Study design: This study is an evaluation of wear and oxidation in retrieved total disc replacements (TDRs). Forty-eight CHARITE TDRs were retrieved from 41 patients after 7.8 years of average implantation. All implants were removed because of intractable back pain and/or facet degeneration. Three unimplanted implants served as controls.
Objective: Our aim was to determine whether gamma-sterilized polyethylene components implanted in the spine oxidize in vivo, and if so, whether polyethylene oxidation has clinical relevance for the long-term performance of TDRs.
Summary of background data: The natural history of polyethylene oxidation following gamma sterilization and long-term implantation in the spine has not yet been investigated.
Methods: Oxidation and oxidation potential were measured at the rim and dome of 47 components using Fourier transform infrared spectroscopy. The wear patterns of each retrieved polyethylene core were analyzed at the rim and dome.
Results: Oxidation was significantly higher at the rim, as compared with the dome of the cores. Hydroperoxide index was also significantly higher at the rim, as compared with the dome. Dome penetration rate was negatively correlated to implantation time (P < 0.0001) but not correlated to oxidation or hydroperoxide index (P > 0.05). Implants with evidence of chronic rim loading had higher rim oxidation.
Conclusion: The data support our hypothesis that, for the historical packaging methods employed by the manufacturer, polyethylene oxidation and oxidation potential were significantly higher at the rim as opposed to the dome. The mechanism is governed by access to oxygen in vivo and may be accelerated under certain combined modes of repeated rim loading. Our findings have clinical significance in cases of chronic impingement, when the rim has to support repeated loading for the lifetime of the implant.