TODAY'S MATERIALS

Cobalt-chromium alloys have been used for dental implants since the 1930s. Their proven record in dentistry, showing that they could be placed in the body safely and were corrosion-resistant, made them an obvious family of materials to explore for joint replacements.

Sir John Charnley used cobalt-chromium alloy in his pioneering hip implants in the 1960s and it is still the most common metal used for replacement joints.

There are several types of alloy used to make implants. If the implant is shaped by casting, the alloy is 59 - 69% cobalt, 27 - 30% chromium, and 5 - 7% molybdenum. If it is forged, then nickel and tungsten may be added.

Milk jugs and shampoo bottles are two familiar forms of polyethylene. The polyethylene used in replacement joints has been altered, however, to give it a higher molecular weight, which means the polymer chain is longer. This increased length gives an increased impact strength to the joint.

The properties of the polyethylene has been changed. It becomes "slippery" and more wear-resistant. Ultra-high-density polyethylene is frequently used to coat the areas of a replacement joint that rub together. However, the particles of polyethylene that rub off as the joint wears can sometimes cause infections.

The first all-ceramic hip replacement was implanted in 1970 by French surgeon Pierre Boutin. Today, ceramics form part of many implants; they are most often used to make the head of the femur in hip replacements.

The most commonly used ceramics are zirconium oxide and aluminium oxide. They wear well, and are light, strong, and corrosion-resistant.

The metal titanium came into widespread commercial use only in the 1950s. Its principal use at the time was in the aircraft industry, because it was light, very strong, and corrosion-resistant.

These are all qualities that are desirable in replacement joints as well as in aircraft. Titanium, usually alloyed with vanadium and aluminium, is now being used for parts of larger replacements, such as the hip.