Light-curing dental resin-based composites: How it works and how you can make it work

In vivo comparison of resin infiltration outcomes under different light conditions: A randomized controlled clinical trial

OBJECTIVE

Success of resin infiltration for the masking of MIH-lesions remains difficult to predict, prompting suggestions to adjust the treatment protocol. This exploratory in vivo study aims to evaluate whether monitoring the resin infiltration process using transmitted light, compared to ambient light, enables a better estimation of when the infiltration process is completed, and to assess how the treatment success, i.e. masking effect and infiltration proportion, is influenced accordingly.

METHODS

15 patients with 19 MIH-lesions, diagnosed according to EAPD diagnostic criteria, were enrolled. MIH-lesions were randomly assigned to two treatment groups. In one group resin infiltration progression was monitored under transmitted light, whereas in the second group ambient light was used, representing the standard procedure. Pictures were taken before infiltration and when progression became evident until infiltration was judged to be completed. Infiltration proportion and color difference between the opacity and surrounding sound enamel were calculated and analyzed using the independent-sample t-test with a level of significance of p < 0.05.

RESULTS

Compared to the ambient light group, the transmitted light group showed a significantly higher infiltration time (17.7 ± 8.2 min vs. 9.3 ± 1.6 min); a significantly higher mean infiltration proportion (97.6 ± 2.8% vs. 83.9 ± 9.7%) and a significantly better masking effect (i.e. lower mean color difference (∆E) between effected and sound enamel: 4.5 ± 2.4 vs. 7.5 ± 3.3).

CONCLUSION

Transillumination-guided resin infiltration required prolonged infiltration time but led to an increased infiltration proportion and improved masking effects.

CLINICAL SIGNIFICANCE

Transillumination guided resin infiltration enables a more accurate judgement as to when the infiltration process has been completed, which leads, through extension of the infiltration time, to a significantly higher mean infiltration proportion and provides favorable esthetic outcomes.

The effect of photoinitiator systems on resin-based composite containing ZnO-nanoparticles

OBJECTIVE

Zinc oxide (ZnO) powder possesses antibacterial activity and although white in color, it can severely reduce the depth of cure (DoC) of resin-based composite (RBC). This study investigated the effect of unary and binary photoinitiator systems on the DoC and degree of conversion (DC) of formulated RBC containing ZnO-nanoparticles.

METHODS

Fourteen RBCs (n = 3/group) were formulated consisting of 50 wt% mixture of monomers (Bis-GMA, TEGDMA, and UDMA) and 50 wt% fillers (inert barium glass powder and silica nanoparticles). ZnO-nanoparticles were added at 0 (control), 0.5, 1, 1.5 and 2 wt%. A unary initiator system consists of camphorquinone (CQ) 0.25, 0.5 and 1 wt% and ethyl 4-(dimethylamino)benzoate (EDMAB) 0.75 wt% or a binary initiator system consisting of diphenyliodonium hexafluorophosphate (DPI) 0.25, 0.5 and 1 wt%, CQ 0.25, 0.5 and 1 wt% and EDMAB 0.75 wt% were added to the monomer mixture. To measure the DoC, each specimen was prepared in a custom-made mold with a slot (16 x 8×2 mm) and a top cover plate, irradiated from one end (40 s), stored dry (37° C, 1 d) and measured at increasing depths using Vickers hardness (0.5 mm intervals). 1 mm thick specimens were prepared to measure DC continuously using FTIR, from zero up to 24 h post-irradiation.

RESULTS

Increasing the concentrations of ZnO led to a significant reduction of DoC (p < 0.05). But most of the binary initiator groups showed significantly higher DoC (p < 0.05). Depth, at 80 % of max VHN, of unary initiator groups reduced from 6.8 mm (ZnO at 0 wt%) to 2.1 mm (ZnO at 2 wt%) and in binary initiator groups from 8.4 mm to 2.3 mm. Groups with lower photoinitiator concentrations (0.25 wt%) showed a significant increase in DoC compared with groups with higher concentrations (1 wt%) (p < 0.05). DC after 24 h was independent of either ZnO concentration or the photoinitiator system (p > 0.05). However, faster conversions were observed in binary initiator groups. The RPmax of binary groups ranged from 8.1 % to 10.1 %/s, and unary groups ranged from 5.2 % to 7.2 %/s.

