Effect of temperature on the degree of conversion and working time of dual-cured resin cements exposed to different curing conditions

Degree of conversion and interfacial adaptation of touch-cure resin cement polymerized by self-curing or dual-curing with reduced light

OBJECTIVES

The first aim of this study was to determine whether there is a difference in degree of conversion (DC) of touch-cure cements polymerized by self-curing with adhesive or dual-curing under reduced light. The second aim was to compare interfacial adaptation of zirconia restoration cemented using touch-cure cements self-cured or dual-cured by reduced light.

METHODS

The DC of touch-cure resin cements with adhesive was measured continuously using Fourier transform infrared spectrometry. Experimental groups differed depending on touch-cure cement. Each group had three subgroups of polymerization method. For subgroup 1, the DC was measured by self-curing. For subgroups 2 and 3, the DCs were measured by dual-curing with reduced light penetrating 3 mm and 1 mm zirconia blocks, respectively. For interfacial adaptation evaluation, Class I cavity was prepared on an extracted third molar, and zirconia restoration was fabricated. The restoration was cemented using the same cement. Groups and subgroups for interfacial adaptation were the same as those of the DC measurement. After thermo-cycling, interfacial adaptation at the tooth-restoration interface was evaluated using swept-source optical coherence tomography imaging.

RESULTS

The DC of touch-cure cement differed depending on the measurement time, resin cement, and polymerization method (p < 0.05). Interfacial adaptation was different depending on the resin cement and polymerization method (p < 0.05).

CONCLUSION

For touch-cure cement, light-curing with higher irradiance presented a higher DC and superior interfacial adaptation than light-curing with lower irradiance or self-curing.

CLINICAL RELEVANCE

Although some adhesives accelerate the self-curing of touch-cure cement, light-curing for touch-cure cement is necessary for zirconia cementation.

The Effect of the Initiator/Activator/Accelerator Ratio on the Degree of Conversion, Film Thickness, Flow, and Cytotoxicity of Dual-Cured Self-Adhesive Resin Cements

Although self-adhesive resin cements are convenient and less technique-sensitive materials for dental clinicians, they exhibit a lower degree of conversion due to acidic components in their composition. Supplementation of the initiator, accelerator, and activator in self-adhesive resin cements has been suggested to compensate for the lower degree of conversion. This study aimed to evaluate the effects of different combinations of self-curing initiators, self-curing activators, and accelerators on the degree of conversion (DC) of self-adhesive resin cements. A dual-cured self-adhesive resin was prepared using six combinations of initiators, activators, and accelerators. The change in the DC over time was evaluated with and without light curing. The film thickness, flow properties, and cytotoxicity of each formulation were assessed. The results showed that all supplemental components had an effect on increasing the DC, but a greater increase in the DC was observed in the following order: activator, accelerator, and initiator. The cytotoxicity of the resin cements was related to the DC values, as resin cements with lower DC values exhibited higher cytotoxicity. The film thickness met the ISO standards for all groups. The results suggest that utilizing an activator is the most effective approach to enhance the DC in self-adhesive resin cement and that cytotoxicity tended to increase with lower DC values, whereas film thickness and flow properties demonstrated no correlation with DC values.

Effects of ceramic thickness, ceramic translucency, and light transmission on light-cured bulk-fill resin composites as luting cement of lithium disilicate based-ceramics

PURPOSE

To assess the effects of ceramic thickness, ceramic translucency, and light transmission on restorative composites used as luting cement for lithium disilicate-based ceramics.

METHODS

Four luting types of cement were tested (n=8); a dual-cured resin cement (Multilink N), a light-cured conventional flowable composite (Tetric N-Flow), and two light-cured bulk-fill flowable composites (Tetric N-Flow Bulk Fill and X-tra base). The 20 s- or 40 s-light (1000 mW/cm2) was transmitted through 1- or 2-mm-thick high- or low-translucency (HT- or LT-) ceramic discs (IPS e.Max press) to reach the 1-mm-thick luting cement. Light transmitted to cement without ceramic served as a control. Vickers hardness number (VHN), flexural strength (FS), fractography, and degree of conversion (DC) were evaluated. One-way and multi-way analysis of variance was conducted to determine the effects of factors on VHN and FS.

RESULTS

Ceramic thickness, light transmission time, and cement type significantly affected the VHN of the luting cement (P < .000). Only Multilink N (LT- and HT-1mm) and Tetric N-Flow (HT-1mm) reached 90% VHN of corresponding control by 20 s-light transmissions, but Tetric N-Flow exhibited lowest VHN and approximately 1/3-1/2 VHN of Multilink N (P < 0.05). X-tra base expressed superior physicochemical properties to Tetric N-Flow Bulk Fill (P < 0.05) and reached >90% VHN of control in all conditions with 40 s-light transmissions except for LT-2 mm. DC, FS, and fractography supported these findings.

CONCLUSIONS

The light-cured bulk-fill composite served as a luting cement for lithium-disilicate-based ceramics in a product-dependent manner. Light transmission time is crucial to ensure sufficient luting cement polymerization.

