Polymerization shrinkage of restorative resins using laser and visible light curing

Análise química e morfológica do esmalte dentário humano tratado com laser argônio durante a colagem ortodôntica

INTRODUÇÃO: as principais utilizações do laser argônio na Ortodontia são a redução do tempo de polimerização durante a colagem ortodôntica e o aumento da resistência à cárie do esmalte dentário. OBJETIVO: o objetivo deste trabalho foi avaliar as alterações químicas e morfológicas do esmalte dentário humano tratado com laser argônio nos parâmetros da colagem ortodôntica. MÉTODOS: quinze primeiros pré-molares hígidos, extraídos por indicação ortodôntica, foram selecionados e seccionados no sentido do longo eixo em dois segmentos iguais. Uma metade de cada elemento dentário foi tratada e a outra permaneceu sem tratamento. Um total de 30 amostras foi analisado, criando o grupo laser (n = 15) e o grupo controle (n = 15). O tratamento foi feito com laser argônio com 250mW de potência por 5 segundos, com densidade de energia de 8J/cm². RESULTADOS: a análise de difração de raios X demonstrou duas fases em ambos os grupos, as fases apatita e monetita. A redução da fase monetita foi significativa após o tratamento com laser, sugerindo maior cristalinidade. A análise de Espectroscopia de Energia Dispersiva (EDS) indicou aumento na razão cálcio-fósforo no grupo laser, compatível com a diminuição da fase monetita. A morfologia superficial do esmalte dentário apresentou-se mais lisa após o tratamento com laser argônio. CONCLUSÕES: o aumento de cristalinidade e a lisura superficial do esmalte no grupo laser são fatores sugestivos de aumento de resistência à cárie no esmalte dentário.

Argon ion laser curing depth effect on a composite resin

The aim of this study was to define optimal power settings as well as curing time associated with evaluating the curing depth of a composite resin as a function of Vickers hardness. The tests were performed with a hybrid composite resin cured with a halogen lamp and argon ion laser, with different exposure times and power settings. The composite resin bulk technique was used using a black polypropylene matrix with thicknesses ranging from 1 to 4 mm and Vickers microhardness was measured on the opposite surface of the light activation. ANOVA and Tukey statistical tests were used. The results showed that the groups activated by the laser for 20 s, at 200 and 250 mW, did not present statistically significant differences regarding the halogen lamp with 1 mm thickness, but the halogen lamp showed better results with thickness values more than 2 mm (p < 0.05).

Effect of chain modifications on the physicomechanical properties of silsesquioxane-based dental nanocomposites

The objective of this study was to evaluate the physicomechanical properties of a series of polyhedral silsesquioxane (SSQ) methacrylate monomers developed for dental applications. The effect of chain modifications on the properties of the SSQ-based monomers was also evaluated. Physicomechanical properties that are investigated include polymerization shrinkage, degree of conversion, hardness, and modulus. Results obtained were compared with unfilled 1:1 (control) bis-GMA/TEGDMA materials (typical monomers used in dental composites). All samples investigated were cured using 400-500 nm light at 500 mW/cm(2) for 40 s. Shrinkage associated with curing and post-gel reactions for all synthesized SSQ compounds were found to range from (0.04 +/- 0.01)% to (0.33 +/- 0.03)% with degree of conversion ranging from (56.68 +/- 2.81)% to (84.53 +/- 2.62)%. At all time intervals, post-gel shrinkage associated with control was found to be significantly greater than all SSQ compounds. No significant difference in degree of conversion was observed for control, and all SSQ compounds except for SSQ attached with eight equivalents of short-chain methacrylate. Mechanical properties associated with SSQ compounds were found to be significantly lower than control. However, through chain modifications, mechanical properties of SSQ compounds can be improved by approximately 50%.

Increases in intrapulpal temperature during polymerization of composite resin

STATEMENT OF PROBLEM

The polymerization of dental composite resins can generate increases in intrapulpal temperature that may damage the pulp. The development of new polymerization devices such as the argon laser makes the assessment of these temperatures important.

PURPOSE

This study compared increases in temperature generated by argon laser and halogen light when polymerizing a bonding system and a composite resin, and also sought to determine whether both types of polymerization lights generate temperature increases below the safe limit of 5.5 degrees C.

