The European Commission reports that ceramic nanocomposites developed in BIOKER could solve the problem of fracture failures in artificial joint implants. This would extend patient mobility and eliminate the high cost of reparative surgery.
Today, more than 500 000 hip- and knee-joint replacements are fitted annually throughout the European Union. Such procedures are extremely successful in restoring mobility to sufferers from arthritic and other degenerative conditions. However, the average lifetime of a hip prosthesis, for example, is around 10 to 15 years – with active and heavyweight patients being particularly prone to premature failure. This clearly poses a quality-of-life problem for younger and other vulnerable recipients. Moreover, revisionary surgery adds a further 70 to 100% to the cost of the original operation, raising EU medical costs by around € 260 million a year.
The hip joint has a ‘ball-and-socket’ structure, in which the spherical head of the thighbone (femur) moves inside a cup-shaped hollow socket (acetabulum) in the pelvis. To duplicate this action, a total hip replacement implant has three parts: a metallic stem fits into the femur and provides stability; a ball replaces the spherical head of the femur; and a cup lines the worn-out hip socket.
Acetabular cups are typically faced with polyethylene having an ultra-high molecular weight, while the femoral heads are made either of cobalt/chromium alloys or of ceramics. Because the wear resistance of the head is a key factor in implant longevity, durable ceramics formed from zirconia or alumina have tended to supersede metallic construction. Yet neither of these materials is optimal for the task, as they are relatively brittle and susceptible to crack propagation at the sites of small surface defects. Even without complete failure, such surface irregularities can also cause wear in the polymer cup, releasing particles of debris that give rise to irritation and possible osteolytic changes.
Concern over the frequency of crack-related breakages in zirconia-based heads caused the US Food and Drug Administration to announce a large-scale recall in 2001. The major supplier of this material has subsequently ceased trading, leaving alumina as the only remaining viable solution.
In the three-year BIOKER project, funded under the European Commission’s GROWTH programme, a consortium of research institutes and industrial partners from three EU countries is investigating the use of zirconia-toughened alumina nanocomposites to form ceramic-ceramic implants with potential life-spans of more than 30 years.
Produced using specially developed processing technology, this material contains numbers of zirconia nanoparticles distributed uniformly among the alumina grains. “After evaluating many different processing variables and mechanically testing a variety of compositions, we eventually selected a formulation containing 2.5 wt% of zirconia,” notes project co-ordinator Ramon Torrecillas, of the Consejo Superior de Investigaciones Científicas (CSIC) in Oviedo, Asturias. “This has a stress intensity threshold much higher than that of either of its individual components. Another advantage is that we do not need to use the stabilisers required for pure zirconia ceramics. Due to reactions taking place in the human body, these were one of the sources of crack generation.”
To date, CSIC has produced the new material in powdered form on a laboratory scale. Barcelona-based partner Ceramica Industrial Montgatina presses the powder into ‘green’ (unsintered) rods for machining into balls and cups. It also employs an innovative pressure casting technique to form green knee components in specially designed plaster moulds. After firing, the parts are hot isostatic pressed and sent to Cheval Frères in Besançon, France for finishing by means of a specially developed three-dimensional machining process, giving a roughness lower than 10 nm.
Italy’s Istituto Ortopedico Rizzoli (IOR) and Insavalor (INSA) in Villeurbanne, France are evaluating the components – conducting tests for citotoxicity, genotoxicity and fatigue behaviour when exposed to simulated body fluids.
“At present we are still facing some difficulties in consistently producing defect-free heads,” Torrecillas admits. “But we are planning to install new milling and spray drying equipment that will enable us to make larger powder batches and carry out more extensive testing. We expect to obtain meaningful results by the end of 2004. At that point we should be in a position to apply for patents on some aspects of the processing.”
Several more years of development and clinical trials would be required before implants based on the BIOKER nanocomposite could enter regular medical service. Their eventual adoption would solve the current material supply problem. In addition, it would make it more feasible – and more affordable – for both young and older European citizens to benefit from what have already been shown to be highly life-enhancing interventions.