Effect of blue and violet light on polymerization shrinkage vectors of a CQ/TPO-containing composite

Impact of Battery Levels of a Cordless LED Curing Unit on Resin Cement under Varied Lithium Disilicate Thicknesses and Translucencies

OBJECTIVES

This study aimed to evaluate the impact of battery levels on the emission of a multi-peak cordless LED light-curing unit (LCU) and the effect on the degree of conversion (DC) and Knoop hardness (KH) of a light-cure resin luting agent activated through varying lithium disilicate (LiS2) ceramic thicknesses and translucencies.

METHODS

High and low translucency LiS2 discs (IPS e.max Press HT and LT, respectively; shade A1) with thickness of 0.5, 1.0, 1.5, and 2.0 mm were fabricated. Resin luting agent specimens (Variolink Esthetic LC) were prepared and cured using a Bluephase G2 LCU at different battery levels (100%, 50%, and 10%) through the LiS2 ceramics. The transmitted irradiance was evaluated using USB4000 MARC, while FTIR and a microhardness tester assessed DC and KH, respectively. After ensuring homoscedasticity, the data wee analyzed using analysis of variance and Tukey HSD test (α=0.05).

RESULTS

The study found strong positive correlations between battery levels and irradiance, particularly with no ceramic interposition and through HT ceramics (R2=0.9471), although this correlation diminished with thicker HT (R2=0.7907) and LT ceramics (R2<0.2980). Both battery levels and ceramic thickness significantly influenced transmitted irradiance (p<0.0001), resulting in lower values with decreased battery levels and increased ceramic thicknesses (p<0.0001). LT ceramics showed lower transmittance than HT. DC was significantly affected by both battery levels and ceramic thicknesses, with generally lower DC values except for LT ceramics at a 10% battery level (p<0.0001). No significant differences in DC were observed between HT and LT translucencies (p=0.548). KH was higher in HT than LT ceramics at 100% and 50% battery levels, with thicker ceramics showing lower KH values at 10% battery level (p<0.0001). Conclusion: Reduced battery levels in cordless LED curing units significantly affect the irradiance, degree of conversion, and hardness of light-curable resin luting agents. Maintaining battery levels above 50% is recommended for optimal performance. Thicker and more opaque ceramics significantly impacted incident irradiance. However, preserving radiant energy could potentially mitigate these limitations.

Short curing time bulk fill composite systems: volumetric shrinkage, degree of conversion and Vickers hardness

This study aimed to evaluate volumetric polymerization shrinkage, degree of conversion and Vickers hardness of four bulk-fill resin composites light-activated with their dedicated light curing units (LCUs). Four groups were evaluated, according to the type of composite and curing mode: Tetric EvoCeram Bulk-fill (TEBO) and Tetric EvoFlow Bulk-fill (TEBF) were light-activated with Bluephase Style 20i (20s, in high-mode), while Tetric Powerfill (TEPO) and Tetric Powerflow (TEPF) were light-activated with Bluephase PowerCure (3s). Volumetric polymerization shrinkage test (n = 6) was performed in standardized box-shaped class-I cavities of extracted third molars (4 x 4 x 4 mm). Teeth were scanned before and after resin composite application by micro-computed tomography, and acquired data were evaluated with Amira software. Degree of conversion (n = 5) was evaluated at the top and bottom surfaces of composite cylindric samples (4 mm diameter, 4 mm thickness) using an FT-IR spectrometer (spectra between 1,500 and 1,800 cm-1, 40 scans at a resolution of 4 cm-1). Three Vickers indentations (50 g / 15 s), spaced 500 μm apart, were performed on the top and bottom composite surfaces and averaged. One-way ANOVA was used for data evaluation. TEPF showed the lowest volumetric polymerization shrinkage (p < 0.05), while the other composites were not significantly different within each other (p > 0.05). All materials presented a significant decrease in degree of conversion and Vickers hardness when compared top to bottom surfaces (p < 0.05). Bottom to top surface ratios for degree of conversion ranged from 0.8 (TEBO and TEPO) to 0.9 (TEBF and TEPF), and from 0.4 (TEPO) to 0.7 (TEBF and TEPF) for hardness. In conclusion, resinous materials present a decrease in hardness and degree of conversion from top to bottom even when a higher power is used, while the flowable material TEPF showed the lowest volumetric shrinkage values compared to the other materials.

Influence of thickness and degree of opacity of lithium disilicate on the degree of conversion and bond strength of resin cements

The aim of this study was to evaluate the influence of various opacities and thicknesses of lithium disilicate on the degree of conversion (DC) of two resin cements and on their bond strength (BS) to the ceramic. Two hundred and forty ceramic samples of lithium disilicate with high translucency (HT), low translucency (LT), and medium opacity (MO) were obtained from IPS e. max CAD in five different thicknesses. Light-cured and dual-cured resin cements were used for DC (n = 9) and BS (n = 8). Cement samples were light-cured under ceramic samples and analyzed using a Raman confocal spectrometer to determine the DC. For BS, resin cement cylinders were fabricated using ceramic samples which were thsen subjected to a microshear bond strength test. The mean values were provided for statistical analysis. The increase in thickness resulted in a decrease in DC for both cements under all experimental conditions, but only affected the BS of the light-cured cement for the MO ceramic. Comparing the opacities, the most translucent ceramics with particular thicknesses exhibited higher DC values than the less translucent ceramics. The LT and MO ceramics with certain thicknesses exhibited the highest BS values than the HT. The dual-cured cement generally showed highest values in both analyses than the light-cured cement. A thicker and more opaque ceramic material can reduce the DC of the cement. The BS decreases with the increasing thickness of the more-opaque ceramics.

