Efficiency of light-emitting diode and halogen units in reducing residual monomers

The effect of light curing time and intensity on the bond strength and fracture resistance of orthodontic adhesive

Background

This study aimed to measure the shear bond strength and compressive strength of orthodontic adhesives at different curing times and intensities.

Methods

Ninety extracted human premolars were used. Orthodontic brackets were bonded on the buccal surface of the teeth with orthodontic adhesive light-cured using VRN-VAFU LED curing light at different curing times (1, 3 and 5 seconds) and intensities (1000, 1600 and 2300 mW/cm2 ). A universal testing machine was used to measure the shear bond strength. The ratio of the adhesive remnant and compressive strength of the orthodontic adhesive, at each curing time at the different intensities, were also evaluated. The results were statistically analyzed using one-way analysis of variance followed by Tukey's test.

Results

The lowest bond strength values (6.4, 9.9 and 12.6 MPa) were recorded with 1000 mW/ cm2 intensity (at all curing times) in comparison with the other intensities (P<0.05). Increasing the curing time significantly increased the bond strength of the orthodontic brackets (P<0.05), except when the curing time was increased from 3 sec to 5 sec at 1600 mW/cm2 intensity. The highest compressive strength values (130.3, 147.1 and 174 MPa) were recorded at 2300 mW/ cm2 intensity (at all curing times) compared to the other intensities (P<0.05). The highest values of the ratio of the adhesive remnants were recorded at 1000 mW/cm2 intensity (at all curing times) compared to the other intensities (P<0.05).

Conclusion

Although, increasing the curing time and\or the curing intensity has a positive effect on the bond strength and compressive strength of the orthodontic adhesive, it might be possible to suggest reducing the curing time of orthodontic adhesive to 1 sec at curing intensity of 2300 mW/cm2.

Evaluation of residual monomer release after polymerization of different restorative materials used in pediatric dentistry

BACKGROUND

The choice of the restorative resin material to be used in pediatric dentistry is of a great importance due to the cytotoxic effects caused by residual monomers. In this study, it was aimed to investigate the amount of residual monomer released over time from different resin-based restorative materials, which are widely used in pediatric dentistry, by using high performance liquid chromatography with photodiode array detector (HPLC-PDA).

METHODS

The compomers in all colors (Twinky Star and Glasiositte A₂), two composites with different hybrid properties (Arabesk-GrandioSO), and RMGIC (Ionolux) samples with 2 × 5 mm diameters were prepared. The samples were polymerized with an LED light unit (CELALUX 2, VOCO, Cuxhaven, Germany) and then finishing-polishing procedures were applied. A total of 156 samples were obtained, 13 samples in each of the 12 groups. The amount of residual monomer (BIS-GMA; HEMA, TEGDMA, UDMA) (µg/mL) released into the 75% ethanol solution was determined at different times, (1st hour, 1st, 7th, 14th, and 21st day) by using HPLC-PDA.

RESULTS

The residual monomer release continued on day 21 and BIS-GMA was the most released monomer in all groups. HEMA release showed a maximum increase in all the materials at day 7. The highest amount of residual monomer was detected in the gold-colored compomer. HEMA and BIS-GMA release from RMGIC was less than others in all time frames.

CONCLUSIONS

The color and composition of resin-based restorative materials affect the amount of residual monomer. Pediatric dentists should prefer gold-colored compomers less than others as a restorative material, especially in deep cavities. More studies are needed about the subject.

Release and toxicity of bisphenol-A (BPA) contained in orthodontic adhesives: A systematic review

OBJECTIVE

The objectives of this systematic review of the literature are to search for BPA release from biomaterials used in orthodontics and to highlight their possible impact on human health.

MATERIALS AND METHODS

Using the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) model, digital and manual searches were conducted in February 2020 in the database of CENTRAL, MEDLINE and EMBASE. Thirteen parameters relating to the release and biological effects of BPA were identified and analysed.

RESULTS

Thirty-one non-randomized clinical trials were included. In general, the level of evidence was low, and the risk of bias ranged from moderate to high. Furthermore, although at different concentrations, most of in vitro and in vivo studies found BPA release from orthodontic adhesives. The lack of standardized protocols and the clinical and methodological heterogeneity of the studies prevented a valid interpretation of the actual results. Concerning the possible toxicity of BPA, no conclusive scientific evidence could be drawn, but it seems that orthodontic biomaterials containing BPA have potential adverse biological effects in humans.

DISCUSSION AND CONCLUSION

Until we have solid evidence from clinical trials, clinicians should consider that orthodontic adhesives containing BPA have potential short- and long-term adverse biological effects in humans. Careful consideration should therefore be given to bonding, polymerization and debonding protocols in orthodontics.

Bisphenol A release from orthodontic adhesives and its correlation with the degree of conversion

INTRODUCTION

Our objective was to quantitatively assess and compare the bisphenol A (BPA) released from an orthodontic adhesive using a light-emitting diode device (LED) or a halogen light-curing unit (HLC) at 3 tip-to-bracket distances (0, 5, and 10 mm) and varying curing times using high-performance liquid chromatography. BPA release with self-etching and moisture-insensitive primers with light-cured and chemically cured composites was also evaluated. BPA release was correlated to the corresponding degree of conversion.

