Page 186 - Clinical relevance of current materials for cranial implants

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Chapter 9
Most related research regarding bone cements is performed in the field of orthopedics, which use the ISO 5833 norm to determine mechanical properties. Since Vertex Self- Curing and NextDent C&B MFH are mostly used as dental acrylics, which have different demands compared to bone cement applications, ISO 20795-1:2013 and 179- 1:2010 norms were applied in this study. ISO 5833 would allow better comparison to the available literature, however, the bone cements are tested in dry conditions after 24 hr. This results in over estimation of the mechanical properties22. The current ISO 5833 standard does do not mimic the conditions or environment in which the material is used clinically and should be revised and preferably harmonized with more realistic dental ISO standards, which use 37°C in water (2 – 7 days).
When comparing the above mentioned materials, NextDent C&B MFH performs better than the other materials on flexural strength and modulus, as well as impact strength. However it also has significantly lower toughness and shows a more brittle behavior, especially compared to Palacos R+G which appears more ductile. This is in line with literature, as an increase in crosslink density lowers the fracture toughness and limits the total crack-tip strain20. NextDent C&B MFH is a poly(dimethacrylate) and therefore has significantly more crosslinks than the other two materials. Due to crosslinking in the materials tested, thermoset polymers with similar chemical compositions may show similar trends to the results presented in this study, although this requires further investigation.
There is no influence of EtO on the molecular weight of PMMA reported in literature, suggesting that EtO does not influence the mechanical properties of PMMA. The results reported in this study show no significant difference between unsterilized and EtO sterilized specimens. However, EtO is a toxic gas and requires a long period - up to fifty days - of degassing9, 12, 23.
This study did not take into consideration the effect of sterilization on biocompatibility of the materials and leaching of potential harmful substances, i.e. unreacted monomer and activator. It should be noted that the powder and liquid components of PMMA used in the operating room are sterilized before use, γ-irradiation is often used to sterilize the powder component8. These properties are crucial for clinical use and should be investigated in future studies.