Does the Translucency of Novel Monolithic CAD/CAM Materials Affect Resin Cement Polymerization with Different Curing Modes?

Light transmittance through resin-matrix composite onlays adhered to resin-matrix cements or flowable composites

OBJECTIVE

The aim of this study was to evaluate the influence of the thickness of resin-matrix composite blocks manufactured by CAD-CAM on the light transmittance towards different resin-matrix cements or flowable composites.

METHODS

Sixty specimens of resin-matrix composite CAD-CAM blocks reinforced with 89 wt% inorganic fillers were cross-sectioned with 2 or 3 mm thicknesses. The specimens were conditioned with adhesive system and divided in groups according to the luting material, namely: two dual-cured resin-matrix cements, two traditional flowable resin-matrix composites, and one thermal-induced flowable resin-matrix composite. Specimens were light-cured at 900 mW/cm2 for 40s. Light transmittance assays were preformed using a spectrophotometer with an integrated monochromator before and after light-curing. Microstructural analysis was performed by optical and scanning electron microscopy (SEM). Nanoindentation tests were performed to evaluate mechanical properties for indirect evaluation of degree of monomers conversion.

RESULTS

Optical and SEM images revealed low thickness values for the cementation interfaces for the traditional flowable resin-matrix composite. The cement thickness increased with the size and content of inorganic fillers. The highest light transmittance was recorded for the onlay blocks cemented with the traditional flowable resin-matrix composites while a group cemented with the dual-cured resin-matrix cement revealed the lowest light transmittance. The elastic modulus and hardness increased for specimens with high content of inorganic fillers as well as it increased in function of the light transmittance.

CONCLUSIONS

The light transmittance of flowable resin-matrix composites was higher than that for resin-matrix cement after cementation to resin-matrix composites blocks. The type, size, and content of inorganic fillers of the luting material affected the thickness of the cement layer and light transmittance through the materials.

CLINICAL RELEVANCE

On chair-side light curing, the transmission of visible light can be interfered by the chemical composition and viscosity of the luting materials. The increase in size and content of inorganic fillers of resin-matrix composites and luting materials can decrease the light transmittance leading to inefficient polymerization.

Comparative evaluation of the shear bond strength of lithium disilicate veneers using one light-cure and two dual-cure resin cement: An in vitro study

Aim

The present in vitro study aimed to comparatively evaluate the shear bond strength (SBS) of one light-cure and two dual-cure resin cement to bond lithium disilicate veneers.

Materials and Methods

Thirty maxillary central incisors (n = 30) were procured and randomly divided into three groups of adhesive/resin cement systems, into groups of 10 each (n = 10); Group A: Adper Single Bond 2/RelyX Veneer Cement, Group B: Prime and Bond NT/Calibra, and Group C: Excite DSC/Variolink II. All the tooth samples were etched and respective bonding agent was applied. Similarly, all the laminate veneer specimens were etched, silanated, and treated with respective bonding agents before cementation with the respective resin cement. The SBS was measured in a universal testing machine with a cross-head speed of 1 mm/min.

Statistical Analysis

Data obtained were analyzed using the one-way analysis of variance and post hoc Tukey's test at a 5% significance level.

Results

The highest SBS was demonstrated by Group C (18.8 ± 0.92 Mpa), followed by Group B (18.4 ± 0.74) Mpa, and the least by Group A (17.4 ± 0.75 MPa). Significant differences were found between Group A, Group B, and Group C, respectively. However, Group B and Group C did not differ significantly from each other.

Conclusions

Dual-cure resin cement have higher SBS than the light-cure variants.