The effect of high-irradiance rapid polymerization on degree of conversion, monomer elution, polymerization shrinkage and porosity of bulk-fill resin composites

OBJECTIVE

The purpose was to compare the degree of conversion (DC), monomer elution (ME), polymerization shrinkage (PS) and porosity of two addition-fragmentation chain transfer (AFCT) modified resin-based composites (RBC) light-cured with rapid- (RP), turbo- (TP) or conventional polymerization (CP) settings.

METHODS

Cylindrical samples (6-mm wide, 4-mm thick) were prepared from Tetric PowerFill (TPF) and Filtek One Bulk (FOB). Four groups were established according to the polymerization settings: 3s-RP, 5s-TP, 10s-CP and 20s-CP. Samples in 1 mm thickness with 20s-CP settings served as controls. The DC at the top and bottom surfaces was measured with micro-Raman spectroscopy. ME was detected with high-performance liquid chromatography. PS and porosity were analyzed by micro-computed tomography. ANOVA and Tukey's post-hoc test, multivariate analysis and partial eta-squared statistics were used to analyze the data (p < 0.05).

RESULTS

FOB showed higher DC values (61.5-77.5 %) at the top compared to TPF (43.5-67.8 %). At the bottom TPF samples achieved higher DCs (39.9-58.5 %) than FOB (18.21-66.18 %). Extending the curing time increased DC (except the top of FOB) and decreased ME. BisGMA release was the highest among the detected monomers from both RBCs. The amount was three-fold more from TPF. The factor Material and Exposure significantly influenced DC and ME. PS (1.8-2.5 %) did not differ among the groups and RBCs except for the lowest value of TPF cured with the 3s_RP setting (p = 0.03). FOB showed 4.5-fold lower porosity (p < 0.001). Significantly higher pore volume was detected after polymerization in 3s_RP (p < 0.001).

SIGNIFICANCE

High-irradiance rapid 3-s curing of AFCT modified RBCs resulted in inferior results for some important material properties. A longer exposure time is recommended in a clinical situation.

Effect of High-radiant Emittance and Short Curing Time on Polymerization Shrinkage Vectors of Bulk Fill Composites

PURPOSE

To evaluate the effect of short curing time using a high-radiant emittance light on polymerization shrinkage vectors in different consistency bulk-fill composites (BFRCs) using micro-computed tomography.

METHODS AND MATERIALS

Radiopaque zirconia fillers were homogeneously incorporated and functioned as radiopaque tracers into two regular-paste: TBFill (Tetric EvoCeram Bulk Fill) and TPFill (Tetric PowerFill), and two flowable (n=6): TBFlow (Tetric EvoFlow Bulk Fill) and TPFlow (Tetric PowerFlow) resin composites. Class I cavities (4 mm depth × 4 mm length × 4 mm width) were 3D-printed and filled in a single increment: TBFill and TBFlow were light-activated using a Bluephase Style 20i (10 seconds in high-mode); TPFill and TPFlow were light-activated using a Bluephase PowerCure (three seconds). The same adhesive system (Adhese Universal) was used for all groups. Microcomputed tomography scans were obtained before and after light-activation. Filler particle movement was identified by polymerization shrinkage vectors at five depths (from 0-4 mm): top, top-middle, middle, middle-bottom and bottom.

RESULTS

TPFlow showed the lowest total vector displacement, followed by TBFlow, TBFill and TPFill, significantly different among each other (p<0.05). Generally, BFRCs showed decreased vector displacement with increased depth, and higher displacement at the top-surface (p<0.05). Qualitative analysis showed a similar pattern of vector magnitude and displacement for groups TBFill and TPFill, with displacement vectors on occlusal (top) surfaces toward the center of the restoration from the top to middle areas, and relatively limited displacement at the bottom. TBFlow and TPFlow showed more displacement on the occlusal (top).

CONCLUSIONS

Short curing time with high-radiant emittance on fast-curing BFRCs was shown to be a feasible option in terms of vector displacement. Flowable BFRCs presented lower vector displacement than their regular-viscosity versions.

Effect of extended light activation and increment thickness on physical properties of conventional and bulk-filled resin-based composites

OBJECTIVES

To evaluate the biaxial flexural strength (BFS), flexural modulus (BFM), and Knoop microhardness (KHN) of incremental and bulk-filled resin-based composites (RBCs) using extended curing exposure times.

MATERIALS AND METHODS

Disc specimens (n = 8; 6-mm diameter) were fabricated using three stacked molds (0.5-mm thick for the top and bottom molds, and a 1-mm-thick center mold for the conventional and 3-mm thick for the bulk-fill RBCs). Conventional (Tetric EvoCeram/TCE and Filtek Z250/FIZ) and bulk-fill RBCs (Tetric EvoCeram Bulk Fill/TBF and Filtek One Bulk Fill Restorative/FOB) were evaluated. The stacked RBC-filled molds were light-cured for (1) the manufacturer-recommended exposure (MRE) duration; (2) 50%, and (3) 100% extension of the MRE. The BFS, BFM, and KHN of the top and bottom discs were measured. BFS and BFM were analyzed by three-way ANOVA (material*curing time*depth) and Tukey's post hoc (α = 0.05). KHN was analyzed by two-way ANOVA (curing time*depth) and Tukey's post hoc (α = 0.05).

