Relationship between shade and depth of cure for light-activated dental composite resins

Influence of Layer Thickness and Shade on the Transmission of Light through Contemporary Resin Composites

BACKGROUND

Material-dependent parameters have an important impact on the efficiency of light polymerization. The present in vitro study aimed to investigate the influence of the increment thickness and shade of nano- and nanohybrid resin composites on the transmission of curing light.

METHODS

Three contemporary resin composites were evaluated: Tetric EvoCeram® (TEC); Venus Diamond® (VD); and Filtek Supreme XTE® (FS XTE). Light transmission (LT) was recorded in accordance with the sample thickness (0.5 to 2.7 mm) and the shade. Polymerized samples were irradiated for 10 s each using the high-power LED curing light Celalux 2 (1900 mW/cm2). LT was simultaneously recorded using the MARC Patient Simulator (MARC-PS).

RESULTS

LT was strongly influenced by the composite layer thickness. For 0.5 mm-thick samples, a mean power density of 735 mW/cm2 was recorded at the bottom side. For the 2.7 mm samples, a mean power density of 107 mW/cm2 was measured. Only LT was markedly reduced in the case of darker shades. From A1 to A4, LT decreased by 39.3% for FS XTE and 50.8% for TEC. Dentin shades of FS XTE and TEC (A2, A4) showed the lowest LT.

CONCLUSIONS

The thickness and shade of resin composite increments strongly influences the transmission of curing light. More precise information about these parameters should be included in the manufacture manual.

Isolated effect of filler particle size on surface properties of experimental resin composites before and after toothbrush abrasion

OBJECTIVE

This study aimed to isolate the relationship between filler size and the surface properties of roughness and gloss before and after toothbrush abrasion for experimental resin-based composites (RBCs) containing uniform spherical fillers.

MATERIALS AND METHODS

Five experimental light-cured RBCs with different spherical filler sizes and three commercial RBCs were studied. Forty specimens were polished using silicon carbide papers. Gloss was measured after 0, 90, 180, and 360 min of simulated toothbrushing, and surface roughness was measured before and after 360 min of toothbrushing. Two-way ANOVA/Tukey's multiple comparison tests were used to compare the RBCs, and the correlation between particle size and surface roughness or gloss was also determined.

RESULTS

After polishing and toothbrushing, RBCs with smaller fillers exhibited significantly higher gloss and lower surface roughness, and RBCs with larger fillers exhibited lower gloss and higher surface roughness. A significant correlation was found between filler particle size and gloss and surface roughness both before and after toothbrush abrasion.

CONCLUSIONS

Gloss of RBCs containing fillers with larger particle sizes was significantly reduced. After toothbrushing abrasion, the surface roughness increased for all RBCs, except those containing the finest-sized fillers. The particle size of the filler is a critical determinant of the surface roughness and gloss of RBCs, after polishing and after toothbrushing.

CLINICAL SIGNIFICANCE

Increased surface roughness caused by toothbrush abrasion reduces the gloss of resin-based composites. Resin-based composites containing finer fillers best maintain glossiness after routine tooth brushing.

Effect of resin composite shade on digital fiber-optic transillumination imaging in vitro

Digital imaging fiber-optic transillumination (DIFOTI) devices have been used to detect caries, a technique without using X-rays. However, the effects of resin composites (RCs) shades on the images acquired with DIFOTI devices have not been investigated. Thus, this study aimed to elucidate the influence of RC shade on the images obtained with DIFOTI technique. Three shades (A1, A3, and Opaque) for each of four flowable RCs were filled on a cavity prepared in a left mandibular first premolar obtained from a donated body. Then, transmission images with a DIFOTI device (DIAGNOcam; KaVo, Biberach, Germany) were acquired, and the average lightness values of the images in the RC and enamel were used to calculate differences between those areas. To clarify the influence of the optical translucency and color on DIFOTI images, the color parameters (L*, a* and b*) of each RC were obtained with black and white backgrounds. The color differences between the backgrounds were calculated as transparency parameter (TP) values. The number of repetitions was set to 10. Differences in the lightness value of the shades varied in each RC. The difference in lightness was significantly associated with the TP value and color parameters of L* (p < 0.01), with negative (R = - 0.81) and positive (R = 0.84) correlations, respectively. In conclusion, DIFOTI images of RCs with high optical translucency resembled those of the natural tooth structure.

Evaluation of physical/mechanical properties of an experimental dental composite modified with a zirconium-based metal-organic framework (MOF) as an innovative dental filler

OBJECTIVES

This study aimed to modify an experimental dental composite using a synthesized nano-structured methacrylated zirconium-based MOF to enhance physical/mechanical properties.

METHODS

The previously known Uio-66-NH2 MOF was first synthesized and post-modified with Glycidyl Methacrylate (GMA). Fourier Transform Infrared (FTIR) Spectroscopy and CHNS analysis confirmed the post-modification reaction. The prepared filler was investigated by XRD, BET, SEM-EDS, and TEM. The experimental composite was prepared by mixing 60% wt. of resin matrix with 40% wt. of fillers, including silanized silica (SS) or Uio-66-NH-Me (UM). The experimental composites' depth of cure (DPC) was investigated in five groups (G1 =40% SS, G2 =30%SS+10%UM, G3 =20%SS+20%UM, G4 =10%SS+30%UM, G5 =40%UM). Then flexural strength(FS), Elastic Modulus(EM), solubility(S), water sorption(WS), degree of conversion(DC), polymerization shrinkage(PS), and polymerization stress(PSR) of the groups with DPC of more than 1 mm were investigated. Finally, the cytotoxicity of composites was studied.

RESULTS

The groups with more than 20% wt. UM, filler (G4, G5) had lesser than 1 mm DPC. Therefore, we investigated three groups' physical and mechanical properties with lower than 20% UM filler (G1-G3). Within these groups, G3 has a higher FS, EM (P < 0.05), and lower WS and S (P < 0.05). DC dropped in G2 and G3 compared to G1 (p < 0.05), but there was no significant difference between G2 and G3 (P = 0.594).

