Some factors influencing the depth of cure of visible light-activated composite resins

Properties of A Model Self-Healing Microcapsule-Based Dental Composite Reinforced with Silica Nanoparticles

Aim: The purpose of this study was to evaluate the mechanical properties of an experimental self-healing dental composite model (SHDC) composed of SiO₂ nanoparticles with varying percentages of triethylene glycol dimethacrylate (TEGDMA) monomer and N,N-dihydroxyethyl-p-toluidine (DHEPT) amine microcapsules. Materials and methods: Microcapsules were prepared by in-situ polymerisation of PUF shells, as explained in our previous work. The model SHDC included bisphenol A glycidyl dimethacrylate (Bis-GMA:TEGDMA) (1:1), 1 wt% phenyl bis(2,4,6-trimethylbenzoyl) phosphine oxide (BAPO), 0.5 wt% benzoyl peroxide (BPO) catalyst, 20 wt% silanised silica dioxide (SiO₂) (15 nm) and (0, 2.5, 5, 7.5, 10 wt%) of microcapsules (120 ± 45 μm). Light transmission, hardness, degree of conversion (DC), flexural strength and elastic modulus of the SHDC model were measured. Results: The degree of conversion of the SHDC ranged from 73 to 76% 24 h after polymerisation. Hardness measurements ranged from 22 to 26 VHN (p > 0.05); however, the flexural strength was adversely affected from 80 to 55 MPa with increasing microcapsules of up to 10 wt% in the composites (p < 0.05). Conclusion: Only flexural strength decreased drastically ~30% with increasing microcapsules (>10 wt%) in the composites. All other measured properties were not significantly affected. Accordingly, we recommend a stronger composite material that could be created by increasing the filler content distribution in order to achieve a hybrid self-healing composite with enhanced mechanical properties.

Thermal, structural and morphological characterization of dental polymers for clinical applications

PURPOSE

Polymers are used in dentistry on a daily basis due to their mechanical, functional and aesthetic properties. However, such biomaterials are subject to deterioration in the oral environment. Thus, this study aimed to evaluate the structural properties of five commonly used dental polymers to determine their best clinical indications.

METHODS

Four hundred-fifty samples of five dental polymers (polyethylenterephthalat - glycol modified (PG), polymethyl methacrylate (PA), ethylene vinyl acetate(E), polycarbonate (PC), polyetheretherketone (PK) were prepared to investigate their thermal, structural and chemical characteristics using energy dispersive spectroscopy (EDS), Fourier transform infrared analysis(FTIR), scanning electron microscopy (SEM), differential scanning calorimetry(DSC), thermogravimetric analysis(TGA), X-ray diffraction(XRD), and Shore D hardness test. Data were analyzed using one-way ANOVA, Tukey's HSD, and Levene's tests (α=0.05).

RESULTS

PK (87.2) and PA (82.4) displayed the highest hardness values and smooth surfaces, as observed with SEM (p<0.001). Silica was detected in PK, PA, and E by EDS and XRD. The highest glass transition temperature was recorded for PC (145.00±2.00°C) and PK (143.00±1.87°C), while the lowest value was measured for E (50.00±2.12°C)(p<0.001).The highest mass loss was detected for PG (91.40±1.40%) by TGA.

CONCLUSIONS

PA and PK polymers can be used for stress-containing treatments due to their mechanical properties. These two materials are also advantageous in terms of plaque accumulation as these polymers reveal smoother surfaces than other groups. Insufficient physical and thermal properties require the use of E with caution and only in limited clinical indications.

Physicomechanical and thermal analysis of bulk-fill and conventional composites

The aim of this study was to evaluate the degree of conversion (DC) and the thermal stability of bulk-fill and conventional composite resins. Eleven composite resin samples were prepared to evaluate the DC, Vickers microhardness (VMH), mass and residue/particle loss, glass transition temperature (Tg), enthalpy, and linear coefficient of thermal expansion (CTE) using infrared spectroscopy (FTIR), microdurometer analyses, thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and dilatometry (DIL). The data were subjected to statistical analysis, with a significance level of 95%. DC and VMH were not influenced by the polymerized side of the sample, and statistical differences were recorded only among the materials. Decomposition temperature, melting, and mass and residue loss were dependent on the material and on the evaluation condition (polymerized and non-polymerized). Tg values were similar between the composites, without statistically significant difference, and CTE ranged from 10.5 to 37.1 (10-6/°C), with no statistical difference between the materials. There was a moderate negative correlation between CTE and the % of load particles, by weight. Most resins had a DC above that which is reported in the literature. TGA, Tg, and CTE analyses showed the thermal behavior of the evaluated composites, providing data for future research, assisting with the choice of material for direct or semidirect restorations, and helping choose the appropriate temperature for increasing the DC of such materials.

Factors affecting polymerization of resin-based composites: A literature review

AIM

The aim of this review was to help clinicians improve their understanding of the polymerization process for resin-based composites (RBC), the effects of different factors on the process and the way in which, when controlled, the process leads to adequately cured RBC restorations.

METHODS

Ten factors and their possible effects on RBC polymerization are reviewed and discussed, with some recommendations to improve that process. These factors include RBC shades, their light curing duration, increment thickness, light unit system used, cavity diameter, cavity location, light curing tip distance from the curing RBC surface, substrate through which the light is cured, filler type, and resin/oral cavity temperature.