Ultra-Translucent Zirconia Laminate Veneers: The Influence of Restoration Thickness and Stump Tooth-Shade

This in vitro study aimed to evaluate the final shade of translucent zirconia laminate veneers with varying thicknesses over teeth with different shades. Seventy-five chairside computer-aided design/computer-aided manufacturing (CAD/CAM) shade A1 third-generation zirconia dental veneers, with thicknesses of 0.50 mm, 0.75 mm, and 1.00 mm, were placed on resin composite teeth with shades ranging from A1 to A4. The laminate veneers were divided into groups based on thickness and background shade. All restorations were evaluated with a color imaging spectrophotometer, to map the veneer surface from A1 to D4. Regardless of the thickness or background shade, all dental veneers showed color alteration from the original shade. Veneers with 0.5 mm thickness tended to display the B1 shade, while veneers with 0.75 mm and 1.0 mm thickness primarily exhibited the B2 shade. The thickness of the laminate veneer and background shade significantly modified the original shade of the zirconia veneer. One-way analysis of variance was performed and a Kruskal-Wallis test was used to determine the significance between the three veneer thicknesses groups. The results indicated that the thinner restorations showed higher values with the color imaging spectrophotometer, suggesting that thinner veneers may result in more consistent color matching. This study underscores the importance of carefully considering thickness and background shade when selecting zirconia laminate veneers, to ensure optimal color matching and overall aesthetic outcomes.

Comparison of the Color Change of Porcelain Laminate Veneers Produced by Different Materials After Luting with Three Resin Cements

OBJECTIVES

This study aimed to compare color change of porcelain laminate veneers fabricated with two lithium disilicate ceramics and a zirconia-reinforced lithium silicate ceramic material after luting with conventional dual-cure, amine-free dual-cure, or light-cure resin cements using artificial accelerated aging (AAA).

METHODS AND MATERIALS

Ninety noncarious human maxillary central incisors were embedded in autopolymerizing acrylic resin blocks to prepare conventional laminate veneers with incisal coverage. Prepared teeth were randomly divided into 3 groups (n=30) to fabricate laminate veneer restorations using: (1) lithium disilicate ceramic, (2) lithium disilicate ceramic with high-density micronization, and (3) zirconia-reinforced lithium silicate ceramic. Impressions of the preparations were taken with a laboratory scanner. Acquired 3D images were processed into a surface tessellation language file. Data were exported for 3D printing on a printer, and laminate veneers were printed in castable wax resin. Ceramic veneers were heat-pressed after investment according to the manufacturer's recommendations and further divided into three groups (n=10) according to luting cement type: (1) light-cure resin cement, (2) amine-free dual-cure resin cement, and (3) conventional dual-cure resin cement. Color measurements were performed from the middle of each specimen with a spectrophotometer before and after AAA for 300 hours with a total energy of 150 kJ/m2. Color changes were calculated with the CIEDE 2000 (ΔE00) formula. Color differences were assessed using two-way analysis of variance (ANOVA) (α=0.05), and the paired t-test was used to compare the L*, a*, and b* parameters in each group before and after aging.

RESULTS

There were no significant differences between the ceramic groups (p>0.05). The color changes of the materials ranged from 2.26 to 3.13. All materials were above the clinically acceptable limit (ΔE00>1.8). The conventional dual-cure resin cement group showed more color change (p>0.05).

CONCLUSIONS

Color changes were observed in all porcelain laminate veneers after artificial accelerated aging. The zirconia-reinforced lithium silicate veneers showed similar color changes as lithium disilicate veneers; the amine-free dual-cure resin cement showed a similar color change as light-cure resin cement after aging.

A Scoping Review on the Polymerization of Resin-Matrix Cements Used in Restorative Dentistry