SIGNIFICANCE

This new filler is an innovative coupling-agent free filler and can be part of dental filler technology itself. It can also introduce a new group of dental fillers based on MOFs, but it still needs a complete investigation to be widely used.

Compositional boundaries for functional dental composites containing calcium orthophosphate particles

OBJECTIVES

To investigate the interrelationships among handling, degree of conversion (DC), mechanical behavior and Ca2+ release of composites containing dicalcium phosphate dihydrate (DCPD, CaHPO4.2H2O), as a function of total inorganic content and DCPD: glass ratio.

METHODS

Twenty-one formulations (1 BisGMA: 1 TEGDMA, in mols) with inorganic fractions ranging from zero to 50 vol% and different DCPD: glass ratios were evaluated for viscosity (parallel plate rheometer, n = 3), DC (near-FTIR spectroscopy, n = 3), fracture toughness/K1C (single-edge notched beam, n = 7-11) and 14-day Ca2+ release (inductively coupled plasma optical emission spectroscopy, n = 3). Data were analyzed by ANOVA/Tukey test (except viscosity, where Kruskal-Wallis/Dunn tests were used, α: 0.05).

RESULTS

Viscosity and DC increased with DCPD: glass ratio among composites with the same inorganic content (p < 0.001). At inorganic fractions of 40 vol% and 50 vol%, keeping DCPD content at a maximum of 30 vol% did not compromise K1C. Ca2+ release showed an exponential relationship with DCPD mass fraction in the formulation (R2 = 0.986). After 14 days, a maximum of 3.8% of the Ca2+ mass in the specimen was released.

CONCLUSION

Formulations containing 30 vol% DCPD and 10-20 vol% glass represent the best compromise between viscosity, K1C and Ca2+ release. Materials with 40 vol% DCPD should not be disregarded, bearing in mind that Ca2+ release will be maximized at the expense of K1C.

Real-time measurement of transmittance changes during photo-polymerization of conventional and bulk-fill composites

This study investigated transmittance changes during photo-polymerization of composites in real-time. The transmittance changes of one conventional micro-hybrid, three conventional nano-hybrid, and four bulk-fill composites were measured before, during, and after photo-polymerization, and the maximum rate of transmittance change was compared with that of polymerization shrinkage. A significant difference in transmittance of composite between before and after photo-polymerization was observed. The transmittance of composites except for one bulk-fill composite increased during photo-polymerization. There was a correlation between the maximum rate of transmittance change and the maximum rate of polymerization shrinkage. The transmittance analysis of composites gives very important information to know for the final aesthetic restoration and allows to evaluate polymerization kinetics.

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.

Toothbrush Abrasion of Restorations Fabricated with Flowable Resin Composites with Different Viscosities In Vitro

The purpose of this study was to examine toothbrush-induced abrasion of resin composite restorations fabricated with flowable resin composites of different viscosities in vitro. In this study, six types of flowable resin composites with different flowability (Beautifil Flow F02, F02; Beautifil Flow F10, F10; Beautifil Flow Plus F00, P00; Beautifil Flow Plus F03, P03; Beautifil Flow Plus X F00, X00; and Beautifil Flow Plus X F03, X03) were used. For the toothbrush abrasion test, the standard cavity (4 mm in diameter and 2 mm in depth) formed on the ceramic block was filled with each flowable resin composite (n = 10) and brushed for up to 40,000 strokes in a suspension containing commercial toothpaste under the conditions of 500 g load, 60 strokes/min, and 30 mm stroke distance. After every 10,000 strokes, the brushed surface of the specimen was impressed with a silicone rubber material. The amount of toothbrush-induced abrasion observed on each impression of the specimen was measured using a wide-area 3D measurement device (n = 10). The viscosity was determined using a cone-and-plate rotational measurement system. Because of the effect of different shear rates on viscosity and clinical use, the values 1.0 and 2.0 s⁻¹ were adopted as data (n = 6). In this study, the results of the toothbrush abrasion test demonstrated no significant differences in the amount of toothbrush-induced abrasion among flowable resin composites used (p > 0.05). No significant correlation was reported between toothbrush-induced abrasion and viscosities of flowable resin composites.

Evaluation of degree of conversion, rate of cure, microhardness, depth of cure, and contraction stress of new nanohybrid composites containing pre-polymerized spherical filler

The aim of the present study was to characterize nanohybrid and nanofilled composites in terms of degree of conversion (DC), rate of cure (RC), microhardness (Vickers hardness number; VHN), depth of cure, and contraction stress (CS). Ceram.X® universal- A3, duo enamel E2, and duo dentin D3 composites were compared to Tetric EvoCeram® and FiltekTMSupreme XTE composites of equivalent dentin and enamel shades under a 40 s photopolymerization protocol. DC was measured by infrared spectroscopy, calculating RC from the kinetic curve. Top and bottom VHN were determined using a Vickers indenter, and bottom/top surface ratio (Vickers hardness ratio; VHR) calculated. CS vs. time was assessed by a universal testing machine and normalized for the specimen bonding area. All materials showed DC  < 60%, Ceram.X® composites reaching higher values than the other composites of corresponding shades. RC at 5 s of photopolymerization was always higher than that at 10 s. All the Ceram.X® composites and the lighter-shaded Tetric EvoCeram® and FiltekTMSupreme XTE composites reached the RC plateau after 25 s, the remaining materials showed a slower kinetic trend. Tetric EvoCeram® and FiltekTMSupreme XTE composites displayed the softest and the hardest surfaces, respectively. Differently from darker-shaded materials, the universal and the three enamel-shaded composites resulted optimally cured (VHR >  80%). The tested composites differed in CS both during and after light cure, Tetric EvoCeram® and FiltekTMSupreme XTE composites displaying the highest and the lowest CS, respectively. Only the Ceram.X® universal-A3 reached a CS plateau value. The tested composites exhibited material-dependent chemo-mechanical properties. Increasing the curing time and/or reducing the composite layer thickness for dentin-shaded composites appears advisable.

