Influence of specimen diameter on the relationship between subsurface depth and hardness of a light-cured resin composite

Effects of various beverages on characteristics of provisional restoration materials

OBJECTIVE

To investigate the effect of common beverages on four currently used provisional restoration materials: Protemp®4, Integrity®, polymethyl methacrylate (PMMA) block, and acrylic resin. Flowable resin composite is included as a control group.

MATERIALS AND METHODS

Each material was formed into disks of 10-mm diameter and 4-mm thickness (N = 40) by loading the material into acrylic molds. The exposed surface in the mold was covered using a glass slide to prevent an oxygen inhibition layer, and polymerization then proceeded. The solidified disks were placed in distilled water for 24 h. These samples (n = 8) were then immersed for 14 days in one of four different beverages: water, orange juice, cola, and coffee. Changes in color dimension, hardness, and roughness were observed and then analyzed using two-way repeated analysis of variance.

RESULTS

The provisional materials had more obvious changes in all three color dimensions than the flowable resin composite. Integrity showed the biggest changes, followed by acrylic resin and PMMA block, whereas Protemp had the smallest changes. The hardness of all the materials significantly decreased after immersion in any of the beverages for 14 days. There were no changes in surface roughness when the materials were immersed in distilled water. The surface roughness of the PMMA block significantly decreased in orange juice whereas that of Integrity and acrylic resin significantly increased in cola.

CONCLUSION

Different kinds of provisional materials had different degrees of staining due to their composition. Moisture had a significant influence on the hardness of materials, and the acidity of cola significantly roughened the surface of the provisional materials.

Impact of adhesive primer and light-curing on polymerization kinetics of self-adhesive resin cement in association with free radical reaction

This study analyzed the impact of adhesive primer and light-curing on the polymerization kinetics of urethane dimethacrylate-based self-adhesive resin cement combined with free radical reaction. Specimens were prepared by mixing the cement paste with or without adhesive primer. Subsequently, specimens were light-cured or set without light-curing. The degree of conversion (DC), Vickers hardness (Hv), and free radical concentrations were repeatedly measured up to 168 h after the curing initiation. Irrespective of the curing procedures, DC, Hv, and free radical concentration rapidly increased during the initial 30 min of curing. The specimens cured with adhesive primer and/or light-curing generally showed higher values of DC, Hv, and radical concentration than those set by chemical curing alone, especially during the initial polymerization phase. Kinetic analysis using a linear mixed model revealed that the adhesive primer had a higher coefficient estimate than light-curing, indicating that the former had a higher impact on the polymerization. Additionally, the adhesive primer alleviated the Hv reduction caused by water and air during the initial polymerization phase, although light-curing hardly prevented the polymerization inhibition. Therefore, we suggest that application of adhesive primer is beneficial to achieve higher degree of conversion and better mechanical properties of self-adhesive resin cements by enhancing free radical reactions.

A Comparative Study of the Mechanical Properties of Selected Dental Composites with a Dual-Curing System with Light-Curing Composites

Dual-curing composites have a wide spectrum of use in practice (rebuilding, reconstruction, and luting). The characterization of this type of material and comparative study of selected mechanical properties with light-cured materials were carried out for this paper. In this study, we used six materials with a dual-cure system—Bulk EZ, Fill-Up!, StarFill 2B, Rebilda DC, MultiCore Flow, Activa Bioactive-Restorative—and three light-cured materials—Filtek Bulk Fill Posterior, Charisma Classic, and G-aenial Universal Flo. The materials were conditioned for 24 h in water at 37 °C before testing. Selected material properties were determined: three-point bending flexural strength, diametral tensile strength, hardness, microhardness, and shrinkage stress. The highest three-point bending flexural strength (TPB) was 137.0 MPa (G-aenial Universal Flo), while the lowest amounted to 86.5 MPa (Activa Bioactive). The diametral tensile strength (DTS) values were in a range from 39.2 MPa (Rebilda DC) to 54.1 MPa (Charisma Classic). The lowest hardness (HV) value of 26 was obtained by the Activa Bioactive material, while the highest values were recorded for Filtek Bulk Fill Posterior and Charisma Classic-53. The shrinkage stress of the tested materials ranged from 6.3 MPa (Charisma Classic) to 13.2 MPa (G-aenial Universal Flo). Dual-curing composites were found to have similar properties to light-cured composites.

