Enamel Microabrasion Combined with in-Office Bleaching in Treating with Moderate to Severe Dental Fluorosis: A Randomized Clinical Trial

Statement of Problem: The stained dental fluorosis affects the beauty and selfconfidence of the patients. How to treat these patients safely and effectively is being explored by the clinicians. Enamel microabrasion has been widely used in treating dental fluorosis, and this method can be combined with at-home bleaching to improve the efficacy. However, this combined treatment was generally applied to patients with mild-to-moderate dental fluorosis. Whether this treatment can be used in severe dental fluorosis is unknown. Additionally, at-home bleaching is patient-applied and timeconsuming. Therefore, microabrasion combined with professional in-office bleaching may be a better choice. However, whether the enamel microabrasion enhances the risk of dental sensitivity of in-office bleaching remain unclear.


Introduction
Dental fluorosis is a special enamel dysplasia caused by excessive intake of fluoride during tooth development and mineralization. It is the most common and prominent symptom in the early stage of chronic fluorosis. The clinical manifestations of dental fluorosis are symmetrical white stripes or patches on the tooth surface [1]. Some serious cases, such as Dean's grading 3 and 4 dental fluorosis, have yellow-brown patches on the tooth surface [2]. The stained dental fluorosis affects the beauty and self-confidence of the patients [3]. Therefore, how to treat these patients safely and effectively is being explored by the clinicians. The clinical treatment methods of dental fluorosis are generally divided into invasive and noninvasive methods. The invasive method is to solve the aesthetic problem of tooth thoroughly by repairing the porcelain veneer. The noninvasive bleaching of teeth is done using peroxide. Because peroxide just can remove the superficial layer of staining, bleaching has not been advocated as a sole treatment of moderate or severe dental fluorosis [4]. But there are some patients who do not require a complete change of stained tooth surface because they cannot pay so much money for the porcelain veneer or they don't want to remove too much enamel. They may prefer to accept bleaching as their treatment method. However, whether bleaching can improve the stained dental fluorosis is unclear.
Bleaching can combine with other treatment to improve the efficacy [5,6]. In recent years, a novel treatment called enamel microabrasion has been used in treating dental fluorosis [7].
Enamel microabrasion technique is to remove/reduce the staining area on the enamel surface using 37% phosphoric acid and pumice [5]. However, no definite conclusion exists regarding whether bleaching combined with enamel microabrasion is effective against moderate or severe dental fluorosis. In addition, bleaching is of two types: at-home tooth bleaching and in-office tooth bleaching. Because the effect of at-home bleaching is patient-applied and timeconsuming, also, due to improper manipulation of the patient, the bleaching agent sometimes overflows from the tray, damaging the gingival tissue [8,9]. Therefore, in-office bleaching by professional clinicians may be the first choice for patients. But compared to at-home bleaching, in-office bleaching may cause more tooth sensitivity [4]. Whether combined with the enamel microabrasion enhances the risk of dental sensitivity of in-office bleaching remains unknown. Thus, the aim of this trial was to observe the efficacy and tooth sensitivity of enamel microabrasion combined with in-office bleaching and in-office bleaching only in treating moderate and severe dental fluorosis

Sample Size Calculation
The sample size was calculated based on a pilot study. A twosided test with alpha of 5% and 80% power was used to detect a difference of 20% between the groups for removing fluorosis stains with Dean's grading 3 and 4. The sample size of each group needed to be at least 25. The sample size calculation formula was as follows In this formula, n is the sample size; the standard normal distribution function Φ is 10; type I error α is 5%; τ is the number of comparisons to be made; and β is type II error; the power of 1 − β is 0.80.

