If this ceramic fulfils its early promise, the team predicts that this material could reduce the need for people with implants to undergo further operations. It could even allow the development of artificial knee joints.
Extending the useful lifetime of artificial joints is a pressing challenge for biomaterials researchers. At present an implant usually needs to be replaced after about ten years.
"Considering [the] ageing population and the growing demand for orthopaedic surgery on younger patients, implants [need] a lifetime of more than 30 years," says materials researcher Jerome Chevalier of the National Institute for Applied Sciences, Lyon, France, and his colleagues.
Chevalier's team combines two ceramics widely used as 'femoral head' implants -- alumina and zirconia, which usually sit in a polymer socket -- to produce a composite material that is stronger than both. Ceramic materials are very hard, so implants do not wear down. Wear debris can react with the body's fluids and cause problems.
But ceramic materials are relatively brittle (so can fracture) and they are susceptible to slow crack growth -- usually starting at a pre-existing fault or defect, such as a scratch in the surface. Pressure or stress at the crack tip extends the fault until the whole implant needs replacing.
So an important measurement in ceramics research is how big this stress needs to be before the crack starts to extend. The higher this 'stress intensity threshold', the better suited the material is to making artificial joints.
The new composite ceramic has a much higher threshold than alumina and zirconia individually (respectively 35% and 60% better). It is also significantly 'tougher' and so less likely to break if an implant patient slips and falls, for example. The team reports its results in the journal
"For the same pre-existing defects, these composites can work at loads two times higher than the [individual materials] without delayed failure," the researchers conclude.
The new material may well show promise in the laboratory but only the human body offers a true test for implants, warns David Hukins, who studies artificial joint materials in the bio-engineering department of Aberdeen University, UK. Other implant materials and designs have been withdrawn following poor performance in patients, he points out, despite passing all relevant tests and trials.
The structure and performance of the new material needs to be refined, the research team admits -- but they are still confident of success. "It will be possible to extend the lifetime of femoral heads, thus contributing to an improved quality of life for a large number of patients," they claim.
Hukins concedes that that the composite may be a "material of promise". Still, the potential benefits will not be available rapidly -- it takes about 20 years for new implants to clear all the necessary safety and performance-testing hurdles.