Comparison of the Depth of Cure of Flowable Composites Polymerized at Variable Increment Thicknesses and Voltages: An In vitro Study

Objectives

The aim of this study is to compare the depth of cure of two composite materials (SDR and Filtek bulk-fill) cured at variable increment depths (2, 4, and 6 mm) and voltages (180 and 220 volts).

Materials and Methods

Each sample of the composite material was packed in a mold of 2 mm, 4 mm, and 6 mm and curing light (quartz tungsten halogen) of optimal intensity was exposed for 20 s at 2 different voltages on each specimen. After curing, the specimens were removed and the composite on the nonexposed end was scraped with a plastic instrument. The remaining composite thickness was measured using a digital Vernier caliper. The reading was divided by half to follow the ISO 4049 method. Independent sample t-test, one-way ANOVA, and linear regression analysis were applied. Level of significance was kept at 0.01.

Results

The mean DOC of SDR and Filtek were 1.93 ± 0.82 and 1.77 ± 0.65 mm. Lowering the voltage from 220 to 180 volts reduced the depth of Filtek from 1.87 ± 0.74 to 1.67 ± 0.54 mm, whereas the DOC of SDR remained unchanged at 1.93 mm at the two voltages. The adjusted R ² for the depth of cure was 0.93 when the increment thickness, voltage, and restorative material were taken together in the regression model.

Conclusions

There was no statistically significant difference between SDR and Filtek for the depth of cure at 2 and 4 mm increments. However, at 6 mm increment, the SDR cured significantly deeper than the Filtek. Around 91% variation in the depth of cure of these composites materials is explained by increment thickness alone.

Fracture Resistance and Marginal Adaptation of Capped and Uncapped Bulk-fill Resin-based Materials

OBJECTIVES

This study tested the fracture resistance of capped and uncapped bulk-fill composite restorations and compared them to a conventional composite. Also, the effect of different radiant exposure was investigated.

METHODS AND MATERIALS

Flowable and high-viscosity bulk-fill composites (SureFil SDR, Filtek Bulk-Fill Posterior, and Tetric N-Ceram Bulk-Fill) and a nanohybrid resin composite (Filtek Z350 XT) were used. Standardized class II cavities were prepared on extracted premolars, and different restoration protocols were used. In protocol 1 (control), restoration was applied using a layering technique; in protocol 2, restoration was applied in bulk with a capping layer; in protocol 3, restoration was applied in bulk without a capping layer; and in protocol 4, restoration was applied in bulk without a capping layer, and the light curing time was extended. After thermocycling, the restorations were examined for marginal gaps and then subjected to the fracture resistance test using a universal testing machine. Statistical analysis was carried out using two-way analysis of variance (ANOVA) followed by one-way ANOVA at a significance level of α = 0.05.

RESULTS

A statistically significant difference in the fracture resistance of the tested materials and protocols was detected. Filtek Bulk-Fill Posterior achieved the highest fracture resistance values regardless of the protocol used, and its results were comparable to those of Filtek Z350. SDR and Tetric N-Ceram Bulk-Fill achieved their highest strengths when a capping layer was added. Tetric N-Ceram Bulk-Fill showed improvement in fracture resistance with extended light curing, while SDR and Tetric N-Ceram Bulk-Fill achieved similar results with the addition of a capping layer. The uncapped bulk-fill group showed more gap-free margins than the capped group.

CONCLUSION

The new high-viscosity bulk-fill composite restorations seem to have adequate fracture resistance. However, the results are material dependent, and some materials perform better with a capping layer and extended light curing.

Depth of cure of high-viscosity bulk-fill and conventional resin composites using varying irradiance exposures with a light-emitting diode curing unit

The purpose of this study was to determine the depth of cure (DOC) of three resin-based composites (RBCs) using varying irradiance exposures with a corded light-emitting diode curing unit. DOCs for Filtek Z250, TPH Spectra, and Tetric EvoCeram Bulk Fill were determined using the International Organization for Standardization (ISO) Standard 4049. The RBCs were light-polymerized using three different power modes and manufacturer-recommended curing times. Irradiance was determined using a spectrometer sensor and the total energy density was calculated for each power mode and concomitant polymerization time. The DOC data were analyzed with a two-way analysis of variance and Tukey's post hoc test. Tetric EvoCeram Bulk Fill produced significantly greater DOCs than TPH and Z250 (P < 0.05) for all three power mode settings. Overall, the DOC of Tetric EvoCeram Bulk Fill was greater than those of TPH and Z250 at all power settings, but the individual RBCs did not show a significant DOC difference among the three power settings (P > 0.05).

A Chemical Approach to Optimizing Bioactive Glass Dental Composites

The chemical microenvironment surrounding dental composites plays a crucial role in controlling the bacteria grown on these specialized surfaces. In this study, we report a scanning electrochemical microscopy (SECM)-based analytic technique to design and optimize metal ion-releasing bioactive glass (BAG) composites, which showed a significant reduction in biofilm growth. SECM allows positioning of the probe without touching the substrate while mapping the chemical parameters in 3-dimensional space above the substrate. Using SECM and a solid-state H⁺ and Ca²⁺ ion-selective microprobe, we determined that the local Ca²⁺ concentration released by different composites was 10 to 224 µM for a BAG particle size of <5 to 150 µm in the presence of artificial saliva at pH 4.5. The local pH was constant above the composites in the same saliva solution. The released amount of Ca²⁺ was determined to be maximal for particles <38 µm and a BAG volume fraction of 0.32. This optimized BAG-resin composite also showed significant inhibition of biofilm growth (24 ± 5 µm) in comparison with resin-only composites (53 ± 6 µm) after Streptococcus mutans bacteria were grown for 3 d in a basal medium mucin solution. Biofilm morphology and its subsequent volume, as determined by the SECM imaging technique, was (0.59 ± 0.38) × 10⁷ µm³ for BAG-resin composites and (1.29 ± 0.53) × 10⁷ µm³ for resin-only composites. This study thus lays the foundation for a new analytic technique for designing dental composites that are based on the chemical microenvironment created by biomaterials to which bacteria have been exposed.