CONCLUSION

The results of the review will guide clinicians toward the best means of providing their patients with successfully cured RBC restorations.

Influence of light-activation protocol on methacrylate resin-composite evaluated by dynamic mechanical analysis and degree of conversion

The aim of this study was to evaluate the degree of conversion (DC) and to identify the viscoelastic properties: storage modulus (E'), loss modulus (E"), tangent delta (tan δ), and glass transition temperature (T g ) of a microhybrid resin-composite light-activated by three different protocols. A Filtek Z250 (3 M ESPE) shade A3 was inserted in a Teflon mold (21 mm × 5 mm × 1 mm for viscoelastic properties; and 5 mm × 1 mm for DC) and light-activated according to the following light-activation protocols: (S) 1,000 mW/cm(2) × 19 s, (HP) 1,400 mW/cm(2) × 14 s, and (PE) 3,200 mW/cm(2) × 6 s, all set up to deliver 19 J/cm(2). Viscoelastic properties was assessed by dynamic mechanical analysis (DMA) (n = 3), performed in single cantilever clamped mode. DC (n = 5) was measured by FTIR on top (T) and bottom (B) surfaces, and the data was submitted to a split-plot one-way ANOVA. For DC, there was a significant effect for surface factor and light-activation protocols factor. Top surface showed higher DC than B in all experimental conditions. Light-activation protocols S and HP resulted in higher DC than PE and were similar between them. Viscoelastic properties (E', E", tan δ, T g ) were not affected by light-activation protocols. It could be concluded that light-activation protocols influenced DC but not influenced the viscoelastic properties.

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

OBJECTIVE

The contraction stress, degree of conversion, and cross-link density (CLD) of the Venus Diamond (low-shrinkage), Filtek P90 (low-shrinkage) and Filtek Z350 XT composites were evaluated after photopolymerization by quartz tungsten halogen or light-emitting diode light curing units.

MATERIALS AND METHODS

Contraction stress measurements were performed on 60 samples fabricated in rings of photoelastic resin. The adhesive was applied and photoactivated, followed by insertion and photoactivation of the composites. The contraction stress (MPa) was measured using a polariscope. The measurements of degree of conversion (%DC) were determined from Fourier transform infrared (FTIR) spectra of the top and bottom surfaces on 60 specimens. Cross-link density was estimated from hardness measurements performed at the top and bottom surfaces on 60 specimens. The Knoop hardness number was measured, and the specimens were placed in absolute ethanol for 24 h. The hardness was again determined and the CLD was estimated from the percentage decrease in hardness (PD) occurring during ethanol exposure for each surface. The contraction stress and PD data were subjected to ANOVA and Tukey's test (5%). The DC data were subjected to one way analysis of variance on ranks followed by pairwise multiple comparisons using Tukey's test (5%).

RESULTS

The Venus Diamond composite exhibited lower contraction stress than other composites, with degrees of conversion similar to those of Filtek Z350 XT at both surfaces, and independent of the light curing unit. The PD value of Venus Diamond was also lower than that for the other composites.

CONCLUSIONS

The low-shrinkage Venus Diamond composite may potentially reduce stress at the restoration/tooth interface.

Effect of polymerization mode and time of adhesive system on microleakage in composite resin restorations

AIM

To quantify the microleakage on restorations carried out with a two-step etch-and-rise adhesive system and nanofilled composite resin after thermal and mechanical loading cycling.

METHODS

Ninety cavities were prepared on proximal surfaces of incisive teeth, and were randomly divided according to the photoactivation time (10, 20, or 30 s) and light-curing mode (Quartz Tungsten Halogen (QTH) lamps, 450 mW/cm(2); Light Emitting Diode (LED) second generation, 1100 mW/cm(2); or LED third generation, 700 mW/cm(2)) of an adhesive system (n = 10). Following restorative procedures and thermal and mechanical loading cycling, the samples were immersed in Methylene Blue for 2 h. The samples were ground, and the powder was prepared for analysis in an absorbance spectrophotometer. All results were statistically analyzed by anova and Tukey's test at the 5% level of significance.

RESULTS

There were no significant statistical difference in microleakage between the light-curing mode (P = 0.1212) and light-curing time (P = 0.2043).

CONCLUSION

Different curing modes and increasing the curing time were not factors that influenced the process of microleakage.

Halogen light versus LED for bracket bonding: shear bond strength

INTRODUCTION: LED light-curing devices seek to provide a cold light activator which allows protocols of material polymerization with shorter duration. OBJECTIVE: The present study aimed to evaluate the shear bond strength of bracket bonding using three types of light-curing devices: One with halogen light (Optilight Plus - Gnatus) and two with LEDs (Optilight CL - Gnatus and Elipar Freelight - 3M/ESPE). RESULTS: Comparing the results by analysis of variance, the Gnatus LED device showed an inferior statistical behavior in relation to other light sources, when activated by a short time. But, when it was used for 40 seconds, the polymerization results were consistent with the other evaluated sources. The device with the best average performance was the halogen light, followed by the 3M/ESPE LED. CONCLUSION: It was concluded that the LEDs may be indicated in orthodontic practice, as long as a protocol is used for the application of light with the activation time of 40 seconds.