Assessment of Degree of Conversion and Volumetric Shrinkage of Novel Self-Adhesive Cement

The degree of monomer conversion and polymerization shrinkage are two of the main reasons for potential adhesion failure between the tooth structure and the restoration substrate. To evaluate the degree of conversion and polymerization shrinkage of a newly developed self-adhesive resin cement, the degree of conversion (DC) was measured using FTIR under different activation modes, temperatures, and times. Volumetric shrinkage was tested using the AcuVol video imaging method. The experimental cement showed a higher DC than other cements under self-curing. The DC of the experimental cement was higher than that of other cements, except SpeedCem Plus under light curing. The experimental cement had a higher DC than other cements, except SpeedCem Plus in some conditions under dual curing. All self-adhesive cements had a higher DC at 37 °C than at 23 °C under self-curing, and there was no statistical difference between 23 °C and 37 °C under light curing. All self-adhesive cements showed a significantly higher DC at 10 min than at 5 min under self-curing. There was no statistical difference between 5 min and 10 min for most cements under dual curing. All self-adhesive cements statistically had the same volumetric shrinkage under light curing and self-curing. The newly developed self-adhesive resin cement exhibited a higher degree of conversion and similar volumetric shrinkage compared to these commercial self-adhesive resin cements.

Effect of degree of conversion on the surface properties of polymerized resin cements used for luting glass fiber posts

STATEMENT OF PROBLEM

Lack of conversion of resin cements for luting glass fiber posts in deeper levels of the root canal may compromise clinical performance.

PURPOSE

The purpose of this in vitro study was to evaluate the effect of the degree of conversion on the surface properties of dual-polymerized resin cements used for cemented glass fiber posts according to the root level.

MATERIAL AND METHODS

Fifty maxillary central incisor teeth were endodontically treated, and glass fiber posts were cemented using the following systems: self-adhesive dual-polymerized resin luting cement (RelyX U200) and dual-polymerizing flowable core and a post luting system (Rebilda DC) with a self-etching adhesive (Futurabond DC). The degree of conversion was determined via attenuated total reflectance and Fourier transform infrared spectroscopy, and surface microhardness and cement film thickness were measured. The intraradicular fit was evaluated with microscopy. All tests were performed in the coronal, middle, and apical third of the root canal. Statistical analysis was done with ANOVA and the multiple comparison Tukey test (α=.05).

RESULTS

The degree of conversion was higher in the coronal third using Rebilda DC (65.3 ±4.8%) than RelyX U200 (38.7 ±5.3%) (P<.05); on both resin cements, these values decreased from the coronal to the apical third (30.9 ±3.7%, 61.9 ±8.7%, respectively). The cement film thickness was higher for RelyX U200 in the 3 thirds than for Rebilda DC; significant differences (P<.05) were recorded in both cementing systems in the coronal and apical thirds. In the middle third, no significant differences were observed (P>.05). The mean surface microhardness values increased in the coronal thirds and decreased with increasing root canal depth. The marginal seal in the coronal thirds and the intraradicular fit in the middle thirds showed closer adaptation; however, some tags were observed in the interface resin cement and radicular dentin. Gap and tag formations were observed in the apical third.

CONCLUSIONS

Depending on the root canal third, the surface properties of dual-polymerized resin cements are influenced by the degree of conversion.

Operator versus material influence on film thickness using adhesive resin cement or pre‐heated resin composite

OBJECTIVE

There is a growing interest in using pre-heated composites instead of dual-cured cements when luting indirect restorations. This study evaluated the film thickness obtained from two pre-heated composites and two resin cements, by two different operators. The influence of the materials and the level of expertise of the operator were analyzed.

MATERIALS AND METHODS

Forty specimens of human dentin and composite discs were prepared and divided into four groups depending on the luting process. Each group was randomly equally divided to be handled by two operators with different levels of experience. Two of the initial four groups were luted using dual-cured cements and the two remaining groups using light-cured pre-heated composites. Specimen discs were cut after luting, and film thickness was measured using a Digital microscope. Data were analyzed using a 2-way ANOVA with the Holm-Sidak pairwise multiple comparison procedure (p < 0.05).

RESULTS

Mean film thickness ranged from 156.16 ± 4.7 to 33.82 ± 0.7 μm. Significant differences (p < 0.001) were noticed between expert and novice results with pre-heated composites.

CONCLUSION

Within the limits of this study, using pre-heated composites as a luting cement requires a better level of expertise to achieve a clinically acceptable film thickness.

CLINICAL SIGNIFICANCE

Using pre-heated composites as luting agent for indirect restorations requires an experimented skill level to achieve a clinically recommended film thickness.

Post-Cure Development of the Degree of Conversion and Mechanical Properties of Dual-Curing Resin Cements

This study investigated the effect of different curing conditions on the degree of conversion and mechanical properties of contemporary dual-curing resin cements. The material specimens were either light-cured directly, light-cured through a 1-mm lithium disilicate glass-ceramic layer, or self-cured. The degree of conversion was measured in 0.1-mm films using Fourier-transform infrared spectroscopy 1 day, 7 days, and 28 days post-cure. Specimens used to study the flexural strength and modulus were prepared according to the ISO 4049 protocol, stored for 28 days post-cure, and subjected to accelerated aging by absolute ethanol immersion. The degree of conversion values ranged between 44.3–77.8%. Flexural strength varied between 11.4–111.1 MPa, while flexural modulus amounted to 0.7–5.5 GPa. The degree of conversion was significantly affected by material type, curing conditions, and post-cure time; however, variations in curing conditions were the least influential factor. A statistically significant effect of curing conditions on the degree of conversion was identified for only one of the five materials tested, whereas the flexural strength and modulus of all tested materials were significantly reduced in the experimental groups that were light-cured through a ceramic layer or self-cured. The effect size analysis showed that mechanical properties were most affected by the material type, while the differences in curing conditions were less influential. A comparison of the degree of conversion and mechanical properties indicated that different curing conditions may lead to significantly different flexural strength and modulus, which are not necessarily accompanied by identifiable variations in the degree of conversion.