Effect of shade, opacity and layer thickness on light transmission through a nano-hybrid dental composite during curing

OBJECTIVE

The aim of this study was to investigate the effect of shade and opacity on the change in light transmission through different thicknesses of a nano-hybrid composite during curing.

MATERIALS AND METHODS

Twelve different shades of Venus Diamond (Heraeus Kulzer) were placed in disk shaped molds with thickness of 1, 2, and 3 mm (n = 3 per group) and cured with an LED light-curing unit. Initial, final and average irradiance, and the total amount of energy passing through the specimen were measured using the MARC Resin Calibrator at every 10s for a total of 40s. The translucency parameter and the contrast ratio were obtained using a chromameter. Results were analyzed with ANOVA/Tukey's test (α = 0.05).

RESULTS

All shades and all thicknesses (up to 3 mm) experienced an increase in light transmittance during curing. The majority of the increase occurred during the initial 10s exposure, with significant increase occurring from subsequent exposures only in thicker specimens (i.e., 3 mm). The increase in irradiance at the bottom during curing was dependent on shade, with darker shades and greater depths of material showing less increase.

CONCLUSIONS

For one specific resin composite formulation, an increase in translucency occurs as cure progresses, and the increase is enhanced for composites with greater lightness and lower contrast ratio.

CLINICAL SIGNIFICANCE

Composites demonstrate increased light transmittance as curing progress, which may improve depth of cure. The thicker composite showed the least increase in light transmission within the same shade. The increase in translucency is enhanced for composites with great lightness and lower contrast ratio.

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.

Polymerization Shrinkage and Depth of Cure of Bulk Fill Flowable Composite Resins

Objective

To evaluate polymerization shrinkage and depth of cure of two bulk fill flowable composites, one nanohybrid composite modified to a flowable consistency, and one standard flowable composite, comparing the scraping method to the Knoop hardness test.

Methods

Two bulk fill flowable composites, SureFil SDR flow (SSF) (Dentsply) and Venus Bulk Fill (VBF) (Heraeus Kulzer), one standard flowable, Filtek Supreme Ultra Flowable (FSUF) (3M/ESPE) (control), and one regular bulk composite that can be made flowable, SonicFill (SF) (Kerr), were used in this study. For polymerization shrinkage (PS), ten 2-mm samples were made for each composite and cured for 20 seconds and shrinkage was measured with a Kaman linometer. For hardness, ten specimens of each composite were made in a 10 × 10-mm mold and cured for 20 seconds; the bottom surface was scraped according to ISO 4049 specification, and the remaining thickness was measured with a micrometer. Hardness samples were prepared at 2-, 3-, 4-, and 5-mm thick ×14-mm diameter, cured for 20 seconds, and polished. After 24 hours of dry storage, a Knoop indenter was applied at 100 g load for 11 seconds. Three readings were made on the top and bottom of each specimen and averaged for each surface to calculate a Knoop hardness value and a bottom/top hardness ratio. One-way analysis of variance and Tukey tests were used to determine significant differences between thicknesses and between test methods for each material.

Results

PS values were 3.43 ± 0.51%, 3.57 ± 0.63%, 4.4 ± 0.79%, and 1.76 ± 0.53% for FSUF, SSF, VBF, and SF, respectively. VBF showed significantly greater shrinkage (4.4 ± 0.79%), followed by FSUF (3.43 ± 0.51%) and SSF (3.57 ± 0.63%), which were similar, and SF (1.76 ± 0.53%), which had significantly less shrinkage (p&lt;0.05). Values for the scraping method for depth of cure were significantly greater for SSF and VBF (&gt;5.0 mm), followed by SF (3.46 ± 0.16 mm) and FSU (2.98 ± 0.22 mm). Knoop top hardness values (KHN) were: VBF 21.55 ± 2.39, FSUF 44.62 ± 1.93, SSF 29.17 ± 0.76, and SF 72.56 ± 2.4 at 2 mm and were not significantly different at 3-, 4-, and 5-mm thick within each material. Ratios for bottom/top values (depth of cure) for 2, 3, 4, and 5 mm were: VBF 0.80 ± 0.1, 0.78 ± 0.03, 0.67 ± 0.10, and 0.59 ± 0.07, respectively; SSF 0.74 ± 0.08, 0.72 ± 0.08, 0.69 ± 0.18, and 0.62 ± 0.08, respectively; SF 0.82 ± 0.05, 0.68 ± 0.05, 0.47 ± 0.04, and 0.21 ± 0.02, respectively; and FSUF 0.56 ± 0.08 at 2 mm and 0.40 ± 0.08 at 3 mm. The bottom/top ratio was .80 or less at all depths and decreased below 0.70 at 4-mm depth for VBF and SSF, at 3 mm for SF and at 2 mm for FSUF.

The effect of interincisal opening, cavity location and operator experience on the energy delivered by a light-curing unit to a simulated dental restoration

BACKGROUND

Curing of resin-based composites depends on the delivery of adequate total energy, which may be operator dependent. Aim To determine the effect of interincisal opening, cavity location and operator experience on the total energy delivered to simulated cavity preparation sites.

