Page 185 - Clinical relevance of current materials for cranial implants
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                                 Effects of sterilization on the mechanical properties of PMMA
In this study Palacos R+G and Vertex Self-Curing have a comparable flexural strength and flexural modules, however, Vertex Self-Curing is more brittle compared to Palacos R+G (Figure 1). According to Perkins and Lewis et al., one should expect that the unnotched impact strength of Palacos R+G exceeds the impact strength of Vertex Self-Curing10,20. However, the experiments showed a decrease of impact strength, suggesting that Palacos R+G fails at impact by a brittle polymer mechanism, e.g. voids in the material. This seems plausible because Palacos R+G has macroscopically visible voids in the material and contains 10% zirconium dioxide21 as filler for radio opacity, which are most probably not chemically incorporated in the matrix and can act as a void.
Beside the composition of the material, the effect of the sterilization procedure was
investigated. In general, autoclave sterilization is one of the most common sterilization
methods4. In this study it caused specimens to deform or exfoliate due to the high
temperatures and pressurized steam, which exceed the glass transition temperature
(Tg) of Palacos R (100°C)21. A material that deforms or exfoliates during sterilization is
not desirable for medical devices, therefore autoclave sterilization was excluded from
further analysis. HPGP and EtO did not tend to significantly change material properties.
In contrast, the flexural strength of all three materials increased significantly following
γ-irradiation. Literature reports a decrease in molecular weight of PMMA upon
γ-irradiation due to chain scission, this directly relates to worsening of the mechanical
properties9-12. γ-irradiation increases the amount of scission, it follows therefore that
it may also increase side-group scission. An increase in flexural strength originating
from additional crosslinking thus seems unlikely, instead it could originate from a 9 change in wettability of the material. These materials show a significant reduction
in flexural strength when immersed in water. A reduction in hydrophilic side-groups due to γ-irradiation induced side-group scission may thus effectively increase the flexural strength compared to the control when both are incubated in water following sterilization, even though the molecular weight is lower.
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