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.

Highlights

  • Light-emitting diodes showed significantly less bisphenol A (BPA) release and greater degree of conversion.

  • BPA release increased and degree of conversion decreased as distance increased and time decreased.

  • Chemically cured composites had less BPA release.

  • Degree of conversion was greatest for chemically cured composites and similar for the other groups.

  • Correlations between BPA release and degree of conversion were strongly negative to weakly positive.

One of the most dramatic changes in the orthodontic specialty in the 1970s was the use of composite resin as a bonding material. Both light-cured and chemically cured composites have been shown to be clinically acceptable and effective. However, curing lights are required for light-cured resins. To obtain the correct irradiance using a halogen lamp, the initial power must be considerably high, producing heat that can cause pulpal damage. As an alternative to halogen light curing units (HLC), light emitting diode (LED) technology has been proposed for curing dental composites. The curing time recommended is between 5 and 20 seconds.

Bisphenol A (BPA) is used as a raw material for the formulation of Bisphenol A diglycidyl dimethacrylate (Bis-GMA). Although bond strength has been evaluated, BPA release has not been extensively studied especially in self-etching primers (SEP) and moisture-insensitive primers (MIP); Eliades et al reported no traces, whereas Gioka et al found that the leachable components involved exclusively the TEGDMA monomer.

In general, the degree of conversion of adhesives modulates the physical and mechanical properties of the material, particularly solubility and degradation. Decreased conversion results in monomer leaching and the release of substances such as plasticizers and polymerization initiators and inhibitors. These can inflict detrimental biologic effects in cell cultures.

The light-curing unit tip should ideally be in direct contact with the resin composite; however, this is not always clinically possible. Inability to place a light tip near the bonding material might reduce intensity and provide a lower degree of polymerization. Hence, the aim of this study was to quantitatively assess and compare the BPA released from an orthodontic adhesive (Transbond XT; 3M Unitek, Monrovia, Calif) when using LEDs and HLCs at 3 tip distances (0, 5, and 10 mm) and varying curing times with high-performance liquid chromatography. BPA release from chemically cured composites was also assessed. The degree of conversion was assessed using Fourier transform infrared spectroscopy. The BPA release was also correlated to the corresponding degree of conversion. The hypothesis was that BPA release and degree of conversion depend on the light-cure tip-to-bracket distance, curing time, and curing mechanism.

Material and methods

We obtained 598 first premolar stainless steel brackets (3M Unitek); 520 of these were used to assess BPA release with high-performance liquid chromatography and 78 for estimating the degree of conversion with the Fourier transform infrared spectrometer (Perkin-Elmer, Norwalk, Conn). For the BPA release, the 520 brackets were divided into 13 groups of 40 each, with groups I, II, and III bonded with Transbond XT (3M Unitek) and cured with an LED (Elipar Free Light 2; 3M ESPE, Seefeld, Germany), with intensity of 1100 to 1200 mW per square centimeter for 20 seconds at light-cure tip-to-bracket distances of 0, 5, and 10 mm, respectively. Groups IV, V, and VI were bonded with Transbond XT and cured with an HLC (Elipar 2500; 3M ESPE) with intensity of 450 to 510 mW per square centimeter for 40 seconds also at light-cure tip-to-bracket distances of 0, 5, and 10 mm, respectively. The varying distances were standardized by following the procedure suggested by Sunitha et al. 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 at a distance of 0 mm with the HLC. Groups XI, XII, and XIII consisted of brackets bonded with SEP (3M Unitek) and Transbond, MIP (3M Unitek) and Transbond, and a chemically cured composite (3M Unitek). The procedure used for assessing BPA release was similar to that described by Eliades et al and subsequently followed by Sunitha et al. Briefly, the bonded brackets were stored in an incubator for 5 minutes at 37°C and 50% relative humidity and immersed in sterile glass tubes containing 15 mL of absolute alcohol (99% v/v) to induce accelerated aging. During the immersion period, the solution was agitated for 10 seconds twice a day. The processed 50 μL of alcohol solution from each group was removed at days 1, 7, 21, and 35 and filtered through 0.2-μm nylon filter paper injected in the instrument and analyzed with high-performance liquid chromatography (model LC2010A-HT; Shimadzu, Kyoto, Japan). The column was calibrated with known concentrations (standards, 0.1-3 ppm) of BPA (Merck, Darmstadt, Germany) in ethanol. The remaining 78 brackets were divided into 13 similar groups of 6 brackets each and used for assessing the degree of conversion. Sample preparation and degree of conversion assessment were similar to those described by Gioka et al. Briefly, after polymerization at room temperature, the bracket-bonded adhesive, which in clinical conditions corresponds to the material in contact with enamel, was measured, and 5 mg of resin discs was placed between 0.01-mm potassium bromide discs. The discs were then transferred to the Fourier transform infrared spectrometer. The degree of conversion was estimated on a relative percentage basis with the 2-frequency method and tangent baseline technique.