RESULTS

Extending the exposure duration did not change the BFS and BFM on the top of the RBCs, but the BFS and KHN increased at the bottom of bulk-fill RBCs. For the conventional RBCs, TCE showed the highest increase on BFS at the bottom, going from 53.6 MPa at T1 to 69.9 at T3. Among the bulk-fill RBCs, FOB presented the highest increase on the bottom BFS (T1: 101.0 ± 19.9 MPa, T3: 147.6 ± 12.9 MPa). For all RBCs and exposure times, BFS and KHN were lower at the bottom. Only FIZ and FOB reached a bottom-to-top hardness ratio of 80%, at T3 and T2.

CONCLUSION

A significant increase on the BFS and KHN on the bottom of bulk-fill RBCs can be observed when the time of exposure to the curing light is double the MRE. However, extended exposure does not eliminate differences on the BFS and KHN between the shallow and deep regions of RBCs. TCE and TBF failed to reach an acceptable B/T hardness ratio at all evaluated exposure times.

CLINICAL RELEVANCE

Mechanical properties of RBCs can be affected by insufficient polymerization, specially at deeper regions of the increment. Therefore, clinicians should consider applying twice the MRE to curing-light to polymerize the maximal increment thickness of bulk-fill RBCs.

Effects of application method on shrinkage vectors and volumetric shrinkage of bulk-fill composites in class-II restorations

OBJECTIVES

Upon initial proximal wall construction, the favorable C-factor of class-II cavities may become unfavorable. This study investigated the application method on bulk-fill resin composite polymerization shrinkage.

METHODS

Occluso-proximal class-II cavities were prepared in 40 molars and bonded with a self-etch adhesive (Adhese Universal). The study groups varied according to the resin composite application: group-1: bulk application, Tetric EvoCeram Bulk Fill (TBF); group-2: proximal wall construction (TBF) and occlusal cavity filling (TBF); group-3: thin flowable liner layer, Tetric EvoFlow Bulk Fill (TEF) and bulk filling (TBF); group-4: flowable liner (TEF), proximal wall (TBF), occlusal cavity (TBF); and group-5: bulk application, SDR (3 mm) and capping layer (TBF, 1 mm). Each resin composite increment was scanned twice using micro-CT (uncured, cured 40 s) at a resolution of 16 µm. Shrinkage vectors and volumetric polymerization shrinkage were evaluated and statistically analyzed (one-way ANOVA). SEM images were used to investigate the tooth-restoration interface.

RESULTS

Shrinkage vectors differed significantly among the groups and were greatest in gp5-fl/SDR (47.6 µm), followed by gp1-TBF (23.8 µm) and least in gp5-fl/SDR+TBF (11.1 µm). Volumetric shrinkage varied significantly with the use of SDR (gp5-fl/SDR: 2.6%) and TEF (gp4-fl/TEF: 2.5%) to TBF (gp4-fl/TEF+wl/TBF: 0.6%) in the incremental application.

SIGNIFICANCE

Building a proximal resin composite wall yielded smaller shrinkage vectors than the bulk application. Applying a thin flowable liner decreased the shrinkage vectors, even more when building a proximal wall. A thin flowable liner is recommended when building a proximal resin composite wall.

Beam Profiling of Dental Light Curing Units Using Different Camera-Based Systems

Objective  This study aimed to perform the beam profile of dental light-curing units (LCUs) using mirrorless and smartphone cameras and correlate it to a camera-based laser beam profiling system.

Materials and Methods  Three LCUs were evaluated (Radii Plus; Bluephase G2; and VALO Cordless). The spectral power of the LCUs was measured by using a spectrophotometer. The light emitted from the LCUs was projected onto a glass diffuser, and the images were recorded by using a mirrorless camera (NEX-F3), a smartphone (iPhone) and a camera-based beam profiler. Bandpass optical-filters were used, and for each LCU, the total spectral power output was integrated to calibrate the images. Statistical analysis was performed by digital image correlation (pixel by pixel) using Pearson’s correlation (α = 0.05; β = 0.2).

Results  The beam profile images showed nonuniform radiant emittance and spectral emission distributions across all the LCUs light tip. A strong correlation was found among cameras (Pearson’s r = 0.91 ± 0.03 with 95% confidence interval [CI]: 0.88–0.94 for the NEX-F3 and Pearson’s r = 0.88 ± 0.04 with 95% CI: 0.84–0.92 for the iPhone).

Conclusion  The standard Ophir beam profile system presented the most accurate distribution, but the mirrorless and smartphone cameras presented a strong correlation in the irradiance distribution of the beam profile images. Alternative cameras can be used to perform light beam profile of dental LCUs, but caution is needed as the type of sensor, image bit depth, and image processing are important to obtain accurate results.

