Effect of High-Irradiance Light-Curing on Micromechanical Properties of Resin Cements

The power of light - From dental materials processing to diagnostics and therapeutics

Harnessing the power of light and its photonic energy is a powerful tool in biomedical applications. Its use ranges from biomaterials processing and fabrication of polymers to diagnostics and therapeutics. Dental light curable materials have evolved over several decades and now offer very fast (≤ 10 s) and reliable polymerization through depth (4-6 mm thick). This has been achieved by developments on two fronts: (1) chemistries with more efficient light absorption characteristics (camphorquinone [CQ], ~30 L mol-1 cm1 [ʎmax 470 nm]; monoacylphosphine oxides [MAPO], ~800 L mol-1 cm-1 [ʎmax 385 nm]; bisacylphosphine oxide [BAPO], ~1,000 L mol-1 cm-1 [ʎmax 385 nm]) as well mechanistically efficient and prolonged radical generation processes during and after light irradiation, and; (2) introducing light curing technologies (light emitting diodes [LEDs] and less common lasers) with higher powers (≤ 2 W), better spectral range using multiple diodes (short: 390-405 nm; intermediate: 410-450 nm; and long: 450-480 nm), and better spatial power distribution (i.e. homogenous irradiance). However, adequate cure of materials falls short for several reasons, including improper selection of materials and lights, limitations in the chemistry of the materials, and limitations in delivering light through depth. Photonic energy has further applications in dentistry which include transillumination for diagnostics, and therapeutic applications that include photodynamic therapy, photobiomodulation, and photodisinfection. Light interactions with materials and biological tissues are complex and it is important to understand the advantages and limitations of these interactions for successful treatment outcomes. This article highlights the advent of photonic technologies in dentistry, its applications, the advantages and limitations, and possible future developments.

Effect of Ceramic and Dentin Thicknesses and Type of Resin-Based Luting Agents on Intrapulpal Temperature Changes during Luting of Ceramic Inlays

The adhesive cementation of ceramic inlays may increase pulpal temperature (PT) and induce pulpal damage due to heat generated by the curing unit and the exothermic reaction of the luting agent (LA). The aim was to measure the PT rise during ceramic inlay cementation by testing different combinations of dentin and ceramic thicknesses and LAs. The PT changes were detected using a thermocouple sensor positioned in the pulp chamber of a mandibular molar. Gradual occlusal reduction obtained dentin thicknesses of 2.5, 2.0, 1.5, and 1.0 mm. Light-cured (LC) and dual-cured (DC) adhesive cements and preheated restorative resin-based composite (RBC) were applied to luting of 2.0, 2.5, 3.0, and 3.5 mm lithium disilicate ceramic blocks. Differential scanning calorimetry was used to compare the thermal conductivity of dentin and ceramic slices. Although ceramic reduced heat delivered by the curing unit, the exothermic reaction of the LAs significantly increased it in each investigated combination (5.4–7.9 °C). Temperature changes were predominantly influenced by dentin thickness followed by LA and ceramic thickness. Thermal conductivity of dentin was 24% lower than that of ceramic, and its thermal capacity was 86% higher. Regardless of the ceramic thickness, adhesive inlay cementation can significantly increase the PT, especially when the remaining dentin thickness is <2 mm.

How the Duration and Mode of Photopolymerization Affect the Mechanical Properties of a Dental Composite Resin

Composite materials are the most common materials in use in modern dentistry. Over the years, the methods of photopolymerization of composite materials have been improved with the use of various devices, such as quartz tungsten halogen lamps (QTHs), light-emitting diode units (LEDs), plasma-arc lamps and argon-ion lasers. This study aimed to compare the mechanical properties of a composite material, depending on the time and mode of photopolymerization. One hundred and forty rectangular specimens (25 × 2 × 2 mm) and forty-two disc-shaped samples (5 mm diameter and 2 mm thickness) were prepared from shade A2 Boston composite resin. Samples were cured using the following seven photopolymerization protocols: four fast-cure modes (full power for 3, 5, 10, and 20 s), two pulse-cure modes (5 and 10 shots of 1 s exposures at full power), and one step-cure mode (soft start with a progressive cycle lasting 9 s). Specimens were subjected to a flexural strength test, Vickers microhardness test, and FTIR spectroscopy test. A 2-factor ANOVA and post-hoc tests were carried out to assess the differences in the flexural strength parameter between the tested groups of samples before and after aging. A mixed-model ANOVA was carried out to assess the differences in the Vickers microhardness parameter between the tested groups of samples before and after aging. The lowest values of flexural strength (p < 0.001) and Vickers microhardness (p < 0.001) were obtained for the 3 s mode for the pre- and post-aging groups. The FTIR mapping tests showed a much more homogeneous chemical structure of the composite after 20 s of continuous irradiation, compared to the sample irradiated for 5 s in the continuous mode. The mode and cure time affects the mechanical properties of the composite resin. Appropriate selection of the cure mode and time ensures better mechanical properties of composite resin. This suggests that the survival of dental restorations within the oral cavity could be extended by using longer photopolymerization durations.

