In vivo oxidation in remelted highly cross-linked retrievals.
BACKGROUND
Elimination of free radicals to prevent oxidation has played a major role in the development and product differentiation of the latest generation of highly cross-linked ultra-high molecular weight polyethylene bearing materials. In the current study, we (1) examined oxidation in a series of retrieved remelted highly cross-linked ultra-high molecular weight polyethylene bearings from a number of device manufacturers and (2) compared the retrieval results with findings for shelf-stored control specimens. The hypothesis was that radiation-cross-linked remelted ultra-high molecular weight polyethylene would maintain oxidative stability in vivo comparable with the stability during shelf storage and in published laboratory aging tests.


METHODS
Fifty remelted highly cross-linked ultra-high molecular weight polyethylene acetabular liners and nineteen remelted highly cross-linked ultra-high molecular weight polyethylene tibial inserts were received after retrieval from twenty-one surgeons from across the U.S. Thirty-two of the retrievals had been in vivo for two years or more. Each was measured for oxidation with use of Fourier transform infrared spectroscopy. A control series of remelted highly cross-linked ultra-high molecular weight polyethylene acetabular liners from three manufacturers was analyzed with electron paramagnetic resonance spectroscopy to measure free radical content and with Fourier transform infrared spectroscopy to measure oxidation initially and after eight to nine years of shelf storage in air.


RESULTS
The never-implanted, shelf-aged controls had no measurable free-radical content initially or after eight to nine years of shelf storage. The never-implanted controls showed no increase in oxidation during shelf storage. Oxidation measurements showed measurable oxidation in 22% of the retrieved remelted highly cross-linked liners and inserts after an average of two years in vivo.


CONCLUSIONS
Because never-implanted remelted highly cross-linked ultra-high molecular weight polyethylene materials had no measurable free-radical concentration and no increase in oxidation during shelf storage, these materials were expected to be oxidation-resistant in vivo. However, some remelted highly cross-linked ultra-high molecular weight polyethylene retrievals showed measurable oxidation after an average of more than two years in vivo. This apparent departure from widely expected behavior requires continued study of the process of in vivo oxidation of ultra-high molecular weight polyethylene materials.
Thayer School of Engineering, Dartmouth College, 8000 Cummings Hall, Hanover, NH 03755, USA. barbara.h.currier@dartmouth.edu
B H
BH
Currier
D W
DW
Van Citters
J H
JH
Currier
J P
JP
Collier
D019544:Equipment Failure Analysis; D006622:Hip Prosthesis / Q000592:standards; D009682:Magnetic Resonance Spectroscopy; D008422:Materials Testing; D010084:Oxidation-Reduction; D011095:Polyethylenes; D017550:Spectroscopy, Fourier Transform Infrared; D013242:Sterilization
D011095:Polyethylenes; C111601:ultra-high molecular weight polyethylene