The human body is generally excellent at repairing itself, but there are parts which, under certain conditions, even the natural maintenance contract cannot reach. Among these is the cartilage that lines the ends of the long bones in joints such as the knee and the hip. This elastic tissue provides a soft, compliant bearing surface which allows for low-friction movement – but unfortunately it is prone to disease. Degenerative osteoarthritis, where the cartilage becomes degraded – causing joints to no longer work easily or painlessly- affects an estimated 40 per cent of the population. A quarter of sufferers are treated medically with joint replacements, where synthetic materials are used instead of bone and cartilage. But researchers now suspect there is a better way of helping. They believe their solution would be an improvement because it only replaces the diseased parts of the cartilage, leaving the bone untouched and, as such, is less invasive. To test this theory, a Leeds University team has built a working model of a knee for experimental laboratory analysis. ’We have built a system that’s a simulation of a natural knee joint,’ said Prof John Fisher, director of the Institute of Medical and Biological Engineering, ’We can load it to make it move like a natural knee joint.’ It can be loaded thousands of times to replicate the lifetime of use it would get in a human. The artificial knee contains samples of natural animal cartilage which will be used to host the tests of portions of recently-developed synthetic cartilage. ’We are interested in replacing a small amount of natural cartilage with a synthetic material,’ said Fisher. ’we are investigating two approaches. The first is a form of hydrogel – a synthetic biogel that has a very high water content with a structure similar to contact lenses. The second is to use patented self-assembling peptide gels of biochemical origin which could be injected into the joint. In both cases we want to see how they interact with natural tissue.’ It is a complex problem that conventional joint replacements do not encounter, simply because the entire joint is substituted. But by taking out only the diseased part of the cartilage, the substitute material must live and work in harmony with the natural tissue that remains. ’There are more interfaces and interactions between the physical parts,’ said Fisher. And in biological materials’ properties and geometries, the loading and motions are more variable than with repeatably manufactured synthetic materials.’ As well as the physical experiments with the simulated joint, the team will use biphasic finite element analysis models ’to dig into the complexity of the structures,’ said Fisher. But why investigate a more complicated approach when thousands of joint replacements have been performed satisfactorily? ’Cartilage substitution therapy offers the chance for earlier intervention, to relieve the pain sooner,’ said Fisher. ’It is minimally invasive and retains more of the healthy tissue. It can be viewed as a step in a continuum of care that may begin with anti-inflammatory medication, with hip replacement as the final stage. Substitution may help delay the need for that final stage.’ Fisher’s team is collaborating with the chemistry department at Leeds, which has patented the self-assembling peptide gel to be used in the experiments. The hydrogel technology is being developed by an industrial partner, while colleagues at Sheffield University are working on the tissue engineering of cartilage. The background work has been completed over several years with funding from the EPSRC which has now awarded it an extra £64,000 for a further year. If the research confirms that replacing just a part of the cartilage rather than the whole joint, it could help reduce the external assistance the body needs to keep it in full working order.