ARTIFICIAL CARTILAGE WITH OPTIMUM ADSORBED FILMS FOR LOW WEAR AND LOW FRICTION

Abstract

INTRODUCTION: The natural synovial joints with very low friction and low wear are likely to operate in the adaptive multimode lubrication mechanism, in which various lubrication modes become effective in various daily activities. On the contrary, in the artificial joints composed of ultra-high molecular weight polyethylene (UHMWPE) and metallic or ceramic components, it is difficult to expect the sufficient fluid film formation to prevent the direct contact between rubbing surfaces, and thus considerable wear of UHMWPE occurs. To improve the longevity of joint prostheses by reduction of wear and friction, it is effective to improve the lubrication mode to fluid film lubrication by the application of compliant materials (1). In this paper, the effectiveness of the compliant artificial cartilage of poly (vinyl alcohol) (PVA) hydrogel of high water content and the existence of the optimum adsorbed films were examined.

MATERIALS AND METHODS: The reciprocating friction apparatus was used to investigate the influence of the lubricants containing proteins on the wear properties. The sliding surfaces are composed of PVA hydrogel and itself or a glass plate. PVA hydrogel was prepared by repeated freezing thawing method. The elastic modulus of PVA hydrogel is 1.2 MPa and equivalent water content is 79%. PVA hydrogel stationary upper specimen have elliptical geometry with diameters of 25mm and 40mm. Lubricants as sodium hyaluronate (HA) or saline solutions with or without serum protein were used. For distinction between albumin and γ-globulin in fluorescent observation for adsorbed film formed on glass plate after tests, albumin and γ-globulin were labeled with Rhodamine-B-isothiocyanate and Fluorescein isothiocyanate isomer I, respectively. Next, to conduct in situ observation of adsorbed film formation, the reciprocating apparatus was constructed on the stage in inverted fluorescent microscope. A sliding pair of a spherical reciprocating upper specimen of PVA hydrogel and flat stationary lower specimen of cover glass was used.

RESULTS AND DISCUSSION: It is noticed in solutions of single protein that an increase in protein concentration increased wear grade. In contrast, in binary protein solution the minimum wear is found at optimum composition such as total content of 2.1 wt% and A/G ratio of 1/2 or 2/1 (A:albumin, G:γ-globulin). The fluorescence images of adsorbed film on rubbed glass plate showed that the adsorbed films formed a layered structure composed of albumin and γ-globulin. On the contrary, in high wear case for higher protein concentration, the adsorbed film was formed as a heterogeneous separated structure. Furthermore, in situ observation of lubricated conjunction clearly indicated that the formation of layered films or heterogeneous films corresponded to lubricant constituents.

In this study, it was shown that the layered adsorbed film formation originated from the optimum composition of proteins in lubricants is effective to maintain low wear and low friction for PVA hydrogel artificial cartilage.