Abstract
Introduction
The metal-on-metal (MoM) total hip prosthesis is widely used. However, the adverse reactions such as pseudotumor around the total hip prosthesis are observed. This is considered the effect of the corrosion of alloy which includes metal ion release and the wear particle generation. As materials for total hip prostheses, cobalt chromium (Co-Cr) alloy is used because of the wear resistance and corrosion resistance. The passive film on the surface of alloy contributes to corrosion resistance. The passive film is removed easily with friction. Therefore, metal ion is released from bare metal. However, this removal of passive film can be restored because of oxidation reaction with neighboring environment. The modular MoM total hip prosthesis such as acetabular component-femoral head or taper junction which connect femoral head and stem have friction interfaces. The friction amplitudes must be different among these interfaces. However, how sliding amplitude affects on removal of a passive film is unclear. The main purpose of this study was to investigate the effect of the sliding amplitude of the reciplocating micromotion on removal and reformation of the passive film of Co-Cr alloy.
Methods
The behavior of the passive film was observed by measuring the electric potential of the alloy. Co-Cr alloy (ASTM F75) pin specimen and common tablet specimen were immersed in simulated body fluid PBS(−) and abraded with friction testing machine. The electronic potential between the pin and the Ag/AgCl reference electrode (RE-1C, ALS, Tokyo, Japan) were measured using a high impedance electrometer (HE-104E, HOKUTO DENKO, Tokyo, Japan). The friction amplitude was chosen from 0.2–2 mm. The reciprocating cycle was 1 Hz. The load of 10 N applied on the pin by a weight.
Results and discussion
The electric potential neared equilibrium before the friction, and the passive film was stable. Electric potential dropped with the onset of friction and gradually increased with the cessation of friction. The potential difference ΔV and a time constant τ of the rise in electric potential after the friction were estimated. Those ΔV and τ were used as the indicator of the passive film destruction and restoration, respectively. At the friction amplitude of 0.2 mm, ΔV was lowest among at the other amplitudes. On the other hand, τ tended to be longer (Fig. 2). It showed that the reformation of passive film was delayed. When the removal of the passive film was repeated on local area of the friction interfaces, it was presumed similar to a phenomenon of the crevice corrosion.
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