Fretting Corrosion Fatigue Study of Modular Joints in Total Hip Replacements by Accelerated In Vitro Testing

The performance of modular head/taper joints in total hip replacements now in use for nearly two decades can be easily termed as a clinical success. However, associated with concerns of metal particulates found in retrieved tissues, a critical review of modularity in hip joints has recently emerged. Evaluation of retrieved modular joints has shown some surface morphological changes and debris generated at the mating surfaces which has not been demonstrated equally by any in vitro laboratory testing.

In this investigation, an experimental setup was developed to conduct fretting corrosion tests of modular head/taper assemblies in an aggressive environment to accelerate the in vivo phenomena. The fretting corrosion tests were conducted in a low pH Ringer's solution at 50°C. A maximum cyclic load of 5.34 kN was applied at 5 Hz on simulated modular head/taper assemblies mounted in a 15° valgus anatomic orientation. Ti-6Al-4V and Co-Cr- Mo alloy tapers and Co-Cr-Mo alloy (cast or wrought) and zirconia ceramic femoral heads in various combinations were evaluated. The morphology evaluation of taper and head bore surfaces after 10 million cycles revealed features such as etching of structure, preferential leaching and fretting in congruence to those reported for retrieved modular hip joints. Nitrogen diffusion surface hardening of Ti-6Al-4V alloy tapers resulted in an increase in resistance to fretting corrosion-induced changes, and a reduction in generated debris. A zirconia ceramic femoral head tested on either a Ti-6Al-4V or a Co-Cr-Mo alloy taper showed an enhanced resistance to both mechanical and chemical phenomena.