Page 184 - Clinical relevance of current materials for cranial implants

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Chapter 9
DISCUSSION
PMMA-based polymers have been used for many years in medical devices with their specific formulations and applications. No reports on a systematic investigation of the mechanical properties using the same ISO standards are available (bone cement (ISO 5833) or dental (ISO 20795-1:2013 and ISO 179-1:2010)), making comparison between the reported values difficult. Literature reports the flexural strength (56.3 MPa), flexural modulus (2213 MPa), the toughness (2.03 MPa m1/2), and total fracture work (897 J m-2) of Palacos R+G16, 17. The impact strength reported for Palacos R without gentamicin was 4.1 kJ m-2 18. For Vertex Self-Curing the flexural strength (79.6 MPa) and flexural modulus (2.38 GPa) are reported19. Currently, there is no data available on the mechanical properties of NextDent C&B MFH. These values reported in literature are in line with the findings presented in this study.
From the measured mechanical properties of the different materials the following trends were observed (I) an increase in flexural strength (σ) resulted in decreased toughness (Kmax and Wf), (II) an increase in flexural strength (σ) resulted in increased impact strength (AcU) and (III) an increase in toughness (Kmax and Wf) resulted in decreased impact strength (AcU). The latter is contradicting with finding of Lewis and Mladsi 18, where a positive correlation was found between the toughness (Kmax) and impact strength (AcU). The toughness and impact strength are two independent properties, which are related to the ductile or brittle nature of the material20. Brittle polymers fail through nucleation of voids and initiation and propagation of brittle cracks resulting in catastrophic failure. The polymers have yield strengths higher than their ultimate or breaking strengths, and thus a low crack initiation and low crack propagation energy in impact20. Ductile polymers fail by crazing or matrix shear yielding. Both mechanisms lead to high crack initiation energy, but to a low propagation energy at impact. As a result one can expect a high unnotched impact strength, but a low notched impact strength20.