Fretting-corrosion of materials used as orthopaedic implants

In the biomedical field, about 6% of hip total joint prostheses have to be replaced after 9 years because of a loosening of the femoral stem. One of the main causes of a new surgical intervention is attributed to the phenomenon of fretting-corrosion, i.e. wear under small movements (lower than 100 μm) in corrosive aqueous medium. To understand this degradation, the fretting between 316L steel and PMMA has been investigated to simulate the fretting between femoral stem and bone cement. First, fretting in air has been studied as a reference environment. No significant wear was observed on the stainless steel. PMMA suffers a wear that exhibits a linear evolution as a function of the cumulated dissipated energy. Third body evolution could explain the particular ‘W’ shape of the active wear track in PMMA, in dry conditions. In aerated Ringerʹs solution, at free corrosion potential, stainless steel suffers significant wear damage, and the wear volume increases linearly with time but not with dissipated energy. Moreover the optical observations and three-dimensional (3-D) profilometry show a ‘W’ shape of the wear track of 316L. A corrosion mechanism involving a crevice effect enhanced by fretting allows to explain the location of the maximum damage zones. Finally, the effect of electrochemical potential on the behaviour of 316L/PMMA contacts has been studied by recording current intensity and cumulated dissipated energy under potentiodynamic conditions. Dissipated energy exhibited a reproducible variation with potential. Lubrication regimes and effect of potential on the surface charges could account for such a behaviour.