Corrosion testing of modular hip implants

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 saline 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.