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Wear and corrosion damage of biomedical alloys alters the structure and electrochemical properties of the surface heterogeneously. It was hypothesized that local regions on the same surface systematically differ from one another in terms of their impedance characteristics. To test this hypothesis, CoCrMo disks exposed to electrosurgical and inflammatory-species-driven damage were characterized using a localized impedance technique, nearfield electrochemical impedance spectroscopy (NEIS), to assess point-specific surface integrity in response to applied damage. It was found that electrosurgical damage, as may arise during primary arthroplasty and revision surgeries, and hydrogen peroxide concentrations of 5-10 mM significantly alter the corrosion susceptibility of the local surface compared to the as-polished CoCrMo surface. A CoCrMo retrieved neck taper (Goldberg score of 4) was scored in different local regions on the basis of visual appearance, and it was found that there is a direct relationship between increasing debris coverage and decreasing impedance, with the global surface impedance closest to the most severely scored local region. This noninvasive method, which uses a millielectrode configuration to test localized regions, can measure the heterogeneous electrochemical impedance of an implant surface and be tailored to assess specific damage and corrosion mechanisms revealed on retrieval surfaces.

Retrieval studies show that metal-on-metal acetabular shell-liner tapers are susceptible to corrosion, which is hypothesized to arise from mechanically-assisted crevice corrosion (MACC). The role of materials on MACC of acetabular tapers has not been previously studied. In vitro tests of seating, pushout, and fretting corrosion performance of acetabular tapers are presented to assess the role of material combinations (Ti-6Al-4V shells, HC CoCrMo, LC CoCrMo, and 316L SS liners). The acetabular tapers were wet-assembled to a seating load of 1,000 N. The liner load-displacement seating mechanics were measured. Fretting corrosion currents were evaluated using a uniaxial incremental cyclic compression test up to 4,000 N, with the load applied at a 55° angle to the taper interface. Fretting currents, fretting onset loads, taper disengagement strength were measured and load-displacement plots were obtained. Pushout tests were also performed pre- and post-fretting corrosion. The average liner seating displacements varied from 134 to 226 μm across groups. Fretting currents at 3,600 N cyclic load were low and ranged between 0.05 and 0.27 μA and were independent of material combination (p > 0.05), reflecting small amounts of fretting. Fretting corrosion onset loads were between 1,800 and 2,100 N, and did not differ across groups (p > 0.05). Pushout loads were 27-43% of the maximum load applied. Fretting corrosion levels were very low for all material combinations and not different from one another. The seating and pushout responses were also not material dependent. The low fretting currents measured imply that MACC may not be a major cause for acetabular taper corrosion. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 38:00-111089, 2020.

The mechanically-assisted crevice corrosion performance of head-neck modular tapers is a significant concern in orthopedic biomaterials. Fretting crevice corrosion processes in modular tapers are thought to be influenced by a wide array of factors including seating mechanics of the junction, hence there is a need for in vitro test methods that can assess their performance. This study presented a test method to directly measure the load-displacement seating mechanics of modular tapers and used this method to compare the seating mechanics for different tapers, moisture, seating loads and seating rates. Seating mechanics were explored whereby the instantaneous load-displacement behavior of the head seating onto the neck is captured and used to define the mechanics of seating. Two distinct taper design/material combinations were assembled wet or dry using axially applied loads (500, 1000, 2000 and 4000 N) at two loading rates of 100 and 104 N/s (n = 5 for each condition) using a servohydraulic test frame. The results showed that pull-off strength scaled with seating load and ranged between 43-68% of seating load depending on sample and wetness. Tapers seated wet had higher pull-off strengths (2200 N +/− 300 N) than those seated dry (1800 N +/− 200 N, P