| What is the difference between a Birmingham Hip
Resurfacing (BHR) and a total hip replacement (THR)?
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
|
