Introduction
The objective of this study was to test the hypothesis that toxic metallic ions in silver solder used in orthodontics are released into saliva.
Methods
The sample included 60 children, 8 to 14 years of age, divided into 2 groups (n = 30 in each group): the control group and the study group (needing maxillary expansion with the hyrax appliance). For analysis of the release of metallic ions, saliva samples from each patient were collected at 6 times in both groups: before placement of the appliance and 10 minutes, 24 hours, and 7, 30, and 60 days after placement. The analysis of saliva was performed by atomic absorption spectrophotometry in a graphite oven to determine the concentrations of cadmium, copper, zinc, and silver ions. Statistical analyses were performed by the mixed model for repeated-measures covariance structure of the symmetrical component type, at P <0.05.
Results
The ion concentrations in the control group had low values for cadmium, copper, and zinc ( P >0.05); for silver ions, the values did not reach the detection limit. In the study group, all ions showed expressive concentrations at 10 minutes after placement of the appliance ( P <0.001), with the highest mean for copper (70.60 μg/L) and the lowest mean for zinc (0.07 μg/L). Comparison between groups revealed significant differences for copper (all periods), zinc (10 minutes, 24 hours, 7 and 30 days), and for cadmium only at 10 minutes.
Conclusions
Great amounts of these ions were released, with the highest concentrations immediately after placement of the appliance.
The biocompatibility of some materials in orthodontics, such as metals, acrylics, latex, and dyes, has been a matter of concern for a long time. In relation to the metals, most alloys have similar compositions as stainless steel, and many devices such as facemasks, bands, and brackets use some type of solder in the fabrication process. The solder alloys most commonly used are made of silver, copper, and zinc. According to the International Register of Potentially Toxic Chemicals of the United Nations Environment Program, these metallic ions might be dangerous chemical products, because they are included in the list of substances and processes with great risks for human life.
Although interactions between an alloy and living tissues could be measured in many ways, the release of elements from alloys in the oral cavity is the primary focus of this study, since adverse biologic effects such as allergies and inflammations have been attributed to this process. Several studies have addressed the release of metallic ions from orthodontic brackets, especially iron, chromium, and nickel, which are the main products of corrosion of stainless steel. Even though these 3 elements determine adverse effects, nickel has drawn special attention because it is the main component of most metallic alloys in orthodontics and is an important nutrient in the diet. However, studies have demonstrated that other ions in silver solder can be released into the oral cavity, and exposure to them might cause several adverse effects with direct toxic alterations in an acute or a chronic manner. Metallic ions such as cadmium, copper, silver, and zinc are potentially dangerous chemicals that pose high risks, because of their carcinogenic potentials to the lungs, prostate gland, and kidneys, in addition to alterations in the hematopoietic, urinary, and digestive systems.
Despite our knowledge of the effects of metallic ions in the environment or some industrialized products, the presence of these ions in solders used in orthodontics and consequently in the oral cavity is almost not addressed in the literature, suggesting that they are not a source of contamination. Also, studies on this issue have presented few results of in-vitro studies and even fewer from in-vivo studies, leaving unanswered questions about their toxicity. Thus, in this study, we evaluated the toxicity of silver solder used in orthodontics by measuring the release of cadmium, copper, zinc, and silver in saliva.
Material and methods
This study was revised and approved by the Institutional Review Board of the Pontifical Catholic University, Porto Alegre, Rio Grande do Sul, Brazil, under protocol n. 1148/05-CEP.
The sample was composed of 60 subjects, according to the following inclusion criteria: age between 8 and 14 years, no metallic restorations, and no orthodontic appliances.
The sequence for fabrication of the hyrax appliance was the following: (1) placement of separating elastics (Dentaurum, Ispringen, Germany) on the proximal aspects of first deciduous molars or first premolars and maxillary first permanent molars; (2) selection of orthodontic bands (Morelli, São Paulo, SP, Brazil) and transfer impression with alginate (Jeltrate, Dentsply, Milford, Del); (3) fixation of bands with wax (Kerr, Los Angeles, Calif) in the impression; (4) pouring with special white stone (Mossoró, Guarulhos, SP, Brazil); (5) fabrication of the hyrax appliance by using round 1-mm archwire (Morelli, São Paulo, SP, Brazil), an expanding screw (11 mm), silver solder, and flow (Morelli, São Paulo, SP, Brazil); (6) washing the appliances with a brush and soap and then drying them; (7) prophylaxis and placement of cotton rolls in the maxillary arch; (8) cementation of the hyrax appliance with Ketac Cem (3M ESPE, St Paul, Minn).