Does Resin Cement Type and Cement Preheating Influence the Marginal and Internal Fit of Lithium Disilicate Single Crowns?

This paper assesses the effect of cement type and cement preheating on the marginal and internal fit of lithium disilicate single crown. Methods: 40 maxillary premolars were selected, restored with lithium disilicate single crowns. Teeth were randomly assigned into four groups (n = 10) based on cement type (Panavia SA or LinkForce) and preheating temperature (25 °C or 54 °C). After fabrication of the restoration, cements were incubated at 25 °C or 54 °C for 24 h, and each crown was cemented to its corresponding tooth. After 24 h, all specimens were thermally aged to (10,000 thermal cycles between 5 °C and 55 °C), then load cycled for 240,000 cycles. Each specimen was then sectioned in bucco-palatal direction and inspected under a stereomicroscope at x45 magnification for marginal and internal fit evaluation. The data were statistically analyzed (significance at p ≤ 0.05 level). Results: At the mid-buccal finish line, mid-buccal wall, palatal cusp, mid-palatal wall, mid-palatal finish line, and palatal margin measuring points, there was a significant difference (p ≤ 0.05) between the lithium disilicate group cemented with Panavia SA at 25 °C and the group cemented with LinkForce at 25 °C, while there was no significant difference (p > 0.05) at the other points. At all measuring points, except at the palatal cusp tip (p = 0.948) and palatal margin (p = 0.103), there was a statistically significant difference (p ≤ 0.05) between the lithium disilicate group cemented with Panavia SA at 54 °C and the group cemented with LinkForce at 54 °C. Regardless of cement preheating, statistically significant differences were found in the buccal cusp tip, central groove, palatal cusp tip, and mid-palatal wall (p ≤ 0.05) in the lithium disilicate group cemented with Panavia SA at 25 °C and 54 °C, as well as the mid-palatal chamfer finish line and palatal margin in the LinkForce group cemented with Panavia SA at 25 °C and 54 °C. At the other measurement points, however, there was no significant difference (p > 0.05). Conclusions: The type of resin cement affects the internal and marginal fit of lithium disilicate crowns. At most measuring points, the cement preheating does not improve the internal and marginal fit of all lithium disilicate crowns.

Improvement on properties of experimental resin cements containing an iodonium salt cured under challenging polymerization conditions

OBJECTIVE

The use of resin cements in clinical practice entails photopolymerization through prosthetic devices, which precludes light penetration. The objective of this study was to modify experimental resin cements (ERCs) with diphenyliodonium hexafluorophosphate (DPI) in an attempt to improve chemical and mechanical properties of materials cured with reduced irradiance and final radiant exposure.

METHODS

A co-monomer base containing a 1:1 mass ratio of 2.2-bis[4-(2-hydroxy-3-methacryloxypropoxy)phenyl]propane (bis-GMA) and triethyleneglycol dimethacrylate (TEGDMA) was prepared, with 1mol% of camphorquinone and 2mol% of ethyl 4-(dimethylamino)benzoate as initiator system. The resin was divided into 4 fractions according to the DPI concentrations (0, 0.5, 1 and 2mol%). The challenging polymerization condition was simulated performing the light activation (12, 23 and 46s) through a ceramic block (3mm thick). The irradiance was assessed with a calibrated spectrometer (1320mW/cm²), resulting in three levels of radiant exposure (0.58, 1.1 and 2.2J/cm²). The polymerization kinetics was evaluated in real-time using a spectrometer (Near-IR). Water sorption and solubility was analyzed and the cohesive strength of resins obtained through the microtensile test. Polymerization stress was assessed by Bioman method.

RESULTS

Resins containing DPI had higher degree of conversion and rate of polymerization than the control (without DPI). The use of DPI reduced water sorption and solubility, and led to higher cohesive strength compared to resins without the iodonium salt. However, the stress of polymerization was higher for experimental resins with DPI.

SIGNIFICANCE

Even under remarkably reduced irradiance, cements containing a ternary initiating system with an iodonium salt can present an optimal degree of conversion and chemical/mechanical properties.

Time-dependent degree of conversion, Martens parameters, and flexural strength of different dual-polymerizing resin composite luting materials

Objective

To investigate the degree of conversion (DC), Martens hardness (HM), elastic indentation modulus (E_IT), and biaxial flexural strength (BFS) of six dual-polymerizing resin composite luting materials initially and after 2 and 7 days of aging.

Materials and methods

Specimens fabricated from Bifix QM (BIF; VOCO), Calibra Ceram (CAL; Dentsply Sirona), DuoCem (DUO; Coltène/Whaledent), G-CEM LinkForce (GCE; GC Europe), PANAVIA V5 (PAN; Kuraray Europe), and Variolink Esthetic DC (VAR; Ivoclar Vivadent) (n = 12 per material) were light-polymerized through 1 mm thick discs (Celtra Duo, Dentsply Sirona). DC, HM, and E_IT were recorded directly after fabrication, and after 2 and 7 days of aging. As a final test, BFS was measured. Univariate ANOVAs, Kruskal–Wallis, Mann–Whitney U, Friedman, and Wilcoxon tests, and Weibull modulus were computed (p < 0.05).

