With the acceleration of the aging process and the increasing trend of joint diseases in younger ages, the number of patients with joint diseases continues to grow. Artificial joint replacement is one of the effective means to treat joint diseases. The wear particles of the artificial joint prosthesis will cause osteolysis, which in return triggers aseptic loosening in the prosthesis itself. Implants survive 10~15 years in vivo to date. Good lubrications are important reasons to reduce friction and wear of artificial joints. Therefore, clarifying the lubrication mechanism is of great significance to prolong its service life.
The synovial fluid (SF), containing the albumin, γ-globulin, hyaluronic acid (HA) and phospholipids (PLs), plays a crucial role in the lubrication of artificial joints. SF is an essential lubricant for natural joints. However, the joint replacements generally introduce a change in the composition and concentration of SF, which influences the tribological behavior of artificial joints. Especially, the total protein concentration of SF of osteoarthritic, rheumatoid arthritis, and periprosthetic is significantly higher than concentrations currently recommended by the International Organisation for Standardisation (ISO) standards. In addition, the presence of proteins in the lubricant is a precondition for wear rates of ultra-high molecular weight polyethylene (UHMWPE) similar to those reported clinically. Moreover, total protein concentration, albumin to γ-globulin ratio, and hyaluronic acid as well as lipids in the physiological concentration range are also important.
The classical elastohydrodynamic lubrication (EHL) may not be completely used to describe the lubrication mechanism of artificial joints because of the illiquid adsorbed film formed on the surface of the prosthesis. It is known that the absorbed film has a prominent influence on the lubrication of artificial joints. The lubrication of a single component may not fully reflect the lubrication mechanism. In addition, it is extremely important to further study the coupling of components of SF. Furthermore, in order to prolong the service life of artificial joints, it also needs to research the stability and long-term lubrication ability of the adsorbed film, which will significantly affect the friction and wear of implants.
The recent researches about tribological properties of artificial joints have achieved remarkable results. Firstly, the relationship between different types and concentration of SF compositions and the macroscopic friction and wear law were established. Secondly, the contribution of each component and its coupling effect on lubrication film was clarified, and the lubrication mechanism of SF components on the artificial joint was revealed. The current paper reviews the effects of the albumin, γ-globulin, hyaluronic acid, and lipids as well as the mixture of each component on the tribological behavior of artificial joints. The current study provides a certain theoretical basis for the exploration of artificial SF, and is of great significance to prolong the service life of artificial joints.
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