The fundamental difference between a BHR and a conventional total hip replacement (THR) is in the femoral (thigh) side. A THR has a long stem inserted into the canal in the thigh bone. Hence the natural femoral head and part of the neck are removed and weight is transmitted through the stem directly into the upper third of the thigh bone. In a resurfacing, the aim is to preserve most of the femoral head and neck. The resurfacing femoral component therefore has a thin (3 to 4 mm) hollow ball surface that directly transmits weight to the femoral head bone underneath it and a very small stem that is not designed to transmit weight. The socket component can be similar in both the THR and a BHR.
There are other differences between a Conventional THR and a BHR. Conventional THR sockets are made of polyethylene (PE). Wear-debris generated from PE wear leads to loosening of the components. This is the primary cause of long-term failure of conventional THRs. PE wear and loosening are directly related to activity. Hence these THRs do not last well in younger and more active patients. In addition, the minimum thickness needed for a PE socket required that the femoral head had to be smaller in diameter than what a metal-metal joint would permit. A small diameter head has the potential to dislocate more readily than a normal hip and therefore the dislocation rates with conventional THRs are greater than those with BHRs.
However, the beneficial effects of a metal-metal joint have now been transferred from the resurfacing technology to replacements as well. These large diameter metal-metal THRs are showing great promise in reducing wear and dislocation rates. The only difference between a BHR and such large diameter metal-metal joints is the long stem. In a BHR, the absence of a long stem makes a revision, should this ever become necessary in the future, easier.
What is a BHR made of? The ball (femoral) and socket (acetabular) components of a BHR are made of a tried and tested alloy of cobalt and chromium. This has been in use in orthopaedic surgery for over 70 years. The original alloy is an as-cast high-carbon alloy. The higher carbon content precipitates in the alloy as carbides which have the hardness of ceramics. They give the metal the needed resistance to wear. These carbides can be depleted by heat treatments in the later stages of manufacture – a process that is used in some other brands of resurfacings. They no longer enjoy the same wear resistance as an as-cast device such as the BHR. Who is a BHR suitable for? A resurfacing is suitable for the treatment of a hip with severe arthritis when the femoral head bone quality is good. It is used more often in young and active patients than older and less active patients because conventional replacements do not last long in young and active patients. Further, a younger patient is more likely to need a revision of an artificial hip at some stage later in life, and it is easier to successfully revise a BHR How long will the implant last? The development of modern resurfacings was based on the secrets of success gleaned from successful historic metal-metal hip replacements which proved their wear resistance, durability and biocompatibility over several decades. The era of modern metal-metal hip resurfacings started in 1991 when Mr McMinn pioneered them. The early models were prototypes that gave precious further information on the best design and material combinations that would make resurfacing successful. The hybrid fixed model turned out to be better than the others Review of the surviving hips amongst the early models reveals that some of them are still going strong in spite of heavy usage over the past 15 years. Hybrid fixation was used in all later models and they were made even more reliable using more advanced techniques. The Birmingham Hip Resurfacing was released in 1997. In Mr. McMinn’s series of nearly 2700 BHRs to date, the failure rate has been 1.2 at nearly 9 years. In the younger age group (under 55 years) with osteoarthritis, the failure rate of Mr. McMinn’s resurfacings is 0.2(survival 99.8 at 11 years follow-up. The failure rate with a traditional hip replacement in this age group by comparison is 19at 10 years and 67at 16 years according to the Swedish Hip Arthroplasty Register.
This information was taken directly from the McMinn Centre Website