The effect of stress state and surface condition on the corrosion behavior of surgical implant alloys
Description
The vast majority of studies concerning the corrosion of implant alloys have tested materials in the polished, unstressed condition. However, most implants are either statically or dynamically loaded. Also, a current trend in orthopaedics is towards the use of implants with grit blasted or porous-coated surfaces. This dissertation research was undertaken to study the effect of surface condition and stress state on the corrosion behavior of implant alloys Commonly used implant alloys: polished, grit blasted, and porous-coated Co-Cr-Mo and Ti-6Al-4V ELI and polished 316L stainless steel, were tested electrochemically using linear polarization, AC impedance, and potentiodynamic polarization techniques. These techniques allowed a variety of measures, of both scientific and clinical interest, to be used to evaluate corrosion behavior. The AC impedance technique was able to detect fractures of the sintered bonds of the porous layer. The alloys were tested under a variety of loading conditions: statically stressed into the plastic region, dynamically stressed to levels typical in pattern and amplitude to those occurring in total hip prostheses during level walking, and dynamically stressed to overload levels Both surface and stress state affected the electrochemical behavior. Compared to the zero stress controls, all non-zero stress states caused stress enhanced ion release (SEIR) with grit blasted Co-Cr-Mo, and all but the dynamic overload stress state caused SEIR with porous-coated Ti-6Al-4V ELI. Statically stressing all Co-Cr-Mo surfaces, and polished 316L also caused SEIR Dynamically stressing 316L to typical levels also caused SEIR, as did dynamic overload stress states with all Co-Cr-Mo surfaces, polished Ti-6Al-4V ELI, and 316L. Dynamic overload stress was more damaging to the passive layers of all Co-Cr-Mo surfaces, grit blasted Ti-6Al-4V ELI, and 316L than was static stress, even though the magnitude of the static loading was much higher. SEIR was found at levels below the yield stress with all of these materials Inferring the in vivo situation from unstressed, polished material can lead to gross underestimates of the amount of material released, by as much as a factor of 38