MATERIAL AND METHODS

Thermocouples linked to a temperature reading system were positioned in the pulp chamber of 10 extracted bovine incisors. Class V cavities were prepared, etched, and filled with a 1-bottle bonding system (Single Bond) and composite resin (Z-100). The test groups were as follows (n = 5 for all groups): halogen light for bonding system (HB); halogen light for composite resin (HC); argon laser for bonding system (LB), and argon laser for composite resin (LC). The polymerization parameters were halogen light operated at 600 mW/cm2 for 40 seconds, which served as control, and argon laser operated at 200 mW for 10 seconds. Data were analyzed by a 2-way (light versus material) analysis of variance (ANOVA) (alpha = .05).

RESULTS

The average temperature increases were 2.35 degrees C (HB), 2.69 degrees C (HC), 1.25 degrees C (LB), and 1.5 degrees C (LC). Significant differences between halogen light and argon laser (P = .002), but not between composite and bonding system, were demonstrated.

CONCLUSIONS

The argon laser produced significantly lower increases in pulpal temperature than the halogen light, independent of the thickness of the polymerized material.

Hygroscopic expansion of a compomer and a composite on artificial gap reduction

OBJECTIVE

This study compared the effect of water sorption on the extent of marginal gap reduction in a compomer (Dyract AP, Dentsply) and a composite (Spectrum, Dentsply) over a 12-week storage period.

MATERIALS AND METHODS

Artificial gaps were created in 40 borosilicate glass cylinders, each 5mm deep and with an internal diameter of 5.5mm. Half of the internal bonding surface of each cylinder was sandblasted, silanised and coated with a dentine adhesive (Prime&Bond NT, Dentsply). Twenty partially bonded cylinders were incrementally filled with Dyract AP (AP) and the rest with Spectrum (S). For each material, 10 specimens were stored in de-ionised water (W), and 10 (control) in non-aqueous silicone fluid (O) at 37 degrees C. The dimension of the same maximum gap created in each specimen was repeatedly measured at 0, 1, 2, 4, 6, 8, 10 and 12 weeks, using a light microscope under incident light at 570x magnification. Gap widths in each of the four groups (n=10) were statistically compared. Correlations between mean gap width reduction and storage time were also examined.

RESULTS

Friedman repeated measures ANOVA on ranks revealed significant differences (p<0.001) among the gap widths measured at different time intervals in groups AP-W and S-W. No significant differences (p>0.05) were found in groups AP-O and S-O. Tukey's multiple comparison test indicated that no significant differences (p>0.05) were detectable beyond the sixth week in AP-W and the fourth week in S-W. Linear regression analyses showed that mean gap widths decreased exponentially with time for AP-W (r=0.97) and S-W (r=0.90). From the slopes of the regression lines, the rate of marginal gap reduction in AP-W was 4.6 times faster than S-W.

CONCLUSION

Marginal gap reduction in both the materials are directly attributed to water sorption and that the reduction is larger and more rapid in Dyract AP.

Human teeth exposed to argon laser irradiation: determination of power-time-temperature working conditions

OBJECTIVE

This study was conducted to establish the operating parameters of the argon laser without thermal damage to the pulp tissue for clinical applications.

SUMMARY BACKGROUND DATA

Previous studies have mainly compared the temperature modifications of the pulp chamber in a very limited situation, where a complete view of the thermal history cannot be obtained nor even extrapolated to new applications.

METHODS

We used samples of molar and premolar tooth where a class V cavity was prepared and illuminated with an argon laser at different power levels, fixing the exposition area for all cases. Situations including open cavity and teeth restoration were analyzed. High-precision thermistors were placed in four different positions, one of which was inside the pulp chamber. The temperature evolution was monitored continuously by an interfaced computer during all laser exposure. Special attention was paid to the intrapulpal temperature variation because it is considered the most vulnerable thermal region. The temperature time evolution allowed the determination of the operating conditions (power-time-temperature variation) in which the use of the argon laser causes no pulpal damage. As a function of temperature variation, we divided the whole parameter space (power-time-temperature) into zones and the optimum zone of operation was determined.

CONCLUSIONS

We created a diagram called power-time-temperature (PTT) where zones of temperature increased under laser irradiation allow the verification of which condition is safe for clinical laser application. The results have a broad use when this type of analysis is applicable.