Light-curing units used in dentistry: Effect of their characteristics on temperature development in teeth

This study aimed to investigate pulp chamber and surface temperature development using different LED light curing units (LCUs). Eight brands of LED-LCUs were tested in a laboratory bench model. The pulp chamber and surface temperature were recorded with a type T thermocouple and infrared cameras, respectively. The highest pulp chamber and surface temperature increase was 6.1±0.3°C and 20.1±1.7°C, respectively. Wide-spectrum LED-LCUs produced higher pulp chamber temperature increase at 0 mm and 2 mm but lower at 4 mm. Narrow-spectrum LED-LCUs produced higher surface temperature increase. LED-LCU featuring modulated output mode resulted in lower increase in pulp chamber temperature but higher on surface temperature. LED-LCU with light guide tip delivering an inhomogeneous beam caused higher increase in temperature on the surface and in the pulp chamber. LED-LCUs with different spectral emission, output mode and light guide tip design contributed to different temperature development in the pulp chamber and at the surface of teeth.

Awareness and Utilization of Bulk-Fill Composites among Dental Practitioners in Saudi Arabia

Objective:

The aim of this study was to assess the knowledge and utilization of Bulk-Fill (BF) resin composites among dental practitioners in Saudi Arabia.

Materials and Methods:

An online survey was distributed through Twitter, Instagram and WhatsApp applications among dentists in Saudi Arabia. A 31-item questionnaire covering personal data, general knowledge and utilization of BF composites was used. Responses were collected and analyzed for trends. A knowledge scale was developed based on answers with specific weight for each correct answer provided by the participants. A score of ≥ 50% was considered as a satisfactory knowledge level for participants. Statistical analysis was conducted using One-sample Z- and Chi-square tests followed by Bonferroni correction at 0.05 significance level.

Results:

The total number of participants of the survey was 183, of which 41.5% had some knowledge and utilized BF composites in their practice. Only 9.84% of the participants score ≥ 50% on the knowledge scale. A significantly low proportion recognized accurately the compositional difference between BF and conventional resin composite as well as the minimum irradiance values needed for proper polymerization of BF. There was no effect for gender or years of experience on the knowledge or utilization of BF materials (p-value = 0.172).

Conclusion:

The proportions of practitioners with adequate knowledge and utilization of BF materials were very low. Additional focus must be provided in order to expose graduating dentists to advances in resin composite formulations.

Flexural strength and microhardness of bulk-fill restorative materials

BACKGROUND

Bulk-fill materials can facilitate the restorative procedure mainly for deep and wide posterior cavities. The purpose of this study was to evaluate flexural strength (biaxial flexural strength [BFS]) and microhardness (Knoop microhardness [KHN]) at different depths of bulk-fill materials.

METHODS

Five bulk-fill materials were tested: two light-curable composite resins, one dual-cure composite, one bioactive restorative, and a high-viscosity glass ionomer. A conventional composite was used as control. BFS and KHN were tested at different depths. Data was analyzed by two- and one-way ANOVAs, respectively and Tukey's post-hoc (α=0.05).

RESULTS

The high-viscosity glass ionomer material presented the lowest BFS at all depths. KHN for the two light-curable and the dual-cure bulk-fill resin composites was reduced following an increase in restoration depth, while the conventional composite, the bioactive material, and the high-viscosity glass ionomer were not affected.

CONCLUSION

There are differences in the properties of the tested materials at 4 mm depth, showing that the studied properties of some materials vary according to the cavity depth, although the results are material dependent.

CLINICAL SIGNIFICANCE

Mechanical properties of light-cured, bulk-fill materials may be affected by inadequate polymerization. Clinicians should consider complementary strategies to achieve adequate polymerization at high-increment depths.

The Effect of Irradiance on the Degree of Conversion and Volumetric Polymerization Shrinkage of Different Bulk-Fill Resin-Based Composites: An In Vitro Study

Objective  The influence of different light-emitting diode (LED) curing light intensities on the degree of conversion (DC) and volumetric polymerization shrinkage (VPS) of bulk-fill resin-based composite (RBC) restorative materials was evaluated.

Materials and Methods  Twenty-four specimens of each RBC material (Filtek one bulk-fill posterior, Reveal HD Bulk, Tetric N-Ceram, and Filtek Z350) were prepared. The RBCs were shaped in molds and cured using an LED curing light unit at high-intensity (1,200 mW/cm ² ) for 20 seconds and low-intensity (650 mW/cm ² ) for 40 seconds Fourier-transform infrared (FTIR) spectroscopy was used to determine the DC and microcomputed tomography was used to evaluate VPS. Data were analyzed using one- and two-way ANOVA, independent t -test, and Tukey’s and Scheffe’s post hoc multiple comparison tests.

Results  With high-intensity curing light, Reveal HD showed the highest DC (85.689 ± 6.811%) and Tetric N-Ceram the lowest (52.60 ± 9.38%). There was no statistical difference in VPS when using high- or low-intensity curing light. The highest VPS was observed for Reveal HD (2.834–3.193%); there was no statistical difference ( p > 0.05) among the other RBCs.

Conclusion  Curing light intensities do not significantly influence the VPS of RBC materials. Reveal HD bulk cured with high-intensity light had the highest DC.

Effect of Shortened Light-Curing Modes on Bulk-Fill Resin Composites

CLINICAL RELEVANCE

Shortened light curing does not affect volumetric polymerization shrinkage or cohesive tensile strength but negatively affects the shear bond strength of some bulk-fill resin composites. When performing shortened light curing, clinicians should be aware of the light output of their light-curing units.

