Introduction
The aims of the study were to evaluate and compare the bisphenol A (BPA) levels in saliva in patients using vacuum-formed retainers or Hawley retainers.
Methods
Forty-five patients who had completed their fixed orthodontic treatment and were entering the retention phase were randomly allocated into 1 of 3 groups: vacuum-formed retainer, Hawley retainer fabricated by heat cure, and Hawley retainer fabricated by chemical cure. Saliva samples were collected immediately before placement, within 1 hour after placement, 1 week and 1 month after placement. Statistical analyses were performed by using 2-way analysis of variance and post-hoc multiple-comparisons Tukey HSD tests.
Results
Statistically significant BPA levels in saliva were found for all groups ( P ≤0.05). The highest levels were noted in the vacuum-formed retainer group, followed by Hawley retainers fabricated by chemical cure; the lowest levels were found with Hawley retainers fabricated by heat cure.
Conclusions
With many options available for removable retainers, clinicians should consider the BPA release from these retainers; a Hawley retainer fabricated by heat cure is a favorable choice.
Highlights
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Retainers fabricated from thermoplastic materials could release BPA.
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BPA in saliva was assessed with vacuum-formed, heat-, and chemical-cure retainers.
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BPA levels increased in all groups after placement of the retainer.
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BPA level in saliva was greatest for vacuum-formed retainers.
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BPA level in salive was lowest for heat-cure Hawleys retainers.
Retention, described as “secondary orthodontic treatment,” is a key step to successful treatment. To prevent relapse, almost every person who has orthodontic treatment will require some type of retention. Commonly used removable retainers are vacuum-formed retainers (VFRs) and Hawley retainers; they differ from each other in composition and fabrication. Although Hawley retainers can be fabricated using either heat cure or chemical cure acrylic and are composed of polymethyl methacrylate, VFRs, introduced in the 1970s by Ponitz, are composed of polypropylene/polyethylene. Hawley retainers have an inherent disadvantage of unaesthetic display of wires on the labial surface of teeth, but VFRs have certain advantages such as durability, esthetics, ease of maintenance, reduced fabrication time and cost, ability to produce minor tooth movement, and ability to serve as carriers for bleaching solutions.
The unease about the release of chemicals from various appliances has grown over recent decades. The World Health Organization in 2011 listed endocrine disruptive chemicals; one of them was bisphenol A (BPA), a synthetic compound that has gained medical attention because of its estrogenic action.
The increased emphasis on BPA release can be attributed to the fact that it affects various organs, has teratogenic effects even at a low dose, results in early onset of puberty, causes feminization in males, and also has carcinogenic effects. Animal studies have shown the presence of BPA in body fluids including urine, adult and fetal blood, amniotic fluid, placental tissues, breast milk, and saliva.
Chemicals such as hydroquinone, present in the liquid component of the Hawley retainer’s base-plate material, are BPA-related products. BPA is added to polymers to make them clear and tough. Therefore, VFRs fabricated of clear thermoplastic material could release BPA. The possible BPA release from these materials would be into the saliva; hence, saliva was chosen as a medium for the analysis of BPA. The added advantages are that it is a noninvasive method of collection and the first diluent of the human body, and it allows long periods of analysis.
Although in-vitro and in-vivo studies have been performed to assess BPA release from composite resins, pit-and-fissure sealants, and lingual bonded retainers, to the best of our knowledge, VFRs and Hawley retainers have not been studied especially in an in-vivo environment. Heat-cure and chemical-cure materials were chosen because of their different methods of processing, the amount of residual monomer, and the filler. There are also overlapping indications for removable retainers, but few studies have evaluated the biocompatibility of retainers.
Thus, the aims of this study were to evaluate and compare the salivary levels of BPA released from VFRs and Hawley retainers.
Material and methods
Forty-five patients (power, 90%; α = 0.05) from the Department of Orthodontics at Sri Ramachandra University, Porur, Chennai, India, who had completed their fixed orthodontic treatment and required removable retainers were selected for this study. After debonding, the cleanup procedure was performed using an 8-fluted tungsten carbide bur on a high-speed hand piece. These patients were randomly allocated to 3 groups of 15 each. Patients in group I were given VFRs, those in group II received Hawley retainers processed by the heat-cure method, and those in group III had Hawley retainers processed by the chemical-cure method. The same type of retainer was used for both arches. The retainers were inserted 24 hours after debonding. Regular oral hygiene instructions were given to the patients, and they were advised to wear the retainers at all times except during eating and brushing. They were also instructed to maintain charts to note the duration of wear of the retainer.
The VFR (Essix ACE Plastic; Dentsply International, Sarasota, Fla) material had a thickness of 1 mm and was formed according to the manufacturer’s instructions with a Biostar vacuum thermoforming system (model 3010; Scheu-Dental, Iserlohn, Germany). For the Hawley retainer processed by heat cure (DPI Heat Cure; DPI India, Mumbai, India), the compression molding technique was used. The powder-liquid ratio was 3:1, and the heat cure was processed at a temperature of 100°C for 1.5 hours. It was then bench cured for 2 hours before deflasking. For the Hawley retainer processed by the chemical-cure method (DPI Cold Cure; DPI India), the “sprinkle on” technique was used. The uniformity of the thickness of the base plate was ensured by measuring it with a Vernier caliper. The retainers were trimmed to remove the excess material and polished with pumice before insertion.