In dentistry, clinicians mainly use dual-cured or light-cured resin-matrix cements to achieve a proper polymerization of the organic matrix leading to enhanced physical properties of the cement. However, several parameters can affect the polymerization of resin-matrix cements. The main aim of the present study was to perform a scoping review on the degree of conversion (DC) of the organic matrix, the polymerization, and the light transmittance of different resin-matrix cements used in dentistry. A search was performed on PubMed using a combination of the following key terms: degree of conversion, resin cements, light transmittance, polymerization, light curing, and thickness. Articles in the English language published up to November 2022 were selected. The selected studies' results demonstrated that restorative structures with a thickness higher than 1.5 mm decrease the light irradiance towards the resin-matrix cement. A decrease in light transmission provides a low energy absorption through the resin cement leading to a low DC percentage. On the other hand, the highest DC percentages, ranging between 55 and 75%, have been reported for dual-cured resin-matrix cements, although the polymerization mode and exposure time also influence the DC of monomers. Thus, the polymerization of resin-matrix cements can be optimized taking into account different parameters of light-curing, such as adequate light distance, irradiance, exposure time, equipment, and wavelength. Then, optimum physical properties are achieved that provide a long-term clinical performance of the cemented restorative materials.

Effect of the thickness of CAD-CAM materials on the shear bond strength of light-polymerized resin cement

This study aimed to investigate the effect of the thickness of computer aided design-computer aided manufacturing (CAD-CAM) restorative materials on the bond strength of light-polymerized resin cement. Ninety specimens were prepared from three different CAD-CAM materials (a leucite-based glass-ceramic [Empress CAD], a polymer-infiltrated ceramic network [Vita Enamic], and a zirconia-reinforced lithium silicate glass-ceramic [Vita Suprinity]) in different thicknesses (1, 2, and 3 mm). One surface of each specimen was treated by using a single-step self-etching ceramic primer (Monobond Etch & Prime). Light-polymerized resin cement was bonded to treated surfaces by exposure to a light-emitting diode curing unit from the untreated side of the samples. Shear bond strength (SBS) between the ceramic and the resin cement was measured by using a universal testing machine. The leucite-based glass-ceramic had higher SBS values than the other materials at each thickness. For each material, 1 mm-thick specimens had the highest SBS values. The difference between the SBS values of 2 mm- and 3 mm-thick polymer-infiltrated ceramic network was nonsignificant, whereas the SBS values of 2 mm-thick leucite-based glass-ceramic and the zirconia-reinforced lithium silicate glass-ceramic were significantly higher than those of the corresponding 3 mm-thick specimens. The choice of the material and its thickness may be highly important for clinical success when light-polymerized resin cements are used for cementation.

The influence of inorganic fillers on the light transmission through resin-matrix composites during the light-curing procedure: an integrative review

PURPOSE

The objective of this study was to perform an integrative review on the effect the inorganic fillers on the light transmission through the resin-matrix composites during the light-curing procedure.

METHOD

A bibliographic review was performed on PubMed using the following search terms: "fillers" OR "particle" AND "light curing" OR "polymerization" AND "light transmission" OR "light absorption" OR "light irradiance" OR "light attenuation" OR "light diffusion" AND "resin composite." The search involved articles published in English language in the last 10 years.

RESULTS

Selected studies reported a decrease in biaxial strength and hardness in traditional resin-matrix composites in function of the depth of polymerization. However, there were no significant differences in biaxial strength and hardness recorded along the polymerization depth of Bulk-Fill™ composites. Strength and hardness were enhanced by increasing the size and content of inorganic fillers although some studies revealed a progressive decrease in the degree of conversion on increasing silica particle size. The translucency of glass-ceramic spherical fillers promoted light diffusion mainly in critical situations such as in the case of deep proximal regions of resin-matrix composites.

CONCLUSIONS

The amount of light transmitted through the resin-matrix composites is influenced by the size, content, microstructure, and shape of the inorganic filler particles. The decrease of the degree of conversion affects negatively the physical and mechanical properties of the resin-matrix composites.

CLINICAL RELEVANCE

The type and content of inorganic fillers in the chemical composition of resin-matrix composites do affect their polymerization. As a consequence, the clinical performance of resin-matrix composites can be compromised leading to variable physical properties and degradation. The polymerization mode of resin-matrix composites can be improved according to the type of inorganic fillers in their chemical composition.

