Evaluation of the fretting corrosion behavior of the proximal pad taper ofa modular hip design

Fretting corrosion tests were conducted on Ti-6Al-4V modular 'pad taper' te st specimens of the. Richards Modular Hip System (RMHS) in an environment c ontaining deionized water. The primary objective was to evaluate and charac terize the fretting debris production at the taper junction between the ste m body and the proximal pad components of the RMHS by measuring metal (Ti) ion concentrations in solution during fretting tests. The fretting test par ameters used in the, research program were intended to simulate the hip pro sthesis loading conditions. The loading parameters were: cyclic fatigue loa d = 2.0 kN (R = 0.1), normal load = 0.2 kN. These loads were selected based on standardized ISO hip stem loading conditions which in turn Were based o n anticipated in vivo loading conditions. Ten RMHS pad taper fretting tests were performed. Each test was performed for 5 million fretting cycles at a frequency of 6 Hz. A solution sample was removed every 1 million cycles wi thout interrupting the fretting test. Each solution sample was analyzed for Ti ions in parts-per-million (ppm). The solution analysis test results ind icated that in six cases there were no detectable ions (< 0.05 ppm) in solu tion. In three cases Ti ions in solution were found to gradually increase w ith the number of fretting fatigue cycles to a maximum of 0.39, 0.35, and 0 .62 ppm after 5 million cycles. This indicated 'moderate' fretting between the pad taper components. However, in one case Ti ions in. solution were fo und to be 9.72 ppm after 5 million fretting fatigue cycles indicating compa ratively more fretting. Subsequently, scanning electron microscope (SEM) an alysis and pull-off force analysis of the test specimens were performed. Th ese results correlated fairly well with the ion release data. The greater t he ion release, the greater the fretting damage observed on the test specim ens and the less the force required to separate the pad taper specimens. Th is study, however, could not identify a particular contributing factor for the increased fretting damage in one case. (C) 1999 Elsevier Science S.A. A ll rights reserved.