Sterilization and strength of 70/30 polylactide cages: e-beam versus ethylene oxide.

STUDY DESIGN

In vitro and in vivo studies on the degradation of 70/30 poly(L,DL-lactide) (PLDLLA) cages.

OBJECTIVE

To evaluate the effect of e-beam and ethylene oxide sterilization on degradation and strength.

SUMMARY OF BACKGROUND DATA

e-beam-sterilized PLDLLA cages were shown to maintain mechanical strength for at least 6 months during degradation studies in vitro. Yet failure of the cages was observed after only 3 months in vivo. We hypothesized that degradation characteristics and mechanical strength could be improved by sterilizing the cages through ethylene oxide (EtO) instead of e-beam.

METHODS

PLDLLA cages were sterilized either by e-beam or EtO, and degraded in phosphate-buffered saline. Each month, cages were compressed until failure. Inherent viscosity was determined as a measure of degradation. For the in vivo evaluation, e-beam- or EtO-sterilized cages were implanted at L3-L4 in a standardized goat model. After 3 or 6 months, retrieved segments were scanned by high-resolution magnetic resonance imaging. Also, inherent viscosity of the polymer was measured.

RESULTS

e-beam sterilization strongly decreased inherent viscosity of PLDLLA compared with EtO sterilization, but initial strength was only affected marginally. After 6 months, the strength of the e-beam-sterilized cages dropped, while that of EtO-sterilized cages was maintained. Degradation in vivo was slightly faster than in vitro. In both groups, however, mechanical failure occurred at 3 months after implantation.

CONCLUSIONS

Inherent viscosity decreases with degradation time, but strength only decreases when inherent viscosity is below a certain threshold. Above this threshold, mechanical strength is a property of the polymer and independent of inherent viscosity. e-beam sterilization strongly decreases inherent viscosity and thus advances mechanical degradation. EtO sterilization delays degradation but does not increase initial strength. Early failure of PLDLLA cages in the goat model thus is unrelated to sterilization method and requires further study.