In vivo degradation of polyethylene liners after gamma sterilization in air.
BACKGROUND
Ultra-high molecular weight polyethylene degrades during storage in air following gamma sterilization, but the extent of in vivo degradation remains unclear. The purpose of this study was to quantify the extent to which the mechanical properties and oxidation of conventional polyethylene acetabular liners treated with gamma sterilization in air change in vivo.


METHODS
Fourteen modular cementless acetabular liners were revised at an average of 10.3 years (range, 5.9 to 13.5 years) after implantation. All liners, which had been machined from GUR 415 resin, had been gamma-sterilized in air; the average shelf life was 0.3 year (range, 0.0 to 0.8 year). After removal, the components were expeditiously frozen to minimize ex vivo changes to the polyethylene prior to characterization. The average duration between freezing and testing was 0.6 year. Mechanical properties and oxidation were measured with use of the small-punch test and Fourier transform infrared spectroscopy, respectively, in the loaded and unloaded regions of the liners.


RESULTS
There was substantial regional variation in the mechanical properties and oxidation of the retrieved liners. The ultimate load was observed to vary by >90% near the surface. On the average, the rim and the unloaded bearing showed evidence of severe oxidation near the surface after long-term in vivo aging, but these trends were not typically observed on the loaded bearing surface or near the backside of the liners.


CONCLUSIONS
The mechanical properties of polyethylene that has been gamma-sterilized in air may decrease substantially in vivo, depending on the location in the liner. The most severe oxidation was observed at the rim, suggesting that the femoral head inhibits access of oxygen-containing body fluids to the bearing surface. This is perhaps why in vivo oxidation has not been associated with clinical performance to date.
Kurtz
Steven M
SM
Implant Research Center, School of Biomedical Engineering, Science, and Health Systems and Department of Materials Engineering, Drexel University, 3141 Chestnut Street, Philadelphia, PA 19104, USA. skurtz@drexel.edu
Rimnac
Clare M
CM
Hozack
William J
WJ
Turner
Joseph
J
Marcolongo
Michele
M
Goldberg
Victor M
VM
Kraay
Matthew J
MJ
Edidin
Avram A
AA
D000077:Acetabulum; D019644:Arthroplasty, Replacement, Hip / Q000009:adverse effects; D001672:Biocompatible Materials / Q000009:adverse effects*; D001696:Biomechanical Phenomena; D019544:Equipment Failure Analysis; D005720:Gamma Rays / Q000009:adverse effects; D006622:Hip Prosthesis / Q000009:adverse effects*; D006801:Humans; D010084:Oxidation-Reduction; D011095:Polyethylenes / Q000009:adverse effects*; D011475:Prosthesis Failure; D012086:Reoperation; D013242:Sterilization / Q000379:methods*
D001672:Biocompatible Materials; D011095:Polyethylenes; C111601:ultra-high molecular weight polyethylene