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

Abstract

Fretting corrosion tests were conducted on Ti–6Al–4V modular `pad taper' test specimens of the Richards Modular Hip System (RMHS) in an environment containing deionized water. The primary objective was to evaluate and characterize the fretting debris production at the taper junction between the stem body and the proximal pad components of the RMHS by measuring metal (Ti) ion concentrations in solution during fretting tests. The fretting test parameters used in the research program were intended to simulate the hip prosthesis loading conditions. The loading parameters were: cyclic fatigue load=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 on 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 without interrupting the fretting test. Each solution sample was analyzed for Ti ions in parts-per-million (ppm). The solution analysis test results indicated that in six cases there were no detectable ions (<0.05 ppm) in solution. In three cases Ti ions in solution were found to gradually increase with 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 found to be 9.72 ppm after 5 million fretting fatigue cycles indicating comparatively more fretting. Subsequently, scanning electron microscope (SEM) analysis and pull-off force analysis of the test specimens were performed. These results correlated fairly well with the ion release data. The greater the ion release, the greater the fretting damage observed on the test specimens and the less the force required to separate the pad taper specimens. This study, however, could not identify a particular contributing factor for the increased fretting damage in one case.