Smith & Nephew | Birmingham Hip Resurfacing | Which Medical Device
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The Birmingham Hip Resurfacing likely needs no introduction, as the device which kicked off the hip resurfacing revival of the last ten years. The device has been successfully marketed as a solution for younger patients requiring hip replacement, preserving bone on the femoral side. The expansion of the market for hip resurfacing lead many manufacturers to develop their own systems, none of which have been as successful. Concerns about large metal-on-metal bearings have had an impact on the use of hip resurfacings (MHRA alert 2010, National Joint Registry 2010), but enthusiasts still promote this device for carefully selected patients.
The suggested advantages of any hip resurfacing are:
As time and experience has grown, so has the evidence base for these assertions. Clearly bone is preserved on the femoral side, and the revision of the femoral component to a stemmed device if required (for example after hip fracture) is well established. On the acetabular side, concerns about the need to remove more bone to accommodate the larger femoral head have been addressed in a number of studies which have shown that similar quantities of bone are removed as when implanting a standard uncemented socket (Moonot P, 2008 ). Revising a well-fixed Birmingham Socket can be challenging although devices like Zimmer’s Explant may be helpful (Sandiford NA, 2008)
In one registry-based study, better function has been reported across all domains of the SF36 for patients undergoing hip resurfacing compared with standard age-matched controls undergoing hip replacement (Lingard et al, 2009). A pilot study has shown improved stair climbing ability after resurfacing compared with total hip replacement (Shrader et al 2009), and another higher activity levels postoperatively (Zyweil MG et al 2009). A further study reports the majority of patients are able to continue in the same employment after Birmingham Hip Resurfacing (Malek IA et al 2010).
Stability of a large diameter metal on metal bearing is better than for smaller sized articulations, but preservation of the femoral neck increases the risk of impingement. However, revisions for dislocation following BHR are unusual (Carrothers AD et al, 2010)
Risks include
Femoral neck fracture is a recognised risk after hip resurfacing, with reported rates of around 1% (Carrothers AD, 2010). The major technical issue linked to femoral neck fracture is notching, because of undersizing or translation of the femoral component inferiorly. Bone mineral density initially falls over 6-12 weeks then increases over the year after surgery (Cooke NJ, et al 2009). Late fractures after trauma have been reported.
The issue of adverse tissue reactions to metal wear debris and the formation of pseudotumours has gained prominence in recent years and lead to the MHRA release about metal-on-metal bearings (MHRA 2010). Some devices may be more susceptible than others to this, but the risk associated with the BHR appears to be low. Serum metal ion concentrations are related to the kind of device used and the socket inclination (Langton et al 2009).
The device is an as-cast cobalt-chrome molybdenum metal on metal device which has been unchanged since its introduction to the market in 1997, other than for the introduction of 2mm increments in sizes. It has a dual radius design, meaning that the socket is thicker at the base than the periphery and the outside of the socket subtends an angle of 180 degrees, whereas the inside subtends an angle of 170 degrees (Muirhead-Allwod et al 2008). The metallurgical properties of the bearing and the geometry of the socket are important in the success of the device. The porous surface on the back of the socket is cast with the socket and has a hydroxyapatite coating. There are two anti-rotation fins which are placed inferiorly, and the device comes mounted on an introducer, held to the socket by braided wires which are removed after implantation.
The seventh report of the UK National Joint Registry confirmed the Birmingham Hip Resurfacing as the UK market leader, but the number of resurfacings had declined from 5,707 in 2008 to 4,099 in 2009 (NJR 2010). The 5 year revision rate for the Birmingham Hip Resurfacing was 4.3% compared with 12% for the ASR (now withdrawn from sale). Results from a surgeon designer report a 98.0% survival rate at ten years for male patients (Treacy et al, 2010).
Craig Gerrand, Orthopaedic Editor.
Read our roundup of hip resurfacing devices.
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