Results

While CAL presented low DC, HM, E_IT, and BFS values, DUO and BIF showed high results. Highest Weibull moduli were observed for VAR and DUO. DC and Martens parameters increased between the initial measurement and 2 days of aging, while aging for 7 days provided no further improvement.

Conclusions

The choice of dual-polymerizing resin composite luting material plays an important role regarding chemical and mechanical properties, especially with patients sensitive to toxicological issues. DUO may be recommended for bonding fixed dental prostheses, as it demonstrated significantly highest and reliable results regarding DC, HM, and BFS. As DC and HM showed an increase in the first 48 h, it may be assumed that the polymerization reaction is not completed directly after initial polymerization, which is of practical importance to dentists and patients.

Clinical relevance

The chemical and mechanical properties of dual-polymerizing resin composite luting materials influence the overall stability and long-term performance of the restoration.

Characterization of Experimental Short-Fiber-Reinforced Dual-Cure Core Build-Up Resin Composites

As a core build-up material, dual-cured (DC) resin-based composites are becoming popular. The aim of this research was to investigate specific physical and handling properties of new experimental short-fiber-reinforced DC resin composites (SFRCs) in comparison to different commercial, conventional DC materials (e.g., Gradia Core, Rebilda DC, LuxaCore Z, and Visalys^® CemCore). Degree of monomer conversion (DC%) was determined by FTIR-spectrometry using either self- or light-curing mode. The flexural strength, modulus, and fracture toughness were calculated through a three-point bending setup. Viscosity was analyzed at room (22 °C) and mouth (35 °C) temperatures with a rotating disk rheometer. The surface microstructure of each resin composite was examined with scanning electron microscopy (SEM). Data were statistically analyzed with analysis of variance ANOVA (p = 0.05). The curing mode showed significant (p < 0.05) effect on the DC% and flexural properties of tested DC resin composites and differences were material dependent. SFRC exhibited the highest fracture toughness (2.3 MPa m^1/2) values and LuxaCore showed the lowest values (1 MPa m^1/2) among the tested materials (p < 0.05). After light curing, Gradia Core and SFRCs showed the highest flexural properties (p < 0.05), while the other resin composites had comparable values. The novel DC short-fiber-reinforced core build-up resin composite demonstrated super fracture toughness compared to the tested DC conventional resin composites.

Development of dual-cured resin cements with long working time, high conversion in absence of light and reduced polymerization stress

OBJECTIVE

This study evaluated the properties of experimental dual-cured cements containing thiourethane (TU) and low concentrations of p-Tolyldiethanolamnie (DHEPT) and benzoyl peroxide (BPO) as chemical initiators.

METHODS

BisGMA/TEGDMA-based dual-cured cement was formulated with 1.0 wt% DHEPT and 0.75 wt% BPO as initiators and used as control. The concentration of BPO was adjusted to 0.1 wt% in catalyst paste of experimental cements, and two base pastes containing TU and 0.5 wt% or 0.25 wt% of DHEPT were formulated. The rheological behavior and kinetics of polymerization of cements were assessed in the absence of light activation. The kinetics of polymerization was also evaluated for cements light-activated immediately or 5 min after the start of mixing. Polymerization stress, flexural strength and elastic modulus (n = 5) were also evaluated under these conditions.

RESULTS

Cements with TU presented lower viscosity than the control, improved working time (0.25% DHEPT > 0.5% DHEPT) and higher conversion in the absence of light-activation. Delaying the light-activation reduced the maximum rate of polymerization (Rpmax) but did not affect the conversion or stress. The addition of TU increased the Rpmax and conversion, and reduced the stress when compared to the control, without affecting the flexural strength. Except for the control with delayed light-activation (highest values), the other experimental conditions yielded similar modulus.

SIGNIFICANCE

Adding TU and using a low concentration of DHEPT/BPO resulted in dual-cured cements with longer working time, reduced polymerization stress and increased conversion even in the absence of light, with no significant effect on the mechanical properties.

Initial curing characteristics of composite cements under ceramic restorations

PURPOSE

To assess the degree of conversion (DC) of dual-curing composite cements when cured through ceramic-veneered zirconia disks.

METHODS

Portions of mixed cement, either G-CEM LinkForce (GC), Panavia V5 (Kuraray Noritake) or ResiCEM (Shofu), were placed on the ATR crystal of a Fourier Transform Infrared Spectroscope (FTIR; iS50, Thermo Scientific) and squeezed to a 100-µm film thickness using a microscopy cover glass. DC (%) of the composite cements applied in self-curing mode was measured in the dark at 37°C. Following the dual-curing mode, the cements were light-cured directly (positive control) or through a ceramic-veneered zirconia disk (0.5-mm thick zirconia with a 1.0-mm thick veneering ceramic) for 40 sec using two light-curing units (G-Light Prima 2, GC; PenCure, Morita). Per experimental group, 5 tests were conducted to measure DC in self-cure and dual-cure mode (n=5). FTIR spectra of the composite cement films were acquired to determine DC every min up to 30 min. DC of the composite cements was statistically compared using two-way repeated-measures ANOVA (α=0.05).

RESULTS

For all cements investigated, the self-curing mode resulted in significantly lower DC at 10, 20 and 30 min than the light-curing mode. When the composite cements were light-cured through the zirconia disk, DC at 30 min dropped significantly for ResiCem (Shofu), while not for Panavia V5 (Kuraray Noritake) and G-CEM LinkForce (GC).