Load-to-Failure Resistance and Optical Characteristics of Nano-Lithium Disilicate Ceramic after Different Aging Processes

The aim of this study was to compare the load-to-failure resistance and optical properties of nano-lithium disilicate (NLD) with lithium disilicate (LDS) and zirconia-reinforced lithium silicate (ZLS) in different aging processes. Thirty crowns were milled from NLD, LDS, and ZLS (n = 10). All crowns were subjected to thermomechanical aging and loaded until catastrophic failure. Ten specimens from each material were prepared in two different thicknesses (0.7 mm and 1.5 mm, n = 5), and color coordinates were measured before and after coffee thermocycling. Color differences (ΔE00) and relative translucency parameter (RTP) were calculated. Data were analyzed by using ANOVA and Bonferroni-corrected t-tests (α = 0.05). ZLS had the highest load-to-failure resistance (p ≤ 0.002), while the difference between LDS and NLD was nonsignificant (p = 0.776). The interaction between material type and thickness affected ΔE00 (p < 0.001). Among the 0.7 mm thick specimens, ZLS had the lowest ΔE00 (p < 0.001). Furthermore, 1.5 mm thick ZLS had lower ΔE00 than that of 1.5 mm thick LDS (p = 0.036). Other than ZLS (p = 0.078), 0.7 mm thick specimens had higher ΔE00 (p < 0.001). The interaction between material type, thickness, and thermocycling affected RTP (p < 0.001). Thinner specimens presented higher RTP (p < 0.001). NLD and LDS had higher RTP than ZLS (p ≤ 0.036). However, 0.7 mm thick specimens had similar RTP after coffee thermocycling (p ≥ 0.265). Coffee thermocycling reduced the RTP values of 0.7 mm thick NLD (p = 0.032) and LDS (p = 0.008). NLD may endure the occlusal forces present in the posterior region. However, long-term coffee consumption may impair the esthetics of restorations particularly when thin NLD is used.

Influence of inhomogeneity of the polymerization light beam on the microhardness of resin cement under a CAD-CAM block

STATEMENT OF PROBLEM

The beam profile of a light polymerization unit shows an inhomogeneous distribution. Therefore, the light passing through indirect restorations may be inhomogeneous and affect the polymerization of the resin cement.

PURPOSE

The purpose of this in vitro study was to investigate the effects of the inhomogeneous distribution of irradiance passing through a computer-aided design and computer-aided manufacturing (CAD-CAM) block on the microhardness of resin cement.

MATERIAL AND METHODS

IPS e.max CAD (A3 LT, A3 HT), Celtra Duo (A3 LT), LAVA Ultimate (A3 LT), and Vita Enamic (A3 T) blocks were tested and cut into 1.0-, 1.5-, 2.0-, and 4.0-mm thicknesses (N=100). The resin cements were Variolink N base (light-polymerized) and RelyX U200 (dual-polymerized). The light transmission, irradiance, and beam profile of each block were measured. For microhardness measurement, 5 points (-4 mm, -2 mm, 0 mm, +2 mm, and +4 mm) that coincided with the distance from the center to the periphery of the tip were marked on the specimen's surface. At each point, microhardness was measured 24 hours after polymerization. Repeated measured 1-way ANOVA with the LSD test was performed to analyze the effect of measuring points on the microhardness (α=.05).

RESULTS

The microhardness of the resin cements decreased with an increase of the CAD-CAM block thickness (P<.05). Resin cements under a 1-mm CAD-CAM block showed relatively uniform microhardness, whereas those under 2-mm and 4-mm blocks showed inhomogeneous microhardness (P<.05).

CONCLUSIONS

Inhomogeneous light transmission from a light polymerization unit through CAD-CAM blocks resulted in the nonuniform microhardness of resin cement.

Optical behaviors, surface treatment, adhesion, and clinical indications of zirconia-reinforced lithium silicate (ZLS): A narrative review

OBJECTIVES

The present narrative review was focused on the optical properties, surface treatment, adhesion, and clinical indications of zirconia-reinforced lithium silicate ceramics (ZLS) for Computer-aided design / Computer-aided manufacturing (CAD/CAM) technologies.