SUMMARY

Purpose: To evaluate volumetric polymerization shrinkage (VPS), shear bond strength (SBS) to dentin, and cohesive tensile strength (CTS) of bulk-fill resin composites (BFRCs) light activated by different modes.Methods and Materials: Six groups were evaluated: Tetric EvoCeram bulk fill + high mode (10 seconds; TEC H10), Tetric EvoFlow bulk fill + high mode (TEF H10), experimental bulk fill + high mode (TEE H10), Tetric EvoCeram bulk fill + turbo mode (five seconds; TEC T5), Tetric EvoFlow bulk fill + turbo mode (TEF T5), and experimental bulk fill + turbo mode (TEE T5). Bluephase Style 20i and Adhese Universal Vivapen were used for all groups. All BFRC samples were built up on human molar bur-prepared occlusal cavities. VPS% and location were evaluated through micro-computed tomography. SBS and CTS tests were performed 24 hours after storage or after 5000 thermal cycles; fracture mode was analyzed for SBS.Results: Both TEC H10 and TEE H10 presented lower VPS% than TEF H10. However, no significant differences were observed with the turbo-curing mode. No differences were observed for the same BFRC within curing modes. Occlusal shrinkage was mostly observed. Regarding SBS, thermal cycling (TC) affected all groups. Without TC, all groups showed higher SBS values for high mode than turbo mode, while with TC, only TEC showed decreased SBS from high mode to turbo modes; modes of fracture were predominantly adhesive. For CTS, TC affected all groups except TEE H10. In general, no differences were observed between groups when comparing the curing modes.Conclusions: Increased light output with a shortened curing time did not jeopardize the VPS and SBS properties of the BFRCs, although a decreased SBS was observed in some groups. TEE generally showed similar or improved values for the tested properties in a shortened light-curing time. The VPS was mostly affected by the materials tested, whereas the SBS was affected by the materials, curing modes, and TC. The CTS was not affected by the curing modes.

Color stability of a bulk-fill composite resin light-cured at different distances

The aim of this in vitro study was to evaluate the color stability of a bulk-fill (Filtek One Bulk Fill, 3M ESPE) and a conventional (Filtek Z350 XT, 3M ESPE) composite resin light-cured at different distances, before and after being submitted to staining with a coffee solution. Sixty specimens of each composite resin were prepared and light-cured at distances of 0, 2 and 4 mm, using a LED light-curing unit (Valo, Ultradent). The specimens were separated (n = 10) for immersion in either distilled water or coffee solution (10 minutes a day for 8 days) to stimulate staining. Color evaluations were performed before and after immersion in the solutions, according to CIELab (△Eab), CIEDE2000 (△E00) and the Whiteness Index for Dentistry (△WID). Kruskal Wallis and Dunn tests, Mann-Whitney tests and Wilcoxon test were applied (α = 5 %). The a* value for conventional composite resin showed a significant increase after immersion in coffee and distilled water (p < 0.05). Both composite resins showed greater b* values when immersed in coffee than in distilled water, with no significant difference among the light-activation distances (p > 0.05). There was no significant difference for L* among the light-activation distances; both resins showed significant decrease in L* after immersion in coffee (p < 0.05). Color change (△Eab, △E00) and difference in whiteness (△WID) were higher for conventional resin when immersed in the coffee solution at all the light-activation distances. Conventional composite resin presented a higher staining value than bulk-fill composite resin, regardless of the light-activation distance.

Influence of beam homogenization on bond strength of adhesives to dentin

OBJECTIVE

This study evaluated the effect of beam homogeneity on the microtensile bond strength (μTBS) of two adhesive resins to dentin.

METHODS

One polywave light-emitting-diode (LED) LCU (Bluephase Style, Ivoclar Vivadent AG) was used with two different light guides: a regular tip (RT, 1010 mW/cm² emittance) and a homogenizer tip (HT, 946 mW/cm² emittance). The emission spectra and beam profiles were measured from both light guides. Extracted third molars were prepared for μTBS evaluation using two adhesive systems: Excite F (EXF) and Adhese Universal (ADU). Bond strength was calculated for each specimen (n = 10) at locations that correlated with the output of the two LED chips emitting blue (455 nm) and the one chip that emitted violet light (409 nm) after 24-hs and after one-year water-storage. The μTBS was analyzed using a four-way analysis of variance (factors: adhesive system, light guide, LED wavelength, and storage time) and post-hoc Tukey test (α = 0.05).

RESULTS

EXF always delivered a higher μTBS than ADU (p < 0.0001), with the μTBS of ADU being about 20% lower than EXF. The light guide (p = 0.0259) and storage time (p = 0.0009) significantly influenced the μTBS. The LED wavelengths had no influence on the μTBS (p > 0.05).

SIGNIFICANCE

Homogeneity of the emitted light beam was associated with higher 24-h μTBS to dentin, regardless of the adhesive tested. Also, differences in the composition of adhesives can affect their compatibility with restorative composites and their ability to maintain bonding over one year.

Microcomputed tomography evaluation of cement film thickness of veneers and crowns made with conventional and 3D printed provisional materials

OBJECTIVE

To evaluate, through microcomputed tomography (μCT), the cement film thickness of veneers and crowns made with different provisional materials.