Neurotic personality trait as a predictor in the prognosis of composite restorations: A 24-month clinical follow up study

The role of personality traits in modulating the incidence and progression of medical disease conditions are well documented, however, there is a paucity of information for its effects on dental health conditions and specifically on the prognosis of restorative dental materials. This study aims to evaluate the clinical performance of Micro-hybrid and Nano-ceramic composite restorations among patients with different personality traits. A total of 323 patients, indicated to receive operative treatment at a University Dental College Hospital, were invited to participate in this study. Consenting patients were requested to complete the Big Five Inventory (BFI-44 Item) personality questionnaire and were evaluated by a psychiatrist for categorizing the participants based on their personality traits. Out of the recruited patients, 124 patients falling in to the dominant trait of Agreeableness (n = 62) and Neuroticism (n = 62) were included in the study for further investigation. Next, patients from the Agreeableness (Group A) and the Neuroticism personality trait group (Group N) were randomly divided into two subgroups each—sub group Am (n = 44) and Nm (n = 48) for Micro-hybrid composite restorations and Sub group An (n = 42) and Nn (n = 47) for Nano-ceramic composite restorations. Two trained and calibrated dentists prepared the cavities according to previously published methodology. The restorations were evaluated at baseline (immediately after restoration), 6-months, 12-months and 24-months intervals by two blinded independent dental professionals for anatomical form, secondary caries, color match, retention, marginal adaptation, surface texture, marginal discoloration and post-operative sensitivity. There is no statistically significant difference noted in various parameters of restoration performance between Micro-hybrid composite and Nano-ceramic composite compared among ‘agreeableness’ personality group and among ‘neuroticism’ personality group after controlling the personality trait factor. Higher ‘Neuroticism’ individuals had higher restoration deterioration in color matching and surface texture when compared to higher ‘Agreeableness’ trait individuals. Regression analysis showed no effect of gender or cavity size on the outcome of results. Assessment of personality traits may serve as a useful tool during treatment planning which would aid clinicians in choosing suitable restorative dental material and prosthesis design according to individual patient’s physiological and functional needs, thereby overall improving the quality of treatment provided.

Bioactivity and Physico-Chemical Properties of Dental Composites Functionalized with Nano- vs. Micro-Sized Bioactive Glass

Bioactive resin composites can contribute to the prevention of secondary caries, which is one of the main reasons for failure of contemporary dental restorations. This study investigated the effect of particle size of bioactive glass 45S5 on chemical and physical composite properties. Four experimental composites were prepared by admixing the following fillers into a commercial flowable composite: (1) 15 wt% of micro-sized bioactive glass, (2) 15 wt% of nano-sized bioactive glass, (3) a combination of micro- (7.5 wt%) and nano-sized (7.5 wt%) bioactive glass, and (4) 15 wt% of micro-sized inert barium glass. Hydroxyapatite precipitation and pH rise in phosphate-buffered saline were evaluated during 28 days. Degree of conversion and Knoop microhardness were measured 24 h after specimen preparation and after 28 days of phosphate-buffered saline immersion. Data were analyzed using non-parametric statistics (Kruskal-Wallis and Wilcoxon tests) at an overall level of significance of 5%. Downsizing the bioactive glass particles from micro- to nano-size considerably improved their capability to increase pH. The effect of nano-sized bioactive glass on degree of conversion and Knoop microhardness was similar to that of micro-sized bioactive glass. Composites containing nano-sized bioactive glass formed a more uniform hydroxyapatite layer after phosphate-buffered saline immersion than composites containing exclusively micro-sized particles. Partial replacement of nano- by micro-sized bioactive glass in the hybrid composite did not impair its reactivity, degree of conversion (p > 0.05), and Knoop microhardness (p > 0.05). It is concluded that downsizing bioactive glass particles to nano-size improves the alkalizing potential of experimental composites with no negative effects on their fundamental properties.