Degree of conversion of resin-cements (light-cured/dual-cured) under different thicknesses of vitreous ceramics: systematic review

PURPOSE

This systematic review synthesized and analyzed the scientific evidence on the degree of conversion (DC) obtained by Fourier-transform infrared spectroscopy (FTIR) of light-cured and dual-cured resinous cements, photopolymerized under different thicknesses of vitreous ceramics.

STUDY SELECTION

The study protocol of this systematic review was registered at the International Prospective Register of Systematic Reviews (PROSPERO) (CRD42017069319). A comprehensive search (PubMed/MEDLINE, Scopus, EMBASE, and LILACS) was performed for papers including an in vitro design and indexed from January 2007 to December 2020 according to the study purposes. A quality appraisal (specific instrument) and descriptive analysis of the articles that met the inclusion criteria were conducted.

RESULTS

Nine included studies were analyzed. Two of them used feldspathic ceramics, six used lithium disilicate, and one used both (comparing different types and opacities of ceramics). Three studies found a higher DC in dual cements, while one did not find any significant differences, and five studies found a higher DC in light-cured resin cements. Light-cured cements showed a better DC in relation to dual-cured cements in vitreous ceramic restorations with thicknesses up to 2 mm.

CONCLUSION

According to the findings, the use of good photoactivation is the most relevant variable to achieve an adequate DC in light-cured and dual-cured resin cements. The use of vitreous ceramic restorations with a thickness of less than 2 mm (light-curing cements) shows a better DC. Standardized in vitro studies are required to generate accurate scientific evidence.

Evaluation of irradiance and radiant exposure on the polymerization and mechanical properties of a resin composite

The objective of the present study was to evaluate the effect of irradiance and radiant exposure on the chemical-mechanical properties of a resin composite. A micro-hybrid resin composite (Clearfil AP-X, Kuraray) was investigated under two different irradiances: low (300 mW/cm2) and high (800 mW/cm2) and radiant exposures: 8 and 16 J/cm2. Four groups, named Low 8 J/cm2, High 8 J/cm2, Low 16 J/cm2, and High 16 J/cm2 were tested, and their flexural strengths, elastic moduli, depths of cure, and degrees of conversion were evaluated. Data were analyzed using two-way ANOVA and Tukey's test. A multiple linear regression model was used to correlate the irradiance and radiant exposure with dependent variables (α = 0.05). Irradiance and radiant exposure were found statistically significant for all dependent variables. The interaction between the factors was statistically significant only for the degree of conversion and elastic modulus. Group Low 16 J/cm2 exhibited a significantly superior performance in all the evaluated properties. Barring the degree of conversion, no significant differences were observed among the properties evaluated between the Low 8 J/cm2 and High 8 J/cm2 groups. The adjusted R2 values were high for the depth of cure and degree of conversion (0.58 and 0.96, respectively). Both irradiance and radiant exposure parameters play an important role in establishing the final properties of a micro-hybrid resin composite. Irradiance has a greater influence under higher radiant exposures.

Could light-curing time, post-space region and cyclic fatigue affect the nanomechanical behavior of a dual-curing cement for fiber post luting?

OBJECTIVE

To evaluate the effects of curing time, post-space region and cyclic fatigue on the micromechanical properties of a fiber-post luting cement. The null hypotheses were that (1) curing time, (2) fatigue and (3) post-space region does not affect the nanoindentation modulus and hardness of the dual-curing cement.

