Abstract
In joint prostheses where ultra-high molecular weight polyethylene (UHMWPE) is used as bearing material, efficacious treatments such as crosslinking, addition of vitamin E and the grafting of phospholipid polymer are known to improve wear resistance. Under severe conditions of various daily activities, however, friction and wear problems in such prostheses have not yet been completely solved. In contrast, extremely low friction and minimum wear have been maintained for a lifetime in healthy natural synovial joints containing articular cartilage with superior lubricity. Accordingly, joint prostheses containing artificial hydrogel cartilage with properties similar to those of articular cartilage are expected to show superior tribological functions. In establishing the function of artificial hydrogel cartilage as a novel material for joint prostheses, the tribological properties of hydrogel materials used and synergistic performance with synovia constituents are both important. In this study, the influence of synovia constituents on friction and wear in artificial hydrogels was examined in reciprocating test and compared with that for articular cartilage.
As biocompatible artificial hydrogel cartilage materials, three poly(vinyl alcohol) (PVA) hydrogels were prepared using the repeated freeze-thawing (FT) method, the cast-drying (CD) method and hybrid method for CD on FT, which are physically crosslinked with hydrogen bonding but differ in terms of structure and mechanical properties. First the frictional behavior of the PVA hydrogels and articular cartilage as ellipsoidal specimens was examined in reciprocating tests against a glass plate with a sliding speed of 20 mm/s under constant continuous loading. As shown in Fig.1, the three hydrogels exhibited different frictional behaviors in a saline solution. It is noteworthy that the hybrid gel maintained very low friction until the end of test. The CD gel showed slightly higher friction and a gradual increase. Meanwhile, the FT gel showed initial medium friction and a gradual increase echoing the time-dependent behavior of natural articular cartilage. Based on these observations, focus was placed on FT gel and articular cartilage to examine how synovia constituents influence friction and wear in these hydrogel materials.
In human body, lubricating constituents in synovial fluids such as hyaluronic acid, proteins, glycoproteins and phospholipids are considered to reduce the coefficient of friction in solid-to-solid interaction. Here, the effects of hyaluronic acid (HA, molecular weight: 9.2×105), serum proteins and phospholipid were examined. Dipalmitoylphosphatidylcholine (DPPC) was used as a typical phospholipid. As indicated in Fig.2 for repeated reciprocating tests, addition of HA alone was effective particularly for PVA-FT hydrogel. The combination of HA and DPPC was more effective in reduction of friction. The simulated synovial fluid (composed of HA 0.5 wt%, DPPC 0.01 wt%, albumin(Alb) 1.4 wt% and gamma-globulin (g-glob) 0.7 wt%) exhibited both low friction and minimum wear. The rubbing surfaces of articular cartilage and FT gel after tests are shown in Fig.3. On the articular cartilage surface, gel-like surface layer existed. On the FT gel surface, the original texture was observed without damage.
These results indicate the importance of synovia constituents for the clinical application of artificial hydrogel cartilage in joint prostheses.
