Abstract: BACKGROUND: Leukoplakia is a common adverse reaction to traditional orthodontic treatment. There are still some controversies about the difference between light-transmitting clear and fixed aligners in the periodontal health status, incidence of leukoplakia, saliva and related bacteria in patients undergoing orthodontic treatment.
OBJECTIVE: To compare the effects of clear and fixed aligners on the smooth surface of orthodontic teeth using Universal Visual Scoring System. 
METHODS: A total of 80 patients admitted to Department of Orthodontics, Second Hospital of Hebei Medical University from January 2018 to December 2022 were selected and randomly divided into a control group (n=40) and an observation group (n=40) using a random number table method. The control group received orthodontic treatment with fixed aligners, and the observation group received orthodontic treatment with clear aligners. Before treatment and after 6, 12, and 24 months of treatment, the labial or buccal sides of teeth 13, 33, 16, and 36 were selected for each patient as the research subjects, and the visual scoring system was used to evaluate the white spot lesions. 
RESULTS AND CONCLUSION: (1) Before treatment and after 6, 12, and 24 months of treatment, there was no significant difference in the color of tooth white spots and the severity of caries between the two groups of patients (P > 0.05). (2) For tooth 13, after 12 months of treatment, the color grade and caries severity of white spots were both higher than before treatment (P < 0.05). For tooth 33, the color grade and caries severity after 12 months of treatment were both higher than before treatment (P < 0.05). The severity of caries after 12 months of treatment was higher than that after 6 months of treatment (P < 0.05). For tooth 16, the color grade of white spots and the severity of caries after 12 months of treatment were both higher than those after 6 months of treatment (P < 0.05). (3) Multi-level ordered multi-classification Logistic regression model analysis showed that the color and severity of white spots on different teeth had little correlation with the type of appliance and were greatly affected by the correction time factor. Among them, the color change and severity of the initial white spots on teeth 33 and 13 were statistically significant at 12 and 24 months of treatment. (4) It is not possible to conclude that there is a difference in the effect of fixed and clear aligners on the white spot lesions of smooth surfaces of the teeth, but an increase in the risk of dental caries with the increase in the duration of treatment can be detected.

Key words: white spot lesions, Universal Visual Scoring System, fixed aligners, clear aligners, orthodontic treatment