DATA/SOURCES

A literature search was performed by 3 calibrated independent researchers on PubMed, Scopus, Embase, Google Scholar, Dynamed, and Open Grey. The criteria for inclusion were: 1) papers addressing at least one of the following variables about ZLS: optical properties, surface treatment, adhesion, and clinical indications; 2) in vitro, in silico, or in vivo studies; 3) case reports; 4) systematic reviews. The exclusion criteria were: 1) animal studies; 2) non-dental studies; 3) studies only focusing on ZLS used in the heat-pressed process.

STUDY SELECTION

98 records among in vitro studies and case reports were included.

CONCLUSIONS

Despite the promising microstructure characteristics of ZLS, increased translucency compared to lithium disilicate ceramics (LS₂) was not proven, but acceptable color changes and stability were reported. Mechanical polishing was the most effective method to reduce surface roughness. Moreover, machinability and handling of ZLS resulted harder than LS₂. Conventional acid etching procedures seemed effective in conditioning ZLS surface, but no protocol has been established yet. Besides, silane-coupling and dual-curing resin cements were recommended.

CLINICAL SIGNIFICANCE

ZLSs can be used for anterior and posterior fixed single-unit CAD/CAM restorations onto both natural teeth and implants, but do not seem to represent a viable treatment option for endocrowns onto posterior teeth or fixed dental prostheses.

The influence of zirconia veneer thickness on the degree of conversion of resin-matrix cements: an integrative review

OBJECTIVE

The main aim of this study was to conduct an integrative review on the influence of the zirconia veneer thickness on the degree of conversion of resin-matrix cements.

MATERIALS AND METHOD

An electronic search was performed on PubMed using a combination of the following search items: zirconia, thickness, veneer, degree of conversion, resin cement, light curing, and polymerization. Articles published in the English language, up to July 2020, were included regarding the influence of ceramic veneer thickness on the degree of conversion of resin-matrix cements. Randomized controlled trials and prospective cohort studies were also evaluated.

RESULTS

Of the 21 selected studies, 9 investigated the light-curing effect, while five other articles evaluated the ceramic translucency. Three studies evaluated the degree of conversion of the resin-matrix cement while four articles assessed the veneer thickness. Results revealed a significant decrease of light transmission through the zirconia with a thickness ranging from 0.1 up to 1.5 mm. However, the ultra-thin thickness around 0.1 and 0.3 mm allowed a full polymerization of the dual-curing resin-matrix cement resulting in the integrity of the interface properties. The light-curing process of resin-matrix cements is also affected by the shade, chemical composition, and microstructure of zirconia and resin cement. Optimal conditions of light-curing are required to reach the threshold intensity of light and energy for polymerization of resin-matrix cements.

CONCLUSIONS

The increase in zirconia veneer thickness negatively affects the degree of conversion of resin-matrix cements. Also, shade and microstructure are key factor to improve the light curing of resin cements.

CLINICAL RELEVANCE

Clinicians should consider the zirconia thickness on resin-based cementation since a higher veneer thickness can negatively affect the light irradiation intensity towards the dual-curing resin-matrix cement. Thus, the degree of conversion of the resin-matrix cement can decrease leading to a low chemical stability (e.g., color instability) and poor mechanical properties.