CONCLUSIONS

Self-curing slows down polymerization but does not reach for all composite cements the highest (light-cured) DC. Ceramic-veneered zirconia-based restorations may affect DC of some composite cements.

A Review of Dental Cements

This review provides an in-depth comparison of advantages and disadvantages of different types of dental cements as they are used for cementing base metal alloy crowns in dogs.

Influence of the composition and shades of ceramics on light transmission and degree of conversion of dual-cured resin cements

Objective

Since the transmittance of ceramics can influence the degree of conversion (DC) of resin cements, ceramics composition and shade should be considered in the selection of resin cement. This in vitro study aimed to evaluate the effect of the transmittance of different composition, opacities and shades of ceramics on the degree of conversion of two dual-cured resin cements.

Methodology

Sixty discs were prepared from low translucency (LT) and medium opacity (MO) lithium disilicate ceramic, and zirconia ceramic (Z). Each group was subdivided into 5 subgroups (n=4) in shades A2, A3.5, B2, C2 and D3. The transmittance measurement was performed in a spectrophotometer. The Variolink II and Rely X U200 resin cements were photoactivated by LED (1400 mW/cm2) for 40 s through the ceramic discs and without the discs (control group). The DC was measured with infrared FTIR spectroscopy, immediately after light activation. Data were analyzed with Kruskall-Wallis and one-way ANOVA, following post-hoc comparisons by Tukey test and Pearson’s correlation test (P<0.05).

Results

LT ceramic exhibited higher transmittance values compared to MO and Z ceramics. LTA2 and LTB2 showed statistically higher transmittance values compared to MOA2, MOA3.5 and ZA3.5. For Variolink II, the ceramic interposition did not influence the DC, since there were no statistical differences between groups with ceramic interposition and the control group. For Rely X U200 cement, the interposition of some ceramics types/shades (LTA3.5, MOA2, MOA3.5 and ZA3.5) significantly decreased the DC values compared to control group. A positive correlation was found between the ceramic transmittance and DC values of both tested cements. Conclusions. The transmittance and DC values of the cements were influenced by composition and shades of the ceramics. The higher the transmittance of ceramics, the higher the DC values for both cements.

Influence of Ambient Temperature and Light-curing Moment on Polymerization Shrinkage and Strength of Resin Composite Cements

Objective:

The purpose of this study was to establish a clinically appropriate light-curing moment for resin composite cements while achieving the highest indirect tensile strength and lowest polymerization shrinkage.

Methods and Materials:

Polymerization shrinkage of seven resin composite cements (Multilink Automix, Multilink Speed Cem, RelyX Ultimate, RelyX Unicem 2 Automix, Panavia V5, Panavia SA plus, VITA Adiva F-Cem) was measured at ambient temperatures of 23°C and 37°C. Testing was done for autopolymerized and light-cured specimens after light application at either 1, 5, or 10 minutes after mixing. Indirect tensile strength of all cements was measured after 24 hours of storage at temperatures of 23°C and 37°C, for autopolymerized and light-cured specimens after light application 1, 5, or 10 minutes after mixing. To illustrate filler size and microstructures, SEM images of all cements were captured. Statistical analysis was performed with one-way ANOVA followed by post hoc Fisher LSD test (α=0.05).

Results:

Final polymerization shrinkage of the resin composite cements ranged from 3.2% to 7.0%. An increase in temperature from 23°C to 37°C as well as the light-curing moment resulted in material dependent effects on the polymerization shrinkage and indirect tensile strength of the cements. Polymerization shrinkage of the cements did not correlate with the indirect tensile strength of the cement in the respective groups. Highest indirect tensile strengths were observed for the materials containing a homogeneous distribution of fillers with a size of about 1 μm (Multilink Automix, Panavia V5, VITA Adiva F-Cem).

Conclusion:

The magnitude of the effect of light-curing moment and temperature increase on polymerization shrinkage and indirect tensile strength of resin composite cements is material dependent and cannot be generalized.

Influence of glass-based dental ceramic type and thickness with identical shade on the light transmittance and the degree of conversion of resin cement

The purpose of this study was to assess the influence of the types and thicknesses of glass ceramic plates on light transmittance and compare the degrees of conversion (DC) of resin cement under the ceramic materials. Three ceramic plates with thicknesses of 0.5, 1.0, 2.0, and 4.0 mm were fabricated from each of five commercial ceramic blocks in shade A2: high-translucency and low-translucency IPS Empress CAD (Emp_HT and Emp_LT); high-translucency and low-translucency IPS e.max CAD (Emx_HT and Emx_LT); and Vita Mark II (Vita). The translucency parameter was obtained using a colorimeter. The light transmittance rate was measured using a photodetector attached to an optical power meter. The DC of a resin cement (Variolink N) underneath the ceramic plates was examined by Fourier transform infrared spectroscopy. The translucency parameter, light transmittance rate, and DC showed significant differences by ceramic type and thickness (P < 0.05). The Emp_HT specimens showed the highest light transmission and DCs, and the Emx_LT showed the least light transmission and the lowest DCs. The high-translucency Empress showed significantly higher DCs than the low-translucency types (P < 0.05), but there was no significant difference in e.max (P > 0.05). Both type and thickness of the glass ceramics significantly influenced the light transmittance and DC of the light-cured resin cement beneath the ceramic of the same shade.