DESIGN

Three cohorts were included: junior dental nurses, senior dental nurses and qualified dentists (N=5, each cohort). Each operator (participant) followed the same procedure and light-cured two simulated restorations in a MARC patient simulator using a Demi light-curing unit for 20 seconds in each of the following situations: left upper second molar (UL7), interincisal opening at both 25 mm and 45 mm; upper central incisor (UR1), interincisal opening at 45mm. The light energy delivered by each operator in each situation was recorded. Five readings for each operator were taken at each interincisal distance. Statistical comparisons of delivered energy (J/cm2) between interincisal openings, location and groups in the total energy delivered were performed using the Kruskal-Wallis nonparametric test: alpha = 0.05.

RESULTS

Less total energy was delivered to the posterior cavity at 25mm (12.0 +/- 5.3 J/cm2) than at 45mm (16.9 +/- 5.6 J/cm2) by all operators (P < 0.05). At 45 mm, less total energy was delivered to the posterior cavity compared to the anterior cavity (25.1 +/- 7.4 J/cm2; P < 0.05). There was no statistically significant difference between junior nurses and qualified dentists (P > 0.05) but there was a significant difference in the total energy delivered between senior nurses (20.1 +/- 7.8 J/cm2) and junior nurses (17.5 +/- 7.6 J/cm2) and between senior nurses and qualified dentists (16.6 +/- 8.7 J/cm2) (P < 0.05).

CONCLUSIONS

Interincisal mouth opening, location of the cavity and operator experience affected the total energy delivered to cavities in a simulated clinical environment.

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.

The effect of disposable infection control sleeves on the total energy delivered by dental LCUs

AIM

This study compared the effect of seven different disposable cross-infection control sleeves on the total energy delivered (J/cm2) by four different dental light-curing units (LCUs).

METHODS

Four LCUs and seven disposable sleeves (six proprietary brands and a general-purpose cellophane wrap) were used. Anterior and posterior tooth position was included as a third factor. Total energy delivered was measured for each LCU without a sleeve (control) and with each sleeve and for each tooth position. Curing time was 10 seconds, as recommended by the LCU manufacturers. Measurements were taken 10 times for each sleeve/LCU/tooth position and means (SD) calculated. Data were analysed using three-way analysis of variance (ANOVA). The null hypothesis was that none of the sleeves would, at a 0.1% level, significantly affect the total energy delivered.

RESULTS

All sleeves significantly reduced the total energy delivered (P < 0.001). Cellophane wrap (cling film) had the smallest effect on total energy. Total energy delivered was most strongly influenced by tooth position and LCU model.

CONCLUSIONS

Sleeves significantly reduce the total energy delivered compared to unsleeved LCUs, but to a degree that may not be clinically important. Cling film reduced total energy delivered to a significantly lesser extent than did proprietary bands of sleeve. In order to determine adequacy of delivered total energy, sleeved LCUs should be tested prior to clinical use.

Surface Hardness of Resin Cement Polymerized under Different Ceramic Materials

Objectives. To evaluate the surface hardness of two light-cured resin cements polymerized under different ceramic discs. Methods. 40 experimental groups of 2 light-cured resin cement specimens (Variolink Veneer and NX3) were prepared and polymerized under 5 different ceramic discs (IPS e.max Press HT, LT, MO, HO, and Cercon) of 4 thicknesses (0.5, 1.0, 1.5, and 2.0 mm), Those directly activated of both resin cements were used as control. After light activation and 37°C storage in an incubator, Knoop hardness measurements were obtained at the bottom. The data were analyzed with three-way ANOVA, t-test, and one-way ANOVA. Results. The KHN of NX3 was of significantly higher than that of Variolink Veneer (P < 0.05). The KHN of resin cement polymerized under different ceramic types and thicknesses was significant difference (P < 0.05). Conclusion. Resin cements polymerized under different ceramic materials and thicknesses showed statistically significant differences in KHN.

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.

General dental practitioners' knowledge of polymerisation of resin-based composite restorations and light curing unit technology

OBJECTIVE

Clinical successful use of resin-based composite restorations (RBCs) depends on knowledge of material and light curing unit (LCU) related factors. The purpose of this study was to evaluate general dental practitioners' knowledge of polymerisation of RBCs and LCU technology.

METHODS

Members of the Active Research Group of the Faculty of General Dental Practice (UK) in England, Scotland and Wales engaged in primary dental care were sent a letter introducing the study and asking for their cooperation, followed by an email containing a link to the online survey questionnaire, hosted on Surveymonkey.com. The questionnaire enquired about current LCUs, and asked a series of questions on material science.

RESULTS

Sixty-six percent of the 274 members contacted responded. Fifty-seven percent used LED units, 25% quartz tungsten halogen (QTH), and 1% plasma arc (missing: 17%). Thirty percent reported having access to a radiometer. Appropriate responses regarding the degree of conversion of composite and adhesive materials were given by 32% and 23% respectively, and 22% agreed that LED and QTH LCUs had comparable efficiency in polymerising composites. Thirty-three percent were aware that RBCs eluted substances that may have adverse local or systemic consequences. Fifty-eight percent stated that if polymerisation of RBC is slowed down, polymerisation stress will be lower, and 43% said that polymerisation shrinkage will be reduced if the degree of conversion is reduced. Knowledge (measured by appropriate responses to these questions) was not related to years since qualification (r=-0.05, n=168, p=0.53).

CONCLUSION

The study suggests that dentists' knowledge of curing RBC restorations and LCUs is poor. This indicates that there is a need for training and guidance in this aspect of primary dental care.

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.

Depth of cure of dental composites submitted to different light-curing modes

Objective:

This study evaluated the depth of cure of five dental composites submitted to different light-curing modes.

