Evaluation of contraction stress, conversion degree, and cross-link density in low-shrinkage composites

Effects of Light Attenuation through Dental Tissues on Cure Depth of Composite Resins

Objective

Polymerization of light-cured resin-based materials is well documented; however, the intensity of the activating light can be reduced by passage through air, dental structure, or restoration compromising the physical properties of the restoration. The aim of this study was to evaluate the depth of cure of different light cured composite resins polymerized directly or transdental, through enamel and enamel/dentin tissues.

Material and methods

Five composite resins were selected for this experiment: SureFil SDR, Dentsply (SDR), Filtek Supreme Plus, 3M ESPE (FSP), Aelite LS, Bisco (ALS), Filtek LS, 3M ESPE (FLS), and TPH, Dentsply (TPH). Thirty specimens of each material were prepared with 2- or 4-mm thickness. The specimens were light-cured (Elipar 2500, 3M ESPE) for 40 sec using three different protocols: direct or transdental, through a disc of enamel with 1 mm of thickness, and a disc of enamel and dentin with 2 mm of thickness. Eight Vickers microhardness (VH) measurements were taken from each specimen, four on top and four on bottom surface (Micromet, Buehler, 100 g per 15 sec). Data was analyzed with ANOVA three-way, Tukey HSD post-hoc (α = .05).

Results

Bottom surfaces of specimens exhibited statistically significant lower Vickers microhardness than the top surfaces for all composite resin evaluated, regardless of the curing conditions, except for the SDR when direct light-cured. Transdental light curing through enamel/dentin layer, significantly decreased VH (P<0.05) on the bottom surface of all composite groups.

Conclusion

The results of this study showed that light-curing attenuation of dental structures negatively affect the micro-hardness of composite resins.

New generation bulk-fill resin composites: Effects on mechanical strength and fracture reliability

PURPOSE

To investigate the mechanical performance and fracture reliability of new generation, bulk-fill resin composites of different viscosities.

METHODS

Forty sound maxillary premolars were prepared into Class I cavities comprised of 5 mm width × 5 mm length × 5 mm thickness. The teeth were randomly allocated into four groups (n = 10) according to the restorative material: Negative control - without restoration; Positive control - conventional resin composite (Opallis; FGM) was applied using increments of up to 2.0 mm-thick; Bulk-Regular - bulk-fill resin composite of regular viscosity (Opus Bulk Fill; FGM) was applied using a single increment of 5 mm-thick; and Bulk-Flow - a low-viscosity bulk-fill resin composite (Opus Bulk Fill Flow; FGM) was applied as the first increment with ∼3.5 mm-thick, followed by two final increments of Opallis (∼1.5 mm-thick). The teeth were stored at 37 °C, for 24 h, and submitted to a mechanical testing machine (DL500; EMIC) under a compressive loading. Work of fracture (W_f) was also obtained. All data were analyzed using ANOVA and Tukey (α = 5%). Reliability of restorations and probability of failure were analyzed by Weibull analysis.

RESULTS

The non-restored teeth showed the weakest behavior of the study. All the restored groups demonstrated similar mechanical properties to each other (p ≥ 0.242). The positive and negative controls failed exclusively within the cohesiveness of enamel/dentin, whereas the bulk-fill-based restorations showed a mixture of cohesive and mixed failures. The restored groups showed an overall similar reliability, although the Bulk-Regular group demonstrated greater characteristic strength than the positive control.

CONCLUSION

The novel bulk-fill resin composites of low and regular viscosities show promising application in the restoration of Class I cavities in premolars, demonstrating similar mechanical performance and reliability as compared with restorations prepared using conventional resin composites. From the bulk-fill materials, the version with regular viscosity presented the greatest compliant behavior of the study.

Polymerization shrinkage stress of resin-based dental materials: A systematic review and meta-analyses of composition strategies

PURPOSE

A systematic review was conducted to determine whether there were composition strategies available to reduce and control polymerization shrinkage stress development in resin-based restorative dental materials.

DATA SOURCES

This report was reported in accordance with the PRISMA Statement. Two reviewers performed a literature search up to December 2016, without restriction of the year of publication, in seven databases: PubMed, Web of Science, Scopus, SciELO, LILACS, IBECS, and BBO.

STUDY SELECTION

Only laboratory studies that evaluated polymerization shrinkage stress by direct testing were included. Pilot studies, reviews and in vitro studies that evaluated polymerization shrinkage stress by indirect methods (e.g., microleakage or cuspal deflection measurements), finite elemental analysis, or theoretical and mathematical models were excluded. Of the 6113 eligible articles, 62 studies were included in the qualitative analysis, and the meta-analysis was performed with 58 studies. The composition strategy was subdivided according to the modified part of the material: filler phase, coupling agent, or resin matrix. A global comparison was performed with random-effects models (α = 0.05). The only subgroup that did not show a statistical difference between the alternative strategy and the control was 'the use of alternative photo-initiators' (p = 0.29).