Four samples of stimulated whole saliva were collected from each patient at the following time intervals: T0, just before placement of the retainer; T1, within 1 hour after placement of the retainer; T2, 7 days after placement of the retainer; and T3, 30 days after placement of the retainer.
The 180 samples collected from 45 patients at 4 time intervals were stored at −80°C in a freezer (ULT freezer; Thermo Electron, Madison, Wis). The BPA levels in the saliva were estimated according to the method described by Eliades et al using high-performance liquid chromatography. Briefly, the sample was processed by mixing and vortexing 5 mL of saliva with an equal quantity of ethanol. The samples were centrifuged at 3000 rpm for 15 minutes to settle the particulate matter. From the processed sample, 50 μL was collected and injected in the instrument and analyzed with high-performance liquid chromatography. These assays were performed in triplicate for each time period, and the results were averaged.
Statistical analysis
The collected data were analyzed with SPSS software for Windows (version 16.0; SPSS, Chicago, Ill). For the multivariate analysis, 1-way analysis of variance with the Tukey post-hoc test was used. For repeated measures (T0, T1, T2, and T3), analysis of variance with adjustment for multiple comparisons was used. To control for type I error, the Bonferroni test was used.
Results
BPA levels in saliva were greater in groups I and III and least in group II ( Table I ; Fig ). In group I, there were an increase in the BPA levels from T0 to T1 (+1.20 ppm) and a further increase from T1 to T2 (+1.18 ppm) followed by a decrease from T2 to T3 (−2.18 ppm). However, in groups II and III, there was an increase in the BPA from T0 to T1 followed by a decrease from T1 to T2 but an increase from T2 to T3 ( Table II ). Statistical significance ( P ≤0.05) between all time points was observed only in group III ( Table III ).
| Retainer | Mean ± SD (ppm) |
|---|---|
| T0 | |
| Group I | 0.00001 ± 0.00001 |
| Group II | 0.00006 ± 0.00004 |
| Group III | 0.00009 ± 0.00006 |
| T1 | |
| Group I | 1.20236 ± 0.35643 |
| Group II | 0.00091 ± 0.00081 |
| Group III | 0.06031 ± 0.02550 |
| T2 | |
| Group I | 2.38420 ± 1.79714 |
| Group II | 0.00045 ± 0.00008 |
| Group III | 0.00363 ± 0.00050 |
| T3 | |
| Group I | 0.20396 ± 0.08709 |
| Group II | 0.00067 ± 0.001410 |
| Group III | 0.00934 ± 0.00237 |
| Dependent variable | Mean difference (ppm) | SE | Significance | |
|---|---|---|---|---|
| T0 | ||||
| Group I | Group II | −0.0000452045 | 1.5489E-05 | 0.02 |
| Group III | Group I | −0.0000812473 | 1.5489E-05 | 0 |
| Group II | Group III | −3.60428E–05 | 1.5489E-05 | 0.06 |
| T1 | ||||
| Group I | Group II | 1.2014493801 | 0.07533513 | 0 |
| Group III | Group I | 1.1420526197 | 0.07533513 | 0 |
| Group II | Group III | −0.059396761 | 0.07533513 | 0.71 |
| T2 | ||||
| Group I | Group II | 2.3837453839 | 0.37887135 | 0 |
| Group III | Group I | 2.3805638183 | 0.37887135 | 0 |
| Group II | Group III | −0.003181566 | 0.37887135 | 1 |
| T3 | ||||
| Group I | Group II | 0.2032902702 | 0.01837029 | 0 |
| Group III | Group I | 0.1946176207 | 0.01837029 | 0 |
| Group II | Group III | −0.00867265 | 0.01837029 | 0.89 |
| T0/T1 | T0/T2 | T0/T3 | T1/T2 | T1/T3 | T2/T3 | |
|---|---|---|---|---|---|---|
| Group I | ||||||
| Mean difference | −1.202 | −2.384 | −0.204 | −1.18 | 0.998 | 2.180 |
| SE | 0.092 | 0.464 | 0.022 | 0.43 | 0.098 | 0.457 |
| Significance | 0 | 0.001 | 0 | 0.094 | 0 | 0.002 |
| Group II | ||||||
| Mean difference | −0.001 | 0.000 | −0 | 0 | 0 | 0 |
| SE | 0 | 0 | 0 | 0 | 0 | 0 |
| Significance | 0.007 | 0 | 0.665 | 0.264 | 1 | 1 |
| Group III | ||||||
| Mean difference | −0.060 | −0.004 | −0.009 | 0.057 | 0.051 | −0.006 |
| SE | 0.007 | 0 | 0.001 | 0.007 | 0.007 | 0.001 |
| Significance | 0 | 0 | 0 | 0 | 0 | 0 |
Supplemental data reporting the results for each patient are also available.
Results
BPA levels in saliva were greater in groups I and III and least in group II ( Table I ; Fig ). In group I, there were an increase in the BPA levels from T0 to T1 (+1.20 ppm) and a further increase from T1 to T2 (+1.18 ppm) followed by a decrease from T2 to T3 (−2.18 ppm). However, in groups II and III, there was an increase in the BPA from T0 to T1 followed by a decrease from T1 to T2 but an increase from T2 to T3 ( Table II ). Statistical significance ( P ≤0.05) between all time points was observed only in group III ( Table III ).