Light Transmittance and Polymerization of Bulk-Fill Composite Materials Doped with Bioactive Micro-Fillers

This study investigated the effect of bioactive micro-fillers on the light transmittance and polymerization of three commercially available bulk-fill resin composites. These were mixed with 20 wt% bioactive glass 45S5, Portland cement, inert dental barium glass, or nothing (controls). Composites were photo-activated and light transmittance through 4 mm thick specimens was measured in real time. Moreover, degree of conversion (DC) and Knoop hardness (KHN) were assessed. Light transmittance of all bulk-fill composites significantly decreased (p < 0.05) with addition of 20 wt% bioactive glass 45S5 but not when inert barium glass was added. For bulk-fill composites modified with Portland cement, light irradiance dropped below the detection limit at 4 mm depth. The DC at the top surface of the specimens was not affected by addition of bioactive or inert micro-fillers. The bottom-to-top ratio of both DC and KHN surpassed 80% for bulk-fill composites modified with 20 wt% bioactive or inert glass fillers but fell below 20% when the composites were modified with Portland cement. In contrast to Portland cement, the addition of 20 wt% bioactive glass maintains adequate polymerization of bulk-fill composites placed at 4 mm thickness, despite a decrease in light transmittance compared to the unmodified materials.

Comparisons of ISO depth of cure for a resin composite in stainless-steel and natural-tooth molds

The purpose of this study was to compare the depth of cure (DOC) of a resin-based composite (RBC) using the ISO DOC protocol with stainless-steel and molar-tooth molds (4 mm cylindrical cavity). The tooth mold included testing with and without the occlusal surface being covered with black tape around the cavity opening. The RBC was cured with either halogen (HAL) or light-emitting diode (LED) light. The results showed that specimens made in the non-taped tooth mold had DOCs that were significantly greater (28%-35%) than those in the stainless-steel mold. The taped tooth mold also produced significantly greater DOCs, but only by 6%-8%. Knoop hardness (KNH) measurements along the central axis of the RBC specimens showed that depths for 80% of maximum hardness were substantially greater than those determined by the ISO DOC protocol but were limited to the center and quickly dropped below 80% in a lateral direction. The KHN mapping for each of the three molds found that the ISO DOCs could validate a KHN of ≥80% across the RBC to the periphery, only for the non-taped tooth mold. This was due to light incident on the tooth surrounding the RBC being scattered into the RBC.

Influence of water aging on surface hardness of low-shrinkage light-cured composite resins

This study evaluated the microhardness of four types of low-shrinkage composite resins and two types of universal composite resins with either 12 or 16 J/cm² light power energy. Three disks were made for each group (n = 3) for a total of 36 specimens. The specimens were prepared by condensing the composite resin into a circular copper mold (diameter: 6 mm; height: 2 mm) and polymerizing with 700 mW/cm² light power density. The microhardness values of the resin specimens were measured using a Vickers hardness tester after different storage durations. Z250 and Clearfil Majesty Posterior composites showed softer subsurfaces when comparing the 24 h samples for all six types. Conversely, Kalore GC and Admira (AD) composites showed harder subsurfaces during the 24 h samples. All the composite resins showed significant differences (P < 0.05) in microhardness values at one of the aging times when they were polymerized with either 12 or 16 J/cm² light power energy. Composite resin AD had a higher microhardness value after polymerization with 12 J/cm² than with 16 J/cm². The results indicated that low-shrinkage composite resins have better subsurface characteristics, and the light power energy of 16 J/cm² is better for the polymerization of most composites.

Filler Content, Surface Microhardness, and Rheological Properties of Various Flowable Resin Composites

Objectives:

The objectives of this study were to determine the filler content, the surface microhardness (at baseline and after immersion in water for 2 years), and the rheological properties of various flowable resin composites.

Methods:

Three flowable resin composites (Grandioso Heavy Flow [GHF], Grandio Flow [GRF], Filtek Supreme XTE Flow [XTE]), one pit and fissure sealant resin composite (ClinPro [CLI]), and three experimental flowable resin composites with the same matrix and a variable filler content (EXPA, EXPB, EXPC) were tested. The filler content was determined by calcination. The Vickers surface microhardness was determined after polymerization and then after immersion in distilled water at 37°C for 7, 60, 180, 360, and 720 days. The rheological measurements were performed using a dynamic shear rheometer.