CONCLUSION

Modification of the resin matrix made the largest contribution to minimizing stress development. The technology used for decreasing stress in the formulation of low-shrinkage and bulk-fill materials was shown to be a promising application for reducing and controlling stress development.

Pulp Chamber Heating: An In Vitro Study Evaluating Different Light Sources and Resin Composite Layers

Abstract The aim of the present in vitro study was to evaluate the temperature variation inside the pulp chamber during light-activation of the adhesive and resin composite layers with different light sources. Cavities measuring 8x10 mm were prepared on the buccal surface of bovine incisors, leaving a remaining dentin thickness of 1 mm. Specimens were placed in a 37±1 °C water bath to standardize the temperature. The temperature in the pulp chamber was measured every 10 s during 40 s of light activation of the adhesive system (SBMP-3M/ESPE) and in the three consecutive 1-mm-thick layers of resin composite (Z250-3M/ESPE). Three light source devices were evaluated: Elipar 2500 (QTH), LD Max (LED low irradiance) and VALO (LED high irradiance). The results were submitted to one-way ANOVA with repeated measures and Tukey's test, both with p<0.001. The exothermic reaction warming was observed in the Z250 increments, but not in the SBMP. The high irradiance LED showed a higher temperature average (42.7±1.56 °C), followed by the quartz-tungsten-halogen light (40.6±0.67 °C) and the lower irradiance LED (37.8±0.12 °C). Higher temperature increases were observed with the adhesive and the first resin composite increment light-activation, regardless of the employed light source. From the second increment of Z250, the restorative material acted as a dispersive structure of heat, reducing temperature increases. Regardless the light source and restorative step, the temperature increased with the irradiation time. It may be concluded that the light source, irradiation time and resin composite thickness interfered in the temperature variation inside the pulp chamber.

Influence of Battery Level of a Cordless LED Unit on the Properties of a Nanofilled Composite Resin

The properties of composite resins can be influenced by light activation, depending primarily on the performance of the curing unit. The aim of this study was to evaluate how different battery levels of a cordless light-emitting diode (LED) unit influence the properties of a nanofilled composite resin. First, the battery voltage and light intensity of the cordless LED unit were individually checked for all light-curing cycles. Then, composite resin discs were prepared and light-cured at different battery levels: high level (HL, 100%), medium level (ML, 50%), and low level (LL, 10%). The degree of conversion, diametral tensile strength, sorption, and solubility of the specimens were tested. Data were checked for homoscedasticity and submitted to one-way analysis of variance followed by Tukey honestly significant difference and Pearson correlation tests (p<0.05). The battery voltage and light intensity varied significantly among the groups (p<0.001). The LL group presented a lower degree of conversion than the HL and ML groups (p<0.001), which shower similar results (p=0.182). Lower diametral tensile strength was also verified for the LL group when compared with the HL and ML groups (p<0.001), which presented no difference (p=0.052). Positive correlation was observed between the light intensity and the parameters studied, with the exception of sorption and solubility (p<0.001). The ML and LL groups showed higher sorption than the HL group (p <0.001), but no difference was verified between the first two groups (p=0.535). No significant differences were found for solubility between the ML and LL groups (p=0.104), but the HL group presented lower values (p<0.001). The different battery levels of the cordless LED curing unit influenced all the properties of the nanofilled composite resin evaluated.

Push-Out Bond Strength of Restorations with Bulk-Fill, Flow, and Conventional Resin Composites

The aim of this study was to evaluate the bond strengths of composite restorations made with different filler amounts and resin composites that were photoactivated using a light-emitting diode (LED). Thirty bovine incisors were selected, and a conical cavity was prepared in the facial surface of each tooth. All preparations were etched with Scotchbond Etching Gel, the Adper Scotchbond Multipurpose Plus adhesive system was applied followed by photoactivation, and the cavities were filled with a single increment of Filtek Z350 XT, Filtek Z350 XT Flow, or bulk-fill X-tra fil resin composite (n = 10) followed by photoactivation. A push-out test to determine bond strength was conducted using a universal testing machine. Data (MPa) were submitted to Student's t-test at a 5% significance level. After the test, the fractured specimens were examined using an optical microscope under magnification (10x). Although all three composites demonstrated a high prevalence of adhesive failures, the bond strength values of the different resin composites photoactivated by LED showed that the X-tra fil resin composite had a lower bond strength than the Filtek Z350 XT and Filtek Z350 XT Flow resin composites.