MATERIALS AND METHODS

48 premolars were endodontically treated and a class I cavity and 8 mm deep post space was prepared. Fiber posts were luted with a universal, dualized adhesive system and a dual-curing cement following manufacturer's instructions. Specimens were divided into three groups (16 specimens for each group) according to light-curing time (no light-curing, 20 s light-curing and 120 s light-curing), which was performed with a LED lamp at 1000 mW/cm 2. The coronal part of the cavity was restored using a nano-filled resin composite. After 24 h, 8 specimens for each group were randomly extract in order to undergo to fatigue test in wet condition through a chewing simulator, while the other specimens were kept in distilled water as benchmark. All the restored teeth were then sectioned in 1 mm thick slices perpendicularly to the fiber post axis. Specimen slices were classified in coronal and apical to be tested through a nanoindenter. Data were analyzed through Kruskal-Wallis test with a significance level of 1%, in order to evaluate the influence of treatments (i.e., curing time and cyclic loading) on the micromechanical properties of the tested luting cement.

RESULTS

Both fatigue and curing time significantly influenced nanoindentation modulus and hardness of dual-curing cement (p < 0.01). No significant differences were reported for post space region. A significant interaction was found among the analyzed factors (p < 0.01).

SIGNIFICANCE

120 s light-curing time is recommended in order to achieve optimal mechanical proprieties, independently from post space region and cyclic fatigue. As matter of fact, 120 s light-curing allowed to prevent strain hardening induced by the fatigue simulation.

Hardness of Resin Cements Polymerized through Glass-Ceramic Veneers

(1) Background: The aim of this study is to evaluate the hardness of resin cements polymerized through ceramic disks under different process factors (ceramic type and thickness, light-polymerization units and polymerization time); (2) Method: Three types of ceramic blocks were used (IPS e.max CAD; Celtra Duo; VITABLOCS). Ceramic disks measuring 0.5 mm, 1.0 mm and 1.5 mm were cut from commercial blocks. Two resin cements (Rely X Veneer and Variolink Esthetic) were polymerized through the ceramic specimens using distinct light-polymerization units (Deep-cure; Blue-phase) and time intervals (10 and 20 s). Hardness of cement specimens was measured using microhardness tester with a Knoop indenter. Data were statistically analyzed using factorial ANOVA (α = 5%); (3) Results: Mean microhardness of Rely X Veneer cement was significantly higher than that of Variolink Esthetic. Deep-cure resulted in higher mean microhardness values compared to Blue-phase at 0.5- and 1-mm specimen thicknesses. Moreover, a direct correlation was found between polymerization time and hardness of resin cement; (4) Conclusions: Surface hardness was affected by resin cement type and ceramic thickness, and not affected by ceramic types, within evaluated conditions. Increasing light-polymerization time significantly increased the hardness of the cement.

High-Power LED Units Currently Available for Dental Resin-Based Materials-A Review

The pursuit of less time-consuming procedures led to the development of high-power light-curing-units (LCU) to light-cure dental-resin-based-materials. This review aims to describe high-power light-emitting-diode (LED)-LCUs, by a bibliometric systematization of in vitro and in vivo studies. The research-question, by PICO model, aimed to assess the current knowledge on dentistry-based high-power LED-LCUs by analyzing to what extent their use can promote adverse events on materials and patients' oral condition when compared to low-power LED-LCUs, on daily dental practice. PubMed and B-on database search focused on high-power (≥2000 mW/cm2) LED-LCUs outputs. Studies assessing performance of high-power LED-LCUs for light-curing dental-resin-based-materials were included. From 1822 screened articles, 21 fulfilled the inclusion criteria. Thirty-two marketed units with high levels of radiant emittance (≥2000 mW/cm2 up to 6000 mW/cm2) were identified. Most output values vary on 2000-3000 mW/cm2. The highest output found was 6000 mW/cm2, in FlashMax™P3. Reports suggest that light-curing protocols with lower emittance irradiance and longer exposure outperforms all other combination, however in some clinical procedures high-power LED-LCUs are advocated when compared to low-power LED-LCUs. Moreover, long time exposures and over-curing can be dangerous to the biological vital pulp, and other oral tissues. Evidence showing that high-power LCUs are the best clinical option is still very scarce.