How well a dental filling or crown performs depends not only on the strength of ceramic material used but also on its thickness and type. Sun-Young Kim of Seoul National University and colleagues investigated how the thickness and type of glass-based ceramic affected how well it transmitted light and thus how well resin cement underneath it hardened. Orthodontists use a special ‘curing’ light to harden resin cement and create a strong bond between the ceramic material and the underlying mouth tissue. They shined light on different types of ceramic plates of similar shade but varying thickness and found that both their ability to transmit light and how well the resin cement underneath them hardened varied significantly. They conclude that orthodontists should not only choose ceramics for dental treatments based on their inherent strength, but should also consider how well they transmit light.

Influence of curing protocol and ceramic composition on the degree of conversion of resin cement

OBJECTIVE

Due to increasing of aesthetic demand, ceramic crowns are widely used in different situations. However, to obtain long-term prognosis of restorations, a good conversion of resin cement is necessary. To evaluate the degree of conversion (DC) of one light-cure and two dual-cure resin cements under a simulated clinical cementation of ceramic crowns.

MATERIAL AND METHODS

Prepared teeth were randomly split according to the ceramic's material, resin cement and curing protocol. The crowns were cemented as per manufacturer's directions and photoactivated either from occlusal suface only for 60 s; or from the buccal, occlusal and lingual surfaces, with an exposure time of 20 s on each aspect. After cementation, the specimens were stored in deionized water at 37°C for 7 days. Specimens were transversally sectioned from occlusal to cervical surfaces and the DC was determined along the cement line with three measurements taken and averaged from the buccal, lingual and approximal aspects using micro-Raman spectroscopy (Alpha 300R/WITec®). Data were analyzed by 3-way ANOVA and Tukey test at =5%.

RESULTS

Statistical analysis showed significant differences among cements, curing protocols and ceramic type (p<0.001). The curing protocol 3x20 resulted in higher DC for all tested conditions; lower DC was observed for Zr ceramic crowns; Duolink resin cement culminated in higher DC regardless ceramic composition and curing protocol.

CONCLUSION

The DC of resin cement layers was dependent on the curing protocol and type of ceramic.

Delayed photo-activation and addition of thio-urethane: Impact on polymerization kinetics and stress of dual-cured resin cements

OBJECTIVE

1) to determine the moment during the redox polymerization reaction of dual cure cements at which to photo-activate the material in order to reduce the polymerization stress, and 2) to evaluate possible synergistic effects between adding chain transfer agents and delayed photo-activation.

METHODS

The two pastes of an experimental dual-cure material were mixed, and the polymerization kinetics of the redox phase was followed. The moment when the material reached its maximum rate of redox polymerization (MRRP) of cement was determined. The degree of conversion (DC) and maximum rates of polymerization (Rp_max) were assessed for materials where: the photoactivation immediately followed material mixing, at MRRP, 1min before and 1min after MRRP. Thio-urethane (TU) additives were synthesized and added to the cement (20% wt), which was then cured under the same conditions. The polymerization kinetics was evaluated for both cements photo-activated immediately or at MRRP, followed by measurements of polymerization stress, flexural strength (FS) and elastic modulus (EM). Knoop hardness was measured before and after ethanol storage.

RESULTS

Photo-activating the cement at or after MRRP reduced the Rp_max and the polymerization stress. Addition of TU promoted additional and more significant reduction, while not affecting the Rp_max. Greater hardness loss was observed for cements with TU, but the final hardness was similar for all experimental conditions. Addition of TU slightly reduced the EM and did not affect the FS.

CONCLUSION

Delayed photo-activation and addition of TU significantly reduce the polymerization stress of dual-cured cements.

Effect of Insufficient Light Exposure on Polymerization Kinetics of Conventional and Self-adhesive Dual-cure Resin Cements

Objectives: The purpose of this study was to investigate the influence of insufficient light exposure on the polymerization of conventional and self-adhesive dual-cure resin cements under ceramic restorations.

Methods: Two conventional dual-cure resin cements (Rely-X ARC, Duolink) and two self-adhesive resin cements (Rely-X U200, Maxcem Elite) were polymerized under different curing modes (dual-cure or self-cure), curing times (20 and 120 seconds), and thickness of a ceramic overlay (2 and 4 mm). Polymerization kinetics was measured by Fourier transform infrared spectroscopy for the initial 10 minutes and after 24 hours. Data were analyzed using mixed model analysis of variance (ANOVA), one-way ANOVA/Student-Newman-Keuls post hoc test, and paired t-test (α=0.05).

Results: When light-curing time was set to 20 seconds, the presence of the ceramic block significantly affected the degree of conversion (DC) of all resin cements. Especially, the DC of the groups with 20 seconds of light-curing time under 4 mm of ceramic thickness was even lower than that of the self-cured groups at 24 hours after polymerization (p&lt;0.05). However, when light-curing time was set to 120 seconds, a similar DC compared with the group with direct light exposure (p&gt;0.05) was achieved in all dual-cure groups except Maxcem Elite, at 24 hours after polymerization.

Conclusions: For both conventional and self-adhesive dual-cure resin cements, insufficient light exposure (20 seconds of light-curing time) through thick ceramic restoration (4 mm thick) resulted in a DC even lower than that of self-curing alone.

