Introduction
Our objective was to evaluate and compare the salivary levels of nickel and chromium before and 1, 7, and 30 days after placement of conventional and self-ligating appliance systems.
Methods
Twenty women were randomly divided into 2 groups. Patients in group 1 had conventional brackets bonded to their teeth; in group 2, self-ligating brackets were bonded. Four samples of unstimulated saliva were collected from each patient 1 hour before placement of the fixed appliance, and 1, 7, and 30 days after placement of the appliance. The chemical analyses for nickel and chromium levels were performed with an atomic absorption spectrometer (PerkinElmer, Shelton, Conn). Two-way repeated measures analysis of variance by ranks (Friedman test) were used to test the statistical significance of differences in the concentrations of nickel and chromium before and after placement of the appliances. Post-hoc pair-wise comparisons among groups of the same element were calculated by using the Wilcoxon signed rank test. A value of P ≤0.05 was considered significant.
Results
Nickel and chromium released into saliva from conventional and self-ligating brackets progressively increased from days 1 to 7 and then decreased at day 30. Nickel release was less, and chromium release was greater in the conventional bracket group.
Conclusions
Both the conventional and the self-ligating brackets did not seem to affect significantly the nickel and chromium concentrations in saliva during the first month of treatment.
Stainless steel alloys containing 8% to 12% nickel and 17% to 22% chromium are generally used in orthodontic appliances. Nickel is added to stabilize the austenitic phase, improve the anticorrosive property of the alloy, and decrease the ductility; chromium is added to facilitate the formation of an anticorrosive passive film. Nickel and chromium can cause hypersensitivity, dermatitis, asthma, and cytotoxicity, have a significant carcinogenic and mutagenic potential, and at nontoxic concentrations induce DNA alterations.
A major concern has been the performance of alloys in the environment in which they are intended to function: the oral cavity. The oral environment is particularly ideal for biodegradation of metals because of its ionic, thermal, microbiologic, and enzymatic properties. The biodegradation of metals usually occurs by electrochemical breakdown. Orthodontic alloys emit electrogalvanic currents with saliva as the medium, leading to a release of metal ions.
Brackets used for orthodontic therapy are made of various alloys. They are subject to increased corrosion in the oral cavity because they are constantly exposed to the patient’s saliva. The conventional bracket used in fixed appliance therapy has undergone many modifications. One such modification was the self-ligating bracket that eliminates the problems and risks of stainless steel or elastic ligatures while saving chair time. Several designs of self-ligating brackets have emerged, each seeking the best combination of friction and control. One design uses a “closing door” to the bracket opening, converting an open-sided slot to a rectangular tube when closed. Another design uses a spring clip to seat the archwire completely, maintaining the clip in contact with the archwire. The SmartClip bracket’s self-ligating mechanism consists of 2 nickel-titanium (NiTi) clips that open and close through elastic deformation of the material when the archwire exerts a force on the clip. This clip could be a source of additional metal release, and the nickel and chromium released from this system have not been studied in vivo to date. Hence, the aims of this in-vivo study were to evaluate and compare the salivary levels of nickel and chromium before and 1, 7, and 30 days after placement of either conventional fixed appliances or self-ligating appliances.
Material and methods
Twenty women of Dravidian ancestry, between the ages of 18 and 25 years (mean, 21 years 6 months) were selected for this study. Each patient had a full complement of teeth (with all third molars erupted) and minimal crowding (<2.5 mm), required first premolar extractions as part of the treatment plan, and had no metallic fillings or restorations. The patients were randomly divided into 2 groups of 10 each. In group 1 (conventional bracket group), preadjusted edgewise brackets (MBT prescription, Victory metal brackets, 3M Unitek, Monrovia, Calif) were bonded to the teeth. In group 2, self-ligating brackets (SmartClip self-ligating appliance system, MBT prescription, 3M Unitek) were bonded to the teeth.
The brackets in both groups were cured with Transbond XT (3M Unitek). The first permanent molars in both arches were banded with Tru-Chrome band material (Rocky Mountain Orthodontics, Denver, Colo). Maxillary triple and mandibular double buccal tubes were welded to these bands, which were cemented with glass ionomer cement (GC Fuji CEM, GC Corporation, Tokyo, Japan). The patients in both groups had 16 brackets and 4 bands each, and a 0.016-in NiTi wire (3M Unitek) was used as the initial archwire along with passive canine lacebacks. In group I, ligation was achieved with elastomeric modules (clear power O’s, Ormco, Glendora, Calif); in group II. they were self-ligated. No patient had palatal or lingual appliances, additional welding, or extraoral orthodontic appliances.
The plaque index and the gingival index were used to assess oral hygiene statuses 1 hour before treatment and also 1, 7, and 30 days after placement of the fixed appliance.
Unstimulated saliva was collected from all subjects. About 5 mL of saliva was collected into a 15-mL sterile Eppendorf test tube. Four samples of saliva were collected from each patient at the following times: 1 hour before placement and 1, 7, and 30 days after placement of the appliance. The saliva samples for day 30 were collected while the 0.016-in NiTi wire was still in place and before any adjustments were made. The samples were stored at −80°C in a freezer (ULT; Thermo Electron, Asheville, NC). Nickel and chromium were estimated as suggested by Singh et al. In brief, processing was done by using 0.5 ml of saliva that was transferred to a 2-mL plastic test tube with a micropipette. The saliva samples were centrifuged at 3000 rpm (refrigerated centrifuge; Eppendorf, Hauppauge, NY) for 5 minutes to settle the particulate matter.