Randomization
This study was a randomized, within-person, triple-blind clinical trial, in which the patient, evaluator, and statistician were masked to the group assignment. A third researcher, who was not involved in the evaluation process, was responsible for the randomization process. The patients were randomized into two treatment groups using a simple randomization process: the

Microabrasive Protocol
The teeth of 30 patients in group 1 were micro-grinded; 37%

Bleaching Protocol
The patients in both the groups received in-office bleaching.
They received lip balm oil; their mouths were opened with a mouth opener, and a wet silver was put in the oral vestibule. -Light-curing resin protector (Beyond) was used to isolate gingival tissue from the bleaching area. Then, 35% hydrogen peroxide gel (Beyond) was applied to the enamel surface (15min) three times according to the manufacturer's protocol. At the end of bleaching, the whitening gel was rinsed thoroughly with an air/water spray for 30s and airdried.

Evaluation of Therapeutic Efficiency
This trial evaluated the effect on patients immediately after bleaching to avoid the effects of daily eating habits of patients, such as drinking coffee, and other factors on the treatment results.
The area of fluoride staining was still measured by the blinded evaluator who collected digital photos of patients and used image software to obtain the area ratio of fluoride staining, which was compared with the fluoride staining area ratio of the baseline. The improvement in fluoride staining was compared between the two groups using the difference in the fluoride staining area before and after the treatment. Meanwhile, the patients were evaluated using VAS1 ranging from 1 (stain not removed at all) to 7 (stain totally removed) to evaluate the improvement in fluoride staining after treatment [13].

Tooth Sensitivity Evaluation
The VAS2 and sensitivity rate were used to assess the patient's sensitivity. The VAS2 ranked from 1 (no tooth or gingival sensitivity) to 5 (severe tooth or gingival sensitivity) [14]. When the patient's VAS2 score was 0, it was considered insensitive. The other scores were sensitive, and the ratio of the sensitive population to the total population was the patient's sensitivity rate.

Statistical Analysis
After using the Shapiro-Wilk test to check the normality distribution of patient age, area ratio (a) of fluoride staining, and the difference between the fluorosis area ratio before and after the treatment (Δa), the paired t test or Wilcoxon signed-rank test was used to compare a before and after the treatment. The t test or Mann-Whitney U test was used to compare patients' age and Δa. The Mann-Whitney U test was used to compare ranked data Dean's index, VAS1, and VAS2. The Fisher exact probability method was used to analyze patient sex and sensitivity rate. All calculations were completed by a blinded statistician using SPSS 20.0(IBM, USA), and the significance was set at 5%.

Result
A total of 60 patients were included in this study ( Figure 2).
The baseline characteristics of patients are shown in (Table 1). No significant difference was found in age (P = 0.534), gender ratio (P = 0.143) and Dean's index (P = 0.557).

Therapeutic Efficiency Area Ratio of Fluoride Staining
Before treatment, no significant difference was found between the two groups (P = 0.145). Both treatments reduced the area of fluoride staining (P < 0.001) ( Table 2  Visual Analog Scale 1: The two groups of patients thought that the treatment had improved fluoride staining. The patients in group 1 were more satisfied with the improvement in fluoride staining compared with the patients in group 2 (P < 0.001) ( Table 3).

Tooth Sensitivity
Visual Analog Scale 2: The tooth sensitivity rate of patients in group 1 was higher than that of patients in group 2 (P = 0.043) ( Table 3).
Tooth Sensitivity Rate: Four patients (13.33%) in group 1 had no tooth sensitivity at all, and 6 (20%) in group 2 did not show tooth sensitivity. No significant difference was found in the sensitivity rate between the two groups (P = 0.731).

