Date of Award
Doctor of Philosophy (PhD)
Jeremy L Gilbert
Modular metal-on-metal acetabular tapers comprise two overarching design categories: a titanium shell and a metal/ceramic/polymer liner pair, or a dual-mobility articulation design with an added polymeric insert containing a freely rotating femoral head. Both designs often include a metal-on-metal (MoM) or modular dual-mobility (MDM) shell-liner interfaces, which is prone to corrosion and associated local effects. The goals of this dissertation were to document and distinguish between corrosion modes at this modular junction, to design in vitro test methods to characterize mechanical modes, and to examine their interactions with macrophages, which represent a critical part of the immune response. Through retrieval analysis, we established that MoM acetabular tapers are less susceptible to mechanically-assisted corrosion (MAC). Retrieved CoCrMo liners revealed a prevalence of chemically driven corrosion damage modes (intergranular corrosion (IGC), pitting, phase boundary corrosion (PBC)) over mechanically-driven ones (fretting corrosion, large-scale wear, stress cracking). Importantly, corrosion was detected outside engagement regions, where no physical interfacial contact can occur, or tight crevice exists. Cellular remnants were also identified within these tight crevice-like taper surfaces, which fit a narrow size distribution similar to macrophages. In vitro testing revealed that contact areas between shell-liner pairs remained a fraction of the total taper surface area, which translated to low currents (under 1µA) generated over a short-term incremental cyclic loading test. An electrochemical impedance spectroscopy (EIS)-based method was used to examine resulting impedance changes in retrieved CoCrMo liner surfaces associated with specific surface features, associated with IGC, oxide deposits and PBC. Indeed, surface impedance was correlated to the type of surface damage. Impedance analysis proved to be a valuable tool for identifying corrosion modes and adds a quantitative method to the retrieval analysis paradigm. Finally, we developed assays to examine macrophage viability outcomes in response to different corrosion and fretting corrosion stimuli. Ongoing fretting corrosion, and the associated debris generation and negative potential shifts seemed to be the most influential factors on macrophage mortality. In a separate study, where CoCrMo alloy was corroded at high anodic potentials to generate ions and debris, we found macrophage mortality was affected in a dose-dependent manner, primarily by chromium ions in culture media, whereas solid cobalt oxide debris particles killed macrophages in a proximity-dependent manner. A sublethal dose (LD50) of metal ions was found to induce a weak proinflammatory response, measured by cytokine (TNF-α, interleukin (IL)-1β, IFN-γ and IL-27) and chemokine expression, at 24 h. We also detected comparable levels of IL-13, which is a known mediator of the anti-inflammatory M2 phenotype in macrophages. With the results described in this study, we can conclude that chemically driven corrosion modes dominate acetabular taper corrosion over mechanical modes. Corrosion byproducts might also initiate adverse local reactions by inducing cell death and activation of macrophages. Thus, MoM acetabular junctions represent a significant factor in total hip arthroplasty outcomes. Since MDM device usage is on the rise owing to its excellent outcomes in high dislocation risk patients, understanding possible corrosion and associated phagocytic responses proves to be crucial in ensuring satisfactory patient outcomes and to make prudent decisions during surgical planning in terms of device and material choices.
Shenoy, Aarti, "Understanding Corrosion in Modular Acetabular Tapers: Retrieval Analysis, In Vitro Testing and Cell-Material Interactions" (2020). All Dissertations. 2725.