Enamel-dentin caries, also known as hidden caries, is a phenomenon that leads toformation of highly mineralized, strengthened enamel surfaces, under which the loss ofmineral may progress gradually and the carious lesion might extend into dentin without aclinically visible crack at the enamel surface. There is no consensus on the cause ofenamel-dentin caries. It has been suggested that, despite the indubitable benefits, thehigh remineralization capacity of fluoride might mask dentin caries, mainly for thosewho have lifelong exposure to water fluoridation. An alternative hypothesis is that thephenomenon occurs as a result of interactions between enamel and dentin7,13,14.
digora for windows crack 24
The radiodensity (in pixels) of the samples was determined with the resident software provided by the manufacturer. The Digora system has a windows-based software, Digora for Windows 2.5 Rev 0 (Soredex, Helsinki, Finland), which is capable to measure density curves of digital radiographies obtained by x-ray impregnation on the image phosphor plate. The radiodensity of each radiographed structure or material was obtained by clicking with the software cursor right above the digital image. Each digital image had it radiodensity measured immediately after scanning, without any modification in contrast or brightness. This software shows data concerning the highest and the lowest radiodensity of the sample, and an average value, which was considered to be the sample's initial radiodensity. Since each sample was subjected to three exposures the sample's final radiodensity was considered to be the mean of those values.
Figures 4 and 5 are SEM images of Ortho Glass LC at the manufacturer's recommended P/L and the 50% reduced P/L ratio, respectively. Figures 6 and 7 of Vitro Cem at the manufacturer's recommended P/L ratio and 50% reduced P/L ratio, respectively. For the RMGICs, reduction in liquid at 50% P/L resulted in poor interaction between matrix and powder particles with clear signs of crack development. For the conventional GICs, reduction in liquid at 50% P/L ratio resulted in a greater presence of salt matrix and its close interaction with powder particles without the development of cracks, which could be seen at the manufacturer's recommended P/L.
Generally, chain displacement can occur at sufficiently high stress in deformable polymers, than in brittle polymeric and ceramic materials, which undergo crack propagation at high stress3,19. Mitsuhashi, et al.15 (2003) showed that when the P/L ratio is decreased and the matrix volume increased as a consequence, the characteristics of the resin matrix are significantly emphasized for the RMGICs. It is expected that greater presence of resin in RMGICs, by reduction of P/L ratio, would render a material with more viscoelastic behavior, greater strain capacity and possibly greater resistance to stress development by load application. However, even if a more viscoelastic behavior is expected, the tendency for generating materials which withstand less load application is real because in the cement mixtures, the volume fraction of the matrix, which has weak mechanical strength, increases at lower P/L ratios. Irrespective of the P/L ratio, this study showed that RMGICs' strength was always higher than that of conventional GICs (Table 1), possibly due to the greater expected viscoelastic behavior of RMGICs and the higher cohesive strength of resin matrix versus salt matrix23. On the other hand, DTS was reduced only for RMGICs when 50% reduced P/L ratio was employed (Table 1).
When comparing RMGICs and GICs, Yamazaki, et al.24 (2006) showed they possess similar viscoelastic behavior, irrespective of the polymeric character of RMGICs. Then, it is possible that resin and salt matrixes act similarly, in these materials, with a higher probability of enabling plastic deformation for the former23. However, Mitsuhashi, et al.15 (2003) found that the fracture toughness of the RMGICs is not greatly influenced by the P/L ratio, as it is for GICs. Only at high reductions in P/L ratio, the fracture toughness started to decrease for RMGICs15. With lower fracture toughness materials became more brittle, reducing plastic behavior and also the resistance against crack propagation15. Thus, differently from GICs, a significant alteration in RMGICs properties is only expected with high reduction of powder amount, because the acid-base reaction of the powder particle and liquid is critical for many physical properties of hardened materials15. The SEM analysis showed an integrated microstructure for RMGICs with manufacturer's recommended P/L ratio (Fig 3), but the same was not true for 50% reduced P/L ratio, which showed a greater presence of matrix per volume, unreacted particles and crack development, recognized as a signal of the jeopardized interaction between matrix and powder particles (Figure 5). The high reduction in P/L ratio, as performed in the present study, resulted in significantly lower DTS values just for RMGICs, and not for conventional GICs (Table 1). In spite of that, for all materials, a decrease in DTS occurred with the reduction in P/L ratio. There are two possibilities to explain this result. One is that limitations of the experimental design did not allow differences between manufacturer's recommended P/L ratio and 50% reduced P/L ratio of the materials to be observed. Larger sample sizes may possibly be necessary to demonstrate statistically significant differences. The other possibility is closely related to the setting reactions of each type of material.
The assumption that both the salt matrix and the resinous matrix have a determinant relationship in the overall strength of RMGICs1,26 was confirmed in this study, but needs more specific studies to be proved. The reduction in P/L ratio for GICs did not result in significantly lower DTS, which mean that the polyacid is able to react with normally unreacted glass particles and the greater presence of salt matrix does not significantly reduces strength. SEM analysis showed a lower presence of salt matrix in manufacturer's recommended P/L ratio (Figure 6) and clear signs of cracks around unreacted powder particles, which mean a high tendency for brittle characteristics in these ionomers. At 50% reduced P/L ratio (Figure 7) a greater presence of salt matrix can be seen, with smaller particles and no signals of cracks. These observations led to the assumption that even if a better interaction between matrix and powder particles is seen when more liquid is added to the mixture, GICs' DTS can be considered more stable when alterations in P/L ratio are performed, since different morphological structures are formed and can resist differently to load application. However, the reduction in absolute DTS means for Vitro Molar and Vitro Fil should be taken into consideration, meaning that more salt matrix can possibly result in poor mechanical properties with time. Further studies are necessary to prove this hypothesis. The use of the manufacturers' recommended P/L ratio is always advisable since properties will be preserved. 2ff7e9595c
תגובות