of them leading to new problems (e.g. labral tears, microinstability).
3. Patients returning to their active lifestyles more quickly and staying active longer.

Joint preserving surgery of the hip, e.g. arthroscopic surgery for femoracetabular impingement, is a growing sector in the orthopaedic market and achieving points 1-3 above will increase the uptake of this surgery. If this is achieved, further impact will be:

1. Delay the need for hip replacement. The ageing population is a governmental concern because this increases the cost of healthcare at a time when extra funding is not available due to economic stagnation. The cost to the NHS of a hip replacement (including surgery, implant, hospital stay and rehabilitation) is approximately £10,000, neglecting the economic cost of lost working days. By making a small repair, before the joint is irreparably damaged, joint preserving techniques allow a far less invasive and traumatic surgery, less rehabilitation and if patients can be discharged the same day, could be performed at a fraction of the cost. But this will only be achieved if the surgery does not accelerate what it is seeking to delay, hence the importance of this work.
2. Opportunity for industry to provide novel ligament repair/fixation instrumentation, ligament repair/replacement materials and tensioning devices. The UK has a strong orthopaedic industry base, and is well placed to spot these opportunities for economic benefit.

The primary aim of our research is to investigate the function of the hip capsular ligaments with the natural hip intact (i.e. joint preserving surgery). However, a secondary goal is to also consider ligament function after total hip replacement (THR). If we are able to design a ligament repair method that works with THR/resurfacing to prevent edge loading, and this is adopted by surgeons, the impact of this secondary goal will be:

1. Improved proprioception and activity levels for patients after total hip replacement
2. Reduced pain after total hip replacement - this is currently the second most common cause of revision.
3. New opportunities in THR/resurfacing implant design. Edge loading been identified as one of the chief failure mechanisms in metal on metal bearings through (a) high wear and (b) high frictional torque transferred to taper connections leading to fretting corrosion. This same mechanism may be responsible for other problems that we are not yet aware of for commercially available products launched in the past 10 years (i.e. for which there is not long term clinical data), for example, we know it causes squeaking in transformation toughened ceramic bearings, but the long term implication of this is unknown. This will lead to new opportunities for implant design because they won't have to be designed to withstand this adverse loading mechanism.