MATERIAL AND METHODS

A veneer and a crown preparation were performed on a central incisor and a second molar of a dental model, respectively, scanned with an intraoral scanner, and the .stl files were exported to an LCD-based SLA three-dimensional (3D)-Printer. Twenty-four preparations were 3D-printed for each veneer and crown and divided into four groups (n = 6/group): (a) Acrylic resin (Acrílico Marche); (b) Bisacrylic resin (Protemp 4); (c) PMMA computer-aided design and computer-aided manufacturing (CAD-CAM) (Vipiblock); and (d) 3D-printed resin for provisional restorations (Raydent C&B for temporary crown and bridge). Veneers and crowns restorations were performed and cemented with a flowable composite. Each specimen was scanned with a μCT apparatus, files were imported for data analysis, and cement film thickness was quantitatively measured. Data were analyzed by 2-way ANOVA and Tukey post-hoc tests (α = .05).

RESULTS

Crowns presented a thicker cementation film than veneers (P < .05).The bisacrylic resin showed the smallest veneer film thickness, similar to the acrylic resin (P = .151), which was not significantly different than the PMMA CAD/CAM material (P = .153). The 3D printed provisional material showed the thicker film, different than all other materials (P < .05). The bisacrylic resin showed a cement film thickness with a high number of voids in its surface. For crowns cementation, the 3D printed provisional material showed the thicker cementation film, different than all other materials (P < .05).

CONCLUSIONS

Different provisional materials present different film thicknesses. The 3D printed provisional material showed the highest veneer and crown film thicknesses. Veneers film thicknesses were smaller than crowns for all provisional materials.

CLINICAL SIGNIFICANCE

The 3D printed provisional material studied can be satisfactorily used, presenting appropriate adaptation with the tooth preparation, however, it shows the highest cement film thickness for both veneers and crowns cementations when compared with other provisional materials. A better internal fit, or smaller cement film thickness is obtained by CAD/CAM materials, acrylic and bisacrylic resins. Veneer cementation showed a smaller cement film thickness compared with crown cementation for all provisional materials.

The effect of different light curing units on Vickers microhardness and degree of conversion of flowable resin composites

This study aimed to assess the influence of different light curing units (LCUs) on the polymerization of various flowable resin composites. Three LCUs (Optilux 501, Elipar™ DeepCure-L LED and Bluephase^®20i) and eight flowable resin composites: MI FIL Flow, Estelite Flow Quick, Estelite Universal Flow (medium), Estelite Universal Flow (super low), Beautifil Flow Plus, Clearfil Majesty ES Flow, Filtek Supreme Ultra flowable and TetricEvo Flow were tested. For Vickers microhardness (VHN) test and degree of conversion (DC), specimens were prepared and polymerized for 20 s. VHN test was performed at top surfaces (3 indentations) and DC for each specimen was measured using Fourier transform infrared (FT-IR) spectroscopy after 24 h dry storage in dark at 37˚C. The data were analyzed with 2-way ANOVA and t-test with Bonferroni correction. DC and hardness values showed a relationship between materials and LCUs. The curing efficacy of LCU type may depend on the material composition.

Effects of extending duration of exposure to curing light and different measurement methods on depth-of-cure analyses of conventional and bulk-fill composites

This study evaluated the effect of extending the duration of exposure to curing light on the depth of cure of two conventional (RBC1-conventional and RBC2-conventional) and two bulk-fill (RBC1-bulk and RBC2-bulk) resin composites. Polywave and single-peak photocuring units were used. Cylinder-shaped specimens were exposed to curing light either for the time period recommended by the manufacturer or twice the length of that time, and depth of cure was estimated using manual scraping (similar to the ISO-4049 standard) and solvent immersion techniques. Depth of cure was analyzed, using two-way ANOVA, for the factors measurement method and exposure time. For RBC1-conventional and RBC1-bulk, the solvent immersion technique estimated a greater depth of cure than did manual scraping; for RBC1-conventional, both techniques and both light-exposure time periods resulted in a depth of cure of >2 mm; and for RBC1-bulk, only the solvent method after photocuring for twice the manufacturer's recommended time resulted in a depth of cure of 5 mm. For RBC2-conventional and RBC2-bulk, neither technique nor exposure time resulted in estimated depths of cure that matched those indicated by the manufacturer. The results suggest that extending the duration of photopolymerization increases depth of cure. Also, calculation of depth of cure can vary according to the measurement technique used.

Influence of Multiple Peak Light-emitting-diode Curing Unit Beam Homogenization Tips on Microhardness of Resin Composites

This study evaluated the effect of light curing unit (LCU) guide type (regular or homogenizing) on top and bottom microhardness of conventional and bulk-fill resin-based composites (RBCs). A polywave light-emitting-diode (LED) LCU (Bluephase Style, Ivoclar Vivadent AG) was used with two different light guides: a regular tip (RT, 935 mW/cm2 emittance) and a homogenizer tip (HT, 851 mW/cm2 emittance). Two conventional RBCs (Herculite Ultra [HER], Kerr Corp; Tetric EvoCeram [TEC], Ivoclar Vivadent AG) and two bulk-fill RBCs (SonicFill [SOF], Kerr Corp; Tetric EvoCeram Bulk Fill [TBF], Ivoclar Vivadent AG) were tested. Disc-shaped samples (10 mm Ø), 2-mm thick for conventional composites and 4-mm thick for bulk-fill composites were prepared. Samples were light cured according to manufacturer-recommended times. Knoop microhardness values (KHN) were obtained on the top and bottom surfaces of each specimen at locations correlated with the output of the three LED chips emitting blue (456 nm) or violet light (409 nm). Beam profile analysis using both light guides was also performed. Microhardness of each composite was analyzed using three-way analysis of variance and Tukey honestly significant difference post hoc test (α=0.05). Beam profile images showed better light distribution across the surface of the HT light guide. Use of the HT decreased KHN of HER at the locations of the blue LED chips at bottom of the sample but had no effect on the top surface. For TEC, use of HT increased KHN of all three LED locations at the top surface. Use of the HT increased KHN of SOF at locations corresponding to one of the blue and the violet LED chips at the bottom surface. For TBF, HT increased KHN at all top surface locations. All RBCs showed higher mean KHN at the top compared with the bottom surfaces. In general, all composites presented a higher KHN at the blue LED areas regardless of the surface or the tip used. Results suggest that the homogenizer light guide resulted in significantly increased microhardness at the top, in composite resins containing alternative photoinitiators; however, that effect was not observed at the bottom surfaces.

