Current sterilization and packaging methods for polyethylene

Gamma sterilization in an air environment can induce oxidation in polyethylene. Oxidation can lead to polyethylene embrittlement, compromising mechanical integrity and clinical performance of polyethylene bearings. For these reasons, orthopaedic manufacturers have modified their methods of sterilizing and packaging polyethylene. Two alternative approaches have emerged: sterilization by non-radiation methods and sterilization by gamma irradiation in inert environments. The current study presents a prognosis for clinical performance of polyethylene sterilized with new methods, based on material property analyses (oxidation levels, mechanical properties, crosslink density) of never implanted and retrieved bearings. Data from bearings that were never implanted which were sterilized with the new methods and shelf aged as many as 3 years, show negligible oxidation, ductility above 400%, and ultimate tensile strength near 50 MPa, all exceeding specifications of the American Society for Testing and Materials. There are significant differences in crosslink density (swell ratio) depending on the sterilization method. Retrievals indicate that bearings sterilized with these new methods are performing well clinically and that the majority are not changing with time. The current study suggests that the shelf oxidation problem has been addressed by these new sterilization techniques and that clinical performance at short followup is acceptable. However, long-term clinical performance must be evaluated in the future.