Effect of ceramic shade on the degree of conversion of a dual-cure resin cement analyzed by FTIR

OBJECTIVES

The aim of this research was to evaluate the degree of monomer conversion of different resin cement shades when photocured under different feldspathic ceramic shades. The photocuring time was also evaluated as well as the translucency of each ceramic shade.

METHODS

Three VITA VM7 ceramic shades (Base Dentin 0M1, Base Dentin 2M2 and Base Dentin 5M3) were used to determine the translucency percentage. A spectrophotometer MiniScan was used to measure the opacity percentage of each specimen (2-mm-thick) and then the translucency was calculated. To measure the degree of conversion (DC), the resin cement (Variolink II; A3 Yellow and transparent) specimens (thickness: 100 μm) were photocured under a ceramic block (2-mm-thick) for 20 or 40s. Specimens photocured without the ceramic block were used as control. Sixteen groups (n=3) were evaluated. Micro-ATR/FTIR spectrometry was used to evaluate the extent of polymerization of all specimens after 24h. The %DC was calculated of experimentally polymerized versus maximally polymerized composite.

RESULTS

The translucency percentages of 0M1, 2M2 and 5M3 ceramics were 12.41 (1.02)%, 5.75 (1.91)% and 1.07 (0.03)%, respectively. The %DC of both resin cement shades cured under ceramic 5M3 was significantly lower than the other groups (p<0.05). The %DC of 0M1 groups exhibited no significant difference from 2M2 groups (p>0.05), with the exception of the transparent cement photocured for 40s.

CONCLUSION

Photocuring under 2mm ceramic showed that the increase in chroma saturation significantly decreased Variolink II resin cement %DC (100-μm-thick).

Evaluation of the light energy transmission and bottom/top rate in silorane and methacrylate-based composites with different photoactivation protocols

AIM

This study investigated the influence of different composite resin organic matrix (methacrylate - Filtek Z350 XT and silorane - Filtek P90) on light energy transmission through the composite and bottom/top rate.

MATERIALS AND METHODS

A light-emitting diode (New Blue Phase), light-curing unit was used with different photoactivation protocols (high-continuous mode - HCM, 1400 mW/cm2 for 20 seconds; low-continuous mode - LCM , 700 mW/cm2 for 40 seconds; and soft-start mode - SSM, 140 mW/cm2 for 5s followed by 39 seconds for 700 mW/cm2). Twenty specimens were prepared for each composite. The light energy transmission through the composite was calculated (n=10). The bottom/top rate of the same specimen was calculated (n=10). The data were compared by Tukey's test in different tests (light energy transmission through the composite and bottom/top rate).

RESULTS

The light energy transmission through the Filtek Z350 XT composite (HCM - 576 mW/cm2, LCM - 238 mW/cm2, SSM - 232 mW/cm2) did not show statistical difference when compared with Filtek P90 composite (HCM - 572 mW/cm2, LCM - 233 mW/cm2, SSM - 230 mW/cm2). The bottom/top rate of the Filtek Z350 XT composite (HCM - 88.98%, LCM - 90.94%, SSM - 89.92%) was statistically higher than that of the Filtek P90 composite (HCM-77.29%, LCM-77.51%, SSM- 77.79%).

CONCLUSION

Light energy transmission through the composite was not influenced by the use of different dental composite restoratives. However, the bottom/top rate of the composites was influenced by the use of different dental composite restoratives.

CLINICAL SIGNIFICANCE

Insufficiently polymerized composite resin may present a large number of problems. For this reason, dental composite resins should have the similar deep surface polymerization as the top surface in dental restorations.

Light energy transmission through composite influenced by material shades

This study investigated the influence of different composite resin shades on light energy transmission through the composite, hardness and cross-link density (CLD). The composite Filtek Z250 was used in shades A1, A2, A3, A3.5 and A4. A quartz tungsten halogen light curing unit was used at an irradiance of 900 mW/cm(2). Thirty specimens were made for each shade. Light energy transmission that passed through the composite was calculated (n=10). Differences in Knoop hardness between the top and bottom (DKH) of the same specimen were calculated (n=10). The Knoop hardness value for each surface was recorded as the average of three indentations (KHN(1)). Thereafter, the specimens were soaked in absolute ethanol for 24hr at room temperature, and hardness was again determined (KHN(2)). The CLD was estimated by the softening effect produced by ethanol, i.e., by decrease in hardness. The percentage of decrease in KHN(2) compared with KHN(1) (PD) in the same specimen was then calculated for both surfaces (n=10). The data were submitted to an ANOVA in different tests (Light energy transmission, Knoop hardness and CLD). The irradiance of light that passed through composite shade A1 (408 mW/cm(2)) was statistically greater than that through shade A2 (376 mW/cm(2)), and was greater through A2 than A3 (359 mW/cm(2)) and through A3 than A3.5 (327 mW/cm(2)); A3.5 showed no statistical difference when compared to A4 (324 mW/cm(2)). The DKH of A4 (20.56%) was not statistically higher than that of A3.5 (20.14%), which was greater than that of A3 (14.08%), A2 (11.65%) and A1 (9.06%). There was no statistical difference in CLD. Darker shades had a significant influence on light energy transmission through dental resin composite and its hardness. However, CLD was not affected by darker dental composite shades.

