Objective: This in vitro study examined the effect of light intensity variations on several properties of two restorative systems. Materials and Methods: Two resin composite systems (Tetric, Ivoclar Vivadent, Schaan, Liechtenstein; 2100, 3M Dental Products Division, St. Paul, Minnesota) and two dentin bonding agents (Syntac, Ivoclar Vivadent; Scotchbond Multi‐Purpose, 3M Dental) were used. To evaluate marginal gap formation in dentin cavities (with a C factor of 2.5), 48 bovine incisors were embedded in epoxy resin. The labial enamel was removed, and the dentin surface was polished to 600‐grit. Preparations were made in dentin and restored using one of four combinations of bonding system and composite. The composite was polymerized with either a high‐intensity lamp (450 mW/cm2) or a low‐intensity lamp (250 mW/cm2). After storage in water for approximately 60 minutes, the surfaces of the samples were prepared for electron microscopy analysis. Also, the flexural strength and modulus of each composite were determined at 30 minutes and 24 hours after polymerization with either the high‐ or low‐intensity light. Volumetric shrinkage was measured using the density method at four time periods for both light intensities and for wet versus dry storage. Finally, Vickers hardness profiles were evaluated to determine depth of cure. Results: Marginal analysis showed that 2100, when used with either Scotch‐bond Multi‐Purpose or Syntac, had an increased incidence of gap formation when polymerized with the high‐intensity light. Tetric had better marginal adaptation than 2100, when used with either dentin bonding agent, and with both light intensities. The flexural strength and modulus of both composites were lower with low‐intensity light‐curing. There were no differences in volumetric shrinkage or depth of cure related to curing method for either composite. Conclusions: The results of this study suggest that marginal quality may be reduced when a high‐intensity light is used to cure composite. Also, the light intensity influences development of the elastic modulus and, therefore, affects shrinkage stress in the composite. The two composites tested in this study responded differently to the different methods of light‐curing. In general, Z100 showed inferior properties than Tetric when cured with either high‐ or low‐intensity light, for both marginal integrity and the flexural modulus and strength. Finally, with a polymerization time of 40 seconds, the composites were equally well cured to a depth of 4.5 mm, regardless of the light intensity used.
This study analyzed the degree of conversion, temperature increase and polymerization shrinkage of two hybrid composite materials polymerized with a halogen lamp using three illumination modes and a photopolymerization device based on blue light emitting diodes. The degree of conversion of Tetric Ceram (TC) (Ivoclar Vivadent) and Filtek Z 250 (F) (3M/ESPE) was measured by Fourier transformation infrared spectroscopy at the surface and 2-mm depth; temperature rise was measured by digital multimeter, and linear polymerization shrinkage was measured during cure by digital laser interferometry. Composite samples were illuminated by quartz-tungsten-halogen curing unit (QTH) (Astralis 7, Ivoclar Vivadent) under the following modes: "high power" (HH) 40 seconds at 750 mW/cm2, "low power" (HL) 40 seconds at 400 mW/cm2 and "pulse/soft-start" (HP) increasing from 150 to 400 mW/cm2 during 15 seconds followed by 25 seconds pulsating between 400 and 750 mW/cm2 in 2-second intervals and by light emitting diodes (LED) (Lux-o-Max, Akeda Dental) with emitted intensity 10 seconds at 50 mW/cm2 and 30 seconds at 150 mW/cm2. A significantly higher temperature increase was obtained for both materials using the HH curing mode of halogen light compared to the HP and HL modes and the LED curing unit after 40 seconds. Significantly lower temperature values after 10-second illumination were obtained when LED was used compared to all halogen modes. For all curing modes, there was no significant difference in temperature rise between 20 and 40 seconds of illumination. Results for the degree of conversion measurements show that there is a significant difference in the case of illumination of resin composite samples with LED at the surface and 2 mm depth. For polymerization shrinkage, lower values after 40 seconds were obtained using LED compared to QTH.
A heat-pressed technique (IPS Empress, Ivoclar) has been described to construct single unit crowns, inlay/onlays and veneers using precerammed and precoloured glass-ceramic ingots. The aim of the study was to evaluate the strength of materials obtained using this technique. Strengths were determined by means of shell (disc rupture) tests. Seven groups of 10 specimens each were prepared with different combinations of core, incisal and glaze firing. Shell strength values of these seven groups varied between 167 +/- 18 and 81 +/- 9 MPa. After analysis, results showed that there was no statistical difference between groups when the core porcelain was in tension. However, there was a significant difference between those groups in which the lower surface was either core or incisal porcelain. The component thicknesses of the core and incisal materials did not change the strength of the complete specimen.
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