The influence of the resin-based cement layer on ceramic-dentin bond strength

The purpose of the study was to measure the cement thickness obtained when ceramic rods were luted to dentin and to analyze the relation between cement thickness and the previously published tensile bond strength of similar test specimens. In addition, the ISO standard 4049:2019 method was used to determine the film thickness of the used cements. Zirconia (n = 100) and lithium disilicate (n = 50) rods were cemented to bovine dentin using one of five different resin-based cements. The ceramic-dentin test specimens were cut into two slices and the cement thickness was measured using a scanning electron microscope and compared to the bond strength values of similar specimens already published. The mean cement thickness recorded for ceramic rods cemented to dentin was in the range 20-40 μm, which was larger than the cement film thickness found by the ISO method. The cement film thickness determined according to ISO standard methods did not concur with the results obtained when cementing ceramic rods to dentin. For cementing ceramic restorations, a cement thickness in the range 25-35 μm seems to be favorable for the bond strength.

Minimum Radiant Exposure and Irradiance for Triggering Adequate Polymerization of a Photo-Polymerized Resin Cement

This study aimed to establish the minimum radiant exposure and irradiance to trigger an adequate polymerization of a photo-polymerized resin cement. In total, 220 disc-shaped specimens (diameter of 10 mm and thickness of 0.1 mm) were fabricated using a photo-polymerized resin cement (Variolink N-transparent, Ivoclar Vivadent). To investigate the minimum radiant exposure, the specimens were polymerized with radiant exposures of 1, 2, 3, 4, 5, 6, and 18 J/cm² (n = 20). During polymerization, the irradiance was maintained at 200 mW/cm². To investigate the minimum irradiance, the specimens were polymerized with irradiances of 50, 100, 150, and 200 mW/cm² (n = 20). During polymerization, the radiant exposure was maintained at the previously determined minimum radiant exposure. The Vickers microhardness (HV) and degree of conversion (DC) of the carbon double bond of the specimens were measured to determine the degree of polymerization of the specimens. The results were analyzed using one-way analysis of variance (ANOVA) and Tukey’s test (p < 0.05). In the investigation of the minimum radiant exposure, the HV and DC of the specimens polymerized with a radiant exposure from 1 to 5 J/cm² were significantly lower than those with 18 J/cm² (all p < 0.05). However, no significant difference in HV and DC was found between the specimens polymerized with 6 J/cm² and 18 J/cm² (p > 0.05). In the investigation of the minimum irradiance, the specimens polymerized with an irradiance of 50 mW/cm² had significantly lower HV and DC than the specimens polymerized with an irradiance of 200 mW/cm² (p < 0.05). However, no significant difference in the HV and DC was found among the specimens cured with irradiances of 100, 150, and 200 mW/cm² (p > 0.05). In conclusion, the minimum radiant exposure and irradiance to trigger an adequate polymerization of the light-cured resin cement were 6 J/cm² and 100 mW/cm², respectively.

Resin viscosity determines the condition for a valid exposure reciprocity law in dental composites

OBJECTIVE

To provide conditions for the validity of the exposure reciprocity law as it pertains to the photopolymerization of dimethacrylate-based dental composites.

METHODS

Composites made from different mass ratios of resin blends (Bis-GMA/TEGDMA and UDMA/TEGDMA) and silanized micro-sized glass fillers were used. All the composites used camphorquinone and ethyl 4-dimethylaminobenzoate as the photo initiator system. A cantilever beam-based instrument (NIST SRI 6005) coupled with NIR spectroscopy and a microprobe thermocouple was used to simultaneously measure the degree of conversion (DC), the polymerization stress (PS) due to the shrinkage, and the temperature change (TC) in real time during the photocuring process. The instrument has an integrated LED light curing unit providing irradiances ranging from 0.01W/cm² to 4W/cm² at a peak wavelength of 460nm (blue light). Vickers hardness of the composites was also measured.

RESULTS

For every dental composite there exists a minimum radiant exposure required for an adequate polymerization (i.e., insignificant increase in polymerization with any further increase in the radiant exposure). This minimum predominantly depends on the resin viscosity of composite and can be predicted using an empirical equation established based on the test results. If the radiant exposure is above this minimum, the exposure reciprocity law is valid with respect to DC for high-fill composites (filler contents >50% by mass) while invalid for low-fill composites (that are clinically irrelevant).

SIGNIFICANCE

The study promotes better understanding on the applicability of the exposure reciprocity law for dental composites. It also provides a guidance for altering the radiant exposure, with the clinically available curing light unit, needed to adequately cure the dental composite in question.