METHODS

Our sample consisted of 598 stainless steel first premolar brackets. Of these, 520 were used for assessing BPA release and divided into 13 groups of 40 each. In groups I, II, and III, the composite was cured with the LED for 20 seconds at distances of 0, 5, and 10 mm, respectively. Groups IV, V, and VI were cured with the HLC for 40 seconds at the same 3 distances. Groups VII and VIII were cured for 5 and 10 seconds with the LED, and groups IX and X were cured for 10 and 20 seconds with the HLC at 0-mm distance. Groups XI, XII, and XIII consisted of brackets bonded with a self-etching primer and Transbond (3M Unitek, Monrovia, Calif), with a moisture-insensitive primer and Transbond, and with a chemically cured composite. The remaining 78 brackets were also divided into 13 groups and used for assessing the degree of conversion.

RESULTS

The LED devices demonstrated significantly less BPA release and greater degrees of conversion (P <0.05). For both units, BPA release increased and the degree of conversion decreased as the tip distance increased and curing time decreased. The chemically cured group showed significantly less BPA release (P <0.05). Among the light-cured composites, those cured according to the manufacturers' recommendations (40 seconds and 0-mm distance for the HLC unit) released less BPA than did the self-etching primer and the moisture-insensitive primer. The degree of conversion was greatest for the chemically cured composite, whereas it was similar for the conventional, self-etching primer, and moisture-insensitive primer groups. However, correlations ranged from strongly negative to weakly positive between BPA release and degree of conversion.

CONCLUSIONS

Clinicians should consider using LEDs in clinical practice and should keep the light-cure tip as close to the bracket as clinically possible. Curing time should be according to the manufacturer's recommendations. These steps will ensure less BPA release and a greater degree of conversion. Since chemically cured composites had less BPA release and a greater degree of conversion, they can be considered superior to light-cured composites in this aspect.

Time reduction of light curing: Influence on conversion degree and microhardness of orthodontic composites

INTRODUCTION

The aim of this study was to assess the influence of curing time and power on the degree of conversion and surface microhardness of 3 orthodontic composites.

METHODS

One hundred eighty discs, 6 mm in diameter, were divided into 3 groups of 60 samples according to the composite used-Transbond XT (3M Unitek, Monrovia, Calif), Opal Bond MV (Ultradent, South Jordan, Utah), and Transbond Plus Color Change (3M Unitek)- and each group was further divided into 3 subgroups (n = 20). Five samples were used to measure conversion, and 15 were used to measure microhardness. A light-emitting diode curing unit with multiwavelength emission of broad light was used for curing at 3 power levels (530, 760, and 1520 mW) and 3 times (8.5, 6, and 3 seconds), always totaling 4.56 joules. Five specimens from each subgroup were ground and mixed with potassium bromide to produce 8-mm tablets to be compared with 5 others made similarly with the respective noncured composite. These were placed into a spectrometer, and software was used for analysis. A microhardness tester was used to take Knoop hardness (KHN) measurements in 15 discs of each subgroup. The data were analyzed with 2 analysis of variance tests at 2 levels.

RESULTS

Differences were found in the conversion degree of the composites cured at different times and powers (P <0.01). The composites showed similar degrees of conversion when light cured at 8.5 seconds (80.7%) and 6 seconds (79.0%), but not at 3 seconds (75.0%). The conversion degrees of the composites were different, with group 3 (87.2%) higher than group 2 (83.5%), which was higher than group 1 (64.0%). Differences in microhardness were also found (P <0.01), with lower microhardness at 8.5 seconds (35.2 KHN), but no difference was observed between 6 seconds (41.6 KHN) and 3 seconds (42.8 KHN). Group 3 had the highest surface microhardness (35.9 KHN) compared with group 2 (33.7 KHN) and group 1 (30.0 KHN).

CONCLUSIONS

Curing time can be reduced up to 6 seconds by increasing the power, with a slight decrease in the degree of conversion at 3 seconds; the decrease has a positive effect on the surface microhardness.

Effect of light-activation with different light-curing units and time intervals on resin cement bond strength to intraradicular dentin

The aim of this study was to assess the bond strength of a resin cement to intraradicular dentin varying the light-curing unit and the moment at which the light was applied. Post spaces of endodontically treated canines were prepared. The roots were distributed into 6 groups (n=10) according to the light-curing unit and the moment of light exposure: I) Quartz tungsten halogen-600 mW/cm² (QTH) + immediate light activation (t0); II) QTH + light activation after 10 min (t10); III) Light-emitting diodes (LED)-800 mW/cm² (LED-800)+ t0; IV) LED-800 + t10; V) LED-1,500 mW/cm² (LED-1500)+ t0; VI) LED-1500 + t10. After post cementation, slices from coronal, middle and apical post/root regions were submitted to the push-out test and failure evaluation. It was verified that LED-800 (4.40 ± 3.00 MPa) and LED-1500 (4.67 ± 3.04 MPa) provided bond strength statistically superior to QTH (3.13 ± 1.76 MPa) (p<0.05), and did not differ from each other (p>0.05). There was no significant difference between t0 and t10 (p>0.05). Coronal post/root region (4.75 ± 3.10 MPa) presented significantly higher bond strength than the apical (3.32 ± 2.30 MPa) (p<0.05) and middle regions (4.14 ± 2.99 MPa) showed intermediate values. Adhesive failures were predominant when using QTH. Adhesive and mixed failures occurred more frequently in the apical region. Higher adhesion of the resin cement to intraradicular dentin was observed in the coronal region with LED light-activation, regardless of the moment of light exposure.