Compliant layer knee bearings. Part II: Preliminary wear assessment

Compliant layer knee joints potentially offer an alternative bearing to conventional joints and may give longevity. In the work reported here a constant load simulator has been used to examine wear rates and wear mechanisms in flat and congruent compliant layer total knee replacements.Joints were tested up to 2 million cycles with bovine calf serum as lubricant. Following visual examination, the main feature observed at the macro level was burnishing of the contacting areas, implying that the bearings operated with some degree of fluid film lubrication. Under microscopic examination some embedded debris (size <20 μm) was seen to be pressed into the compliant layer and the pits created had been polished, having smoothed edges. This confirms one of the perceived functional benefits of the compliant layer model whereby third body debris is expected to be embedded into the compliant layer rather than being retained as an abrasive agent in the articulating space. Delamination of the compliant layer from the hard backing and other severe fatigue related phenomena were not observed for either the flat or congruent compliant layer inserts as observed in other studies.It was shown that gravimetric wear measurement per the cleaning protocol completed did not present a reliable method for measuring the wear of hydrophilic polyurethane bearings with more detailed studies required to confirm findings. However, surface profilometry was seen to correlate with observed wear mechanisms. The bearings did exhibit some scratching and it was proposed that the scratching was caused by asperity contact or by third body wear debris such as calcified proteins that precipitated out of solution. It was also clear that the static constant load applied by the wear simulator did not fully compromise the lubrication by cancelling out squeeze film effects and the results obtained corresponded to other published work where dynamic loading was used.