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Management of Post-orthodontic White Spot Lesions and Subsequent Enamel Discoloration with Two Microabrasion Techniques.

Statement of the Problem Demineralization of enamel adjacent to orthodontic appliances frequently occurs, commonly due to insufficient oral hygiene. Purpose The aim of this study was to compare two microabrasion techniques on improving the white spot lesions as well as subsequent enamel discoloration. Materials and Method Sixty extracted premolar teeth without caries and hypoplasia were selected for this study. White spot lesions were artificially induced on the buccal surface of each tooth. Teeth were randomly assigned to three treatment groups, each treated with pumice powder as the control, microabrasion with 18% HCl, and microabrasion with 37% H3PO4. Subsequently, the three groups were daily immersed for five minutes in a tea-coffee solution for a period of one week. Colorimetric evaluation was done before and after formation of white spot lesions, after microabrasion, and after immersion in the colored solution; then the color differences (∆E) were calculated. Statistical analysis was performed by multiple measurement analysis and the Tukey’s test. Results This study showed that ∆E between the stages of white spot formation and microabrasion for H3PO4 was more than other groups and for the pumice powder group it was less than the others. Furthermore, there was a significant difference between ∆E of the three study groups (p= 0.017). Additionally, ∆E after placing the teeth in the colored solution and microabrasion was the highest for the HCl group and the lowest for the pumice powder group. There was also a significant difference between the three groups (p= 0.000). Conclusion Pumice powder alone had similar effects as 18% HCl on removing the white spot lesions. Nevertheless, 18% HCl makes the enamel susceptible for subsequent color staining more than the other microabrasion methods.


Introduction
White spot lesions are subsurface enamel porosities caused by enamel demineralization. [1] According to Ogaard et al. white spot lesions are the most prevalent iatrogenic side effect of orthodontic treatment. These lesions develop as a result of prolonged plaque accumu-lation on the enamel surfaces adjacent to orthodontic devices, commonly due to poor oral hygiene. [2][3] After active orthodontic treatment, demineralization process is normally expected to decelerate due to alterations in these local factors. Some white spot lesions may sometimes remineralize and return to normal or at least to a visually acceptable appearance. However, some white spot lesions persist and become unsightly.
In severe cases, white spot lesions may progress to enamel carious lesions which require restorative treatment. Hence, early diagnosis and treatment of these lesions by dentists would be of particular importance. [4] Various treatments have been proposed to improve the appearance of white spot lesions including restorative procedures, improvement of remineralization using high concentrations of topical fluoride or casein phosphopeptide amorphous calcium phosphate (CPP-ACP), chewing gum to promote remineralization, microabrasion, and argon-laser irradiation. [5][6] Microabrasion technique involves application of hydrochloric acid and pumice in form of a paste, to the affected enamel surfaces, which removes about 100µ of the enamel through a combination of erosion and abrasion. In this technique, polished enamel does not have the typical enamel surface appearance because no interprismatic space exists in microabraded enamel.
This highly polished enamel surface, compared with intact natural enamel, is more resistant to bacterial colonization and demineralization. Therefore, this technique would not place the patient at risk of demineralization and its progression. [7][8] In this regard, Willis et al. concluded that microabrasion with hydrochloric acid is an appropriate technique to employ prior to initiating more aggressive cosmetic dentistry. [9] Another study has shown that the application of microabrasion technique with hydrochloric acid and pumice resulted in 83% reduction in the size of white spot lesions after the treatment. Therefore, the researchers concluded that microabrasion technique is an effec- revealed that the mean surface roughness by hydrochloric acid was statistically lower than phosphoric acid (p< 0.001). Deeper demineralization depth (p< 0.003) and a larger total demineralization area were observed in teeth treated with hydrochloric acid as compared with those treated with phosphoric acid ( p < 0.005). This study showed that application of phosphoric acid is safer and more convenient to perform. [12] The effects of the two microabrasion methodsusing 18% hydrochloric acid and 37% phosphoric acid on improvement of post-orthodontic white spot lesions and subsequent enamel discoloration has not been investigated yet, therefore, our study aims on comparison of these two techniques.

