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Biomaterials in TMJ replacement833Table 2: Influence of the opposing material on wear of polymer materials in hip prostheses60Material Amount of wear Non-modified UHMWPE Ceramic head: 0.098–0.03 mm/yMetallic head: 0.12–0.25 mm/yHylamer Ceramic head: 0.15–0.33 mg/million cyclesMetallic head: 0.13–0.4 mm/yHDPE Ceramic head: 0.072 mm/yMetallic head: 0.076 mm/yHDPE, high density polyethylene; UHMWPE, ultra-high-molecular-weight polyethyleneA second problem with UHMWPE is shelf aging, a process of oxidative degradation (chain scission) that is initiated by reactive free radicals generated by common γ-irradiation sterilization procedures. As a result of chain scission, the mechanical properties change during storage and implantation with a loss of mechanical strength, and wear resistance is diminished. This problem has been partially solved by exposing the PE during sterilization to a neutral gas or vacuum atmosphere, resulting in significantly reduced oxidation of the surface layer. As a result, the effects of long term shelf aging after irradiation have been limited to a decrease in fracture and fatigue resistance.(62,63) Another solution to this problem can be found in the use of antioxidants such as vitamin E, creating vitamin E-stabilized UHMWPE. By incorporating α-tocopherol in UHMWPE, the material’s oxidation resistance is increased, as vitamin E is capable of interacting with free radicals, actively preventing oxidative degradation. This incorporation is possible by either blending α-tocopherol in UHMWPE powder, or by diffusing vitamin E into UHMWPE.(63) While vitamin E is added after radiation crosslinking in the latter, this is not the case for the mixed blend. As a result, as the concentration of α-tocopherol increases, the efficiency of crosslinking is diminished. As such both the concentration of α-tocopherol and radiation dosage have to be optimized to achieve both optimal wear- and oxidation-resistance.(63,64) Oral et al.(64) concluded that vitamin E should not exceed 0.3wt%, with 0.1wt% being more optimal to achieve a similar crosslink density compared to untreated UHMWPE. Along with the addition of vitamin E, an increase in radiation dose is needed to achieve a crosslink density approaching that of a blend without Nikolas de Meurechy NW.indd 83 05-06-2024 10:14