Statistical analysis

Statistical analysis for the BPA release was performed using 2-way analysis of variance (ANOVA), post hoc multiple comparisons Tukey HSD tests, and the paired t test. The samples obtained from the degree of conversion were analyzed separately with 1-way ANOVA. The Pearson correlation was used to assess the relationship between BPA release and degree of conversion on day 1.

Results

The BPA release increased from day 1 to day 21, after which the levels mostly decreased ( Table I ). The least BPA release was observed with chemically cured group (group XIII) and then with the LED and HLC groups when used according to the manufacturers’ recommendations (groups I and IV). Greater releases of BPA were observed for the 10-mm LED and HLC tip distances (groups III and VI); the LED group showed less BPA release than did the HLC group for corresponding tip distances ( Table I ). There was a significant reduction ( P <0.05) in BPA for the LED group when compared with corresponding curing times and tip distances with the HLC group ( Table II ). The magnitudes of BPA release were more or less similar between the SEP and MIP groups ( Table III ).

Table I
Means and standard deviations (in ppm) of BPA levels on days 1, 7, 21, and 35
Group Day 1 Day 7 Day 21 Day 35
Mean ± SD Mean ± SD Mean ± SD Mean ± SD
Group I: LED, 0 mm distance, 20 s curing time 5.30 ± 0.01 7.11 ± 0.00 9.29 ± 0.00 8.97 ± 0.00
Group II: LED, 5 mm distance, 20 s curing time 7.46 ± 0.00 11.84 ± 0.00 12.63 ± 0.00 11.78 ± 0.00
Group III: LED, 10 mm distance, 20 s curing time 10.62 ± 0.01 12.39 ± 0.00 14.72 ± 0.00 13.56 ± 0.00
Group IV: HLC, 0 mm distance, 40 s curing time 12.52 ± 0.01 14.50 ± 0.07 16.00 ± 0.02 16.23 ± 0.00
Group V: HLC, 5 mm distance, 40 s curing time 16.50 ± 0.03 19.61 ± 0.03 21.56 ± 0.01 20.58 ± 0.02
Group VI: HLC, 10 mm distance, 40 s curing time 17.71 ± 0.03 23.98 ± 0.01 25.28 ± 0.00 23.34 ± 0.01
Group VII: LED, 0 mm distance, 5 s curing time 14.92 ± 0.00 16.10 ± 0.00 19.52 ± 0.00 18.21 ± 0.00
Group VIII: LED, 0 mm distance, 10 s curing time 12.49 ± 0.00 14.63 ± 0.01 16.66 ± 0.00 15.67 ± 0.00
Group IX: HLC, 0 mm distance, 10 s curing time 17.73 ± 0.00 21.35 ± 0.00 23.77 ± 0.00 21.10 ± 0.00
Group X: HLC, 0 mm distance, 20 s curing time 16.73 ± 0.00 19.49 ± 0.01 21.94 ± 0.00 18.86 ± 0.00
Group XI: SEP and Transbond 23.70 ± 0.14 24.26 ± 0.15 24.25 ± 0.26 22.25 ± 0.15
Group XII: MIP and Transbond 20.37 ± 0.12 21.25 ± 0.26 22.25 ± 0.19 17.35 ± 0.23
Group XIII: chemically cured composite 0.18 ± 0.00 0.27 ± 0.00 0.32 ± 0.01 0.29 ± 0.00