Results:

The determined filler contents differed from the manufacturers' data for all the materials. The materials with the highest filler content presented the highest microhardness, but filler content did not appear to be the only influencing parameter. With respect to the values recorded after photopolymerization, the values were maintained or increased after 720 days compared with the initial microhardness values, except for GHF. For the values measured after immersion for 7 days, an increase in microhardness was observed for all the materials over time. All the materials were non-Newtonian, with shear-thinning behavior. At all the shear speeds, GRF presented a lower viscosity to GHF and XTE.

Conclusions:

GRF presented a low viscosity before photopolymerization, associated with high filler content, thereby providing a good compromise between spreadability and mechanical properties after photopolymerization.

Effects of Different Temperatures and Storage Time on the Degree of Conversion and Microhardness of Resin-based Composites

OBJECTIVE

Dental materials are often made at room temperature, whereas clinically they are made in the mouth. This study evaluated the effects of temperature on the degree of conversion (DC) and Knoop microhardness (KHN).

MATERIALS AND METHODS

Two types of resin-based composites (RBCs) were light-cured using a light-emitting diode (LED) light-curing unit. The resin specimens were centered on an Attenuated Total Reflectance Fourier transform infrared (FT-IR) plate heated to 23°C or 33°C. The DC of the resin was calculated after 120 seconds, the specimens were removed, and the KHN was tested at the bottom of the specimens both immediately, after 24 hours, and after 7 days storage in distilled water in complete darkness at 37°C. The effects of different temperatures on the DC and KHN with their storage time were compared by analysis of variance and Fisher's protected least significant difference post hoc multiple comparison tests (p < 0.05).

RESULTS

Increasing the temperature had a significant and positive effect on the DC and KHN for immediate values of the RBCs. Greater conversion and hardness occurred when the curing temperature was increased from 23°C to 33°C. The KHN increased significantly after 24 hours of storage. There was a linear relationship between DC and KHN (R(2) = 0.86) within the range of DC and KHN studied.

CONCLUSION

The physical properties of dental materials can be expected to be better when made in the mouth than when they are made in a laboratory at room temperature.

Effect of High Irradiance on Depth of Cure of a Conventional and a Bulk Fill Resin-based Composite

OBJECTIVES

This study evaluated the effect of using three commercial light curing units (LCUs) delivering a range of irradiance values, but delivering similar radiant exposures on the depth of cure of two different resin-based composites (RBCs).

METHODS

A conventional hybrid RBC (Z100 shade A2, 3M ESPE) or a bulk fill RBC (Tetric EvoCeram Bulk Fill shade IVA, Ivoclar Vivadent) was packed into a 10-mm deep semicircular metal mold with a 2-mm internal radius. The RBC was exposed to light from a plasma-arc-curing (PAC) light (Sapphire Plus, DenMat) for five seconds, a quartz-tungsten-halogen (QTH) light (Optilux 501, Kerr) for 40 seconds, or a light-emitting-diode (LED) light (S10, 3M ESPE) for 20 seconds and 40 seconds (control). The Knoop microhardness was then measured as soon as possible at the top surface and at three points every 0.5 mm down from the surface. For each RBC, a repeated measures analysis of variance (ANOVA) model was used to predict the Knoop hardness in a manner analogous to a standard regression model. This predicted value was used to determine at what depth the RBC reached 80% of the mean hardness achieved at the top surface with any light.

RESULTS

The PAC light delivered an irradiance and radiant exposure of 7328 mW/cm(2) and 36.6 J/cm(2), respectively, to the RBCs; the QTH light delivered 936 mW/cm(2) and 37.4 J/cm(2) and in 20 seconds the LED light delivered 1825 mW/cm(2) and 36.5 J/cm(2). In 40 seconds, the control LED light delivered a radiant exposure of 73.0 J/cm(2). For Z100, using 80% of the maximum hardness at the top surface as the criteria for adequate curing, all light exposure conditions achieved the 2.0-mm depth of cure claimed by the manufacturer. The LED light used for 40 seconds achieved the greatest depth of cure (5.0 mm), and the PAC light used for five seconds, the least (2.5 mm). Tetric EvoCeram Bulk Fill achieved a 3.5-mm depth of cure when the broad-spectrum QTH light was used for 40 seconds delivering 37.4 J/cm(2). It required a 40-second exposure time with the narrow-spectrum LED, delivering approximately 73 J/cm(2) to reach a depth of cure of 4 mm.