Influence of feldspar ceramic thickness on the properties of resin cements and restorative set

This in vitro study evaluated the influence of feldspathic ceramic thickness on the properties of light- and dual-cured resin cements. For each cement (RelyX Veneer, Allcem Veneer, RelyX Ultimate, and Allcem Dual), three ceramic specimens were prepared, with seven thicknesses for each (0.5, 1.0, 1.5, 2.0, 2.5, 3.0, and 3.5 mm). The degree of conversion, Vickers microhardness, irradiance power, color variation (ΔE₀₀ ), and translucency parameters were assessed. Microhardness and irradiance power were analyzed using analysis of variance (ANOVA) with post hoc Tukey's test, while ΔE₀₀ , translucency parameters, and degree of conversion were analyzed using ANOVA of ranks with post hoc Duncan's Multiple Range Test. The relationship between each of the dependent variables (degree of conversion, ΔE₀₀ , and translucency parameter tests) and the specimen thickness was described using linear regression for each of the four resin cements. The significance level for all analyses was set at 5%. RelyX Ultimate yielded the lowest degree of conversion values among all resin cements. Allcem Veneer produced the lowest microhardness values, without statistical differences between thicknesses, of up to 1 mm. Allcem Dual produced the highest ΔE₀₀ and translucency parameter values. Feldspathic ceramic thickness influenced the mechanical properties of resin cements and optical aspects of the restorative set.

The effect of thickness and translucency of polymer-infiltrated ceramic-network material on degree of conversion of resin cements

PURPOSE

The aim of the present study was to determine the degree of conversion of light- and dual-cured resin cements used in the cementation of all-ceramic restorations under different thicknesses of translucent (T) and high-translucent (HT) polymer-infiltrated ceramic-network (PICN) material.

MATERIALS AND METHODS

T and HT PICN blocks were prepared at 0.5, 1.0, 1.5, and 2.0 mm thicknesses (n=80). Resin cement samples were prepared with a diameter of 6 mm and a thickness of 100 µm. Light-cured resin cement was polymerized for 30 seconds, and dual-cure resin cement was polymerized for 20 seconds (n=180). Fourier transform infrared spectroscopy (FTIR) was used for degree of conversion measurements. The obtained data were analyzed with ANOVA and Tukey HSD, and independent t-test.

RESULTS

As a result of FTIR analysis, the degree of conversion of the light-cured resin cement prepared under 1.5- and 2.0-mm-thick T and HT ceramics was found to be lower than that of the control group. Regarding the degree of conversion of the dual-cured resin cement group, there was no significant difference from the control group.

CONCLUSION

Within the limitation of present study, it can be concluded that using of dual cure resin cement can be suggested for cementation of PICN material, especially for thicknesses of 1.5 mm and above.

A Pilot Study About the Effect of Laser-Induced Fluorescence on Color and Translucency of Human Enamel During Tooth Bleaching

Objective: To probe into the effect of laser-induced fluorescence (LIF) on color and translucency of human enamel during tooth bleaching. Materials and methods: Twenty enamel slabs were randomly assigned to be whitened by acidic 30% hydrogen peroxide (HP), neutral 30% HP, alkaline 30% HP, and distilled water, respectively, monitored by a colorimeter and Raman spectrometer simultaneously. Afterward, the parameter differences of color, translucency, Raman relative intensity, and LIF intensity between baseline and post-treatment of each bleaching cycle were calculated. Results: The results demonstrated that the three bleaching groups resulted in increasingly prominent whitening outcome over time compared with control group, and no statistical difference was detected between them. Accordingly, the bleaching groups also engendered a same decrease tendency in fluorescence intensity (FI). However, less demineralization effect occurred on the enamel surface in neutral HP group. The correlation analysis further excluded the effect of demineralization on all the optical parameters (p > 0.05). Besides, various degrees of dependency were detected between FI and translucency parameter (TP), masking effect (ME), C*ab, W*, b*. In addition, ΔFI was associated with parameters of ΔC*ab, ΔW*, Δb*, ΔE, Δa*, and ΔME. ΔFI% was correlated with ΔC*ab, Δb*, ΔW*, and ΔE values. Conclusions: Thirty percent HP with different pH values could result in same variation tendency of enamel color, translucency, and FI. Plus, FI showed a strong association with enamel color and translucency alteration, which is promising for future application as a nondestructive testing method to evaluate bleaching effect and might be a novel way to investigate tooth bleaching mechanism.