Material and Methods:

Canal-shaped cavities with 5mm of length were prepared on the buccal surfaces of extracted third molars, and restored using P-60, A-110, Admira, Z-250 and Supreme resin composites. Materials were light-cured from the top, according to three modes (Group 1- Conventional (C): 500 mW/cm²/ 40 s; Group 2 – Soft-Start (SS):250 mW/cm²/ 20 s + 500 mW/cm²/ 20 s + 500 mW/cm²/ 10 s and Group 3 – LED: 250 mW/cm²/ 40 s). After that, cavity longitudinal surfaces were polished and marked with a millimeter scale of 4mm of length. Depth of cure was evaluated by means of Knoop hardness number (KHN), so that five indentations were performed at each millimeter. Original data were submitted to three-way ANOVA and Fisher's LSD test (α = 0.01).

Results:

All materials presented a significant reduction on KHN from first to third millimeter. Regarding depth of cure, the results obtained for Conventional and Soft-Start modes were similar, but statistically superiors to those found for group 3 (LED). Conclusion: This performance may be related to the differences among energy densities obtained with different light-curing modes.

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.

Effects of irradiance, wavelength, and thermal emission of different light curing units on the Knoop and Vickers hardness of a composite resin

The aim of this study was to evaluate the effects of irradiance, light emission wavelength, and heating of different light curing units on the Knoop and Vickers hardness of a hybrid composite resin. The specimens were irradiated during 40 s with ten different light curing units, LEDs, and halogen lights. The spectral emission of each light curing unit was assessed by a spectrometer, the irradiance was measured by two commercial radiometers, and the heating measured with a thermocouple. After 48 h of storage in a dark recipient under a 100% humidity condition, the Knoop and Vickers hardness tests were carried out. The hardness results were analyzed by ANOVA, and Tukey HSD test (p < 0.05). The results showed that the surface hardness of the composite resin depends not only on the irradiance, but strongly on the emission wavelength and heating of the light curing units. It was observed, a linear correlation between the conversion degree and radiant exposure. In addition, it is suggested that the well known base to top surface hardness ratio convention of 80-90% is not appropriate to evaluate curing efficiency of composites, since the top surface is not always sufficiently polymerized.

Influence of the Feldspathic Ceramic Thickness and Shade on the Microhardness of Dual Resin Cement

Polymerization of dual resin cements is significantly more affected by thickness of feldspathic ceramic restorations than by shade.

Effects of different sizes of occlusal metal on curing depth of light-cured orthodontic band cement

INTRODUCTION

Sufficient penetration and quantity of light are necessary to cure band cement. It is doubtful whether this occurs when bands are cemented to teeth restored with large pieces of occlusal metal. The purpose of this in-vitro investigation was to examine the curing depth and degree of conversion from monomer to polymer of light-cured adhesives when used to cement bands to ceramic blocks covered with metal restorations.

METHODS

Two resin adhesives--Transbond Plus (3M Unitek, Monrovia, Calf) and Ultra Band-Lok (Reliance Orthodontic Products, Itasca, Ill)--and 3 curing methods were used. The upper surfaces of ceramic blocks (13 x 10 x 15 mm), used instead of human molars, were restored by using metal pieces of different sizes (4 x 8 x 2, 6 x 8 x 2, and 8 x 8 x 2 mm). The curing depth of the cement along the lateral surface of the block was measured at the middle of its width with slide calipers. Fourier transform infrared spectrometry was used to evaluate the degree of conversion of the adhesives.

RESULTS

There was unpolymerized resin in the deep area of the cement on curing with central irradiation from the occlusal surface. However, the amount of unpolymerized resin was significantly (P < .05) decreased with the boundary and circle irradiation methods.

CONCLUSIONS

The curing depth of band cement was significantly affected by the size of the occlusal metal restoration and the irradiation method. Therefore, a suitable irradiation method is essential for complete curing of resin for cementing bands to teeth with large metal restorations.

How light source and product shade influence cure depth for a contemporary composite

Directly placed light activated resin based composite restorations are becoming increasingly popular for restoring cavities in posterior teeth. Marketing and patient as factors well as operator preference are behind this trend. Achieving adequate depth of cure is critical to the success of these restorations. Recently a number of second generation light emitting diode (LED) light activation units have been marketed and the manufacturer of one of these claims that it is capable of curing in half the time of its predecessor. This study tested that claim using a companion composite from the same manufacturer. The relationship between cure depth, shade changes on curing and opacity were also assessed. Under the limitations of the current investigation the results indicated that the second generation LED unit in question met the manufacturer's claim for halving cure time. Depths of cure approached those of a control halogen unit in half the 40 s radiation time of the latter. For the product tested depth of cure is strongly linked to material opacity.

How filler properties, filler fraction, sample thickness and light source affect light attenuation in particulate filled resin composites

OBJECTIVE

It was hypothesized that by standardizing variables such as light sources, filler types and filler surface treatment, it should be possible to use Beer-Lambert's law to predict light absorption in visible light-cured dental composites.

METHODS

Mixture of 50 wt% bisGMA and 50 wt% TEGDMA to which a photo-initiator (0.35 wt% champhorquinone) and a co-initiator (0.7 wt% of dimethylaminoethylmethacrylate) was prepared. Three different filler types (HBB, SBB and KU) were added to that mixture in eight different volume percentage. Filler particles were either silane surface treated or not. Specimens were made with thicknesses of 1-5 mm. Total number of 1200 specimens were made for this study. Light transmission was obtained for halogen source and laser lights, which made the number of observations 2400. The absorbance values of the different materials were analyzed in Matlab with respect to the differences in filler fraction and sample thickness.

RESULTS

The obtained results revealed that of the two light sources, more light was absorbed by the composite when the laser light was used. Among different filler types, the HBB filler absorbed most light and the KU filler the least. There were significant differences (p < 0.05) in light absorption between all three filler types.

SIGNIFICANCE

By comparing the modeled surfaces generated by Matlab for different materials it was possible to determine how different variables such as filler type, filler surface treatment and light source affect light attenuation. The characteristic of incident light affected the light absorbance, meaning that not only the composite's composition needs to be considered in light absorption studies of dental composites.