CONCLUSIONS

When delivering a similar radiant exposure of 37 J/cm(2), the QTH (40 seconds) and LED (20 seconds) units achieved a greater depth of cure than the PAC (five seconds) light. For both resins, the greatest depth of cure was achieved when the LED light was used for 40 seconds delivering 73 J/cm(2) (p<0.05).

Monomer conversion and shrinkage force kinetics of low-viscosity bulk-fill resin composites

OBJECTIVE

To investigate the subsurface degree of conversion (DC) and shrinkage force formation of low-viscosity (flowable) bulk-fill composite materials.

MATERIALS AND METHODS

Three flowable bulk-fill resin composites [SureFil SDR flow (SDR; Dentsply DeTrey), Venus Bulk Fill (VB; Heraeus Kulzer) and x-tra base (XB; VOCO)] and one conventional flowable control composite material [EsthetX flow (EX; Dentsply DeTrey)] were tested. The materials were photoactivated for 20 s at an irradiance of 1170 mW/cm2 and the DC (n=5) was recorded at 0.1-, 1.5- and 4-mm depth using Fourier transform infrared spectroscopy. Shrinkage forces (n=5) of 1.5-mm-thick specimens were continuously recorded for 15 min using a custom-made stress analyzer. Data were statistically analyzed by ANOVA, Tukey's HSD and Bonferroni's post-hoc tests (α=0.05).

RESULTS

SDR generated the significantly lowest shrinkage forces (22.9±1.4 N), but also attained the significantly lowest DC at 1.5-mm depth (67.5±0.8%). The conventional flowable composite EX generated the significantly highest shrinkage forces (40.7±0.7 N) and reached a significantly higher DC (74.4±1.3%) compared to SDR and XB at 1.5-mm depth. The shrinkage force values of VB (29.4±1.1 N) and XB (28.3±0.6 N) were similar (p>0.05). All materials attained significantly higher DC at 4-mm depth than at the near-surface.

CONCLUSION

The tested low-viscosity bulk-fill materials show lower shrinkage force formation than a conventional flowable resin composite at high levels of degree of conversion up to 4-mm incremental thickness.

Influence of photoactivation source on restorative materials and enamel demineralization

OBJECTIVE

The objective of this study was to evaluate the influence of the photoactivation source on the polymerization depth of restorative materials and its effects on resistance to enamel demineralization.

BACKGROUND DATA

Argon-ion laser (AL) irradiation itself provides a reduced depth of caries lesions in sound enamel.

METHODS

Eighteen human teeth were sectioned into 36 blocks and distributed into two groups according to the respective restorative material: resin-modified glass ionomer material (RMGI) (Vitremer-3M ESPE; A3; n=18) and composite resin (CR) (Z350-3M ESPE; n=18). Each group was subdivided into three subgroups and activated by a quartz-tungsten-halogen (QTH) lamp, an AL, or a light-emitting diode (LED) (n=6). Knoop microhardness (KHN) analysis of the materials was evaluated at two different depths: 0 and 1.6 mm from the enamel surface. The blocks were thermocycled and submitted to five demineralization-remineralization cycles at 37°C. The KHN values of the enamel surface (0 mm) were evaluated. The specimens were longitudinally sectioned, and the restorative material was evaluated at a depth of 1.6 mm. Data were evaluated by two way analysis of variance (ANOVA) and Tukey tests (p<0.05). The evaluation of subsuperficial enamel demineralization by KHN analysis was conducted by seven indentations located at 100 μm from the restored cavity. Data were evaluated by three way ANOVA and Tukey tests (p<0.05).

