PubMed · 121561
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S Takahashi, T Miyazawa, K Ishii, S Nagasawa, M Ito. 1979. [Testing composite resins for building abutments].. https://pubmed.ncbi.nlm.nih.gov/121561/
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Polymerization induced by UV-VIS light of composite dental materials produces a solid matrix within which terminal radicals of non-polymerized monomers remain trapped. Electron Spin Resonance (ESR) allowed three different types of radicals to be identified. The analysis of ten normally available commercial products gave information on: (1) the propagation of the conversion reaction as a result of exposure to light; (2) the time necessary for the decay of each type of radical; and (3) the variations with temperature and the effects of shattering on the materials under study. The presence of inorganic filling material slowed the process of polymerization, while it accelerated the decay of radicals. It was suggested that the nature of these processes depended on the composition of the base resin materials, whereas it did not depend on the sizes of the filler particles. Moreover, the complete propagation of the conversion reaction needed a period of light exposure greater than that currently suggested by the manufacturers. The structural stability and the resistance of the composites were confirmed by both the long period of decay and the high temperatures needed to overcome the potential barrier for starting the radical decay process. Finally, the composite shattering investigation indicated that particles removed by surface abrasion experience rapid radical decay, thus reducing the possibility of harmful effects on internal organs.
Significant occlusal wear occurs on posterior composite restorations in areas without direct contact with opponent teeth (contact-free wear) in addition to wear at the contacts. Recent research has indicated that all occlusal posterior composite surfaces wear, and those surfaces show decreasing wear rates over time. This provides more evidence for an earlier theory proposed by Jørgensen et al. (1979) that composite contact-free wear is a function of access and attrition caused by small particles in the food bolus. This hypothesis is now called the "protection theory" or "protection hypothesis" for wear. Macroscopic protection or "sheltering" from wear is provided by cavity preparation walls. Microscopic protection against resin matrix wear is provided by filler particles that are close together. Clinical evidence supporting the protection hypothesis comes from recently confirmed low wear rates for microfill and hybrid composites. The objective of this work was to calculate the minimum inter-particle spacing required for microscopic protection (wear resistance) for composites based on assumptions of packing fractions (0.68) with small particles (0.02 microns radius) for dispersed versus agglomerated conditions. An inter-particle spacing (IPS) of < 0.10 microns was assumed to afford wear protection. The results are that only 1.5-6.0 volume percentage microfiller is theoretically required to generate an IPS of 0.1 micron for composite microscopic protection. However, microfiller particles are suspected to agglomerate into clusters. Modified calculations for that effect indicate that at least 35% filler is required to provide microscopic protection. This effect explains much of the 3 to 5 year clinical research results for microfills and hybrids.(ABSTRACT TRUNCATED AT 250 WORDS)
The on-going search for a biologically acceptable restorative material has brought a confusing variety of composites on the dental market. In the present study, commercially available composites are categorized as a function of their mean particle size, filler distribution, filler content, Young's modulus, surface roughness, compressive strength, surface hardness, and filler morphology. Out of this information, it can be concluded that the materials of choice for restoring posterior cavities at present are the Ultrafine Compact-Filled Composites because their intrinsic surface roughness, Young's modulus and, indirectly, their filler content, compressive strength, and surface hardness are comparable to the same properties of enamel and dentin. The Ultrafine Midway-Filled Composites seem to be very satisfactory materials for anterior use.