Creep Analysis of PLLA

Bioabsorbable fixation device failures occur clinically on occasion, with failures often brittle in nature. However, creep failure may also occur for implants that are subjected to sustained loads whereby the device may slowly deform over time, perhaps leading to fracture. Even without fracture occurring, the device may become too distorted to function. There is little in the literature regarding creep performance of bioabsorbable devices such as plates and screws. This study investigated the creep characteristics of craniofacial plates and screws made of a copolymer of 82% poly-L-lactic acid and 18% polyglycolic acid. Four-hole straight plates were attached to 2 rectangular portions of synthetic bone substrate using 2.0-mm-diameter bioabsorbable screws (2 screws used to attach the plate to each substrate portion). The constructs were submersed in phosphate-buffered saline (pH 7.4, 37°C) and placed in tension with 3 load configurations, that is, 230, 460, and 1140 g, for at least 6 days. Creep rate was constant at a given load and was directly proportional to load (4.7, 14.3, and 33.3 μm/h for 230-, 460-, and 1140-g loads, respectively). The data conformed well to basic creep theory analysis and provided an estimate of the absolute viscosity of the polymer of 8.7 × 10(12) ± 0.81 × 10(12) P (average ± SEM), which is intermediate between that of soft and hard tissue, although closer to the latter. Surgeons and engineers should be aware of the potential for creep to occur when designing bioabsorbable implants and investigating new clinical applications.