Free radical elimination in irradiated UHMWPE through crystal mobility in phase transition to the hexagonal phase

Abstract

Ultra-high molecular weight polyethylene (UHMWPE) is radiation cross-linked to decrease wear in total joint applications. Irradiation decreases the strength of UHMWPE and introduces residual free radicals, which can cause oxidation in the long-term. We advanced a method eliminating the free radicals without a reduction in strength. UHMWPE exhibits a hexagonal phase at high pressure and temperature, where chain mobility in the crystalline phase is increased, leading to the formation of extended chain crystals. We hypothesized that the increased chain mobility during transformation from the orthorhombic to hexagonal phase could be used to eliminate the residual free radicals in irradiated UHMWPE. We eliminated the free radicals in 25-, 65-and 100-kGy irradiated UHMWPE and these materials did not show oxidation after accelerated aging. The ultimate tensile strength and work to failure of 25 and 65-kGy irradiated UHMWPEs were improved significantly while that of 100-kGy irradiated UHMWPE was lower compared to irradiated UHMWPE melted at ambient pressure.

The elimination of free radicals in irradiated UHMWPEs with and without vitamin E stabilization by annealing under pressure

“…26 Another was applying high pressure and temperature to induce the phase transformation from orthorhombic to hexagonal crystal structure without going through the melt phase, again enhancing mobility and demonstrating the elimination of free radicals. 27 …”

Spectroscopic and sub optical band gap properties of e-beam irradiated ultra-high molecular weight polyethylene

“…It is quite obvious that energy band gaps are in direct relation with the number of carbon atoms in the C ¼C cluster (i.e. higher is the number of carbon atoms in a C¼ C cluster lower is the energy band gap), and it has already been established with reasonable understanding that irradiating PE with high energy radiations increases the C ¼C unsaturation within its structure (Costa et al, 2008;Mehmood et al, 2014Mehmood et al, , 2013Oral et al, 2008).…”

Oxidation resistance and abrasive wear resistance of vitamin E stabilized radiation crosslinked ultra‐high molecular weight polyethylene

“…Over time, the free radicals are believed to migrate to lamellar surfaces, where the presence of dissolved air (oxygen) in the amorphous regions can induce oxidation that can further result in chain scission. 12,13 A major drawback of HXLPEs is that both the radiation dose as well as the post-radiation heat treatments decreased several mechanical properties, such as tensile elongation, 10,11,[14][15][16][17] ultimate tensile strength, 10,11,14 J-integral fracture toughness, 14 and resistance to fatigue crack propagation. 7,8 It also accelerates the generation of particulate wear in joint components.…”

“…Post-radiation treatments removed the concern of oxidation due to irradiation and also high wear rate of the implant since HXLPEs have shown a high resistance to wear in laboratory tests 10,11 as well as in early clinical studies. 12,13 A major drawback of HXLPEs is that both the radiation dose as well as the post-radiation heat treatments decreased several mechanical properties, such as tensile elongation, 10,11,[14][15][16][17] ultimate tensile strength, 10,11,14 J-integral fracture toughness, 14 and resistance to fatigue crack propagation. [18][19][20] While irradiation has been unavoidable due to its large increase in wear resistance, post-radiation treatments to reduce free radical content in irradiated HXLPEs can be avoided by incorporation of a stabilizer, such as Vitamin E or other biocompatible antioxidants.…”