Hip
Those who cannot remember the past, are condemned to repeat it. George Santayana 1863 - 1952
There is now an extensive literature on Metal on Metal (MoM) bearings at the hip, much of which has been published in the JBJS (Br). Initial reports of hip resurfacing gave cause for optimism of a bone sparing procedure which would offer extended service with a biomechanical arrangement closely resembling the normal joint.1 Thereafter papers looked at a variety of aspects which might influence outcome, culminating in reports on the reactions to metal debris which now dominate medical and lay literature.
In 2005, 9% of primary hip replacements were hip resurfacings which peaked at 10% in 2006 (National Joint Registry figures). Many surgeons will recall patients demanding the procedure often supporting their case with newspaper cuttings and internet articles on the “everlasting hip”. As the less favourable observations on MoM bearings have appeared the tide has turned with the NJR recording only 3% of hip implantations as resurfacings in 2010.
What happened in the interim? Probably many things; would it be so wrong to suggest that some surgeons thought they must be able to do this operation and subsequently implanted the components badly causing acetabular component edge loading? Did other manufacturers feel they had to have a resurfacing device and in a desire to distinguish their version altered the articulation making it inherently vulnerable to edge loading with the slightest malposition? Was there a leap of faith that said it would be safe to apply a large modular head on a stem to obtain the advantage of the MoM bearing when resurfacing could not be undertaken? I suspect all three are true. It is a shame the indications reported in 2004 were not followed by all. Re-inspection of that paper will remind you the patients were all under 55 years old and 78.6% of them were men.
Where are we heading now? Some companies are introducing more modularity with a separate femoral neck to allow fine adjustment of offset, version and neck length. Is it wise to create another junction with the potential for fretting, corrosion and disassembly? If the anatomy is so bizarre that carefully planned surgery using conventional modular options cannot meet the requirements a bespoke implant would seem a better choice.
There is another area for reflection before there is a stampede to adopt large ceramic on ceramic bearings. How will the trunnion behave in this setting? Just because the coefficient of friction will be lower than in MoM bearings, the forces at the trunnion might still be unfavourable if acetabular malposition causes edge loading to occur.
As a profession we need to be careful and act responsibly for the sake of our patients.
Professor Gareth Scott, Consultant Orthopaedic Surgeon, Royal London Hospital, UK
Reference
1. Daniel J, Pynsent PB, McMinn DJW. Metal-on-metal resurfacing of the hip in patients under the age of 55 years with osteoarthritis. J Bone Joint Surg [Br] 2004; 86-B: 177-184.