Degree of conversion of a resin cement light-cured through ceramic veneers of different thicknesses and types

During the cementation of ceramic veneers the polymerization of resin cements may be jeopardized if the ceramics attenuate the irradiance of the light-curing device. The aim of this study was to evaluate the effect of different types and thicknesses of ceramic veneers on the degree of conversion of a light-cured resin-based cement (RelyX Veneer). The cement was light-cured after interposing ceramic veneers [IPS InLine, IPS Empress Esthetic, IPS e.max LT (low translucency) and IPS e.max HT (high translucency) - Ivoclar Vivadent] of four thicknesses (0.5 mm, 1.0 mm, 1.5 mm and 2.0 mm). As control, the cement was light-cured without interposition of ceramics. The degree of conversion was evaluated by FTIR spectroscopy (n=5). Data were analyzed with one-way ANOVA and Tukey's test (α=0.05). Significant differences were observed among groups (p<0.001). The degree of conversion was similar to the control for all light-cured groups with interposition of ceramics of 0.5 mm and 1.0 mm (p>0.05). Among 1.5-mm-thick veneers, IPS e.max LT was the only one that showed different results from the control (p<0.05). At the thickness of 2.0 mm, only the IPS e.max LT and HT veneers were able to produce cements with degrees of conversion similar to the control (p>0.05). The degree of conversion of the evaluated light-cured resin cement depends on the thickness and type of ceramics employed when veneers thicker than 1.5 mm are cemented.

Correlation between clinical performance and degree of conversion of resin cements: a literature review

Resin-based cements have been frequently employed in clinical practice to lute indirect restorations. However, there are numerous factors that may compromise the clinical performance of those cements. The aim of this literature review is to present and discuss some of the clinical factors that may affect the performance of current resin-based luting systems. Resin cements may have three different curing mechanisms: chemical curing, photo curing or a combination of both. Chemically cured systems are recommended to be used under opaque or thick restorations, due to the reduced access of the light. Photo-cured cements are mainly indicated for translucent veneers, due to the possibility of light transmission through the restoration. Dual-cured are more versatile systems and, theoretically, can be used in either situation, since the presence of both curing mechanisms might guarantee a high degree of conversion (DC) under every condition. However, it has been demonstrated that clinical procedures and characteristics of the materials may have many different implications in the DC of currently available resin cements, affecting their mechanical properties, bond strength to the substrate and the esthetic results of the restoration. Factors such as curing mechanism, choice of adhesive system, indirect restorative material and light-curing device may affect the degree of conversion of the cement and, therefore, have an effect on the clinical performance of resin-based cements. Specific measures are to be taken to ensure a higher DC of the luting system to be used.

Influence of the bracket on bonding and physical behavior of orthodontic resin cements

The aim of the study is to determine the influence of the type of bracket, on bond strength, microhardness and conversion degree (CD) of four resin orthodontic cements. Micro-tensile bond strength (µTBS) test between the bracket base and the cement was carried out on glass-hour-shaped specimens (n=20). Vickers Hardness Number (VHN) and micro-Raman spectra were recorded in situ under the bracket base. Weibull distribution, ANOVA and non-parametric test were applied for data analysis (p<0.05). The highest values of ή as well as the β Weibull parameter were obtained for metallic brackets with Transbond™ plastic brackets with the self-curing cement showing the worst performance. The CD was from 80% to 62.5%.

Effects of Temperature and Aging on Working/Setting Time of Dual-cured Resin Cements

OBJECTIVES

To evaluate the effects of aging and temperature on working time (WT) and setting time (ST) of several dual-cured resin cements.

METHODS

WT and ST were determined with a thermo-controlled stage oscillating rheometer. New cement kits were used for the study. Cements were mixed according to instructions and dispensed on the oscillating stage that was preset at 22°C or 37°C. Rheologic charts were generated from the beginning of mixing until no further oscillation was detected. After initial measurements, cement kits were aged at 37°C for 12 weeks, and WT/ST was determined again at both temperatures. Five samples were read for each material and condition. Data were analyzed with repeated measures analysis of variance and a Tukey test at α=5% for each individual material.

RESULTS

The WT and ST of all cements were significantly affected by temperature and aging (p<0.05). In general, higher temperature accelerated WT/ST, but aging effects were material dependent. Some materials presented reduced WT/ST, whereas others showed increased WT/ST, regardless of the temperature.

CONCLUSIONS

The WT and ST were significantly affected by temperature variation and aging condition. Although temperature changes appeared to affect all materials similarly, aging effects were material dependent.

Degree of conversion of dual-polymerizing cements light polymerized through monolithic zirconia of different thicknesses and types

STATEMENT OF PROBLEM

Monolithic zirconia restorations are increasingly common. Dual-polymerizing cements have been advocated for cementation. The opacious nature of zirconia restoration can attenuate light, compromising optimal resin polymerization and eventually restoration debonding.

PURPOSE

The purpose of this in vitro study was to evaluate the influence of material thickness on light irradiance, radiant exposure, and the degree of monomer conversion (DC) of 2 dual-polymerizing resin cements light-polymerized through different brands of monolithic zirconia.

