5.
BThe Effect of Pressure-Annealing on the Oxidation Resistance of Irradiated UHMWPE
F. D’Angelo, F. Conteduca, T. S. Thornhill and A. Bellare
Introduction
UHMWPE wear debris and related osteolysis are of a great concern for long- lasting joint replacements [1,2].
Radiation crosslinked ultra-high molecular weight polyethylene have a very high resistance to wear [3,4].
In this study, we hypothesized that annealing below the melting temperature at high pressure would be effective in decreasing free radical concentrations, thereby reducing long term oxidation.
Material and Method
UHMWPE (PE): Starting material was a GUR 1050 (Hoechst-Ticona, Bayport, TX) ram-extruded rod stock (PolyHi Solidur, Ft. Wayne, IN), crystallinity 53.8%.
Crosslinked (XPE): The rod stock was subjected to a dose of 50 kGy gamma- irradiation (Isomedix, Northborough, MA).
3 groups of pins of 25mm length and 9mm diameter, stored in water to minimize further exposure to ambient air. A Carver hydraulic press was used to apply the pressure.
• 100, 200, 300, 400 and 500 MPa to irradiated PE specimens at room temperature for 20min followed by pressure release.
• 100, 300 and 500MPa, to irradiated PE specimens at 130ºC for 20min, followed by slow cooled to room temperature and pressure release.
• PE specimens were heated to 130ºC, pressurized to 500MPa, then further heated to 160ºC, 180ºC, 200ºC and 220ºC for 20min, slow cooled to room temperature followed by pressure release.
Oxidation: A Parr bomb reactor filled with oxygen gas at 5atm pressure and 70ºC temperature for 2 weeks (ASTM standard F2003-02).
Sledge Microtome: Thin sections of 100-200mm thickness of all PE samples were prepared using a Leitz Wetzlar (Leica, Nussloch, Germany).
Oxidation Index Fourier Transform Infrared (FTIR) Spectroscopy was performed using a Nicolet Magna 860 spectrometer. The oxidation index, OI, was defined to be the ratio of the area under 1740cm-1 carbonyl and 1370 cm-1 methylene stretching absorbances.
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Results
• FTIR experiments showed the bomb aged 50 kGy PE had a maximum oxidation index (OI) of 1.68 ± 0.13 compared to 0.29 ± 0.09 for the air aged PE (Fig. 1.)
• Group (a): There was no general trend in the maximum oxidation index (OI).
Application of high pressure does not affect resistance to oxidation.
• Group (b): There was a clear increase in the OI. High pressure annealing increases oxidation with increase in pressure at 130ºC.
• Group (c): The maximum OI decreased substantially but there was no trend in the OI as a function of annealing temperatures.
Discussion
• These results can be explained in terms of the phase diagram of PE (Fig. 3), which shows that the melting temperature increases at approximately 20ºC/100MPa of applied pressure from an initial value of 133ºC at atmospheric pressure [5].
• In protocol (a), there was no thermal energy at high pressures to decrease free radical concentrations.
• In protocol (b) the annealing temperature of 130ºC was much lower than Tm at 500MPa, consequently the OI increased with increase in pressure at isothermal annealing conditions.
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Figure 1:
Maximum OI of accelerated-aged irradiated PE versus annealing temperature at 500MPa pressure.
The maximum OI of the air aged PE was subtracted from all bomb aged PE to reflect oxidation solely associated with the bomb aging.
Figure 2:
Plot of Maximum OI of accelerated –aged irradiated PE versus pressure for annealing at room temperature (triangles)and 130ºC (dots connected with dashed lines).
• For protocol (c) the annealing temperature range of 160-220ºC is below the estimated Tm of 233ºC at 500MPa yet there was a substantial decrease in free radical concentrations.
• In conclusions, this study showed that pressure-annealing can be effective in decreasing oxidation in irradiated PE but only at high temperature
References
11. Harris WH, et al. Clin Orthop Rel Res 2001; 393:66-70
12. Greenwald AS, et al. J Bone Joint Surg 2001; 83(A)suppl2-2:68-72 13. McKellop H, et al. J Ortho Res1999;17(2):157-67
14. Muratoglu O, et al. Biomat 1999; 20:1463-1470 15. Kurelec L et al, Macromolecules 2000, 33: 5593-601
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Figure 3:
Schematic of the phase diagram of PE showing temperature and pressures when PE is in the melt state, and as orthorhombic, monoclinic and hexagonal crystals.
