The main features of the hip resurfacing artificial joint are the following:
1. The system comprises of a metal on metal couple or joint with no UHMWP. The socket is usually implanted without cement while the "cap" or femoral component is implanted with cement.
2. The shell used on the socket side is quite thin - only 4-6 mm, but this appears to be sufficient.
3. The "cap" used on the femoral head is virtually the same size as the natural head. In the standard hip replacement operation, the most common used ball sizes are 28 and 32 mm. The larger size may enhance the "feel" of the hip joint for the patient and make it appear more normal than a conventional hip replacement.
4. Because of the large size of the femoral component, the range of motion achieved is usually (but not always) better than that used with standard hip replacement operations. The larger size also makes this artificial joint more resistant to dislocation than a standard hip replacement and reduces the wear forces per unit area making it in theory, less prone to focal wear.
5. Most surgeons believe that given the lack of any UHMWP in the resurfacing joint, it is acceptable for patients to be more vigorous and to carry out activities at a higher level than considered wise after standard hip replacement operations. It goes without saying however, that avoiding repetitive impact activity would "strain" the resurfacing the least and therefore minimises the risk of loosening.
6. There is the potential for the metal on metal resurfacing to outlast by a considerable amount, the usual life expectancy of conventional hip replacement operations. This view is based on theoretical principles and there is no evidence to confirm that is what will happen in practice.
7. It is possible to convert or revise the resurfacing operation to a conventional hip replacement if that should become necessary in the future. If the "cap" or femoral component fails then, it should be possible to retain the implanted socket component and simply place a standard hip replacement femoral component on the thigh bone side to articulate with the socket of the resurfacing system.
8.. Given that the modern form of resurfacing has only been carried out for about 8 years or so, experience with the system must be considered to be limited. Patients must understand that this is an experimental procedure and the outcome in the long term is unknown. There have also been issues raised about the generation of tiny metal wear particles as the effect of these particles on health is unknown. This is notwithstanding the fact that NICE (National Institute for Clinical Excellence) has recommended that the hip resurfacing procedure be offered to all patients under the age of 65 needing a hip replacement operation.
9. As is the case with any surgical procedure, this technique too carries the risk of complications and of things going wrong. There are the usual risks of infection, dislocation, thrombosis and embolism, fracture of the femoral shaft, nerve injury, leg length discrepancy etc. as with the conventional hip replacement. Some patients also report a clunking noise in this hip after surgery with each step and this is probably due to a degree of non congruent movement of the metal bearing surfaces in the early post operative period. The lack of the polyethylene also makes the system more "noisy". However, as the bearings "bed in", some of the clunking seems to get better.
10. There is also one other additional risk with this artificial joint and that is the risk of fracture (a break) of the neck of the femur bone - a risk not present with standard hip replacement. The neck of the femur is that portion of the thigh bone that bridges the ball section with the long shaft of the bone. In the standard hip replacement, most of the neck of the femur is removed during the surgery and replaced by the metal implant. In resurfacing however, the whole of the neck portion is retained. The "cap" that is fit on the ball of the femur finishes just where the neck portion starts. There is therefore an area of increased stress at this point. Further, the surgery itself may cause some weakening of the neck of the femur and that can happen either during the operation itself or later, due to interference with blood supply to this area. This combination of factors can result in a fracture of the bone. If there is already some weakening of the neck of femur prior to the surgery, then the risk becomes greater. The weakening before the surgery is a natural consequence of aging through osteoporosis and greater in women and often quite significant after the age of 65 years. Most surgeons do not therefore advise this type of surgery for patients above 65 or those with evidence of osteoporosis on x-ray. There are however, some patients who may be older than 65 years and yet do not have osteoporosis and in those patients, hip resurfacing may be a viable option.
11. Patients who wish to have this surgery should be aware that, on the rare occasion, it may not be possible to complete the hip resurfacing procedure even if the surgeon has started the operation. This possibility arises from the lack of flexibility available to the surgeon should the soft tissue tension in the ligaments and muscles around the hip be insufficient to provide stability to the artificial joint. The lack of stability would increase the risk of the dislocation in the replaced joint. If this situation were to arise in a standard hip replacement operation, the surgeon can deal with it by using a longer ball on the femur side. This option is not available to him in the resurfacing system because of the design of this joint. If the only resurfacing size available to him does not do the job, then there may be no option but to convert the operation to a standard hip replacement procedure.
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