Abstract
The reduction of both friction and wear is required in existing joint prostheses composed of ultra-high molecular weight polyethylene (UHMWPE) and metallic or ceramic components, or even in Hard-on-Hard joint prostheses. In contrast, the healthy natural synovial joints with rubbing surfaces of articular cartilage are likely to operate at very low friction and low wear for the entire lifetime in the adaptive multimode lubrication mechanism, in which various lubrication modes become effective in various daily activities. Therefore, to establish a similar lubrication mechanism in joint prostheses by the application of compliant artificial cartilage, we conduct various researches to improve lubrication modes resulting in reduction in both friction and wear. In this paper, the effectiveness of the hydrogel artificial cartilage of high water content is discussed from the viewpoint of bionic design to mimic natural synovial joints.
The aim of this paper is to facilitate a function based on multimode lubrication mechanism in joint prostheses similar to natural synovial joints. Firstly, the possibility of full elastohydrodynamic lubrication was evaluated by experimental methods in friction tester and joint simulator. The joint prostheses with compliant rubbing materials or polymer-on-hard joint with better geometrical congruity showed siginificant fluid film formation, but some local intimate contact occurred. Therefore, as the second viewpoint, the effectiveness of adsorbed film formation was examined. The noteworthy phenomena are remarkable reduction in friction for artificial joint with poly(vinyl alcohol) (PVA) hydrogel articular surfaces and a notable increase in friction for artificial joint with polyurethane surface in hyaluronate solutions containing serum proteins. These results indicated that adsorbed protein films can reduce or increase friction and wear depending on probably fluid film thickness.
Other findings of effectiveness of layered adsorbed film and negative effect of heterogeneous adsorbed film are described on the basis of various observation in friction tests.
As the third viewpoint, the importance of biphasic lubrication and hydration lubrication for hydrogel surface with high water content is discussed. In friction tests of natural articular cartilage against glass plate, it was observed that the unloading for 5 min after continuous 30 min rubbing reduced the friction at restarting probably due to biphasic lubrication and/or hydration lubrication after rehydration, where adsorbed films have some influences on friction and wear. For joint prostheses with compliant hydrogel artificial cartilage, similar mechanism is required for surface and bulk structure of artificial cartilage.
In this paper, several important essential points from the bionic design are indicated for development of the next generation for joint prostheses with higher function and better longevity.
Correspondence should be addressed to Diane Przepiorski at ISTA, PO Box 6564, Auburn, CA 95604, USA. Phone: +1 916-454-9884; Fax: +1 916-454-9882; E-mail: ista@pacbell.net