Intensity of quartz-tungsten-halogen light-curing units used in private practice in Toronto

BACKGROUND

The authors conducted a study to determine light intensity and heat/glare measurements of quartztungsten-halogen (QTH) light polymerization units used in dental offices.

METHODS

Research assistants visited 100 dental offices and assessed 214 QTH light units. They recorded each unit's model, age, service history, light intensity and heat/glare emissions.

RESULTS

Mean light intensity was 526 milliwatts per square centimeter (120-1,000 mW/cm2), with 26 units having intensity less than 300 mW/cm2. The mean light unit age was 5.6 years. Light units older than three years had significantly lower output intensities than those that were one, two or three years old. The authors found a wide range of heat/glare measurements (3-300 mW/cm2), with 4.6 percent of the units having values greater than 50 mW/cm2, including three with values of more than 200 mW/cm2. The mean light intensity of units serviced in the preceding year was 539 mW/cm2; it was 418 mW/cm2 for units serviced from one to six years previously.

CONCLUSIONS

Light intensity and heat/glare values varied among the 214 units; some units had values well outside the recommended levels. Each unit's age and service history significantly affected its intensity. An awareness campaign is needed to promote testing, repair or replacement of light polymerization units. Periodic testing of light polymerization units should be considered by regulatory bodies to ensure optimum quality of composite restorations.

CLINICAL IMPLICATIONS

Light polymerization units in some private dental offices in Toronto had intensities that may result in composites restorations with inferior properties. Dentists need to regularly monitor the intensity of the light polymerization units and maintain the units to ensure quality composite restorations.

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

In pilot studies of the relationship between subsurface depth and hardness of a light-cured resin composite, it was found that the resin composite was softer at a depth of 0.5 mm than at, for example, a depth of 1.0 mm. It is possible that the increase in hardness at intermediate subsurface depths compared with the hardness at small depths is due to the heat of polymerization, causing a greater increase in temperature at intermediate depths than at small depths. As the temperature rise increases with volume of the test specimen, it was hypothesized that the increase in hardness would increase with the diameter of the irradiated specimen. The hardness of a resin composite was measured as a function of subsurface depth for cylindrical specimens of 3, 4, and 6 mm diameter. It was found that the resin composite was softer at 0.5 mm than at 1.0-1.5 mm depth independent of specimen diameter. Possible explanations are oxygen inhibition of polymerization and high rate of cure of material at small subsurface depth. It was also found that, corresponding with increasing specimen diameters, the specimens became significantly softer at depths of 3.0 mm, 3.5 mm, and 4.0 mm, respectively. The heat production and reflection of light from the walls of the molds may explain the latter, but not the former finding.

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.

Curing-light intensity and depth of cure of resin-based composites tested according to international standards

BACKGROUND

Several factors control the light curing of a resin-based composite: the composition of the composite, the shade of the composite, the wavelength and bandwidth of the curing light, the distance of the light from the composite, the intensity of the curing light and the irradiation time. The authors investigated the depth of cure of several shades of five brands of resin-based composites when irradiated via light in the 400- to 515-nanometer wavelength bandwidth at the International Organization for Standardization, or ISO, recommended intensity of 300 milliwatts per square centimeter. The resin-based composites were irradiated for the times recommended by the products' manufacturers.

METHODS

The authors used a curing light adjusted to emit 300 mW/cm2 in the 400-nm to 515-nm wavelength bandwidth to polymerize five samples of each composite brand type and shade. They measured depth of cure using a scraping method described in the ISO standard for resin-based composites. Depth of cure was defined as 50 percent of the length of the composite specimen after uncured material was removed by manual scraping. The authors determined a mean from the five samples of each composite brand and shade.

RESULTS

Thirteen (62 percent) of 21 composite materials met the ISO standard depth-of-cure requirement of 1.5 millimeters. Six of the eight remaining materials met the depth-of-cure requirement when the authors doubled the irradiation time recommended by the product manufacturers.

CONCLUSIONS AND CLINICAL IMPLICATIONS

Curing lights with an intensity of 300 mW/cm2 appear to effectively cure most resin-based composite materials when appropriate curing times are used, which, in some cases, are longer than those recommended by the manufacturers. Dentists should verify the depth of cure of a composite material as a baseline measure, and then check depth of cure periodically to confirm light and material performance. The ISO depth-of-cure measurement method can be used for this purpose.

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.

The application of magnetic resonance microimaging to the visible light curing of dental resins. 3. Stray-field nuclear magnetic resonance imaging (STRAFI)

OBJECTIVE

To investigate the application of stray-field nuclear magnetic resonance imaging (STRAFI) to the visible light curing of dental restorative materials. STRAFI can overcome peak broadening associated with the conventional magnetic resonance microimaging (MRM) of glassy polymers, and has the potential to image dental restorative resins at both low and high degrees of conversion.

METHODS

Cylindrical composite specimens were light-cured from one end to produce some that were fully cured throughout their length and others that were fully cured at one end and uncured at the other. A one-dimensional probe was used to measure the magnetisation in 40 microm thick slices at 100 microm intervals along the length of the specimen. A quadrature pulse sequence was applied and the magnetisation decay recorded in a train of eight echoes.

RESULTS

A value for T(2) could be obtained only for the polymer (59+/-16 microms), therefore the echoes were summed to give an approximate indication of the degree of conversion. The echo sum for the monomer was significantly higher than that for the polymer. Differences in composite shade and cure time produced changes in the cure profiles.

SIGNIFICANCE

STRAFI produced measurements for both monomer and polymer in all stages of conversion that allowed cure profiles to be produced. Summing the decay echoes produced a qualitative measure of the condition of the material in the selected slice. The same data can be used to calculate T(2), a quantitative parameter. This first investigation has demonstrated that STRAFI is well suited to polymerisation studies.

