Corrosion of modular tapers in total joint replacements: A critical assessment of design, materials, surface structure, mechanics, electrochemistry, and biology

Modular taper connections are conical junctions between components in orthopaedic implants that provide significant benefits to the patient, surgeon, and hospital. These tapers are used in hips, knees, shoulders, and other locations in the body to connect elements or components of metallic total joint replacements. They typically have at least one of the contacting surfaces composed of metal alloy, which is susceptible to mechanically assisted crevice corrosion (MACC), or fretting crevice corrosion resulting from the loading and exposure to the body environment. This paper describes the elements of design, materials, surface structure, mechanics, biology, and electrochemistry involved in the complex processes associated with MACC. Taper mechanics related to flexural rigidity, surface asperity contact mechanics, assembly mechanics and cyclic loading motions, and their relationship to corrosion are described. Similarly, the electrochemical processes associated with oxide film disruption and repassivation are discussed, and a fretting corrosion equation developed by our group is used to explore titanium (Ti) and cobalt-chromium-molybdenum (CoCrMo) surface behavior. This includes the voltage, loading, and sliding speed dependence of the fretting currents. Finally, the effect of the biological system on corrosion of Ti and CoCrMo is explored to better understand the role of inflammation in altering the reactive oxygen species in the solution chemistry and how this increases the corrosion susceptibility of CoCrMo and Ti alloys.