Comparison of silorane and methacrylate-based composite resins on the curing light transmission

The aim of this study was to investigate the influence of different composite resins - Filtek P90 (silorane-based composite) and Heliomolar (methacrylate-based composite) - on light transmission and decrease in Knoop hardness between the bottom and top of cured specimens. The irradiance of a light-curing unit (LCU) was measured with a power meter (Ophir Optronics; 900 mw/cm2) and spectral distributions were obtained using a spectrometer (USB 2000). Twenty standardized cylindrical specimens (2 mm thick x 7 mm diameter) of each composite resin were obtained by curing using the LCU for 40 s. Light energy transmission through the composite was calculated (n=10). The Knoop hardness number for each surface was recorded as the mean of 3 indentations. The difference in Knoop hardness between the top and bottom (DKH) of the same specimen was calculated (n=10). The irradiance of light that passed through Filtek P90 (272 mW/cm2) was not significantly greater than that the passed through Heliomolar (271 mW/cm2). The DKH of Filtek P90 (25%) was significantly higher than that of Heliomolar (12%). There was a greater degree of subsurface polymerization of the methacrylate-based composite compared to the silorane-based composite.

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.

Influence of the Internal Conditioning of Indirect Restorations of Resin Composite in Relation to Microleakage Using LEDs and QTH Units

An efficient polymerization of resin composite inlays associated with treatment of the surface and luting with dual resinous cement and flowable resin composites presents better longevity of restorations.

Light transmission on dental resin composites

OBJECTIVES

The purposes of this study was: (1) to examine the light transmittance characteristics of two light-cured resin composites, for different thickness, (2) to correlate the light transmittance through the resin composites and the filler contents, and (3) to determine the penetration depth of the light as a function of the wavelength.

METHODS

Two resin composites (Filtek Z250, shade A2 and Filtek Supreme XT, shade A2E) were used. Specimens of six different thicknesses (0.15, 0.25, 0.30, 0.36, 0.47 and 0.75 mm) were prepared (n=3). The transmittance at wavelengths from 400 to 800 nm was measured using a UV-visible spectrophotometer, before and after light polymerization.

RESULTS AND SIGNIFICANCE

Significant differences were found in the wavelength dependence of transmittance between the two materials, and between the unpolymerized and polymerized stages of each resin composite. At lower wavelengths, the light transmittance of the Filtek Supreme XT resin composite was lower than the Filtek Z250. At the higher wavelengths, however, Filtek Supreme XT presented higher light transmittance. For both resin composites, the penetration depth was higher after polymerization. However, Filtek Supreme XT showed a higher gain in transmittance at the 0.15 mm thickness. The difference in light transmittance characteristics of the resin composites may affect their depth of polymerization.

Hardness comparison of bulk-filled/transtooth and incremental-filled/occlusally irradiated composite resins

STATEMENT OF PROBLEM

Use of a bulk-fill/transtooth composite resin insertion/irradiation technique may not provide as well polymerized a restoration as when using a conventional incremental placement/irradiation technique. Little information exists as to how the hardness of restorations produced by the 2 techniques compare.

PURPOSE

The purpose of this study was to determine the effect of composite resin placement and an irradiation technique on the axial hardness at various depths in a Class I composite resin to include the influence of composite resin filler classification and shade.

MATERIAL AND METHODS

Cylindrical Class I preparations were made in 70 recently extracted human molars and restored with either a light (A1) or dark shade (A4) of a microfill, microhybrid, or nanohybrid composite resin, or with a single shade of a translucent material. Half were placed using a conventional 2-mm-thick incremental-fill/occlusal irradiation technique, and half using a bulk-fill/transtooth irradiation method (n=5). Specimens were sectioned occluso-apically and axial Knoop hardness values were obtained at depths of 0.5, 1.0, 1.5, 2.0, 2.5, and 3.0 mm. Hardness at 0.5 mm was used as a control to compare to deeper readings. Statistical analyses consisted of multiple ANOVAs and Dunnett's post-hoc tests performed at appropriately determined significance levels.

RESULTS

For 3 multishaded materials tested, axial hardness values were relatively unaffected by composite resin shade or filler classification for the incremental technique, but were significantly affected by these factors when using the bulk-fill/transtooth irradiation method. A single shade translucent material was not affected in either the bulk or incremental condition.

CONCLUSIONS

Use of a bulk-fill/transtooth irradiation technique for composite resin placement does not result in axial hardness values equivalent to that of an incremental-fill/occlusal irradiation technique.

Composite Depth of Cure Obtained with QTH and LED Units Assessed by Microhardness and Micro-Raman Spectroscopy

Lower depth of cure with the LED unit, compared to the QTH unit, is associated with different light scattering due to differences in spectral emission.

Effect of light curing tip distance and resin shade on microhardness of a hybrid resin composite

Resin composite shades and resin composite polymerization performed with a distanced light tip are factors that can affect polymerization effectiveness. Thisin vitro study aimed to evaluate the influence of curing tip distance and resin shade on the microhardness of a hybrid resin composite (Z250 - 3M ESPE). Forty-five resin composite specimens were randomly prepared and divided into nine experimental groups (n = 5): three curing tip distances (2 mm, 4 mm, and 8 mm) and three resin shades (A1, A3.5, and C2). All samples were polymerized with a continuous output at 550 mW/cm². After 24 hours, Knoop microhardness measurements were obtained on the top and bottom surfaces of the sample, with a load of 25 grams for 10 seconds. Five indentations were performed on each surface of each sample. Results showed that bottom surface samples light-cured at 2 mm and 4 mm presented significantly higher hardness values than samples light-cured at 8 mm. The resin shade A1 presented higher hardness values and was statistically different from C2. The resin shade A3.5 did not present statistical differences from A1 and C2. For the top surface, there were no statistical differences among the curing tip distances. For all experimental conditions, the top surface showed higher hardness values than the bottom surface. It was concluded that light curing tip distance and resin shade are important factors to be considered for obtaining adequate polymerization.

