Crack-growth properties of various elastomers with potential application in small joint prostheses

Silastic small joint spacers for the metacarpophalangeal joint fail catastrophically at a reported rate ranging from 2 to 26%. Although the exact cause of this problem is not known, it is speculated that failure is due to the propagation of flaws generated in the material surface. In addition to wear secondary to bony impingement, these flaws can be introduced through manufacturing, surgical handling, and in vivo frictional wear. In an effort to identify an elastomeric material that will function similarly to Silastic as a self-hinging joint spacer but provide an increased functional life, we have investigated and compared the crack-growth properties of two polyurethanes, ChronoFlex and Medicaflex, and a thermoplastic elastomer, Santoprene, with those of Silastic. The materials were evaluated after sterilization by either ethylene oxide or gamma irradiation in an ASTM standard flexing machine under conditions of high humidity and body temperature both before and after artificially aging. In each case, the materials investigated presented significantly lower crack-growth rates than Silastic (p < 0.001).