Introduction
In this study, we aimed to determine the effect of carbon dioxide laser irradiation on enamel surface microhardness.
Methods
In this single-blind interventional clinical trial, 16 patients needing at least 2 premolars extracted for orthodontic purposes participated. In each subject, 1 premolar was treated with the carbon dioxide laser (beam diameter, 0.2 mm; power, 0.7 W); the other was exposed to a visible guiding light as the control. A t-loop was ligated to the bonded brackets to increase caries risk. After at least 2 months, the teeth were extracted, and the surface microhardness was measured. Scanning electron microscope evaluation was performed on 1 sample from each group. Normal distribution of the data was assessed by the Kolmogorov-Smirnov and Shapiro-Wilks tests. Mean microhardness values of the 2 groups were compared using paired t tests.
Results
The data had normal distributions. Means and standard deviations of the microhardness in the laser-treated and control groups were 301.81 ± 94.29 and 183.9 ± 72.08 Vickers hardness numbers, respectively; this was different significantly ( P <0.001). Scanning electron microscopy showed the enamel surface melting in the laser-treated specimens.
Conclusions
Carbon dioxide laser treatment results in higher enamel surface microhardness around orthodontic brackets. Patients at high risk of caries might benefit from this intervention. Exact control of the laser irradiation parameters is recommended.
Orthodontic treatment with fixed appliances causes an increased risk of enamel demineralization adjacent to orthodontic brackets. However, little information is available about preventive measures that do not rely on patients’ compliance.
During the last 35 years, several in-vitro studies have demonstrated the potential inhibitory effects of laser pretreatment on enamel or root caries-like lesion formation. Different types of lasers with various settings have been used for caries prevention. However, carbon dioxide (CO 2 ) lasers with wavelengths that are similar to the absorption bands of phosphate, carbonate, and hydroxyl groups of both enamel and dentin are the mainstays for inhibition of caries formation. A previous study investigated the inhibitory effect of a CO 2 laser on caries prevention and showed that irradiation in a wavelength of 10.6 μm is the most effective wavelength for this laser.
Various explanations have been given for the alteration of the dental enamel acid reactivity rate by CO 2 laser irradiation. These include but are not restricted to enamel permeability to chemical agents, chemical changes, enamel surface melting that results in reduced enamel permeability and solubility, fusion and recrystallization of enamel crystallites, sealing of the enamel surface, apatite crystals of different shapes and sizes, loss of prismatic structure, and conversion of acid phosphate to pyrophosphate to inhibit demineralization and decrease water and total carbonate content. In addition to these changes, it has been shown that laser irradiation increases fluoride uptake and diminishes the dissolution rate of enamel significantly; this results in synergistic influences of these 2 anticaries methods.
Most studies to date have evaluated the effect of CO 2 lasers on the demineralization process in vitro. To the best of our knowledge, a split-mouth design has not yet been used to study CO 2 laser effect on enamel surface hardness. Ramalho et al found that CO 2 laser irradiation could effectively reduce enamel surface loss caused by citric acid exposure in vitro. This caries prevention might work through enamel surface hardening. Poosti et al in their in-vitro study showed that fractional CO 2 laser irradiation before fluoride therapy could improve surface microhardness and harden demineralized enamel surfaces.
Regarding the inhibitory effects of laser irradiation on enamel demineralization, authors have hypothesized that the application of a laser could increase enamel surface microhardness, which is an indicator of enamel resistance to demineralization.
Because of the lack of studies evaluating the effect of CO 2 laser irradiation on enamel surface microhardness and caries prevention in clinical conditions, we aimed to evaluate the clinical effects of CO 2 laser irradiation on enamel surface microhardness around orthodontic brackets.
Material and methods
Before their participation in this study, the research plan was completely explained to the patients, and they were told that this research would have no effect on their treatment. Informed consent was obtained, and the study was approved by the ethical committee of Hamadan University of Medical Sciences in Iran.