Thermal analysis of dental resins cured with blue light-emitting diodes (LEDs)

Thermal analysis was used to measure the characteristics of dental resins cured with the use of a new light-activation unit equipped with high illuminant blue light-emitting diodes (LEDs). The characteristics were compared with those of resins cured with the use of two conventional halogen lamp units. The prepared base monomer consisted of a mixture of Bis-GMA and TEGDMA (60:40 by weight), with 0.5 wt% CQ/DMPT or CQ/DMAEMA. The two experimental visible-light-cured resins were polymerized for 40 s. Differential scanning calorimetry (DSC) was used to examine the thermal characteristics of the cured resins. The activation energy for the decomposition of the dental resin was calculated from the peaks of the endothermic curves obtained when the specimens were heated at three different rates (5, 10, and 15 C/min) during DSC. The activation energies calculated for the LED-cured specimens were more than 220 kJ/mol; specimens cured with the use of the halogen units had activation energies of less than 192 kJ/mol. The Knoop hardness number (KHN) of the same specimens was measured, and was higher with the blue LED units than with halogen lamp units. Therefore, dental resins cured using blue LEDs have a higher degree of polymerization and more stable three-dimensional structures than those cured with halogen lamps.

Monomer conversion versus flexure strength of a novel dental composite

OBJECTIVES

To quantify the monomer conversion and flexural strength of an experimental oxirane-based composite material (EXL596) compared with two, commercially available, dimethacrylate based restoratives (Z250 and Z100).

METHODS

Fourier-transform infra-red spectroscopy (FTIR) was utilised to evaluate the degree of conversion (DC) (n=5) and biaxial flexure strength (BFS) testing (n=20) was used to analyse flexural strength and associated Weibull moduli (m) of each material following 0.1, 0.5, 1, 4, 24 and 48 h immersion in a lightproof waterbath maintained at 37+/-1 degrees C.

RESULTS

The DC of Z250 and Z100 following 0.1, 0.5 and 1 h post-irradiation was significantly greater than the DC of EXL596 for the same immersion periods. This was manifested as a significant decrease in BFS and associated m of EXL596 compared with Z250 and Z100 for the 0.1, 0.5 and 1 h post-irradiation periods. The DC and BFS of EXL596 were significantly greater than Z250 and Z100 following 24 h immersion.

CONCLUSIONS

Assessment of FTIR spectra, BFS and associated m has provided a useful method in the quantitative analysis of resin-based composite conversion. Identification of the decreased DC of EXL596 compared with Z250 and Z100 was achieved using FTIR. However, decreased conversion rates within the first hour following irradiation of EXL596 may compromise flexural strength properties (associated with a decrease in BFS and m) which may be inadequate under masticatory loading.

Polymerization efficiency of LED curing lights

PURPOSE

The purpose of this study was to compare the curing efficiency of three commercially available light-emitting diode (LED)-based curing lights with that of a quartz tungsten halogen (QTH) curing light by means of hardness testing. In addition, the power density (intensity) and spectral emission of each LED light was compared with the QTH curing light in both the 380- to 520-nm and the 450- to 500-nm spectral ranges.

MATERIALS AND METHODS

A polytetrafluoroethylene mold 2 mm high and 8 mm in diameter was used to prepare five depth-of-cure test specimens for each combination of exposure duration, composite type (Silux Plus [microfill], Z-100 [hybrid]), and curing light (ZAP Dual Curing Light, LumaCure, VersaLux, Optilux 401). After 24 hours, Knoop hardness measurements were made for each side of the specimen, means were calculated, and a bottom/top Knoop hardness (B/T KH) percentage was determined. A value of at least 80% was used to indicate satisfactory polymerization. A linear regression of B/T KH percentage versus exposure duration was performed, and the resulting equation was used to predict the exposure duration required to produce a B/T KH percentage of 80% for the test conditions. The power densities (power/unit area) of the LED curing lights and the QTH curing light (Optilux 401) were measured 1 mm from the target using a laboratory-grade, laser power meter in both the full visible light spectrum range (380-780 nm) and the spectral range (between 450 and 500 nm), using a combination of long- and short-wave edge filters.

RESULTS

The emission spectra of the LED lights more closely mirrored the absorption spectrum of the commonly used photoinitiator camphorquinone. Specifically, 95% of the emission spectrum of the VersaLux, 87% of the LumaCure, 84% of the ZAP LED, and 78% of the ZAP combination LED and QTH fell between 450 and 500 nm. In contrast, only 56% of the emission spectrum of the Optilux 401 halogen lamp fell within this range. However, the power density between 450 and 500 nm was at least four times greater for the halogen lamp than for the purely LED lights. As a result, the LED-based curing lights required from 39 to 61 seconds to cure a 2-mm thick hybrid resin composite and between 83 and 131 seconds to adequately cure a microfill resin composite. By comparison, the QTH light required only 21 and 42 seconds to cure the hybrid and microfill resin composites, respectively.