SIGNIFICANCE

The addition of DPI resulted in an overall increased curing depth, which was enhanced when lower concentrations of photoinitiators were used. Also, DPI resulted in faster conversions. This is desirable in designing antibacterial RBC containing ZnO.

Monomer elution and shrinkage stress analysis of addition-fragmentation chain-transfer-modified resin composites in relation to the curing protocol

OBJECTIVE

The purpose was to compare the effects of rapid (3 s) and conventional (20 s) polymerization protocols (PP) of mono- and multichip LED curing units (LCU) on shrinkage stress (SS) and monomer elution (ME) in bulk-fill resin-based composites (RBC) with and without addition-fragmentation chain-transfer (AFCT) monomer.

METHODS

Cylindrical (5x4mm) specimens were prepared from two RBCs containing different AFCT monomers (Filtek OneBulk-FOB; Tetric PowerFill-TPF) and one without (Tetric EvoCeram Bulk-TEC). After soaking for 3, 10, and 14 days (75 % ethanol), ME was quantified using standard monomers by High-Performance Liquid Chromatography. SS was measured from the start of polymerization to 5 min using a Materials Testing Machine. The radiant exitance of LCUs was measured using a spectrophotometer. ANOVA and Tukey's post-hoc test, multivariate analysis and partial eta-squared statistics were used to analyze the data (p < 0.05).

RESULTS

AFCT-modification significantly decreased ME (p < 0.001). ME was reduced by half by day 10 and by one tenth by the end of the 14-day compared to the 3-day sampling. ME itself was dependent, whereas the percentage of monomers released was independent of the PP used (p > 0.05). FOB showed the lowest SS (p < 0.001), while there was no significant difference between TPF and TEC (p = 0.124). Both ME and SS were significantly influenced by material type and PP.

SIGNIFICANCE

The incorporation of the AFCT monomer reduced ME, but this was inversely related to a decrease in exposure time. SS values reduced by rapid PP in parallel with increasing ME values. The utilization of the AFCT molecule in conjunction with an appropriate resin-, initiator-system is of significant consequence for the kinetics of polymerization and the incorporation of monomers into the network.

Depth of Cure of Resin-Based Composites Irradiated With Three Types of Light-Curing Units at Different Output Intensities

Aim Dental light-curing units (LCUs) are used at Matsumoto Dental University Hospital (MDUH). However, the time of installation of the same type of light irradiator varies, which affects the output intensity of the LCU. The purpose of this study was to evaluate the performance of LCU with different output intensities by comparing the depth of cure (DOC) of resin-based composites (RBCs). Materials and methods The output intensities of three types of LCUs, namely Pencure 2000(Morita, Osaka, Japan), DC BlueLEX Plus (Yoshida, Tokyo, Japan)​​​​​​​, and Candelux (Morita, Osaka, Japan), were measured using a commercial dental radiometer, namely Bluephase Meter II (Ivoclar Vivadent, Schaan, Liechtenstein). The units with the highest and lowest output intensities were selected and used. The RBC, either Body A3 or Opaque A3 (Premise, Kerr, Brea, USA)​​​​​​​, was inserted into a cylindrical mold with an inner diameter of 4 mm and depth of 8 mm, and light irradiation was performed using an intervening polyester strip for 30 s. After removing the unpolymerized portions of the RBC from the mold with a plastic spatula, the long axis of the cured portion was measured with a digital caliper (n=10). Results The highest DOC was observed when the RBCs were cured with Pencure 2000 at 1513 mW/cm2, the highest output intensity. The DOC was significantly greater when the LCU was positioned at 0 mm than at 8 mm from the RBC surface, and the DOC of Body A3 was greater than that of Opaque A3 (p<0.05). Moreover, a positive correlation was observed between output intensity and DOC. The output intensity of LCUs in the same model also varied, which affected the DOC. Conclusion Increasing the output intensity at the tip of the light guide of the LCU also increased the DOC of the RBC. Increasing the irradiation distance from 0 mm to 8 mm decreased the DOC of the RBC. The DOC of the opaque-shade RBC was smaller than that of the body-shade RBC when curing was conducted with the same LCU.

