In the UK, Metal on Metal (MoM) bearings have enjoyed a renaissance as the Birmingham Hip Resurfacing (BHR). This implant is engineered to trap a thin layer of the body's own fluid between the metal bearing surfaces, as oil is used in a car engine. This produces a very low friction bearing that wears very slowly. Resurfacing involves trimming of the ball of the hip to fit the implant rather than removing it and fixing the implant lower down the thigh bone as occurs in conventional hip replacement. Popularity of this implant increased dramatically in the last decade based on the perceived benefits of low wear, resistance to dislocation, and improved activity levels. It was also believed that should the implant fail, then changing it to a total hip replacement would be relatively easy.
This popularity lead to the introduction of hip resurfacing implants from other implant manufacturers, and unfortunately, whilst all hip resurfacing implants look the same superficially, the different metallurgy and manufacturing methods may make them behave very differently. Concerns about hip resurfacing implants have increased in the last two years due to the poor performance and early failure of certain manufacturers’ designs and in patients whose resurfacing components are not perfectly aligned. Hip resurfacing has a higher failure rate in females. Inflammatory problems caused by debris from the metal bearings were also reported, with occasional formation of large inflammatory masses round the hip which damage the surrounding muscles. Whilst these disasters are rare, their consequences are severe.
Hip resurfacing still has a small but valuable place in the treatment of young men with impingement type osteoarthritis. For patients who are prepared to accept the small but additional risks of resurfacing over ceramic hip replacement (namely inflammation from metal debris, bigger scar, femoral neck fracture) then it may give slightly more durable hip function, which athletic patients may benefit from. Established implants manufactured from high carbide cobalt chrome should be used (such as the Birmingham Hip Resurfacing from Smith & Nephew and the Adept from Finsbury) and optimal alignment are both essential to prevent excessive generation of metal debris. Computer-based surgical navigation ensures that excellent implant position is achieved consistently.
The BHR has been in use in its current form since 1997. Despite relatively short follow up, the failure rate has already been shown to be lower than for conventional THR in patients under 55 years old. The commonest reason for re-operation is femoral neck fracture (0.5%), and it is now known that resurfacing is not suitable for patients with or at risk of osteoporosis.