CLINICAL SIGNIFICANCE

The first-generation LED-based curing lights in this study required considerably longer exposure durations than the QTH curing light to adequately polymerize a hybrid and a microfill resin composite.

[Effect of the methods of photoactivation and insertion on the hardness of composite resins]

The purpose of this study was to assess the effect of methods of photoactivation and insertion on the Knoop hardness of the Z100 and Alert composite resins. The specimens were confected in cavities measuring 4 x 4 mm. The insertion of material was carried out by means of two methods: single-portion technique and insertion of two 2-mm-thick layers. When inserted in a single portion, the resin was compressed with a static load of 1 kgf on a glass slab recovered with a polyester strip, in order to remove the excess of material. After the removal of the glass slab and polyester strip, the materials were photoactivated by means of continuous light emitted by a XL 3000 unit with a light intensity of 520 mW/cm2 for 40 seconds; double pulse, with light emission of 150 mW/cm2 for 10 seconds, plus 30 seconds with light intensity of 520 mW/cm2 emitted by a XL 3000 unit; and pulsatile light of 520 mW/cm emitted by the Optilux 400 unit, turned on for 2 seconds and off for 2 seconds, during 60 seconds. The two layers of the material submitted to double insertion were photoactivated in the same conditions as the bulk-inserted material, and the excess of material was also removed from the second layer. After storage in a stove at 37 C and 100% relative humidity for 24 hours, the specimens were embedded in polyester resin, trimmed and polished with sandpaper and diamond slurry. Knoop hardness was assessed in 4 depths with a HMV Shimadzu penetrometer under the load of 50 g during 30 seconds. The data submitted to ANOVA and Tukey's test revealed that Z100 presented greater hardness values; double insertion was better than single insertion; the hardness at the surface was smaller than that at the bottom of the specimen, regardless of other factors. For Z100, all activation methods produced smaller hardness values when double insertion was carried out. Meanwhile, for Alert, continuous light promoted statistical similarity between the insertion techniques. For Z100, there was statistical similarity between the activation methods only when the double-insertion technique was employed. There was statistical similarity between the double-pulse and pulsatile-light techniques for Alert, when the single-portion insertion technique was carried out.

A comparison of polymerization by light-emitting diode and halogen-based light-curing units

BACKGROUND

Light-emitting diode, or LED, technology provides certain advantages over halogen-based light polymerization of resin-based composites. The authors investigated the adequacy of cure of LED light-curing units, or LCUs.

METHODS

The authors used two halogen-based light-curing units (Optilux 400 and 501, Demetron Research Corp., Danbury, Conn.) and two commercially available LED LCUs (LumaCure, LumaLite, Spring Valley, Calif., and VersaLux, Centrix, Shelton, Conn.) to polymerize top surfaces of hybrid (Filtek Z-250, 3M, St. Paul, Minn.) and microfilled (Renamel, Cosmedent, Chicago) resin-based composite specimens. Specimens were indented on their top and bottom surfaces with a Knoop hardness tester and measured for hardness. Bottom:top hardness ratios determined the percentage of cure. The authors separated the data into eight groups (two composites cured with four different lights) with 15 observations per group, for a total sample size of 120.

RESULTS

The authors compared composites and curing lights by a two-way analysis of variance, and results indicated significant main effects. The main effect of composite was statistically significant (P < .0001) when microfilled composite was compared with hybrid composite, regardless of curing light, for all top and bottom hardness measurements, with the hybrid producing much higher hardness measurements overall. The main effect of light was significant as well (P < .0001), regardless of composite type, with the two halogen-based lights producing harder top and bottom composite surfaces than the two LED LCUs.

CONCLUSIONS

The light output of commercially available diodes for resin-based composite polymerization still requires improvement to rival the adequacy of cure of halogen-based LCUs. Additional studies are necessary.

CLINICAL IMPLICATIONS

Commercially available LED LCUs were introduced just in the past year. However, they may not adequately polymerize resin-based composites, which can lead to restoration failures and adverse pulpal responses to unpolymerized monomers.

Effect of exposure intensity and post-cure temperature storage on hardness of contemporary photo-activated composites

OBJECTIVES

The effect of variation in post-exposure storage temperature (18 vs. 37 degrees C) and light intensity (200 vs. 500mW/cm(2)) on micro-hardness of seven light-activated resin composite materials, cured with a Prismetics Mk II (Dentsply) light activation unit, were studied.

METHODS

Hardness values at the upper and lower surfaces of 2mm thick disc shaped specimens of seven light-cured resin composite materials (Herculite XRV and Prodigy/Kerr, Z100 and Silux Plus/3M, TPH/Dentsply, Pertac-Hybrid/Espe, and Charisma/Kulzer), which had been stored dry, were determined 24h after irradiation with a Prismetics Mk II (Dentsply) light activation unit.

RESULTS

Hardness values varied with product, surface, storage temperature, and curing light intensity. In no case did the hardness at the lower surface equal that of the upper surface, and the combination of 500mW/cm(2) intensity and 37 degrees C storage produced the best hardness results at the lower surface.

