Spectroscopic and chromatographic quantification of an antioxidant-stabilized ultrahigh-molecular-weight polyethylene

The oxidative stability of various antioxidant-containing ultrahigh-molecular-weight polyethylene (UHMWPE) formulations has been widely reported. Depending on which specific antioxidant is used, the process by which it is incorporated into UHMWPE, and the amount of the antioxidant incorporated, there could be substantial differences in the material and toxicological properties of the UHMWPE formulation. Pentaerythritol tetrakis (3-[3,5-di tertiary butyl-4-hydroxyphenyl] propionate) (PBHP) has been extensively used as an efficient antioxidant in various applications. However, it has not thus far been used to stabilize UHMWPE in orthopaedic implants. It is therefore important to characterize and verify the concentration and homogeneity of distribution of PBHP in the composition, the chemical consequence of exposure of the antioxidant to gamma irradiation, and to assess the toxicological risk of use by the identification and quantification of leachables before the use of PBHP-containing UHMWPE in implantable devices.

(1) Can the concentration and uniformity of distribution of the antioxidant PBHP in UHMWPE powder and in the consolidated, preirradiated formulation be verified? (2) Can the leachable compounds in the gamma radiation crosslinked PBHP/UHMWPE formulation be identified and quantified?

PBHP in GUR 1020 UHMWPE was quantified by Fourier transform infrared (FTIR) and ultraviolet-visible (UV-Vis) spectroscopy. The chemical byproducts generated by gamma irradiation of PBHP were identified using gas chromatography in conjunction with mass spectrometry followed by a second-stage mass spectrometry (GC-MS/MS). When GC-MS/MS was coupled with Stir Bar Sorptive extraction, leachable components in the UHMWPE formulation were identified and quantified.

The percent concentration of PBHP in UHMWPE powder was confirmed by UV-Vis spectroscopy and the concentration and uniform distribution of PBHP in UHMWPE after consolidation and before radiation crosslinking was verified through FTIR spectroscopy. GC-MS/MS analysis enabled the identification and quantification of 16 gamma irradiation byproducts of PBHP. These 16 compounds were verified as potentially leachable compounds in PBHP-stabilized UHMWPE and were found to be well below the safety threshold concern of 150 ng/device in orthopaedic knee inserts made from PBHP-stabilized UHMWPE.

Spectroscopic analysis has been successfully used to demonstrate the ability to reliably quantify the amount as well as the distribution of PBHP in UHMWPE in orthopaedic bearings. State-of-the-art chemical extraction and analytical techniques have enabled the identification of the gamma radiation-induced byproducts of PBHP and the quantification of these components as leachables from the PBHP-stabilized UHMWPE formulation.

Antioxidant-stabilized UHMWPE materials being considered for orthopaedic bearings must be fully characterized for composition before use because it is apparent that exposure to high doses of gamma radiation would cause the formation of new chemical entities. It is important to verify the identities and quantities of chemical species that could leach out of implanted devices in the long term to enable their toxicological risk assessment.