Fatigue and life prediction for cobalt-chromium stents: A fracture mechanics analysis

To design against premature mechanical failure, most implant devices such as coronary and endovascular stents are assessed on the basis of survival, i.e., if a fatigue life of 108 cycles is required, testing is performed to ascertain whether the device will survive 108 cycles under accelerated in vitro loading conditions. This is a far from satisfactory approach as the safety factors, which essentially tell you how close you are to failure, remain unknown; rather, the probability of fatigue failure should instead be assessed on the basis of testing to failure. In this work, a new damage-tolerant analysis of a cardiovascular stent is presented, where the design life is conservatively evaluated using a fracture mechanics methodology. In addition to enabling estimates of safe in vivo lifetimes to be made, this approach serves to quantify the effect of flaws in terms of their potential effect on device failure, and as such provides a rational basis for quality control.