In conventional Total Hip Replacement designs, a metal ball articulates with a plastic (polyethylene) cup. The plastic cup is not usually hard enough to cause metal ball wear, but the ball does wear the plastic cup away. Thus, in general terms, a plastic bearing will always wear out at some stage, albeit in a predictable and slow manner. Not only do plastic cups eventually fail, the particles of plastic, which are released from the frictional wear, can cause local bone resorption (osteolysis). This loss of bone can often be an issue when revising these types of Total Hip Replacement, and often bone graft (substitute bone) needs to be used to fill the defects. One strategy to slow the wear of the plastic cup is to substitute the metal ball for a smoother ceramic ball, which causes less frictional wear and may prolong the life of the plastic liner. Also, newer harder ‘cross-linked’ plastics have become available which also look to slow the wear rate. We do not yet have long-term data to support this yet, though.
The BHR, and other metal-on-metal (MoM) designs, have recently been subject to much media speculation regarding ‘metal allergy’. Other phrases which have also been used are ALVAL, and most recently ARMD (Adverse Reaction to Metal Debris). As one walks on any type of THR, there will be a small amount of particulate debris released from wear between the ball and cup (or cup liner) as the ball rubs against the cup. In some patients, the metal released into the hip joint (usually cobalt and chrome) can cause a local allergic reaction. This can cause damage to the muscles surrounding the hip, and if very advanced, even the bone. This damage is irreversible, and can also be silent (the patient is not aware of a problem), until quite advanced destruction has occurred. At this point, the results of revision of the components is always disappointing, as although the surgeon can eradicate the metal reaction by removing the components and affected tissue, they may not reconstitute the lost muscle.
My strategy to prolong the life of the Total Hip Replacement is to use a ceramic ball on a ceramic liner (ceramic on ceramic, or CoC). There are many advantages to this coupling. The wear debris does not cause any local reaction in bone or muscle. The wear rates are extremely low and it has an excellent chance of lasting a young patient’s lifetime. The surgeon can use larger heads, which impart greater stability (one of the perceived advantages of MoM). The downsides are that the components are very expensive, and historically there has been reported incidences of the ceramic squeaking or even breaking.
Ceramic components have been used for around twenty years now, however. They have evolved (we are now on the fourth generation) so that they are now much less brittle (difficult to break or shatter) and therefore can withstand much higher impact forces. The incidence of squeaking is extremely low, and does not necessarily mean that there is a problem with the components. However, there was a particular type of CoC THR that was widely used, especially in the USA, which had a very high rate of squeaking. This was due to a design issue of those particular components, but as a result, all CoC components gained a bad name for this. Thankfully, in Europe and the rest of the world, this has not been seen in any significant numbers and so we have continued to be able to use them with great success.
As a result of all of the above, my personal strategy for successfully treating the active patient for whom joint preservation keyhole surgery is not suitable, a bone conserving design coupled with a CoC bearing is the implant of choice. However, all patients are treated on a case-by-case basis, and not everyone is suitable. Rest assured, however, that whatever we decide upon (between the patient and surgeon) will be the best option for you.