Volumetric polymerization shrinkage and its comparison to internal adaptation in bulk fill and conventional composites: A μCT and OCT in vitro analysis

OBJECTIVE

To quantify the volumetric polymerization shrinkage (VPS) of different conventional and bulk fill resin composites, through micro-computed tomography (μCT), and qualitative comparison of gap formation through optical coherence tomography (OCT).

METHODS

Box-shaped class I cavities were prepared in 30 third-molars and divided into 5 groups (n=6): G1- Filtek Z100 (Z100); G2- Tetric Evoceram Bulk Fill (TEC); G3- Tetric EvoFlow Bulk fill (TEF); G4- Filtek Bulk fill (FBU); and G5- Filtek Bulk fill Flowable (FBF). All groups were treated with Adper Single Bond Plus adhesive and light cured (Bluephase 20i). Each tooth was scanned three times using a μCT apparatus: after cavity preparation (empty scan); after cavity filling (uncured scan) and after light curing of the restorations (cured scan). The μCT images were imported into a three-dimensional rendering software, and volumetric polymerization shrinkage percentage was calculated (%) for each sample. In the same images, interfacial gaps in the pulpal floor were qualitatively evaluated. After μCT evaluation, the pulpal floor from each tooth was polished until a thin tooth structure was obtained and OCT images were obtained by scanning the pulpal portion. Gap formation was observed and qualitatively compared to the μCT images.

RESULTS

VPS means ranged from 2.31 to 3.96% for the studied resin composites. The bulk fill materials, either high viscosity or flowable, were not statistically different from each other (p>0.05). The conventional resin composite Z100 presented statistically higher VPS than both high viscosity bulk fill materials studied (p<0.05), although it was statistically similar to the flowable bulk fill materials studied (p>0.05). Both μCT and OCT methodologies enabled gap formation visualization, and images from both technologies could be associated. Gap formation was mostly observed for G1-Z100, G4-FBU, and G5-FBF. VPS% and pulpal gap formation could not be completely associated with each other for all groups and samples. Voids were observed in most of the resin composite fillings, and most VPS were observed in the occlusal area of the samples.

SIGNIFICANCE

Volumetric polymerization shrinkage was material-dependent, although bulk fill materials did not differ from each other. Both μCT and OCT enabled interfacial pulpal gap formation visualization. VPS and gap formation cannot be completely associated with one another.

Polymerization pattern characterization within a resin-based composite cured using different curing units at two distances

OBJECTIVES

To investigate the relationship of the irradiance-beam-profile areas from six different light-curing units (LCUs) with the degree of conversion (DC), microhardness (KH), and cross-link density (CLD) throughout a resin-based composite (RBC) cured at two clinically relevant distances, and to explore the correlations among them.

MATERIALS AND METHODS

A mapping approach was used to measure DC using micro-Raman spectroscopy, KH using a Knoop indentor on a hardness tester, and %KH reduction after ethanol exposure, as an indicator for CLD within a nano-hybrid RBC increment (n = 3) at various depths. These sample composites were cured from two distances while maintaining the radiant exposure, using six different light-curing units: one quartz-tungsten-halogen; two single and three multiple-emission-peak light-emitting-diode units. Irradiance beam profiles were generated for each LCU at both distances, and localized irradiance values were calculated. Points across each depth were analyzed using repeated measures ANOVA. Correlations across multiple specimen locations and associations between beam uniformity corresponding with polymerization measurements were calculated using linear mixed models and Pearson correlation coefficients.

RESULTS

Significant non-uniform polymerization patterns occurred within the specimens at various locations and depths. At 2-mm curing distance, the localized DC = 52.7-76.8%, KH = 39.0-66.7 kg/mm², and %KH reduction = 26.7-57.9%. At 8-mm curing distance, the localized DC = 50.4-78.6%, KH = 40.3-73.7 kg/mm², and %KH reduction = 28.2-56.8%. The localized irradiance values were weakly correlated with the corresponding DC, KH, and %KH reduction, with only a few significant correlations (p < 0.05).

CONCLUSIONS

Although significant differences were observed at each depth within the specimens, the localized irradiance values for all LCUs did not reflect the polymerization pattern and did not seem to have a major influence on polymerization patterns within the RBC, regardless of the curing distance.

CLINICAL RELEVANCE

Commonly used LCUs do not produce uniform polymerization regardless of the curing distance, which may contribute to the risk of RBC fracture.

Effects of boundary condition on shrinkage vectors of a flowable composite in experimental cavity models made of dental substrates

OBJECTIVES

Bond strength to enamel and dentin depends on the bonding approach or condition. This study investigated the effects of the boundary conditions, in terms of the bonding substrate and the bonding condition, on the shrinkage vectors of a flowable composite.

