7.10 Wear: Knee joint arthroplasty

The human knee is the most complex joint in the body providing three rotational and three translational motions under high loads reaching multiples of body weight. Total Knee Replacement (TKR) is booming and projected to reach millions of procedures per year. Knee arthroplasty is a successful, reliable and cost-effective surgery which allows the patient to return to pain-free and close to normal leg function. The improved longevity of implant design and materials was vital for this success, especially the wear resistance of ultra-high molecular-weight polyethylene (UHMWPE) components commonly used as bearing inserts. Wear still poses a risk to the patient, due to the particulate debris which can cause accumulative inflammation (osteolysis) and can result in bone resorption, implant loosening and eventual failure requiring revision surgery. This chapter describes the knee joint and its replacement surgery, and a glimpse of the evolution of TKR, dwelling on the technological factors that have brought success and implant longevity, and the interplay of those factors with wear. The main principles behind in vitro knee wear simulator testing are described, contrasting the displacement control and force control standard TKR wear test methods. Citations are made throughout the text of clinical and in vitro wear test results.