MATERIAL AND METHODS

Dual-polymerizing resin cements (RelyX Ultimate; 3M-ESPE, and Variolink II; Ivoclar, Vivadent) were mixed according to the manufacturers' instructions with a film thickness of 40 μm, placed under a 10 × 10 mm specimen of monolithic zirconia (Prettau Anterior by Zirkonzahn, Katana by Noritake, BruxZir by Glidewell, and Zenostar by Wieland) and a zirconia core control (ICE zirkon by Zirkonzahn) at various thicknesses (0.50, 1.00, 1.50, and 2.00 mm, n = 5 of each thickness). Each specimen was irradiated for 20 seconds (RelyX Ultimate) and 40 seconds (Variolink II) with Elipar S10 (3M-ESPE, 1200 mW/cm(2)). The amount of irradiance and radiant exposure was quantified for each specimen. Fourier transform infrared spectroscopy was used to measure the DC from the bottom surface of the resin. Statistical analysis was performed with 2-way ANOVA and post hoc Tukey honest significant difference (HSD) tests (α = .05).

RESULTS

Light irradiance and radiant exposure decreased as the thickness of the specimen increased (P < .05) regardless of the brand. The ranking from least to highest was BruxZir < ICE zircon = Wieland < Katana = Prettau Anterior. The zirconia brand, thickness, and cement type had a significant effect on the DC (P < .001). The DC decreased significantly as the thickness of the zirconia increased (P < .001). Katana and Prettau Anterior showed the highest DC and BruxZir showed the lowest.

CONCLUSION

The thickness of zirconia affects the DC of resin-based cements. The DC of the resin cements differed significantly between cements and among zirconia brands. More polymerizing time may be needed to deliver sufficient energy through some brands of zirconia.

Examining exposure reciprocity in a resin based composite using high irradiance levels and real-time degree of conversion values

OBJECTIVE

Exposure reciprocity suggests that, as long as the same radiant exposure is delivered, different combinations of irradiance and exposure time will achieve the same degree of resin polymerization. This study examined the validity of exposure reciprocity using real time degree of conversion results from one commercial flowable dental resin. Additionally a new fitting function to describe the polymerization kinetics is proposed.

METHODS

A Plasma Arc Light Curing Unit (LCU) was used to deliver 0.75, 1.2, 1.5, 3.7 or 7.5 W/cm(2) to 2mm thick samples of Tetric EvoFlow (Ivoclar Vivadent). The irradiances and radiant exposures received by the resin were determined using an integrating sphere connected to a fiber-optic spectrometer. The degree of conversion (DC) was recorded at a rate of 8.5 measurements a second at the bottom of the resin using attenuated total reflectance Fourier Transform mid-infrared spectroscopy (FT-MIR). Five specimens were exposed at each irradiance level. The DC reached after 170s and after 5, 10 and 15 J/cm(2) had been delivered was compared using analysis of variance and Fisher's PLSD post hoc multiple comparison tests (alpha=0.05).

RESULTS

The same DC values were not reached after the same radiant exposures of 5, 10 and 15 J/cm(2) had been delivered at an irradiance of 3.7 and 7.5 W/cm(2). Thus exposure reciprocity was not supported for Tetric EvoFlow (p<0.05).

SIGNIFICANCE

For Tetric EvoFlow, there was no significant difference in the DC when 5, 10 and 15J/cm(2) were delivered at irradiance levels of 0.75, 1.2 and 1.5 W/cm(2). The optimum combination of irradiance and exposure time for this commercial dental resin may be close to 1.5 W/cm(2) for 12s.

Effect of Evaporation on the Shelf Life of a Universal Adhesive

Objectives

The purpose of this study was to evaluate how evaporation affects the shelf life of a one-bottle universal adhesive.

Methods

Three different versions of Scotchbond Universal (SBU, 3M ESPE, Seefeld, Germany) were prepared using a weight-loss technique. SBU0 was left open to the air until maximal weight loss was obtained, whereas SBU50 was left open until 50% of evaporation occurred. In contrast, SBU100 was kept closed and was assumed to contain the maximum concentration of all ingredients. The degree of conversion (DC) was determined by using Fourier transform infrared spectroscopy on different substrates (on dentin or glass plate and mixed with dentin powder); ultimate microtensile strength and microtensile bond strength to dentin were measured as well.

Results

DC of the 100% solvent-containing adhesive (SBU100) was higher than that of the 50% (SBU50) and 0% (SBU0) solvent-containing adhesives for all substrates. DC of the adhesive applied onto glass and dehydrated dentin was higher than that applied onto dentin. Even though the ultimate microtensile strength of SBU0 was much higher than that of SBU50 and SBU100, its bond strength to dentin was significantly lower.

Conclusions

Evaporation of adhesive ingredients may jeopardize the shelf life of a one-bottle universal system by reducing the degree of conversion and impairing bond strength. However, negative effects only became evident after more than 50% evaporation.

Effect of agitation and storage temperature on water sorption and solubility of adhesive systems

The purpose of this study was to evaluate the influence of storage temperature and flask agitation on the water sorption (WS) and solubility (SL) of simplified adhesive systems. Seventy-two disc-shaped specimens were prepared according to the adhesive system (water/ethanol-based: Adper Single Bond 2; and water-based: One Coat Bond SL) and experimental conditions tested (mechanical agitation and storage temperature). Statistical analysis (3-way ANOVA, alpha=5%) found significantly greater WS and SL means for the water/ethanol-based system when compared to the water-based. Irrespective of factors studied, significant differences in WS and SL were noted between cold and room temperatures, with greater values been obtained at 1°C, and lower ones at 20°C. Agitation provided increased WS for both materials at all temperatures, but did not affect their SL. The mechanical agitation of the flask may negatively affect the dynamics of diffusion of simplified adhesive systems, even at extremely cold or warm temperatures.