Influence of shade and storage time on the flexural strength, flexural modulus, and hardness of composites used for indirect restorations

STATEMENT OF PROBLEM

Fracture resistance, elastic modulus, and hydrolytic degradation resistance are important properties of indirect composite restorations. Composite systems developed specifically for indirect application are said to have enhanced mechanical properties due to their elevated monomer conversion.

PURPOSE

This study evaluated the influence of shade and the effect of 30-day water storage on the flexural strength, flexural modulus, and hardness of 4 commercially available indirect composite systems and 1 composite used with the direct technique.

MATERIAL AND METHODS

A variety of commercially available indirect resin composites (Artglass, Belleglass, Sculpture, and Targis) and 1 directly placed composite (Z100, control) were used. Specimens made with either incisal or dentin shade (n = 10) were fractured with a 3-point bending test. Pre-failure loads corresponding to specific displacements of the crosshead were used for flexural modulus calculation. Knoop hardness was measured on fragments (n = 3) obtained after the flexural test. Tests were performed after 24 hours and after a 30-day water storage at 37 degrees C. Flexural strength data were analyzed with the Weibull distribution. Flexural modulus and Knoop hardness data were analyzed with 3-way ANOVA and Tukey's post-hoc test (alpha=0.05).

RESULTS

In general, the directly placed composite (Z100) demonstrated flexural strength similar to that of Artglass, Targis, and Sculpture. Belleglass presented the highest flexural strength (221.7 MPa for incisal shade after 24 hours storage; 95% confidence interval: 208.3-235.4). Z100 demonstrated the highest flexural modulus (range: 10.9 +/- 0.6 to 12.0 +/- 0.9 GPa) and Targis the lowest (range: 5.1 +/- 0.5 to 5.9 +/- 0.9 GPa). Sculpture was the only material that showed differences in flexural strength with respect to shade (incisal-24 hours: 149.8 MPa; incisal-30 days: 148.7 MPa; dentin-24 hours: 200.0 MPa; dentin-30 days: 177.9 MPa). The flexural modulus and hardness of the dentin shade of Sculpture were higher than those of the incisal shade after 30 days. Belleglass also showed a significant difference in flexural modulus (dentin-24 hours: 11.1 GPa; incisal-24 hours: 9.6 GPa). The effect of water storage was more evident on hardness since all composite systems softened after 30 days. Prolonged water storage decreased flexural strength only for Artglass-dentin and Z100, both incisal and dentin shades. Water aging did not affect the flexural modulus of any composite tested.

CONCLUSION

In general, indirect composites did not show enhanced mechanical properties compared to the directly placed composite. Property differences due to shade were more evident for Sculpture. Prolonged water storage had a deleterious effect on the hardness of all composites tested. However, water storage did not affect the flexural strength of most of the indirect composites or the flexural modulus of any composite tested.

The suitability of packable resin-based composites for posterior restorations

BACKGROUND

Packable composites, promoted for the restoration of stress-bearing posterior teeth, have captured clinicians' interest.

METHODS

The authors tested three packable composites (Alert, Jeneric/Pentron; Solitaire, Heraeus Kulzer, Wehrheim, Germany; SureFil, Dentsply De Trey, Konstanz, Germany); a new packable organically modified ceramic, or ormocer (Definite, Degussa AG, Hanau, Germany); a hybrid composite (Tetric Ceram, Ivoclar Vivadent, Schaan, Liechtenstein) and an ion-releasing composite (Ariston pHc, Ivoclar Vivadent, Schaan, Liechtenstein). They determined modulus of elasticity according to EN 24049:1993 of the European Committee for Standardization. They measured Vickers hardness using a 200-gram load for 40 seconds. To determine the materials' depth of cure, they used both a scraping method (International Standards Organization standard CD 4049:1997) and a hardness profiling method.

RESULTS

The authors calculated means and standard deviations from 10 replications of each test and used one-way analysis of variance and post hoc Tukey tests (alpha = .05). The materials had significant differences (P < .001) in all characteristics. Solitaire had the significantly lowest elastic modulus and microhardness; Alert had the highest values for these characteristics. Ariston pHc exhibited the significantly lowest depth of cure. There was a significant correlation between the two methods of measuring depth of cure (r2 = 0.9945; P = .021).

CONCLUSIONS

The material group of packable composites is rather inhomogeneous in terms of mechanical and physical data. Our data suggest that bulk curing of packable composites in deep cavities still is not recommendable.

CLINICAL IMPLICATIONS

The clinician needs to select packable composites carefully, as it seems that not all of these materials quality for stress-loaded posterior restorations.

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.

Characterization of resin composites polymerized with plasma arc curing units

OBJECTIVES

Newly developed curing units (plasma arc curing units) operate at relatively high intensity and are claimed to result in optimum properties of resin composites in a short cure time. This study was conducted to determine a number of characteristics of resin composites polymerized by plasma arc curing units.

METHODS

The investigated polymerization characteristics were quantity of remaining double bonds, depth of polymerization, flexural strength and modulus, and wall-to-wall polymerization contraction. The investigated plasma arc curing units were Apollo 95E and 1000 PAC. The conventional curing unit XL 3000 was used as baseline.

RESULTS

Irradiation with Apollo 95E resulted in a higher quantity of remaining double bonds than did XL 3000, whereas the results obtained with 1000 PAC depended on the resin composite. The depth of cure with the plasma arc units was equal to or less than that obtained with the conventional unit, depending on the resin composite. The flexural strength did not depend on the curing unit. The flexural modulus resulting from curing with Apollo 95E was less than that resulting from curing with XL 3000 in 3 out of 4 comparisons. The wall-to-wall polymerization contraction was equal to or less with the plasma arc units than with the conventional unit.

SIGNIFICANCE

Plasma arc curing units make it possible to polymerize resin composite in much shorter times than conventional curing units. However, the polymerization characteristics associated with the units may be less than optimal.