Table II
Post hoc multiple comparisons with the Tukey HSD test, comparisons of BPA release with LED and HLC units on each of the 4 days at light-cure tip-to-bracket distances of 0, 5, and 10 mm and curing times of 5, 10, and 20 seconds for LED and 10, 20, and 40 seconds for HLC ( P <0.05 is significant)
Distance – Distance per day LED HLC Time – Time per day LED Time – Time per day HLC
Mean diff Sig Mean diff Sig Mean diff Sig Mean diff Sig
Day 1 Day 1 Day 1
0 mm 5 mm −2.16 0.00 −3.98 0.00 5 s 10 s 2.43 0.00 10 s 20 s 1.00 0.00
10 mm −5.32 0.00 −5.19 0.00 20 s 9.62 0.00 40 s 5.21 0.00
5 mm 10 mm 2.16 0.00 3.98 0.00 10 s 5 s −2.43 0.00 20 s 10 s −1.00 0.00
10 mm −3.16 0.00 −1.21 0.00 20 s 7.19 0.00 40 s 4.21 0.00
10 mm 0 mm 5.32 0.00 5.19 0.00 20 s 5 s −9.62 0.00 40 s 10 s −5.21 0.00
5 mm 3.16 0.00 1.21 0.00 10 s −7.19 0.00 20 s −4.21 0.00
Day 7 Day 7 Day 7
0 mm 5 mm −4.73 0.00 −5.11 0.00 5 s 10 s 1.46 0.00 10 s 20 s 1.86 0.00
10 mm −5.28 0.00 −9.49 0.00 20 s 8.98 0.00 40 s 6.85 0.00
5 mm 0 mm 4.73 0.00 5.11 0.00 10 s 5 s −1.46 0.00 20 s 10 s −1.86 0.00
10 mm −0.55 0.00 −4.38 0.00 20 s 7.52 0.00 40 s 4.99 0.00
10 mm 0 mm 5.28 0.00 9.49 0.00 20 s 5 s −8.98 0.00 40 s 10 s −6.85 0.00
5 mm 0.55 0.00 4.38 0.00 10 s −7.52 0.00 20 s −4.99 0.00
Day 21 Day 21 Day 21
0 mm 5 mm −3.34 0.00 −5.56 0.00 5 s 10 s 2.86 0.00 10 s 20 s 1.84 0.00
10 mm −5.43 0.00 −9.23 0.00 20 s 10.23 0.00 40 s 7.77 0.00
5 mm 10 mm 3.34 0.00 5.56 0.00 10 s 5 s −2.86 0.00 20 s 10 s −1.84 0.00
10 mm −2.10 0.00 −3.72 0.00 20 s 7.37 0.00 40 s 5.94 0.00
10 mm 0 mm 5.43 0.00 9.28 0.00 20 s 5 s −10.2 0.00 40 s 10 s −7.77 0.00
5 mm 2.10 0.00 3.72 0.00 10 s −7.37 0.00 20 s 5.94 0.00
Day 35 Day 35 Day 35
0 mm 5 mm −2.81 0.00 −4.35 0.00 5 s 10 s 2.54 0.00 10 s 20 s 2.24 0.00
10 mm −4.59 0.00 −7.12 0.00 20 s 9.24 0.00 40 s 4.87 0.00
5 mm 0 mm 2.81 0.00. 4.35 0.00 10 s 5 s −2.54 0.00 20 s 10 s −2.23 0.00
10 mm −1.78 0.00 −2.77 0.00 20 s 6.70 0.00 40 s 2.64 0.00
10 mm 0 mm 4.59 0.00 7.12 0.00 20 s 5 s −9.24 0.00 40 s 10 s −4.87 0.00
5 mm 1.78 0.00 2.77 0.00 10 s −6.70 0.00 20 s −2.64 0.00
diff , Difference; Sig , significance.

Table III
Multiple comparisons for light-cured and chemically cured composites with Tukey HSD on day 1 ( P <0.05 is significant)
Group Mean difference Significance
Group IV Group XI −11.18 0.00
Group XII −7.85 0.00
Group XIII 12.34 0.00
Group XI Group IV 11.18 0.00
Group XII 3.33 0.00
Group XIII 23.52 0.00
Group XII Group IV 7.85 0.00
Group XI −3.33 0.00
Group XIII 20.19 0.00
Group XIII Group IV −12.34 0.00
Group XI −23.52 0.00
Group XII −20.19 0.00

As with BPA release, there was a significant reduction ( P <0.05) in degree of conversion with the LED when compared with corresponding curing times and distances of the HLC. A lower degree of conversion for the light-cured systems when compared with the chemically cured group was observed. The degree of conversion was greatest for the chemically cured group, followed by the MIP group, the conventionally bonded light-cured group, and the SEP group ( Table IV ). Increased distance and decreased curing time led to reduced degrees of conversion ( Tables V-VIII ). Although the degrees of conversion were more or less similar between the SEP and the MIP groups, there was a statistical difference ( Table IX ). The correlations between BPA release and degree of conversion ranged from strongly negative to weakly positive, but statistical significance could not be established for many of the correlations ( Tables X and XI ).

