Abstract
Introduction
Vitamin-E (VE, dl-α-tocopherol) is a powerful antioxidant for highly cross-linked polyethylene (XLPE). It was previously reported that VE-stabilized XLPE succeeded in retaining no measurable oxidation even after accelerated aging tests combined with cyclic loading or lipid absorption. Thus, VE-stabilized XLPE is nowadays recognized worldwide as one of the new standard materials in total hip arthroplasty (THA). However, the effects of such VE addition on physical behavior of polyethylene remain to be fully elucidated by contrast to the clear statement of its chemical role (i.e., the enhanced oxidation resistance) in the published literature. In this presentation, we shall attempt to provide those missing notations and to explore the microstructural and biomechanical role of VE in XLPE acetabular liner on the molecular scale.
Methods
The two different types of XLPE acetabular liners, VE-blended and VE-free (no VE-blended) component (n=3 for each sample), were investigated by means of laser-scanning confocal polarized micro-Raman spectroscopy. In both components, the cross-linking was achieved by electron-beam irradiation with a total dose of 300kGy in vacuum. Raman spectroscopy offers non-destructive, contactless, and high-resolution analyses of polymer morphologies. In this study, we performed an in-depth profiling of crystalline and non-crystalline phase (i.e., amorphous and intermediate phase between crystalline and amorphous regions) percentages and degree of molecular orientation in the above two liners before and after introducing the 10% plastic deformation via uniaxial compression loading at room temperature. These results were also compared to the morphological analyses under the same compression conditions performed on the virgin conventional polyethylene (Virgin liner) without radiation crosslinking as well as VE blending.
Results
In the deformed state, Virgin and VE-blended liner showed a pronounced development of the surface crystalline texture. On the other hand, deformation-induced texturing occurred at much less extent in VE-free liner. According to the results of phase percentages, there was no crystallinity change in VE-blended liner by contrast to the marked increase of crystallinity in Virgin and VE-free liner after compression deformation. Alternatively, amorphous-to-intermediate phase transition was confirmed in VE-blended liner.
Discussion/Conclusion
We found molecular rearrangement and phase transitions in crystalline and non-crystalline phase as a reconstruction process after plastic deformation in the investigated samples, which can be deeply related to their wear and mechanical properties. The morphological comparisons between Virgin and VE-free liner suggested that the intermolecular cross-linked networks in polyethylene highly restricted the molecular chain mobility as evidenced by few texture evolutions in VE-free liner. On the other hand, the comparisons between VE-free/-blended liner indicated that the presence of VE might promote molecular chain mobility even in the cross-linked structure, resulting in the significant surface texturing. These physical and structural aspects of VE blending would imply the possibility of the increased micromechanical wear through the strain-softening and weakening phenomena due to the molecular reorientation during in-vivo service. However, in other words, wear resistance of VE-blended liner might be further maximized by the more rigid control of molecular movements.