Digital manufacturing techniques and the in vitro biocompatibility of acrylic-based occlusal device materials

OBJECTIVES

Material chemistry and workflow variables associated with the fabrication of dental devices may affect the biocompatibility of the dental devices. The purpose of this study was to compare digital and conventional workflow procedures in the manufacturing of acrylic-based occlusal devices by assessing the cytotoxic potential of leakage products.

METHODS

Specimens were manufactured by 3D printing (stereolithography and digital light processing), milling, and autopolymerization. Print specimens were also subjected to different post-curing methods. To assess biocompatibility, a human tongue epithelial cell line was exposed to material-based extracts. Cell viability was measured by MTT assay while Western blot assessed the expression level of selected cytoprotective proteins.

RESULTS

Extracts from the Splint 2.0 material printed with DLP technology and post-cured with the Asiga Flash showed the clearest loss of cell viability. The milled and autopolymerized materials also showed a significant reduction in cell viability. However, by storing the autopolymerized material in dH2O for 12 h, no significant viability loss was observed. Increased levels of cytoprotective proteins were seen in cells exposed to extracts from the print materials and the autopolymerized material. Similarly to the effect on viability loss, storing the autopolymerized material in dH2O for 12 h reduced this effect.

CONCLUSIONS/CLINICAL RELEVANCE

Based on the biocompatibility assessments, clinical outcomes of acrylic-based occlusal device materials may be affected by the choice of manufacturing technique and workflow procedures.

Validation of a human saliva model for the determination of leachable monomers and other chemicals from dental materials

This study aimed to prove the validity of a mixture of chemicals, including salts, small organic molecules, mucin, and α-amylase, as saliva surrogate ("artificial saliva") for assessing leakage of methacrylate monomers and other constituents from dental materials. To achieve this, we developed and validated a liquid chromatography-tandem mass spectrometry (LC-MS/MS) method for the quantification of 2-hydroxyethyl methacrylate (HEMA), triethylene glycol dimethacrylate (TEGDMA), diurethane dimethacrylate (UDMA), bisphenol A glycerolate dimethacrylate (BisGMA), diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide (TPO), bisphenol A (BPA), and five homologues of ethoxylated bisphenol A dimethacrylate (BisEMA EO2-6) in unstimulated and artificial saliva, and compared their concentrations in the two saliva media following either spiking with a mixture of the compounds or incubation of test specimens of printed biomaterials. Test specimens were immersed in unstimulated/artificial saliva, incubated at 37 °C for 24 h, and saliva aliquots were extracted with methanol and subsequently analyzed by LC-MS/MS. The method was validated with regard to matrix effects, linearity, selectivity, lower limits of quantification (LLOQ), precision, bias and combined measurement uncertainty (u'). The performance characteristics of the method were comparable for unstimulated and artificial saliva samples. The combined u' for individual chemicals at a concentration of 10 × LLOQ were within the range of 5.3-14 % for unstimulated saliva and 6.9-16 % for artificial saliva, except for the BisEMA homologues. Combined u' for the latter were 27-74 % in unstimulated saliva, and 27-79 % in artificial saliva. There was no detectable release of BPA from the test specimens, and the TPO concentrations were mainly below the LLOQ. TEGDMA and UDMA were detected in the highest quantities, and at comparable concentrations in the unstimulated and artificial saliva. For all BisEMA homologues, the release was higher in unstimulated saliva than in artificial saliva. The study showed that the artificial saliva model can be a suitable replacement for native saliva, but might underestimate leakage of more lipophilic methacrylates.