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.