Discussion
This trial explored whether in-office bleaching combined with enamel microabrasion could improve the staining of severe dental fluorosis. Compared with in-office bleaching only, the effect of inoffice bleaching combined with enamel microabrasion was better.
Also, the patient satisfaction with the curative effect was higher. In previous studies, bleaching combined with enamel microabrasion were generally applied to patients with mild-to-moderate dental fluorosis [15][16][17]. Most of these studies evaluated the changes in the color of the staining area. For the deeply stained tooth, bleaching combined with enamel microabrasion could not completely remove fluorine. Further, the standard of evaluation was limited [18,19]. Bezerra, et al. [20] measured the enamel staining area using software and found that enamel microabrasion could reduce the fluoride staining area and improve the esthetic appearance of patient teeth [20]. Castro, et al. [6] compared the effect of enamel microabrasion combined with at-home bleaching. The microabrasion of enamel has an important role, but bleaching has little effect on the curative effect [6]. Knösel, et al. [21] also thought that the in-office bleaching was not a suitable treatment for dental fluorosis [21]. This was different from the results of Celik. The combination of enamel microabrasion and bleaching was thought to be better than simple enamel microabrasion. Also, the patient satisfaction was high, indicating an important role of bleaching in improving fluoride staining [22]. The results of this trial suggested that bleaching only improved the fluorosis status of patients, and when it was combined with enamel microabrasion, the whitening effect was enhanced. As the greatest harm of fluoride staining lies in the esthetic impact on patients, their satisfaction with their own esthetic appearance is the key objective of fluoride staining treatment. VAS is the most commonly used method for evaluating the efficacy and side effects of dental fluorosis [13,23].
In this trial, VAS was used to evaluate the whitening effect.
Most patients thought that bleaching could improve their fluoride staining. The patients with bleaching and enamel microabrasion were more satisfied with the curative effect. Besides ensuring efficacy, all treatments need to take into account the safety aspect. In previous studies, the composition of whitening agents was changed, including reducing the concentration of peroxide, increasing antisensitive substances, and so on, hoping to reduce the side effect of whitening, which is tooth sensitivity, without reducing the curative effect [24][25][26][27][28]. Tooth whitening is still considered to be a treatment with minimal damage to the structure of the teeth [29][30][31][32]. At present, enamel microabrasion is widely used in clinical practice, including dental staining for young people [33][34][35]. Although enamel microabrasion is considered safe in removing fluoride staining, [36] some studies found that enamel microabrasion increased the roughness of the enamel surface and led to the loss of part of the enamel [37][38][39]. Pini, et al. observed the morphology of the enamel using confocal laser-scanning microscopy and found that these side effects could be reversed using the polishing procedure or saliva exposure [40,41]. Paic thought that the operator's manipulation was the main factor causing the microabrasion and the polishing of the enamel surface was also important [42]. In this study, no significant difference was found in the sensitivity rate between the two groups, but the results of VAS2 indicated that enamel microabrasion could increase the degree of sensitivity of the patients.
No misoperation occurred during the treatment because the doctor was experienced. The microabrasion of the enamel may cause some damage to the tissue of the teeth [43]. The bleaching treatment was performed immediately after microabrasion. Whether the patient's sensitivity is reduced after grinding for a certain period needs further exploration. In addition, the evaluation of the patient's sensitivity is quite subjective. Some patients are not adapted to the low-rotation handpiece in the process of microabrasion, affecting the judgment of sensitivity. VAS is influenced by personal choice [44]. However, patients' experience is also a factor that should be considered by the doctors in clinical practice. The results of this trial suggested that the individual differences in the degree of acceptance of the treatment must be taken into consideration when choosing the treatment, especially for young people, because of the need to remove the 25-to 200μm enamel [43]. In this trial, the effects of different dietary habits on the results were taken into account. Therefore, only the fluoride staining areas before and after the treatment were compared. Patients with similar dietary habits should be followed up for a long time in the future. Further, this trial was a single-center study. Large-sample multicenter trials are needed to improve the accuracy and credibility of the results.
Both treatments improved the fluoride staining of moderate and severe dental fluorosis. The combination of enamel microabrasion and bleaching could achieve the better curative effect. However, microabrasion should be performed gently, and attention should be paid to the feelings of the patients during the treatment.

Conclusion
In-office bleaching can improve moderate and severe dental fluorosis. The effect of the combination of enamel microabrasion and bleaching is better. The choice of treatment should take into account the patient's esthetic requirement and the individual acceptance of treatment.

Conflict of Interest
The authors declare that they have no conflict of interest.