Long-term effects of gamma-sterilization on degradation of implant materials

Immunogenecity of modified alkane polymers is mediated through TLR1/2 activation

BACKGROUND

With the advancement of biomedical technology, artificial materials have been developed to replace diseased, damaged or nonfunctional body parts. Among such materials, ultra high molecular weight alkane or modified alkyl polymers have been extensively used in heart valves, stents, pacemakers, ear implants, as well as total joint replacement devices. Although much research has been undertaken to design the most non-reactive biologically inert polyethylene derivatives, strong inflammatory responses followed by rejection and failure of the implant have been noted.

METHODOLOGY/PRINCIPAL FINDINGS

Purification of the alkane polymers from the site of inflammation revealed extensive "in vivo" oxidation as detected by fourier transformed infra-red spectroscopy. Herein, we report the novel observation that oxidized alkane polymers induced activation of TLR1/2 pathway as determined by ligand dependent changes in intrinsic tyrosine fluorescence intensity and NF-kappaB luciferase gene assays. Oxidized polymers were very effective in activating dendritic cells and inducing secretion of pro-inflammatory cytokines. Molecular docking of the oxidized alkanes designated ligand specificity and polymeric conformations fitting into the TLR1/2 binding grooves.

CONCLUSION/SIGNIFICANCE

This is the first report of a synthetic polymer activating immune responses through TLR binding.

Electron paramagnetic resonance analyses of surface radical chemistries of γ-sterilized orthopedic materials: Implications pointing to cytotoxicity via wear debris-induced inflammation

In this research, electron paramagnetic resonance (EPR) spin-trapping was utilized to determine if surface radical chemistries occur for gamma (gamma)-sterilized orthopedic materials-ultra-high molecular weight polyethylene (UHMWPE) and the novel, hybrid, diurethane dimethacrylate (DUDMA)-based RHAKOSS. The materials' ability to competitively chelate catalytic ferrous ions (Fe(2+)) or readily reduce ferric ions (Fe(3+)), and hydrogen peroxide (H(2)O(2)) directly, in facilitating the Fenton reaction (FR), is indicative of cytotoxicity. Validations with a radical scavenger aids to confirm a radical mechanism. In conjunction, materials were thermally annealed and characterized by attenuated total reflectance-Fourier-transform infrared (ATR-FTIR) spectroscopy in order to explore accelerated oxidative degradation induced by residual radicals evolving from gamma-sterilization. Particularly, there was a significant decrease in spin-adduct peak areas obtained from the reduction of H(2)O(2) in the presence of RHAKOSS or UHMWPE, evaluated against their respective controls. Additionally, chelated Fe(2+) accelerated the rate of FR. This phenomenon suggests that the materials are not better chelators than the Fe-activating chelator, edta. Neither material had the propensity to readily reduce Fe(3+) to the relevant Fe(2+), as certified by a nonradical mechanism. Alternatively, the false spin-adduct signal acquired when chelated Fe(3+) is employed arises via the nucleophilic addition of water onto the DMPO spin trap. Residual radicals in UHMWPE did not recombine/terminate following thermal annealing in an inert atmosphere. The radicals in RHAKOSS, however, did recombine under mild heating in an oxidizing or inert atmosphere. Both materials displayed quenching of ( )OH; however, for UHMWPE, this mechanism was jointly accountable for its accelerated degradation, evidenced by ATR-FTIR. Quenching of ( )OH by the silica found in RHAKOSS manifested in a competing effect that counterbalanced the observed FR. Implanted RHAKOSS is not likely to promote cytotoxicity and should not degrade, but the damaging effect of gamma sterilization on UHMWPE is a serious dilemma confronting its long-term durability and biocompatibility.

The impact of sterilization method on wear in knee arthroplasty

Sterilization by gamma irradiation in air has been shown to have the potential to accelerate the oxidation of polyethylene components resulting in reduced mechanical properties. In the hip, it has been reported that the occurrence of delamination and cracking in retrieved bearings is significantly different when comparing components sterilized with gamma irradiation in air with components sterilized with ethylene oxide. Using a collection of 1635 retrieved polyethylene knee bearings, this study pursues a similar comparison of sterilization method with clinical wear in the knee. It confirms that retrieved polyethylene knee components that were gamma irradiated in air have a high incidence of delamination and cracking, leading at times to complete wear through of the bearing. Knee components sterilized with ethylene oxide showed no evidence of fatigue damage even after in vivo durations in excess of 15 years.