MATERIALS AND METHODS

An experimental cylindrical cavity (diameter = 6 mm, depth = 3 mm) consisting of the enamel floor and the surrounding dentin cavity walls was prepared for the "enamel-floor" group. Cylindrical cavities of the same dimensions were prepared with access from the occlusal enamel into dentin and served as controls. Each cavity model group was divided and bonded with two bonding conditions (n = 9): a self-etch (Adper Easy Bond, 3M ESPE) and a total-etch approach (OptiBond FL, Kerr). The composite (Tetric EvoFlow, Ivoclar Vivadent) was mixed with glass beads, applied to the cavity, scanned twice by micro-CT (uncured and cured states). The scans were evaluated by rigid registration, sphere segmentation, and registration for computing shrinkage vectors.

RESULTS

The free surface of all restorations moved downward. The shrinkage vectors in the experimental cavity model pointed downward towards the enamel cavity floor, and the net axial movement was downward. In the control group, shrinkage vectors additionally moved upward, away from the cavity floor. The effect of the bonding substrate and the bonding condition was investigated for the shrinkage vectors and the axial movement (univariate ANOVA).

CONCLUSION

The bonding substrate, enamel, influenced the shrinkage vectors' direction, while the bonding condition caused only variations in the magnitude.

CLINICAL RELEVANCE

Bonding to enamel influences shrinkage vectors' direction, while the bonding condition plays only a minor role. Graphical abstract ᅟ.

Photoinitiators in Dentistry: Challenges and Advances

Photopolymerization is used in a wide range of clinical applications in dentistry and the demand for dental materials that can restore form, function and esthetics is increasing rapidly. Simultaneous with this demand is the growing need for photoinitiators that provide effective and efficient in situ polymerization of dental materials using visible light irradiation. This chapter reviews the fundamentals of Type I and II photoinitiators. The advantages and disadvantages of these photoinitiators will be considered with a particular focus on parameters that affect the polymerization process in the oral cavity. The chapter examines recent developments in photoinitiators and opportunities for future research in the design and development of photoinitiators for dental applications. Future research directions that employ computational models in conjunction with iterative synthesis and experimental methods will also be explored in this chapter.

Effect of Thickness on Light Transmission and Vickers Hardness of Five Bulk-fill Resin-based Composites Using Polywave and Single-peak Light-emitting Diode Curing Lights

OBJECTIVES:

This study compared light transmission through different thicknesses of bulk-fill resin-based composites (RBCs) using a polywave and a single-peak light-emitting diode light-curing unit (LCU). The effect on the surface hardness was also evaluated.

METHODS:

Five bulk-fill RBCs were tested. Specimens (n=5) of 1-, 2-, 4-, or 6-mm thickness were photopolymerized for 10 seconds from the top using a polywave (Bluephase Style) or single-peak (Elipar S10) LCU, while a spectrophotometer monitored in real time the transmitted irradiance and radiant exposure reaching the bottom of the specimen. After 24 hours of storage in distilled water at 37°C, the Vickers microhardness (VH) was measured at top and bottom. Results were analyzed using multiple-way analysis of variance, Tukey post hoc tests, and multivariate analysis (α=0.05).

RESULTS:

The choice of LCU had no significant effect on the total amount of light transmitted through the five bulk-fill RBCs at each thickness. There was a significant decrease in the amount of light transmitted as the thickness increased for all RBCs tested with both LCUs ( p<0.001). Effect of LCU on VH was minimal (η_p²=0.010). The 1-, 2-, and 4-mm-thick specimens of SDR, X-tra Fill, and Filtek Bulk Restorative achieved a VH_bottom/top ratio of approximately 80% when either LCU was used.

CONCLUSIONS:

The total amount of light transmitted through the five bulk-fill RBCs was similar at the different thicknesses using either LCU. The polywave LCU used in this study did not enhance the polymerization of the tested bulk-fill RBCs when compared with the single-peak LCU.

Shrinkage vectors of a flowable composite in artificial cavity models with different boundary conditions: Ceramic and Teflon

Polymerization shrinkage of dental resin composites leads to stress build-up at the tooth-restoration interface that predisposes the restoration to debonding. In contrast to the heterogeneity of enamel and dentin, this study investigated the effect of boundary conditions in artificial cavity models such as ceramic and Teflon. Ceramic serves as a homogenous substrate that provides optimal bonding conditions, which we presented in the form of etched and silanized ceramic in addition to an etched, silanized and bonded ceramic cavity. In contrast, the Teflon cavity presented a non-adhesive boundary condition that provided an exaggerated condition of poor bonding as in the case of contamination during the application procedure or a poor bonding substrate such as sclerotic or deep dentin. The greatest 3D shrinkage vectors and movement in the axial direction were observed in the ceramic cavity with the bonding agent followed by the silanized ceramic cavity, and smallest shrinkage vectors and axial movements were observed in the Teflon cavity. The shrinkage vectors in the ceramic cavities exhibited downward movement toward the cavity bottom with great downward shrinkage of the free surface. The shrinkage vectors in the Teflon cavity pointed towards the center of the restoration with lateral movement greater at one side denoting the site of first detachment from the cavity walls. These results proved that the boundary conditions, in terms of bonding substrates, significantly influenced the shrinkage direction.