The 360 glass flasks used for saliva collection were prepared and identified with tags indicating patient number, period of collection, and ions to be analyzed.
For the ion release analysis, saliva samples were collected from each patient at 6 times for both groups: before placement of the appliance, at 10 minutes, 24 hours, and 7, 30, and 60 days after placement of the appliance. The method used for saliva collection is known as the “spitting” method, in which the patient accumulates a certain quantity of saliva in the oral cavity and then spits it into a receptacle.
Collections were made in the morning; the patients were asked to rinse the mouth with deionized water for 30 seconds and spit. Then saliva was accumulated in the mouth for 5 minutes without stimulation and then spit into the previously prepared and identified flask. The collected saliva was maintained in a closed flask at 3°C until the laboratory analysis.
The collected saliva was evaluated by using atomic absorption spectrophotometry in a graphite oven (Analyst 800, Perkin Elmer, Waltham, Mass) to determine the concentrations of copper, zinc, and silver ions; although it was not reported in the composition chart supplied by the manufacturer, cadmium ions were also analyzed, with detection limits of 0.30, 0.005, 0.05, and 0.10 μg per liter, respectively.
Statistical analysis
The mixed model for repeated-measures covariance structure of the symmetrical component type was used with software (version S.0.2, SAS, Cary, NC).
Results
The ion concentrations in the control group showed reduced values for cadmium, copper, and zinc ions ( Tables I-III ), without statistical significance at P >0.05. For silver ions, the values did not reach the detection limit of the spectrophotometer at any period.
| Group | n | Mean (μg/L) ± SD | |
|---|---|---|---|
| Before placement | Control | 30 | 0.15 ± 0.02 |
| Test | 30 | 0.14A ± 0.01 | |
| 10 minutes | Control | 30 | 0.15 ± 0.05 |
| Test | 30 | 0.71B ∗ ± 2.07 | |
| 24 hours | Control | 30 | 0.17 ± 0.05 |
| Test | 30 | 0.31C ∗ ± 0.78 | |
| 7 days | Control | 30 | 0.17 ± 0.04 |
| Test | 30 | 0.25C ∗ ± 0.38 | |
| 30 days | Control | 30 | 0.17 ± 0.76 |
| Test | 30 | 0.18 A.C ± 0.91 | |
| 60 days | Control | 30 | 0.18 ± 0.08 |
| Test | 30 | 0.18 A.C ± 0.24 |
| Group | n | Mean (μg/L) ± SD | |
|---|---|---|---|
| Before placement | Control | 30 | 6.54 ± 7.83 |
| Test | 30 | 16.98A ± 12.71 | |
| 10 minutes | Control | 30 | 4.67 ± 6.70 |
| Test | 30 | 70.60B ∗ ± 40.68 | |
| 24 hours | Control | 30 | 3.81 ± 4.43 |
| Test | 30 | 31.86C ∗ ± 22.14 | |
| 7 days | Control | 30 | 3.84 ± 2.45 |
| Test | 30 | 26.07D ∗ ± 15.30 | |
| 30 days | Control | 30 | 4.65 ± 2.63 |
| Test | 30 | 20.80A ± 14.19 | |
| 60 days | Control | 30 | 5.86 ± 2.81 |
| Test | 30 | 12.63A ±12.34 |
| Group | n | Mean (μg/L) ± SD | |
|---|---|---|---|
| Before placement | Control | 30 | 0.01 ± 0.00 |
| Test | 30 | 0.05A ± 0.03 | |
| 10 minutes | Control | 30 | 0.01± 0.01 |
| Test | 30 | 0.20B ∗ ± 0.28 | |
| 24 hours | Control | 30 | 0.02 ± 0.01 |
| Test | 30 | 0.07C ∗ ± 0.03 | |
| 7 days | Control | 30 | 0.02 ± 0.01 |
| Test | 30 | 0.05D ∗ ± 0.02 | |
| 30 days | Control | 30 | 0.02 ± 0.01 |
| Test | 30 | 0.05E ∗ ± 0.04 | |
| 60 days | Control | 30 | 0.02 ± 0.00 |
| Test | 30 | 0.05A ± 0.06 |
In the study group, all ions showed expressive concentrations at the first 10 minutes after placement of the hyrax appliance ( P <0.001), with higher means for copper ( Table II ) and a tendency for reduction after 24 hours, maintaining the significant difference up to 7 days for cadmium and copper ( Tables I and II ) and up to 30 days for zinc in relation to the initial values ( P <0.001) ( Table III ). For silver, there was no reduction of values to compare the statistical similarity with the initial values ( Table IV ).
