Influence of scratch damage to S phase coated and uncoated orthopaedic grade stainless steel on wear of ultrahigh molecular weight polyethylene

Abstract

The aim of the present paper was to quantify the significance of uncoated and S phase coated medical grade stainless steel (MGSS) surfaces with simulated scratch dimensions, akin to those generated in vivo, on the overall ultrahigh molecular weight polyethylene (UHMWPE) wear factor, i.e. to establish whether or not scratch lips generated under physiologically relevant loading conditions serve to augment the overall wear factor produced by the mean plate asperity heights of the polished metallic counter surfaces. A 29% increase in UHMWPE wear factor was produced when sliding against non-scratched N19 S phase coated MGSS compared to those obtained when apposing uncoated MGSS. This was attributed to the tested coated surfaces having ∼1·7 times more microasperities per 50 μm test length in the direction of sliding contact compared to the uncoated surfaces. A validated experimental procedure for creating scratched metallic surfaces with dimensions (lip heights and groove depths) towards the upper bound of those observed on MGSS explants was used to show that a critical minimum threshold scratch lip height must be reached before it influences the UHMWPE wear factor beyond that due to the interaction with the microasperities alone. The N19 S phase coating effectively reduced the potential detrimental effect of prior scratching by suppressing the height of attainable scratch lip heights, i.e. a mean lip height of 0·23±0·09 μm produced on the N19 S phase coated MGSS was insufficient to alter the mean UHMWPE wear rate, whereas the larger mean scratch lip height of 1·28±0·29 μm produced on the uncoated MGSS caused an increase in the mean UHMWPE wear factor by ∼34%. In addition, two types of UHMWPE wear morphology were identified. Type A comprised wave-like or rippled areas, while type B zones had a less regular roughness. Both zones contained plate-like features (flakes), which are one possible origin of UHMWPE debris. The extent of type A formation was qualitatively observed to increase with increasing UHMWPE wear factor.