RESULTS

Comparing the two restorative materials, the KHN values at the surface (0 mm) were greater for CR, whereas at 1.6 mm, they were greater for RMGI. In addition, there was less development of enamel demineralization around RMGI restorations than CR restorations. Moreover, there were statistically significant differences on subsuperficial enamel demineralization between the two restorative materials and between the three photoactivation methods (p<0.05); RMGI presented the highest KHN values, and QTH and AL presented the lowest.

CONCLUSIONS

The photoactivation source did not influence superficial enamel demineralization, but LED activation positively influenced the subsuperficial microhardness of enamel.

Curing characteristics of a composite. part 2: the effect of curing configuration on depth and distribution of cure

OBJECTIVE

The objective of this study was to examine the effect different configurations of curing would have on the depth and distribution of the cure within each configuration, for a specific resin-based composite (RBC).

METHODS

RBC was cured in a variety of configurations, consisting of 6mm molds of three different colors; large molds that simulated the condition of no mold at all; and 3-6mm diameter molds to check the effect of size. All specimens were cured for 20s with a quartz-halogen lamp and were allowed to cure for 24h in the dark. Transmission measurements were made for these same configurations. Knoop hardness measurements were made across the central plane of some configurations to determine the distribution of curing.

RESULTS

Depths of cure and distribution of curing were significantly affected by changes in configuration. Under the configuration of no mold, the cure extended well beyond the periphery of the light guide due to scattering of the light. When a mold was used, a pronounced effect by the walls resulted in decreased hardness as the mold wall was approached, and the severity of this effect was dependent on the color of the mold. It is believed that this is due to absorption/reflection characteristics of light by the walls, with the white molds showing the least effect. Reducing the diameter of the molds resulted in significant decreases in depth of cure, which are attributed to light absorption by the walls that limits the penetration of light during the curing procedure.

SIGNIFICANCE

Configuration of curing has a significant effect on the depth of cure, but also significantly reduces the cure near the mold wall. This can have clinical ramifications for the cure along a stainless steel matrix band for Class II restorations, and for test procedures in general, where there is no standardization regarding configuration or where measurements are made on specimens.

Surface hardness evaluation of different composite resin materials: influence of sports and energy drinks immersion after a short-term period

OBJECTIVES

This study evaluated the effect of sports and energy drinks on the surface hardness of different composite resin restorative materials over a 1-month period.

MATERIAL AND METHODS

A total of 168 specimens: Compoglass F, Filtek Z250, Filtek Supreme, and Premise were prepared using a customized cylindrical metal mould and they were divided into six groups (N=42; n=7 per group). For the control groups, the specimens were stored in distilled water for 24 hours at 37°C and the water was renewed daily. For the experimental groups, the specimens were immersed in 5 mL of one of the following test solutions: Powerade, Gatorade, X-IR, Burn, and Red Bull, for two minutes daily for up to a 1-month test period and all the solutions were refreshed daily. Surface hardness was measured using a Vickers hardness measuring instrument at baseline, after 1-week and 1-month. Data were statistically analyzed using Multivariate repeated measure ANOVA and Bonferroni's multiple comparison tests (α=0.05).

RESULTS

Multivariate repeated measures ANOVA revealed that there were statistically significant differences in the hardness of the restorative materials in different immersion times (p<0.001) in different solutions (p<0.001). The effect of different solutions on the surface hardness values of the restorative materials was tested using Bonferroni's multiple comparison tests, and it was observed that specimens stored in distilled water demonstrated statistically significant lower mean surface hardness reductions when compared to the specimens immersed in sports and energy drinks after a 1-month evaluation period (p<0.001). The compomer was the most affected by an acidic environment, whereas the composite resin materials were the least affected materials.

CONCLUSIONS

The effect of sports and energy drinks on the surface hardness of a restorative material depends on the duration of exposure time, and the composition of the material.

New insight into the "depth of cure" of dimethacrylate-based dental composites

OBJECTIVES

To demonstrate that determination of the depth of cure of resin-based composites needs to take into account the depth at which the transition between glassy and rubbery states of the resin matrix occurs.

