Alternative bearings

The German surgeon Themistocles Gluck is thought to have been the first person to perform knee replacement surgery on joints destroyed by Tuberculosis back in the 1880s. He used a hinged device, made of ivory. Since then considerable research has focused on the materials that are used. The ‘bearing surface’ of a joint replacement refers to each of the two surfaces that glide against each other as the joint moves. This surface should be very smooth (low friction), and should be inert (so that no biodegradation occurs over time). It should have good wear properties, so that it takes a long time to become worn thin, and any particles produced by long term wear should not cause an immune reaction (where the body ‘rejects’ the material). In addition the bearing material should be tough, so that it does not get broken easily and it needs to be possible to engineer it precisely, to the exact dimensions required. The ivory joint replacements used historically did not have many of these qualities, so it is not surprising that they did not last for very long after implantation.

There is no single perfect bearing surface. For knee replacements, metal (cobalt chrome alloy medical grade stainless steel) is used for the femoral side and plastic (highly crosslinked polyethylene) is used for the tibial side. This is also the most common bearing used for total hip replacement. There is a long track record of these materials being used in combination, and we know that they are safe, with good wear properties. Many joint replacements performed 20 years ago are still working well, and more recent advances mean that the polyethylene used today is both tougher and slower-wearing than it has been previously.

'Metal-on-metal hip replacement' refers to those where both the ball and the socket side of the joint are made from polished metal. This combination has the advantage of being very hard wearing and strong even when a relatively thin layer of metal is used. Therefore it is ideal for resurfacing hip replacements, (which differ from total hip replacements in that just the thin layers of cartilage in the ball and socket are replaced, rather than the whole ball of the hip joint). In a well-functioning joint replacement with metal bearing surfaces, very few wear particles are produced. Unfortunately there have been some implants (now withdrawn from the market) that make more particles, and over time some individuals react badly to these particles. The use of metal on metal bearing surfaces is now reserved for a selected group of people who will benefit from the advantages of hip resurfacing, and implants restricted to those with a proven track record which are known to produce very few wear particles.

'Ceramic-on-ceramic' refers to hip replacements where both the ball and the socket bearing surface is made from a polished ceramic. Of all the bearing surface combinations, ceramic-on-ceramic is the most hard wearing. Therefore in the very young, fit and active person this bearing surface may be recommended. Ceramic is not appropriate for knee replacements or hip resurfacing though as a relatively thick layer is required to give adequate strength. Ceramics are by nature brittle compared to metal, and can crack (fracture) although advances in manufacture mean that ceramic bearings in use today are very strong, and this is a rare complication. Another potential disadvantage of ceramic-on-ceramic bearing surfaces is that there can be an audible squeaking sound from the hip during some movements. This sound does not signify any problem with the joint, but some people are perturbed by it.

'Ceramic-on-polyethylene' refers to hip replacements where the ball is made from ceramic and the socket from polyethylene (plastic). A ceramic ball causes less wear on the polyethylene than a metal ball so this combination may be recommended for younger, active individuals. It has the advantage of avoiding the possibility of squeaking from the hip joint when compared to ‘ceramic-on-ceramic’ bearings.