Component mal-position: the 'Achilles' heel' of bearing surfaces in hip replacement

Total hip replacement (THR) is a very successful procedure, reporting very high success rates. The bearing surface is a key feature of replacement hip joints. It is generally accepted that excessive amounts of wear debris precludes their long-term survivorship. Hip simulator testing has resulted in a new generation of bearing surfaces. In general, these laboratory (in vitro) improvements in wear performance have resulted in a similar improvement in performance in the body (in vivo). However, there have been reports of components that have not performed as well as expected and this has been linked to the mal-positioning of components in the body. This article reviews bearing surface options and considers the effects of component mal-position on each of those bearings. There are three bearing surfaces in common use in THR. The effect of component mal-position in all cases is increased levels of wear. In metal/metal bearings this can result in cup rim wear and elevated blood metal ion levels. In metal (or ceramic)/polyethylene bearings wear can be measured radiographically and can result in increased osteolysis. In ceramic/ceramic bearings mal-positioning can result in stripe wear and squeaking.