Biomechanics and biotribology of UHMWPE artificial hip joints

Well function of hip joints ensures daily movements such as walking, standing, climbing, or lifting. However, joint diseases such as osteoarthritis, rheumatoid arthritis, and trauma often require the natural bearings to be replaced by artificial ones. John Charnley pioneered the first metal-on-polyethylene artificial hip joints in the 1960s, when he articulated a femoral head against the ultrahigh molecular weight polyethylene (UHMWPE) liner. Although ceramic-on-ceramic and metal-on-metal artificial hip joints have been widely used in clinic, the UHMWPE hip implants are most prevailing with great success. Currently, over one million patients accept total hip replacement around the world every year, and the demand remains increasing with the accelerated aging population. However, unlike natural synovial hip joints with excellent elastohydrodynamic lubrication, artificial hip joints overall experience boundary lubrication or mixed lubrication. Under such lubrication conditions, direct contact between femoral head and acetabular liner is inevitable and finally generates extensive micro-wear debris. Then bioreaction of soft tissues rendered by UHMWPE wear particles occurs, which eventually leads to aseptic loosening of hip implants in the long term. In the past decades, much research enhancing wear resistance of the UHMWPE hip implants has been done by polymer scientists, biomedical engineers, orthopedic surgeons, and manufacturers. This chapter aims to review the latest research on wear performance of UHMWPE artificial hip joints from both biomechanics and biotribology.