Materials and Method
Ethics approval was obtained from the Ethics Commit-

White spot elimination
The first group was polished with pumice powder. Microabrasion technique with 18% hydrochloric acid, and 37% phosphoric acid was performed for the second and the third groups, respectively. For all groups, the treatment was done using a prophylactic rubber cap with slight pressure for five seconds. [12] After this stage, L, a, b were again measured and the color differences were compared with the second colorimetery (∆E2).

Staining Stage
The teeth were immersed in a tea-coffee solution for one week. This solution was prepared as follows: 1 g of tea and 1 g of coffee were mixed in 100 cc of boiled water and then filtered. The teeth were placed in the solution daily for five minutes. Subsequent to this stage, the L, a, b values were determined again and color differences were compared with the second colorimetery (∆E3). The teeth from all groups were preserved in Fusayama-Meyer artificial saliva at pH of 7.3 containing KCI (0.4 g/l), NaCl (0.4 g/l), CaCl 2 , 2H 2 O (0.906 g/l), NaH 2 PO 4 , 2H 2 O (0.690 g/l), Na 2 S, 9H 2 O (0.005 g/l), Urea (1 g/l), and distilled water at 37°C.

Statistical analysis
The Data were analyzed by SPSS (version 10.0) and statistical analysis was performed by using the multiple measurement analysis. Tukey's test was done for pairwise comparison between the means. The significant level was set at p< 0.05.

Results
Based on multiple measurement test, there was an interaction effect between the groups and between different stages (p< 0.05). Furthermore, at an error level of 5%, there were significant differences in ∆E values of all study groups (p< 0.05).
The means and standard deviations of ∆E for the three groups have been shown in Table 1. According to this table, no significant difference was observed between ∆E1 of all groups under study (p= 0.106). As shown in Table 2, the mean ∆E2 for the 37% phosphoric acid group was higher than the 18% hydrochloric acid group, while it showed the lowest value for the pumice powder (control) group. Furthermore, there was a significant difference between ∆E2s of the three study groups (p= 0.017). Additionally, ∆E3, which was the mean difference between L, a, b after placing the teeth in the color solution and L, a, b after microabrasion, was the highest for the HCL group and the lowest for the pumice powder group. There was also a significant difference between ∆E3s of the three groups (p= 0.000) ( Table 3).

Discussion
Despite the previous efforts made by many researchers to prevent white spot formation around the fixed orthodontic appliances, this problem is still not resolved. [5] Moreover, it has been shown that enamel microabra-sion is a conservative method to eliminate enamel staining. The application of this method in addition to saving time, if used correctly, will improve teeth appearance without requiring mechanical tooth preparation, pain, and sensitivity. [7] In the present study, we employed two microabra- In the current study, after placing the teeth in a tea-coffee solution, the hydrochloric acid and the pumice powder groups showed the most and the least color changes, respectively. This result, as it has been suggested by Meireles et al., may be due to a deeper demineralization size after hydrochloric acid application.
[12] Color differences greater than two units might be recognized, but most clinical studies set the limit for a visible color change at 3.7 units. The current study used the color difference of 3.7 units as a threshold between an acceptable and unacceptable color change. [19] Furthermore, the calculated color changes in all the stages of assessment and in all groups were higher than 3.7 units that could be recognized by the naked eye.
Finally, it must be noted that the using staining solution, immersion time, dilution with saliva and whether the teeth are cleaned or not during the study would be some of the factors that may determine the severity of enamel staining.

Conclusion
Despite the limitations of this study, microabrasion with pumice powder alone had similar effects as with 18% hydrochloric acid in removing the white spot lesions.
Nevertheless, 18% hydrochloric acid would make the enamel more susceptible to subsequent color staining in comparison with the other microabrasion methods.