IN CONTACT ANALYSIS OF SYNOVIAL FLUID LUBRICATING FILM PROPERTIES

Total hip replacement is a well-established and highly successful treatment for end stage hip arthritis and in recent years there have been significant improvements in prosthetic components. However there are still concerns about performance and component life as implants are increasingly being used in younger and more active patients. Wear of the articular surfaces remains a problem and is known to be a major cause of failure in metal-polymer joints through osteolysis. Although wear is reduced significantly with the new generation of metal-on-metal joints there are concerns about the formation of nano-wear particles which lead to increased levels of chromium and cobalt in the body. Recently this problem has been accentuated by reports of 'pseudo-tumours' which are associated with high metal ion levels. Thus prosthesis wear remains an important area of research and most experimental studies have concentrated on this aspect. Relatively little attention has been paid to analysing the properties of the synovial lubricating film and the mechanisms of film formation, although such knowledge is key to the development of strategies to reduce wear. Wear of prosthetic joints is controlled by the properties of the synovial lubricating film and the nature of the articulating surface. The current proposal will focus on understanding lubrication mechanisms and the role of synovial fluid constituents in artifical hip joints.The proposed study will analyse the chemical and physical properties of synovial fluid lubricating films formed during rubbing. The project will use two analytical methods; In-contact Fluorescence Imaging and Atomic Force Microscopy to analyse the chemical composition, molecular structure and local physical properties (rheology, friction) of SF lubricating films. The analysis will be carried out 'in contact' so the film properties are measured during the lubrication process rather than post-test. The proposed work will provide information on the fundamental lubrication mechanisms occurring in artificial hip joints. The research has important implications for the development of low-wear strategies and new prosthesis designs. The primary beneficiaries will be the NHS, orthopaedic surgeons and their patients as the outcome will be improved joint life and reduced incidence of prosthesis revision. In 2007, the UK performed 10,500 THR revision operations, each of which may cost up to 25K, totalling 255 million per year. Thus a reduction in revision rate, particularly for MoM joints, is an important goal as it is a costly and demanding procedure, which already consumes 10% of the NHS joint replacement budget.The research will also deliver fundamental information of the effect of SF chemistry on joint wear. Such knowledge will enable surgeons to make an informed choice of the most appropriate type of prosthesis for each patient depending on their SF chemistry. The study could also contribute to the development of a SF 'tribo' health check and remedial strategies to improve joint lubrication. Prosthesis manufacturers will also benefit from the proposed research as a detailed understanding of the lubrication process will aid improved design of joints to reduce wear and increase implant life.

The study is on synovial fluid (SF) lubrication mechanisms in prosthetic joints. The findings will contribute directly to improvements in prosthesis design and materials to reduce wear and extend implant life. Such knowledge will benefit other researchers in the field, implant manufacturers, orthopaedic surgeons, the NHS and patients. The research is driven by the recognition that the performance and life of joint implants must be improved to meet clinical and patient expectations; a need emphasised by speakers at the London Hip Meeting 2009. The work will impact the following areas: Health care Total hip or knee replacement is a well-established treatment for end stage arthritis. At present over 130,000 primary hip and knee joint replacement operations per year (2007/2008 entered on the NJR) take place in the UK alone. The outcome of joint replacement is usually very good and overall revision rates are currently less than 2% after 3 years1 increasing to 5% after 5 years3. However for metal-on-metal hips the revision rates are higher than expected approximately 10%. In 2007, the UK performed 10,500 THR revision operations, each of which may cost up to 25K, totalling 255 million per year. Thus a reduction in revision, particularly for MoM joints, is an important goal as it is a costly and demanding procedure, which already consumes 10% of the NHS joint replacement budget. The knowledge from this study will contribute to a reduction in revision rates and extended implant life, through improved design and optimisation of surface properties. The research will also examine the effect of component malposition, SF chemistry or component materials on lubrication condition. Such knowledge will enable surgeons to make an informed choice of the most appropriate type of prosthesis for each patient. The study could also contribute to the development of a SF 'tribo' health check and remedial strategies to improve joint lubrication. The research will be carried out in collaboration with Mr Alister Hart (Consultant Orthopaedic Surgeon and LIRC) New Implant Technologies The proposed research will contribute to development of implant surfaces optimised to promote boundary lubrication and reduce wear. This might be by development of functionalised surfaces and coatings to promote boundary film formation, or the optimisation of surface texture. A second EPSRC proposal is currently being developed and technology (implant manufacturers, coatings) partners will be sought to collaborate on this project. Economic The global orthopaedic market is forecast to be worth $18.8billion by 2010. The forecast for the UK hip implant market in 2007 was $109 million and many of the leading companies (Corin, JRI, Smith and Nephew, Finsbury Orthopaedics) are based here. However as the Datamonitor UK Hip Orthopedics Industry Profile noted Despite the UK possessing a developed pharmaceutical and biotech industry, international companies dominate, with US and German companies holding significant market share. The implant industry is technology driven and to maintain or increase this market share requires continued innovation. Thus the development of new implant designs and the provision of research scientists and engineers to drive this forward are of vital importance. Dissemination of findings The proposed research will be disseminated to the wider academic community, which include engineering, biophysics and medical disciplines, via research papers and presentations at UK and international conferences. The work will also be presented at the AAOS and ORS orthopaedic meetings. The LIRC group meetings, attended by orthopaedic surgeons and UK implant manufacturers, will also provide a forum for reporting and discussion of the findings.