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.

An analysis of hylamer and polyethylene bearings from retrieved acetabular components.

Hylamer and conventional polyethylene acetabular liners of the same design, revised for a variety of reasons, were examined and compared to assess the performance of Hylamer as a bearing material. Clinical damage modes, linear wear rates, oxidation levels, and mechanical properties were measured. In both series, many liners were retrieved for dislocation. Wear/osteolysis was the most common reason for retrieval in the Hylamer series, while none of the conventional polyethylene liners were retrieved for this reason. Nearly all liners exhibited abrasion, burnishing, scratching, and creep. The Hylamer liners had more cracking, delamination, and pitting. The Hylamer liners had an average linear wear rate of 0.32 mm/year, while the conventional polyethylene liners had an average wear rate of 0.20 mm/year. Due to sample size, no statistical difference in wear rate was noted between the two groups. In general, both the Hylamer and conventional polyethylene showed oxidation peaks subsurface, resulting from their exposure to gamma radiation in air. Liners with elevated oxidation had decreased ultimate tensile strength, elongation, and toughness. For given oxidation levels, the corresponding mechanical properties of Hylamer appeared lower than those of conventional polyethylene. The ultimate tensile strength values ranged from 14 to 33 MPa for Hylamer and 19 to 32 MPa for conventional polyethylene. Elongation ranges were 19% to 350% (Hylamer) and 80% to 375% (conventional). The Hylamer retrievals in this study gave initial indications of performance; Hylamer appeared to behave similarly, but not superiorly, to conventional polyethylene, in the early functional period with respect to clinical wear and clinical performance. Both Hylamer and conventional polyethylene liners were degraded by gamma sterilization in air, with Hylamer liners demonstrating greater property changes.