In this single-blind interventional clinical study, patients scheduled for extraction of at least 2 premolars for orthodontic reasons were recruited. Demographic data including age and sex, and the number of decayed, missing, and filled teeth (DMFT) of the participants were documented. The inclusion criteria were age less than 25 years, complete eruption of teeth, no lesions on enamel surfaces, and moderate to good oral hygiene according to the guidelines of Silness and Loe. Only patients with a low caries risk were allowed to participate in the study to prevent bias because of the necessity of using additional preventive measures for those with a high caries risk. Patients with evident enamel lesions or cracks on the buccal surfaces of their teeth and those who did not agree to participate were excluded from the study. Each subject’s teeth were divided into control and test groups. The treated group (n = 16) included the first or second premolar treated with a CO 2 laser (DEKA Laser Technologies, Florence, Italy) with special characteristics (wave length, 10.6 μm; pulse duration, 3 seconds; repetition rate, 5 Hz; beam diameter, 0.2 mm; level, 1.5; power, 0.7 W), and the control group (n = 16) included the contralateral first or second premolar that received no laser treatment. To blind the patients to the right and left laser treatment and also to prevent bias because of left- or right-handed patients, the selection of teeth for the control and treated groups was predetermined randomly and concealed, and the teeth of the control group were exposed to a nontherapeutic light.
To observe the possible surface changes on the enamel after laser therapy, 1 tooth as a sample of each group was prepared for assessment with scanning electron microscopy (SEM). For this reason, 2 teeth (maxillary first premolars of 1 patient) were extracted 1 week after laser treatment (treatment group) and light irradiation (control group). For SEM evaluation, the enamel surfaces of the teeth were dried and sputter-coated with gold. The prepared samples were observed by SEM (Kyky Technology Development, Beijing, China) at 500 and 739 times magnification.
Stainless steel standard orthodontic brackets (Dentaurum, Ispringen, Germany) were bonded on both the right and left premolars. For this reason, the buccal surfaces of the teeth were etched for 15 seconds with 37% phosphoric acid (3M Unitek, Monrovia, Calif), rinsed with water for 30 seconds, and gently air dried for 20 seconds. Transbond XT primer (3M Unitek) was painted on the etched surfaces. The brackets were bonded with Transbond XT adhesive (3M Unitek). Excess resin was removed with an explorer and then polymerized with the Demi LED light-curing system (Kerr, Orange, Calif). The curing time was 40 seconds (20 seconds mesially and 20 seconds distally).
A t-loop made from 0.014-in stainless steel wire was engaged using an elastomeric ring to increase plaque accumulation and hasten the carious process.
A toothbrush (Oral-B, number 35; Proctor & Gamble, Cincinnati, Ohio) and toothpaste (Colgate Total; Colgate-Palmolive, New York, NY) containing 1100 ppm of fluoride were prescribed to all participants. They all were trained in how to brush and advised to do it every night. After at least 2 months, which is a sufficient period for caries formation, the teeth were extracted by an experienced dental surgeon. The extracted teeth were immersed in 10% formalin for 48 hours. The soft-tissue residues were removed, and the teeth were stored in distilled water at room temperature until the experiment. The anatomic crowns were transversely sectioned at the cementoenamel junction of the buccal aspect using a cutting machine (Demco high-speed grinder; CMP Industries LLC, Albany, NY) under water coolant and then mounted in self-curing acrylic resin (GC America, Alsip, Ill).
To evaluate surface microhardness, the teeth were rinsed twice in deionized water. Tooth surface microhardness was evaluated with a Vickers diamond microhardness testing machine (KB Prüftechnik, Hochdorf-Assenheim, Germany). The Vickers tester was used with a load of 100 g for 10 seconds, placing its diamond indenter adjacent to the gingival margin of the bonded brackets. The measurement unit was Vickers hardness number (VHN).
Statistical analysis
Data were gathered and analyzed with SPSS software (version 13.00; SPSS, Chicago, Ill). Normal distribution of data was assessed by the Kolmogorov-Smirnov and Shapiro-Wilks tests. Mean microhardness values of the 2 groups were compared using the paired t test. P <0.05 was considered statistically significant.
Results
Sixteen patients (5 male, 11 female) who needed bilateral extractions of premolars were recruited for this study. First or second premolars in both jaws were included. The patients’ oral hygiene was moderate to good, and DMFT did not exceed 6 ( Table ). Figure 1 shows SEM photographs. Surface melting is obvious in the laser-treated enamel surface.
| Mean | SD | Minimum | Maximum | |
|---|---|---|---|---|
| Age (y) | 15.6 | 3.6 | 12 | 25 |
| DMFT (n) | 3.13 | 2.56 | 0 | 6 |
| Duration ∗ | 75.3 | 34.6 | 27 | 123 |
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