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The in vivo fretting behavior of modular hip prostheses was simulated to determine the effects of material combination and a unique TiN/AlN coating on fretting and corrosion at the taper interface. Fretting current, open-circuit potential (OCP), and quantities of soluble debris were measured to determine the role of mechanically assisted crevice corrosion on fretting and corrosion of modular hip tapers. Test groups consisting of similar-alloy (Co-Cr-Mo head/Co-Cr-Mo neck), mixed-alloy (Co-Cr-Mo head/Ti-6Al-4V neck), and TiN/AlN-coated mixed-alloy modular hip taper couples were used. Loads required to initiate fretting were similar for all test groups and were well below loads produced by walking and other physical activities. Decreases in OCP and increases in fretting current observed during long-term cyclic loading were indicative of fretting and corrosion. Current measured after cessation of cyclic loading suggests that once the conditions for crevice corrosion are established, corrosion can continue in the absence of loading. The chemical, mechanical, and electrochemical measurements, along with microscopic inspections of the taper surfaces indicate that the fretting and corrosion behavior of similar- and mixed-alloy taper couples are similar and that the coated samples are more resistant to fretting and corrosion. The results of this study clearly indicate the role of mechanical loading in the corrosion process, and support the hypothesis of mechanically assisted crevice corrosion.

A new test method was used to rapidly produce a controlled, repeatable scratch on the surface of CoCrMo (ASTM F75) samples, resulting in fracture of the surface oxide. Current transients resulting from ionic dissolution and repassivation of the exposed reactive alloy were measured with the samples potentiostatically held in phosphate-buffered saline. The effects of potential, contact load, pH, aeration, and proteins on the magnitude of the current transients and time constants for repassivation were determined. Using the scratch test apparatus, topographic images of scratched surfaces were constructed and used to measure scratch depth. Aeration had no significant effect on peak currents and time constants owing to the availability of oxygen from the hydrolysis of water. Peak current behavior reflected the transition regions observed in polarization curves. A decrease in peak currents in the presence of albumin may have been due to barrier effects of the adsorbed protein preventing water from reaching the sample surface, or lubrication resulting in less material removed from the surface during scratching. Peak currents and scratch depth increased with load. A model used to predict repassivation behavior was in agreement with experimental results.

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Flexibility in sizing, lower inventory requirements, and the ability to choose materials with optimum physical properties for each component have made modular hip implants the design of choice for most orthopedic surgeons. However, the presence of surface asperities and angular mismatch between the head and neck components can result in the formation of a crevice between the taper surfaces of the assembled couple. This crevice may be large enough to allow fluid ingress and fretting to occur which can lead to crevice and fretting corrosion. This paper describes several test methods used to duplicate the fretting behavior of modular taper components and determine the electrochemical changes that occur as the result of fretting corrosion. Mixed and similar metal modular taper junctions were tested in phosphate buffered sahne solution with and without the application of a cyclic load. Fretting currents, open circuit potential (OCP), and pH, pO2, and [Cl-] of solution trapped inside the spaces between the head and neck components were measured. Metal ion concentrations in fluid trapped inside similar metal taper junctions were measured. The effects of load magnitude on OCP and fretting currents were determined. Test components and trapped solution were inspected and analyzed for signs of corrosion. Results of testing support a hypothesis of mechanically assisted crevice corrosion and are presented.