CONCLUSIONS

Material composition had a significant influence on surface hardness. Only one of the seven products (TPH) produced a mean hardness values at the lower surface >80% of the maximum mean upper surface hardness obtained for the corresponding product at 500mW/cm(2) intensity/37 degrees C storage temperature when subjected to all four test regimes. Despite optimum post-cure storage conditions, 200mW/cm(2) intensity curing for 40s will not produce acceptable hardness at the lower surface of 2mm increments of the majority of products tested.

The application of magnetic resonance microimaging to the visible light curing of dental resins. Part 2. Dynamic imaging by the FLASH-MOVIE pulse sequence

OBJECTIVES

To investigate the application of a rapid NMR imaging pulse sequence, FLASH-MOVIE, to the visible light curing of dental restorative materials.

METHODS

The light guide was applied at one end of a cylindrical specimen of visible light curing unfilled resin and the light directed along the cylinder. During polymerisation an NMR imaging pulse sequence, FLASH-MOVIE, was run at 15s intervals with a 50 ms repetition time. The image of a 1mm thick vertical slice was recorded with a (125 microm)2 pixel size.

RESULTS

Images with good contrast were obtained from all resin monomers. The image intensity from the polymer was indistinguishable from the background intensity. Thus, the progress of light activated polymerisation in the material could be followed in real time through a series of up to 16 images. Initially the image intensity increased in the material closest to the light guide, then decreased over time to zero. Concomitant with this fall, a "cure-front" moved through the specimen.

SIGNIFICANCE

The FLASH-MOVIE NMR pulse sequence applied to microimaging of dental diacrylate resins can be used to obtain a dynamic record of visible light curing. A more refined experimental protocol will be required to apply this unique data to models proposed for this polymerisation mechanism.

Effect of Distance on the Power Density from Two Light Guides

PURPOSE

This study determined the effect of distance on the power density from standard and Turbo light guides (Demetron/Kerr, Danbury, Connecticut).

MATERIALS AND METHODS

Power density was measured from 0 to 10 mm away from the tip of standard 8-mm curved light guides and 13/8-mm Turbo curved light guides. To determine the effect of distance on power density, a polynomial regression line was fitted. The Kolmogorov-Smirnov (K-S) statistic and the Wilcoxon rank sum (WR) tests were used to determine if there was a difference in the rate at which the power density decreased for the standard and Turbo light guides as the distance from the tip increased. Photographs of the light dispersion from each tip were also taken.

RESULTS

At 0 mm, the mean (+/- SD) power density from the two standard light guides was 743 +/- 6.1 mW/cm2 and from the four Turbo light guides was 1128 +/- 22.1 mW/cm2. As the distance from the tip of the light-guide tip increased, the power density decreased, but the rate of decrease was greater from the Turbo light guides than from the standard light guides. At 6 mm the power density from the standard light guides fell to 372 mW/cm2 (50% of the original value) and the power density from the Turbo light guides fell to 263 mW/cm2 (23% of the original value). Both the K-S statistic and the WR sum test indicated that the distribution of light intensities was significantly different from the two light guides (WR p-value = .0246, K-S p-value < .0001). The two estimated polynomials intersected at 3.66 mm, and the 95% prediction intervals intersected at about 2.8 and 4.8 mm. Therefore, beyond 5 mm away from the tip of the light guide, the standard light guides gave higher power density readings than the Turbo light guides. Photographs showed that the light dispersed at a wider angle from the Turbo light guides than from the standard light guide.

CLINICAL SIGNIFICANCE

The design of the light guide of a light curing unit affects light dispersion, power density, and ultimately the dentist's ability to properly cure composite. For these reasons, manufacturers should report the power density at the tip of the light guide and 6 mm from the tip of the light guide, since significant differences exist between light guide designs.

Microdureza de resina composta fotopolimerizável: a cor da matriz experimental pode alterar os resultados dos testes?

O grau de polimerização das resinas compostas, avaliado principalmente pela microdureza vem sendo estudado relacionado com inúmeros fatores. Algumas pesquisas in vitro fazem uso de matrizes como base para a confecção de corpos-de-prova e não dentes naturais. Para as resinas fotopolimerizáveis, a intensidade de luz que atinge o material exerce influência direta no seu grau de polimerização. A cor e/ou transparência dessas matrizes não deveriam induzir a diferenças nos resultados finais de microdureza. Com essa preocupação, este trabalho teve por objetivo avaliar o efeito de duas matrizes de polipropileno utilizadas para testes com resina composta. Uma delas era branca e outra preta. A resina foi inserida e polimerizada em incremento único de 3 mm e em 3 incrementos de 1 mm cada e sua dureza medida a 3 mm de profundidade. Analisando os resultados pode-se concluir que houve diferença estatisticamente significante entre as duas cores, sendo que a microdureza foi menor com a matriz preta. A técnica de inserção e polimerização em incremento único induziu a menor grau de polimerização que a técnica incremental em ambas as matrizes (p < 0,01). Pôde-se obter com esses resultados, que ao utilizar matrizes pretas para confecção de corpos-de-prova de resina composta fotopolimerizável, os valores obtidos, principalmente relacionados com profundidade de polimerização, demonstram o menor grau possível de polimerização, e que depende exclusivamente das propriedades da resina.

A survey of the efficiency of visible light curing units

OBJECTIVES

The purpose of this study was to survey the efficiency of visible light curing units in dental practices across Australia.

METHODS

Survey forms were distributed to representatives of 3M Health Care to complete when visiting dentists in their working areas. The information collected included the type and age of the unit, curing times used, history of maintenance, replacement of components, and the light intensity reading.

