Effect of energy density on low-shrinkage composite resins: diode-pumped solid state laser versus quartz-tungsten-halogen light-curing unit

OBJECTIVE

The purpose of the present study was to evaluate the effect of energy density on the polymerization of low-shrinkage composite resins.

BACKGROUND DATA

The number of photons needs to initiate the polymerization process can be controlled by light intensity and curing time through the form of energy density.

MATERIALS AND METHODS

For the study, two methacrylate-based (Premise [PR] and Venus Diamond [VE]) and one silorane-based (Filtek LS [LS]) composite resins were light cured using a quartz-tungsten-halogen (QTH) light-curing unit (LCU) and a 473 nm diode-pumped solid state (DPSS) laser. Degree of conversion (DC), microhardness, refractive index, and polymerization shrinkage were evaluated under different energy densities. Through the study, the feasibility of DPSS laser as a light source was tested as well.

RESULTS

LS showed the highest DC and refractive index both on the top and bottom surfaces, and the least polymerization shrinkage among the tested specimens. For the same or similar energy density, QTH and DPSS showed insignificant DC difference (p>0.05). On the other hand, for microhardness, except for one case at the bottom surface, QTH and DPSS showed significant difference (p<0.001). DPSS generated slightly lower polymerization shrinkage than that by QTH.

CONCLUSIONS

DC, microhardness, refractive index, and polymerization shrinkage were linearly correlated with energy density. In most cases, there was a strong linear correlation among DC, mirohardness, and refractive index. The DPSS laser of 473 nm could polymerize low-shrinkage composite resins to the level that was achieved by the conventional QTH unit.

Update on Dental Nanocomposites

Dental resin-composites are comprised of a photo-polymerizable organic resin matrix and mixed with silane-treated reinforcing inorganic fillers. In the development of the composites, the three main components can be modified: the inorganic fillers, the organic resin matrix, and the silane coupling agents. The aim of this article is to review recent studies of the development of dental nanocomposites and their clinical applications. In nanocomposites, nanofillers are added and distributed in a dispersed form or as clusters. For increasing the mineral content of the tooth, calcium and phosphate ion-releasing composites and fluoride-releasing nanocomposites were developed by the addition of DCPA-whiskers or TTCP-whiskers or by the use of calcium fluoride or kaolinite. For enhancing mechanical properties, nanocomposites reinforced with nanofibers or nanoparticles were investigated. For reducing polymerization shrinkage, investigators modified the resin matrix by using methacrylate and epoxy functionalized nanocomposites based on silsesquioxane cores or epoxy-resin-based nanocomposites. The effects of silanization were also studied. Clinical consideration of light-curing modes and mechanical properties of nanocomposites, especially strength durability after immersion, was also addressed.

Spectroscopic and quantitative analysis of spiroorthoester synthesis by two-dimensional correlation and multivariate curve resolution methods of NIR data

The spiroorthoester synthesis includes several competitives reactions. A way of determining the reactions that are taking place and their sequential order, is presented. The reaction between the phenylglycidylether and gamma-butyrolactone to obtain a spiroorthoester has been monitored by near-infrared spectroscopy (NIR). In addition to the formation of the corresponding spiroorthoester, some parallel processes can occur. By means of two-dimensional correlation analysis, only one reaction is postulated, the one corresponding to the spiroorthoester formation. This was confirmed by recording the NMR spectra of the final product. Applying multivariate curve resolution-alternating least squares (MCR-ALS) to the NIR spectra obtained during the reaction, it has been possible to obtain the concentration values of the species involved in the reaction. The recovered spectra were compared with the experimentally recorded spectra for the reagents (phenylglycidylether, gamma-butyrolactone) and the final product (spiroorthoester) and the correlation coefficients were, in all cases, higher than 0.990. The maximum and minimum limits associated with the ALS solutions were calculated, making it possible to limit to a considerable extent the ambiguity that is characteristic of these curve resolution methods.

Stability of silorane dental monomers in aqueous systems

Siloranes (silicon-based monomers with oxirane functionality) are investigated as matrix resins for new low shrinkage/stress dental composites. Compounds containing oxirane groups are known to be reactive with water, which could impart instability to the composite.

OBJECTIVE

To test the stability of siloranes by measuring changes in the chemical structure of the oxirane group in aqueous environments.

METHODS

Two siloranes (PH-SIL and TET-SIL) and their 1:1 mixture (SIL-MIX) were evaluated (n=2-3). Siloranes were mixed in aqueous solutions with and without 1% tetrahydrofuran (THF) containing either liver esterase or epoxide hydrolase at pH 7.4, or dilute HCl at pH 1.4. The stability of conventional dioxiranes 3,4-epoxycyclohexyl-methyl-3,4-epoxycyclohexane carboxylate (ECHM-ECHC), and bisphenol A diglycidyl ether (BADGE) were also monitored under similar conditions. NMR was used to estimate the extent of reaction and give structural information about reaction products.

RESULTS

Siloranes were found to be stable for 24h in all aqueous environments tested. In contrast, ECHM-ECHC reacted at pH 1.4 to form species containing oxirane, ester, hydroxyl and carboxylic acid groups. Water hydrolyzed the ester group of ECHM-ECHC in the presence of liver esterase. In the presence of epoxide hydrolase, BADGE oxirane groups were hydrolyzed to diols, hydrolysis ranged from 0 to 34% depending on the aqueous environment.

CONCLUSION

The stability and insolubility of siloranes in biological fluid simulants suggests that these may be more suitable for use in the oral environment than conventional oxirane-functional monomers.