MECHANICALLY ASSISTED CORROSION PERFORMANCE OF METALLIC BIOMATERIALS: IMPLANT RETRIEVAL, MATERIAL ANALYSES AND DEVICE TESTING
Date of Award
Doctor of Philosophy (PhD)
Biomedical and Chemical Engineering
JEREMY L. GILBERT
Crevice Corrosion, Fretting corrosion, Hip Implants, Mechanically Assisted Corrosion, Spinal Implants
Mechanically assisted corrosion (MAC) of metallic biomaterials continues to be a concern for highly-loaded medical devices for spinal, dental, cardiovascular and orthopedic applications. Increasing usage of modularity (multiple-component system) and mixed-alloy metal-metal junctions in orthopedic surgery gives the surgeon increased intraoperative flexibility for choosing optimal components; however, these design changes can accelerate corrosion reactions and significantly impact local biological processes and mechanical integrity of the implants. In order to successfully decrease the potential for premature failure of metallic implants, MAC performance of metallic biomaterials (stainless steel, titanium and cobalt chromium alloy) needs to be investigated and understood.
First, a failure analysis was performed on a retrieved Ti6Al4V neck-stem modular taper which was implanted for 6 years. A new unreported mechanism of in vivo corrosion (oxide induced stress corrosion cracking (OISCC)) which led to the failure of neck-stem taper was proposed. The results showed large penetrating pitting attack where pits ultimately developed into cracks and the cracks propagated due to stresses developed by oxide growth within the crack geometry (seemingly independent of externally applied stresses).
Second, instrumented pin-on-disk fretting corrosion tests were conducted to investigate the effect of mechanical (load, frequency) and electrochemical (potential) factors on fretting corrosion performance of 6 alloy couples made from 316L Stainless Steel (ASTM F138), Titanium alloy-Ti6Al4V ELI (ASTM F136) and Cobalt Chromium alloy-CoCrMo (ASTM F1537). A new theoretical model was also developed to predict voltage excursions typically seen during MAC processes. The results of this study showed that fretting corrosion is affected by alloy combination, normal load, potential and motion conditions at the fretting interfaces. In particular, Ti6Al4V_Ti6Al4V couples exhibited the highest fretting currents, COF and voltage excursions compared to other alloy couples. SS316L containing couples showed the highest susceptibility to fretting-initiated crevice corrosion attack compared to couples of CoCrMo and Ti6Al4V alloy. The developed model was capable of predicting the voltage excursions for all alloy couples tested.
Third, a new performance test method was developed to study the effect of mixing stainless steel, titanium and cobalt chromium alloy components (17 alloy combinations) on fretting corrosion response of spinal screw and rod implants (assembled using ASTM F1717). The hypothesis was assessed as to whether mixed-alloy combinations are different in terms of fretting corrosion behavior than similar-alloy combinations for the same design under similar test conditions. All Ti6Al4V constructs showed the best response (lowest fretting currents) while stainless steel containing constructs were more prone to fretting-initiated crevice corrosion attack and showed severe pitting in SEM analysis. There were no obvious galvanic-based acceleration of corrosion reactions in these tests.
Lastly, a new test protocol and a custom-built test set-up was developed for evaluation of modular hip tapers in terms of electrochemical (fretting corrosion) and mechanical (micromotions) response. A strong correlation between measured corrosion currents and micromotions were seen. This test method can be used to evaluate performance of modular implants for multiple factors including different taper geometries, taper lengths, materials, assembly load and other conditions to assess relative effects of fretting corrosion.
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MALI, SACHIN A., "MECHANICALLY ASSISTED CORROSION PERFORMANCE OF METALLIC BIOMATERIALS: IMPLANT RETRIEVAL, MATERIAL ANALYSES AND DEVICE TESTING" (2015). Dissertations - ALL. 224.