Today there is a new type of metal-on-metal prosthesis used in the ANCA medical centre. (www.wmt.com en www.hipreplacement.com) These are fabricated from a harder cobalt-chromium alloy, and thus consist of a metal-on-metal couple. This new combination creates a differential hardness between the two moving parts. This differential makes that the metal is likely to release less particles into the body and wear less over time. (figure 3 and 4) In all hip prostheses the femoral head wears quicker than the acetabular shell (except the metal-on-polythylene ones (hard-on-soft)). The difference in hardness is a major advantage over the previous metal-on-metal prostheses and even more over the metal-on-polyethylene prostheses.
The metal-on-metal bearings are hard without being brittle, they are more resistant to scratching and wear. There is a debate on the effect of metal ions released by the couple; the eventual effect of these ions isn’t clear yet, but research is being done on that subject. (see also pfd-file on metal-ions)
The femoral heads can be made bigger in diameter using the same sized acetabular shells. This results in (1) a bigger ‘Range of Motion’ of the hip (figure 5) and (2) greater stability. The latter is a crucial factor in the success rate of the implant because the dislocation rate is lower.
It is important to mention to your surgeon if you are known with a metal allergy, i.e. nickel allergy. (see metal-allergy page) It is good to realise that not every prosthesis is suitable for every patient. The surgeon will make the decision with you together, which one will be the best for you.
One of the prostheses used, the Wright Conserve A-class, was given to tennis legend Jimmy Connors (figure 6). Since he has got this new hip, he’s back on the court, playing his favourite game.