Fatigue failure of a coated titanium implant alloy

Forrest, Darren James (1996) Fatigue failure of a coated titanium implant alloy. University of Southampton, Doctoral Thesis.

Abstract

The use of cementless load bearing implants account for half of all hip prostheses currently manufactured, of which ninety percent are titanium alloys, mainly Ti-6Al-4V. Cementless implants require some form of attaching the implant to the bone: hydroxyapatite has shown excellent biocompatability clinically, and promotes early bone apposition. The fatigue strength of titanium alloys is fairly well understood in aerospace applications. In a physiological environment the fatigue properties of medical grade titanium alloys with the pretreatments required for coating adhesion and the effects of coatings have not been fully investigated.

Commercial coating pretreatments include; grit blasting (of variable duration, blasting medium and pressure), polishing (e.g. finely polished, belting, various grits of paper) and cleaning processes. S-N fatigue data was gained for two grit blasting pretreatments, a 600 grit polished surface and a cleaning process used for vacuum plasma spraying (VPS). The grit blasting process which roughens the surface and imparts compressive residual stress, was shown to significantly reduce the fatigue life compared to the polished substrate. The VPS cleaning process gave a large reduction in fatigue life. Fatigue crack micromechanisms were investigated using a replication technique and the effects of residual stress and surface roughness were investigated independently. The residual compressive residual stresses were shown to control the fatigue life of the substrate with and without increased surface roughness.

For air plasma sprayed (APS) hydroxyapatite coatings the effect of coating thickness was investigated: 50μm and 100μm coatings showed no significant reduction in high cycle fatigue, but the application of a 150μm hydroxyapatite coating resulted in a significant reduction in the fatigue life compared to the grit blasted substrate.

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