Abstract
Investigation of how natural joints functioned seemed closely dependent on the lubrication mechanisms involved. This was equally relevant to artificial joints where, if fluid-film lubrication could be generated, the rubbing surfaces would be completely separated by the liquid film which would have the advantage of reducing friction, since this depends only on the shearing of the lubricant film, and reducing wear since the two surfaces would not be touching.
In 1969 when I first entered this research area, hip joints were mainly small diameter (<32mm) made from ultra high molecular polyethylene (UHMWPE) rubbing against stainless steel or cobalt chromium molybdenum alloy (CoCrMo), metal-on-metal joints and alumina-on-alumina joints. A few calculations showed that the small diameter metal-on-metal hips and the UHMWPE acetabular components in combination with any type of head material were unable to produce fluid-film lubrication. Insufficient film-thicknesses could be generated to separate the rough surfaces of the joints so surface-to-surface contact prevented full separation.
Ceramic-on-ceramic was different. This could be polished very smoothly and was hydrophilic so it could draw the water based lubricants (synovial fluid), into the contact region, which in turn generated fluid-films. This meant that with alumina-on alumina, wear was not a problem-but fracture was in some circumstances.
As more was learned about lubrication, large diameter CoCrMo hip resurfacing devices became possible. Whilst small diameter metal-on-metal hip joints were unable to generate fluid-film lubrication, larger diameter hips could, provided the lubricant contained serum (similar to synovial fluid). This was interesting since water based lubricants of similar rheology to synovial fluid (carboxy methyl cellulose, CMC) could not produce fluid-films (Figure 1) even though theory suggested that they should. Thus it was assumed that the proteins present in the natural lubricant were important, but the reason was unknown.
Returning to the original assumptions of Osborne Reynolds in deriving the theory of hydrodynamic lubrication, we see that in order to draw fluid into the contact area, the fluid adjacent to the solid boundary was assumed to be travelling at the same speed as the boundary itself. To do this the lubricant must ‘wet’ the surface or attach to it- but what if the surfaces are hydrophobic? The speed of drawing the fluid into the contact will be lower than the surface speed and so less fluid will be drawn in and the pressure generated will be lower than predicted by theory. So a simple experiment was tried using a large diameter CoCrMo alloy hip resurfacing device where a water-based lubricant (CMC) first had bovine serum added, then a simple detergent to reduce the surface tension. Figure 1 shows clearly that the lubrication improves markedly with the detergent – even more so than the bovine serum. This suggests that the wettability of the surfaces is important.
Another approach to enhancing fluid-film lubrication stems from the concept of elasto-hydrodynamic theory. Here, lower modulus, more compliant surfaces, produce thicker fluid-films for similar entraining velocities and applied loads. Thus we developed compliant hip and knee joints using hydrophilic poly carbonate urethane (PCU) acetabulae against metals or ceramics. These produced phenomenally low coefficients of friction (circa 0.001) and in the knee, wear rates of only 0.06 mm3/million cycles (two orders of magnitude lower than metal on UHMWPE).
Another interesting biomaterial is carbon-fibre reinforced poly-ether-ether-ketone (CFR-PEEK). Very long term hip simulator wear experiments (25 million cycles), showed wear rates which were lower than cross-linked polyethylene (circa 1.5 mm3/million cycles), yet friction was very high (µ=0.2–0.3). Clearly this was not fluid-film lubrication but improvements are being investigated.
