Time-dependent failure in load-bearing polymers : a potential hazard in structural applications of polylactides

Polylactides are commonly praised for their excellent mechanical properties (e.g. a high modulus and yield strength). In combination with their bioresorbability and biocompatibility, they are considered prime candidates for application in load-bearing biomedical implants. Unfoliunately, however, their long-term performance under static load is far from impressive. In a previous in vivo study on degradable polylactide spinal cages in a goat model it was observed that, although short-term mechanical and real-time degradation experiments predicted otherwise, the implants failed prematurely under the specified loads. In this chapter we demonstrate that this premature failure is attributed to the time-dependent character of the material used . The phenomenon is common to all polymers, and finds its origin in stressactivated segmental molecular mobility leading to a steady rate of plastic flow. The main conclusion is that knowledge of the instantaneous strength of a polymeric material is insufficient to predict its long-term performance.