Table IV
Means and standard deviations of the percentages of degree of conversion (n = 6)
Group Mean (%) ± SD
Group I: LED, 0 mm distance, 20 s curing time 26.46 ± 1.56
Group II: LED, 5 mm distance, 20 s curing time 13.48 ± 0.41
Group III: LED, 10 mm distance, 20 s curing time 11.51 ± 0.44
Group IV: HLC, 0 mm distance, 40 s curing time 25.50 ± 0.73
Group V: HLC, 5 mm distance, 40 s curing time 3.28 ± 0.14
Group VI: HLC, 10 mm distance, 40 s curing time 2.68 ± 0.22
Group VII: LED, 0 mm distance, 5 s curing time 4.91 ± 0.39
Group VIII: LED, 0 mm distance, 10 s curing time 10.55 ± 0.52
Group IX: HLC, 0 mm distance, 10 s curing time 2.30 ± 0.12
Group X: HLC, 0 mm distance, 20 s curing time 5.33 ± 0.42
Group XI: SEP and Transbond 24.66 ± 0.57
Group XII: MIP and Transbond 26.15 ± 0.71
Group XIII: chemically cured composite 89.54 ± 1.56

Table V
Post hoc multiple comparisons Tukey HSD test: comparison of percentage of DC using LED and HLC at 0, 5, and 10 mm light cure tip-bracket distances at constant curing times of 20 and 40 seconds, and comparison of percentages of DC for LED (5, 10, and 20 seconds) and HLC (10, 20, and 40 seconds) at 0 mm light cure tip-bracket distance ( P <0.05 is significant)
Group – Group for distance LED, 20 s HLC, 40 s Group – Group for time LED at 5, 10, and 20 s HLC at 10, 20, and 40 s
Mean diff Sig Mean diff Sig Mean diff Sig Mean diff Sig
0 mm 5 mm 12.98 0.00 22.22 0.00 5 s 10 s −5.64 0.00 −3.03 0.00
10 mm 14.95 0.00 22.82 0.00 20 s −21.55 0.00 −23.20 0.00
5 mm 0 mm −12.98 0.00 −22.2 0.00 10 s 5 s 5.64 0.00 3.03 0.00
10 mm 1.97 0.01 0.60 0.43 20 s −15.91 0.00 −20.17 0.00
10 mm 0 mm −14.95 0.00 −22.82 0.00 20 s 5 s 21.55 0.00 23.20 0.00
5 mm −1.97 0.01 −0.60 0.43 10 s 15.91 0.00 20.17 0.00
diff , Difference; Sig , significance.

P <0.05 is significant.

Table VI
Paired t test for degree of conversion with HLC (40 seconds) and LED (20 seconds) for tip distances of 0, 5, and 10 mm ( P <0.05 is significant)
t test for equality of means
Levene test for equality of variances t Df Sig 2 tail Mean diff 95% CI
F Sig Lower Upper
0 mm, equal variances assumed 0.42 0.53 −1.148 10 0.28 −0.976 −2.87 0.92
Equal variances not assumed −1.148 9.856 0.28 −0.976 −2.87 0.922
5 mm, equal variances assumed 2.59 0.14 −56.388 10 0.00 −10.21 −10.61 −9.80
Equal variances not assumed −56.388 6.189 0.00 −10.21 −10.65 −9.77
10 mm, equal variances assumed 2.25 0.16 −43.401 10 0.00 −8.83 −9.28 −8.37
Equal variances not assumed −43.401 7.486 0.00 −8.83 −9.30 −8.35
Diff , Difference; Sig, significance.

Table VII
Paired t test for degree of conversion with LED (5, 10, and 20 seconds) and HLC (10, 20, and 40 seconds) at 0-mm tip distance ( P <0.05 is significant)
t test for equality of means
Levene test for equality of variances t Df Sig 2 tail Mean diff 95% CI
F Sig Lower Upper
10 s HLC and 5 s LED
Equal variances assumed 3.448 0.09 −15.227 10 0.00 −2.61 −2.99 −2.23
Equal variances not assumed −15.227 6.022 0.00 −2.61 −3.03 −2.19
20 s HLC and 10 s LED
Equal variances assumed 0.497 0.50 −18.936 10 0.00 −5.22 −5.83 −4.60
Equal variances not assumed −18.936 9.652 0.00 −5.22 −5.83 −4.60
40 s HLC and 20 s LED
Equal variances assumed 0.415 0.53 −1.148 10 0.278 −0.977 −2.87 0.92
Equal variances not assumed −1.148 9.856 0.278 −0.977 −2.88 0.92
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Apr 6, 2017 | Posted by in Orthodontics | Comments Off on Bisphenol A release from orthodontic adhesives and its correlation with the degree of conversion
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