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Metal release mechanisms in hip replacement

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The release of metal ions from metallic implant materials is influenced by chemical, biological and mechanical factors.

From the corrosion point of view, Co-28Cr-6Mo implant alloys (similarly to all materials used in the human body) show a relatively high resistance to dissolution, due to the spontaneous formation of a highly protective passive film on the alloy surface. However, as in all cases of passive metals, damage to the passive film by either chemical or mechanical means increases the release of metal ions to the surroundings. In hip implants, micromotion leads to continuous mechanical activation of the surface (i.e. removal of the protective passive film)—typically followed by a fast repassivation reaction. Regarding metal ion release into the surroundings, it is not only the generation of wear debris that is crucial, but also the electrochemical behavior of the alloy and its constituents during the repassivation process. Considering the electrochemistry of the Co-Cr-Mo alloy and of the alloying elements themselves, it becomes clear that not only the intensity of metal dissolution, but also the dissolution mode, can vary in different kinds of surroundings.

For instance, under the chemical conditions usually prevailing in the body, no stable Cobalt oxides exist and thus formation of soluble Cobalt ions instead of solid Cobalt oxides is favored.

On the other hand, Cr(III) oxides are very stable, and therefore substantial dissolution of Cr (of solely electrochemical origin) can be expected to happen only under sufficiently high oxidizing conditions, in the electrochemical stability region of soluble Cr(VI) species. Taking both of these factors into consideration, selective dissolution of Co from a Co-Cr-Mo alloy can be expected (and has been observed under laboratory conditions) during activation/repassivation cycles. The electrochemical conditions that prevail can thus influence the concentration and the chemical speciation of the released alloying elements, the latter also being of importance for subsequent biological reactions. In the worst case, the disturbance of the biological equilibria by release of metal species may be such that metal dissolution rates are further increased as a result. In such a case, an autocatalytic failure mode would be observed.

Source:

Acta Orthopaedica 2006; 77 (5): 695–696

Sannakaisa Virtanen, Institute for Surface Science and Corrosion, Department of Materials Science, University of Erlangen-Nuremberg, Erlangen, Germany
virtanen@ww.uni-erlangen.de

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