RESULTS

Of the 214 light curing units surveyed, approximately 27% recorded a light intensity of 200 mW cm-2 or less, a level regarded as inadequate to cure a 2-mm thick increment of composite resin. An additional 26% registered an output of between 201 and 399 mW m-2. This level would be considered acceptable with additional curing time; however, 44% of practitioners were curing for 20 s or less. A negative correlation was found between the age of the unit and the intensity recorded. Nearly 50% of respondents had never checked the light output of their unit.

CONCLUSIONS

The results indicate that just over one-half of the light curing units surveyed were not functioning satisfactorily. An obvious reduction in intensity was noted with the older units. There is a substantial lack of awareness among dentists of the need for maintenance and regular checking of the light intensity of these units.

The effect of curing light variations on bulk curing and wall-to-wall quality of two types and various shades of resin composites

OBJECTIVES

This study evaluated the influence of light intensity and irradiation time variations on the curing efficacy of two types and various shades of resin composites and the effect of reduced light intensity on the preservation of wall-to-wall continuity.

MATERIALS AND METHODS

Three microfilled composites (in three different shades) and one hybrid composite were used in this study. Polymerization shrinkage, and the hardness and adaptation of adhesive restorations in dentin cavities were determined at light intensities of 175 and 700 mW/cm2 and irradiation times of 10 and 60 s. Data were compared using in a general linear model analysis.

RESULTS

Shrinkage measurements were the indication of conversion and conversion rate. Reduced intensity slowed down the rate of polymerization but did not reduce the conversion as long as an irradiation time of 60 s was employed. High-energy irradiation caused increased separation of the composite from the tooth structure. On the basis of obtaining optimal conversion and adaption, it was demonstrated that the irradiation time to be more effective than irradiation energy.

SIGNIFICANCE

Light-cured composites require an understanding of their structure, pigmentation and irradiation parameters to obtain optimal performance. High intensity light-curing does not necessarily lead to optimal quality.

Light-activated restorative materials: a method of determining effective radiation times

The aim of this investigation was to develop a test method that allows an operator to determine the appropriate radiation time for any light-activated material, irrespective of the characteristics of the light-activation unit or material to be cured. A computer-based radiometer was used to monitor the radiation energy transmitted through a number of light-activated materials during irradiation, and a method was devised to determine the predicted radiation time for any given material/shade/light unit combination based upon the change in transmission against time that occurred as the material polymerized. Additional test samples were cured for the predicted radiation times and upper and lower surface microhardness measurements were taken to verify the validity of the test method. Predicted times varied from 16 to 44 s for 2 mm thick samples of the materials/shade/light-activation units used. This method may allow operators to determine appropriate radiation times for any new light-activated restorative they decide to use with their own light-activation unit.

Depth of cure of radiation-activated composite restoratives--influence of shade and opacity

Inadequate depth of cure may reduce the longevity of visible light-activated composite restorations. Radiation-activated composites, originally marketed for the restoration of anterior teeth, have been developed and refined for restoring occlusal and proximal cavities in posterior teeth. Clinical factors such as the accessibility of the light source, the direction of the light, intervening tooth tissue and cavity depth may limit depth of cure. Unfortunately, a hard top surface is no indication of adequate polymerization at the base of the cavity. The current investigation used a penetrometer to evaluate the depth of cure of a range of recently introduced 'universal' composite materials and assess the influence of shade and opacity on depth of cure. Depth of cure of the enamel opacity is always greater than that of the same shade of dentine for each manufacturer's product range.

A classification of dental composites according to their morphological and mechanical characteristics

The on-going search for a biologically acceptable restorative material has brought a confusing variety of composites on the dental market. In the present study, commercially available composites are categorized as a function of their mean particle size, filler distribution, filler content, Young's modulus, surface roughness, compressive strength, surface hardness, and filler morphology. Out of this information, it can be concluded that the materials of choice for restoring posterior cavities at present are the Ultrafine Compact-Filled Composites because their intrinsic surface roughness, Young's modulus and, indirectly, their filler content, compressive strength, and surface hardness are comparable to the same properties of enamel and dentin. The Ultrafine Midway-Filled Composites seem to be very satisfactory materials for anterior use.

Direct posterior composite resin restorations: a review. 2. Clinical technique

Despite the increased use of composite resin to restore posterior teeth, there is evidence that clinicians should be selective in their use of these restorations. This paper describes the clinical technique--preparatory procedures, preparation of the cavity, preparation for placement of composite resin, placement of composite resin and finishing of the restoration--for the relatively conservative use of composite resin in posterior teeth and reviews the literature to discuss briefly many of the controversial aspects of the technique.

Report of the Committee on Scientific Investigation of the American Academy of Restorative Dentistry

Subjects of the past decade in the dental literature are reflected in this year's Committee report. We note the decrease in the prevalence of caries, the influence of dental implants, the advancements in dental materials, and the continued efforts to control adhesive events in the oral cavity. This year we included comments from and about many significant review articles published this past year. The Committee continues to be concerned about the quality of some of the work reported and the quality of the reporting. We have attempted to select the distinguished work, that which provides new information to our profession. The subjects covered include pulp biology, caries prevention, periodontics, implants, craniomandibular function and dysfunction, occlusion, and dental materials.