M. Roach, S. Williamson, L. Zardiackas, University of Mississippi Medical Center, Jackson, MS
Austenitic stainless steels have been widely used since the early 1920’s for dental and orthopedic implant applications primarily because of their high strengths, good corrosion resistance, cost effectiveness, and material availability. In vivo, implants are subjected to discontinuous cyclic loading often over a period of many years and nearly all failures are due to fatigue. This study examined the smooth and notched corrosion fatigue characteristics of a series of commercially available cold worked austenitic stainless steel implant alloys including BioDur™ 108 low nickel steel, 316L, and 22-13-5 in a saline environment at physiological temperatures. Corrosion fatigue testing was performed in tension-tension on smooth samples of each alloy in Ringer’s solution at 37°C. Fatigue testing followed the guidelines of ASTM F1801 at 1Hz using the given R value of 0.053. After the completion of fatigue testing, analysis of the fracture surfaces were examined using stereo imaging techniques in SEM. BioDur™ 108 revealed a substantially lower notched fatigue run-out compared to the smooth fatigue response. Fracture analysis showed a quasi-cleavage fracture mechanism consisting of cleavage facets, secondary cracking, and typical striated ductile fatigue. 316L was the least notch sensitive, but also showed the lowest smooth sample fatigue strength. Fracture analysis on 316L revealed typical ductile fatigue fracture with substantial secondary cracking. The 22-13-5 steel exhibited the most notch sensitivity becoming increasingly more sensitive at higher cycle counts. Fracture analysis on 22-13-5 showed very finely striated ductile fatigue fracture with secondary cracking. In conclusion, each of the three stainless steel alloys tested exhibited a suppressed S/N curve for the notch samples compared to the one for the smooth samples. These results suggest BioDur™ 108 may be especially beneficial for high strength applications where the surgeon may have to bend the implant material or where it may be scratched during implantation.
Summary: This study examined the smooth and notched corrosion fatigue characteristics of a series of commercially available cold worked austenitic stainless steel implant alloys including BioDur™ 108 low nickel steel, 316L, and 22-13-5 in a saline environment at physiological temperatures. Corrosion fatigue testing was performed in tension-tension on smooth samples of each alloy in Ringer’s solution at 37°C. Each of the three stainless steel alloys tested exhibited a suppressed S/N curve for the notch samples compared to the one for the smooth samples.