METHODS

A commercially available nano-hybrid composite (Grandio) in a thick layer was light cured from one side for 10 or 40 s. Samples were analyzed by Vickers indentation, Raman spectroscopy, atomic force microscopy, electron paramagnetic imaging and differential scanning calorimetry to measure the evolution of the following properties with depth: microhardness, degree of conversion, elastic modulus of the resin matrix, trapped free radical concentration and glass transition temperature. These measurements were compared to the composite thickness remaining after scraping off the uncured, soft composite.

RESULTS

There was a progressive decrease in the degree of conversion and microhardness with depth as both properties still exhibited 80% of their upper surface values at 4 and 3.8 mm, respectively, for 10 s samples, and 5.6 and 4.8 mm, respectively, for 40 s samples. In contrast, there was a rapid decrease in elastic modulus at around 2.4 mm for the 10 s samples and 3.0 mm for the 40 s samples. A similar decrease was observed for concentrations of propagating radicals at 2 mm, but not for concentrations of allylic radicals, which decreased progressively. Whereas the upper composite layers presented a glass transition temperature - for 10 s, 55°C (±4) at 1 mm, 56.3°C (±2.3) at 2 mm; for 40 s, 62.3°C (±0.6) at 1 mm, 62°C (±1) at 2 mm, 62°C (±1.7) at 3 mm - the deeper layers did not display any glass transition. The thickness remaining after scraping off the soft composite was 7.01 (±0.07 mm) for 10 s samples and 9.48 (±0.22 mm) for 40 s samples.

SIGNIFICANCE

Appropriate methods show that the organic matrix of resin-based composite shifts from a glassy to a gel state at a certain depth. Hence, we propose a new definition for the "depth of cure" as the depth at which the resin matrix switches from a glassy to a rubbery state. Properties currently used to evaluate depth of cure (microhardness, degree of conversion or scraping methods) fail to detect this transition, which results in overestimation of the depth of cure.

Influence of light curing and sample thickness on microhardness of a composite resin

The aim of this in vitro study was to evaluate the influence of light-curing units and different sample thicknesses on the microhardness of a composite resin. Composite resin specimens were randomly prepared and assigned to nine experimental groups (n = 5): considering three light-curing units (conventional quartz tungsten halogen [QTH]: 550 mW/cm² – 20 s; high irradiance QTH: 1160 mW/cm² – 10 s; and light-emitting diode [LED]: 360 mW/cm² – 40 s) and three sample thicknesses (0.5 mm, 1 mm, and 2 mm). All samples were polymerized with the light tip 8 mm away from the specimen. Knoop microhardness was then measured on the top and bottom surfaces of each sample. The top surfaces, with some exceptions, were almost similar; however, in relation to the bottom surfaces, statistical differences were found between curing units and thicknesses. In all experimental groups, the 0.5-mm-thick increments showed microhardness values statistically higher than those observed for 1- and -2-mm increments. The conventional and LED units showed higher hardness mean values and were statistically different from the high irradiance unit. In all experimental groups, microhardness mean values obtained for the top surface were higher than those observed for the bottom surface. In conclusion, higher levels of irradiance or thinner increments would help improve hybrid composite resin polymerization.

Halogen and LED light curing of composite: temperature increase and Knoop hardness

This study assessed the Knoop hardness and temperature increase provided by three light curing units when using (1) the manufacturers' recommended times of photo-activation and (2) standardizing total energy density. One halogen--XL2500 (3M/ESPE)--and two light-emitting diode (LED) curing units--Freelight (3M/ESPE) and Ultrablue IS (DMC)--were used. A type-K thermocouple registered the temperature change produced by the composite photo-activation in a mold. Twenty-four hours after the photo-activation procedures, the composite specimens were submitted to a hardness test. Both temperature increase and hardness data were submitted to ANOVA and Tukey's test (5% significance). Using the first set of photo-activation conditions, the halogen unit produced a statistically higher temperature increase than did both LED units, and the Freelight LED resulted in a lower hardness than did the other curing units. When applying the second set of photo-activation conditions, the two LED units produced statistically greater temperature increase than did the halogen unit, whereas there were no statistical differences in hardness among the curing units.