Approximately 90% of all hip replacements are undertaken for osteoarthritis. There are many reasons why a joint develops arthritis but the changes are remarkably consistent. The smooth articular cartilage that covers the rubbing surface of the bone wears away. The underlying bone stiffens with small areas of cystic degeneration and new bone forms around the joint margins. Joint movement becomes restricted and painful. The irreversible changes are relatively superficial so for many years surgeons have reasoned that a replacement only really needs to replace the worn cartilage and a few millimetres of the underlying bone.
Osteoarthritic femoral head removed during a standard hip replacement
When knee replacement is undertaken, only the residual cartilage and a thin layer of the underlying bone is removed. For knees this strategy works extremely well and almost all knee replacements are surface replacements.
Side-on (lateral) x-ray view of a knee replacement
A number of Hip resurfacing designs were developed and tested in the 1970’s. The results were disappointing because the materials were not sufficiently durable.
1970’s Hip Resurfacing implants
In the late 1990’s, the Conserve plus was developed by Harlum Amstutz in the US and Birmingham Resurfacing was developed by Derek McMinn in the UK.
These were made of cast cobalt chrome and were known as metal-on-metal resurfacings.
The results of these designs were much better. Most Orthopaedic manuafcturers developed similar products and by 2008 about a tenth of all hip replacements were of the resurfacing variety.
The Birmingham Resurfacing (BHR)
The Conserve Plus
The Birmingham Resurfacing has been used since the late 1990’s and there are a growing number of people with Birmingham Resurfacing hips that have been in situ for over twenty years and continue to function without problems. Indeed, the Birmingham resurfacing hip now has an ODEP 13A* rating which means that there is a better than 94% chance that a Birmingham resurfacing hip replacement will not need to be revised within 13 years of being implanted. Unfortunately, this is not the case for all patients and it has been found that the Birmingham Resurfacing works better for men than women and works better for people whose natural femoral head is over 48mm in diameter.
When hip resurfacing works, there are two advantages over conventional stemmed hip replacements. Firstly, the ball of the replacement (the femoral head) is the same diameter as the natural joint and this makes the joint more stable than conventional stemmed hip replacements (where the new ball has a smaller diameter than the natural femoral head). Secondly, patients with resurfaced hips can continue running and undertake vigorous sporting activities that involve repetitive impact loading. Such activities are usually discouraged for patients with conventional stemmed hip replacements.
Unfortunately, two problems continue to beset Hip Resurfacing. Firstly, the shell that is fitted to the femoral head and the shell that is fitted to the acetabular socket in the pelvis are both made of cast cobalt chrome. This means that two metal surfaces are rubbing against each other. Over the years, billions of metal particles are abraded and shed into the joint. The number of particles that are generated depends on the diameter of the bearing surfaces, the geometry of the surfaces, the positioning of the implants and the amount of activity that the person undertakes. Some people are particularly sensitive to the metal particles and suffer an adverse reaction to the metal debris. In extreme cases, the bone adjacent to the implants dies and the muscles and tendons that surround the joint are destroyed, with devastating consequences. These problems affect women considerably more often than men. In consequence, very few Orthopaedic surgeons continue to undertake hip resurfacing and most of the surgeons who continue to undertake these operations only do so for men under sixty who are determined to undertake vigorous sporting activity and are willing to accept the risk of an adverse reaction to the metal debris.
MRI scan showing expansive pseudotumour formation around a metal-on-metal Resurfacing Hip replacement.
The second challenge for surgeons undertaking hip resurfacing is that the femoral head is only trimmed rather than being removed. In consequence, a more extensile dissection is required to get the retained femoral head out of the way so that the surgeon can access and implant a new hip socket. In an era where minimally invasive techniques are gaining popularity, surgeons are learning to undertake hip replacements with less collateral damage to the surrounding soft tissue envelope and patients are enjoying a faster post-operative recovery. This makes the more invasive resurfacing procedure more of a challenge for the patients and it takes longer for the soft tissues to settle after surgery.
Extensile soft tissue dissection for hip resurfacing to enable retention of femoral head and access to acetabular socket.
There are a number of ongoing initiatives to resolve the problems of metal-on-metal bearings. A number of companies are now undertaking trials of resurfacing hips that are made of ceramics rather than metal. Another strategy that is being tried is to is to use a highly cross-linked polyethylene for the bearing surface in the socket. While polyethylene sockets were tried in 1970’s resurfacing implants, modern polyethylene is far more durable and may avoid the rapid wear that beset the earlier designs.
Ceramic on ceramic Resurfacing
Developed by Prof Justin Cobb at Imperial College London.
Despite the continuing challenges, hip Resurfacing remains an alternative for patients who are male, under sixty and wish to undertake vigorous sporting activities. The London Hip Practice surgeons are happy to discuss this option with patients who understand the additional risks associated with Hip Resurfacing and Prof Field continues to undertake this operation for appropriate patients. Hopefully, the results of ongoing clinical trials will make Hip Resurfacing a good option for a greater number of patients.