The chemical analyses for nickel and chromium were performed with an atomic absorption spectrometer (PerkinElmer, Shelton, Conn) attached to a graphite furnace (HGA 900; PerkinElmer, Rodgau, Germany). The technique makes use of the wavelengths of light specifically absorbed by an element. Standard calibration curves were made for nickel and chromium by using several solutions of known concentrations (20–100 ppm).
We injected 20 μL of the sample directly into the graphite tube from an automated micropipette and sample exchanger. The tube was heated electrically by passing a current of 5 mA through it in a programmed series of steps that included 15 seconds at 120°C to evaporate the solvent, 6 seconds at 950°C to drive off any volatile organic material and char the sample to ash, and 5 seconds at 2100°C to vaporize and atomize the elements. Heating the tube to 2300°C made it ready for the next sample.
A beam of electromagnetic radiation from a hollow cathode lamp specific for nickel (232 nm) and chromium (357.9 nm) was passed through the vaporized sample by using argon gas. The metal atoms in the sample absorb some of the irradiation. This gave the absorbance of the sample, which enabled the estimation of the nickel and chromium concentration in the unknown sample from the standard calibration curves. The concentrations of nickel and chromium were expressed in micrograms per liter. Each sample was analyzed in triplicate, and the mean value was calculated.
Statistical analysis
Statistical analysis was done with the Statistical Package for the Social Sciences (version 15, SPSS, Chicago, Ill). Two-way repeated measures analysis of variance (ANOVA) by ranks (Friedman Test) were used to test the statistical significance of the differences in concentrations of each metal before and after placement. Post-hoc pair-wise comparisons among groups of the same element were calculated by using the Wilcoxon signed rank test. A value of P ≤0.05 was considered significant.
Results
The mean salivary nickel levels for the conventional (group 1) and self-ligation (group 2) groups are given in Table I . The mean salivary chromium levels for these groups are given in Table II . Great variations in the concentrations of both nickel and chromium were observed in the groups.
| Period | Nickel | Chromium | ||
|---|---|---|---|---|
| Group 1 | Group 2 | Group 1 | Group 2 | |
| BP | 0.68 ± 0.51 | 0.68 ± 1.39 | 5.19 ± 5.38 | 6.06 ± 3.50 |
| 1D | 1.95 ± 0.87 | 2.72 ± 2.30 | 21.78 ±10.09 | 10.65 ± 3.71 |
| 7D | 2.89 ± 1.68 | 4.95 ± 3.11 | 36.69 ± 14.86 | 14.34 ± 4.19 |
| 30D | 1.18 ± 0.83 | 1.12 ± 1.29 | 8.98 ± 6.31 | 6.31 ± 4.16 |
| Pairs | Nickel | Chromium | ||||||
|---|---|---|---|---|---|---|---|---|
| Group 1 | Group 2 | Group 1 | Group 2 | |||||
| z value | P value | z value | P value | z value | P value | z value | P value | |
| BP/1D | −3.920 | 0.000 ∗ | −2.803 | 0.005 ∗ | −3.920 | 0.000 ∗ | −2.803 | 0.005 ∗ |
| BP/7D | −3.920 | 0.000 ∗ | −2.803 | 0.005 ∗ | −3.920 | 0.000 ∗ | −2.803 | 0.005 ∗ |
| BP/30D | −1.982 | 0.048 ∗ | −1.376 | 0.169 | −2.025 | 0.043 ∗ | −2.380 | 0.017 ∗ |
| 1D/7D | −3.920 | 0.000 ∗ | −2.803 | 0.005 ∗ | −3.920 | 0.000 ∗ | −2.803 | 0.005 ∗ |
| 1D/30D | −3.696 | 0.000 ∗ | −2.293 | 0.022 ∗ | −3.823 | 0.000 ∗ | −2.803 | 0.005 ∗ |
| 7D/30D | −3.920 | 0.000 ∗ | −2.803 | 0.005 ∗ | −3.920 | 0.000 ∗ | −2.803 | 0.005 ∗ |
The nickel concentrations varied from 0.689 to 2.895 μg/L for group 1 and from 0.680 to 4.950 μg/L for group 2. The chromium concentrations varied from 5.192 to 36.696 μg/L for group 1 and from 6.068 to 14.344 μg/L for group 2.
The release of nickel and chromium into saliva from conventional and self-ligating brackets progressively increased from days 1 to 7 and then decreased at day 30. There was a greater release of nickel from the self-ligating brackets when compared with the conventional brackets. There was a greater release of chromium from the conventional group when compared with the self-ligating group. Greater amounts of chromium were released when compared to nickel.
Statistically significant differences were observed when comparing the levels before placement, 1 day after placement, 7 days after placement, and 30 days after placement of both elements with the Friedman test ( Table III ). Differences of nickel and chromium released in both groups during the various time periods were statistically significant when compared with the independent sample t test ( Table IV ).
| Parameter | Nickel | Chromium | ||
|---|---|---|---|---|
| t value | P value | t value | P value | |
| DBP1D | 1.327 | 0.201 | −4.379 | 0.000 ∗ |
| DBP7D | 2.014 | 0.050 ∗ | −4.703 | 0.000 ∗ |
| DBP30D | −0.106 | 0.917 | −2.236 | 0.038 ∗ |
| D1D7D | 2.289 | 0.034 ∗ | −3.315 | 0.004 ∗ |
| D1D30D | −1.871 | 0.078 | 3.155 | 0.005 ∗ |
| D7D30D | −2.570 | 0.019 ∗ | 4.167 | 0.001 ∗ |
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