The removal method of biofilm on orthodontic bracket surfaces and its influence on enamel demineralization in vitro

Background Enamel demineralization may occur during orthodontic treatment, especially in patients with poor oral hygiene. This may be related to biofilm accumulation on teeth and orthodontic appliances. This study aimed to determine the distribution pattern of biofilm on the surface of orthodontic brackets, determine whether reducing biofilm on the bracket surface reduces enamel demineralization, and explore the necessity of removing biofilm on brackets in caries environments. Methods Scanning electron microscopy was used to observe the distribution of biofilm on brackets in vivo for more than 2 years (n = 40) and in vitro for 42 days (n = 40). A bracket model coated with Streptococcus mutans biofilm cultured for 48 h was examined from experimental day 0 (D0). Three experimental groups were formed based on cleaning methods: a blank group, a toothbrush group, and an ultrasonic group (n = 5/group). Toothbrushes were used to clean the enamel every 12 h in all groups. The brackets of the blank group were only washed with phosphate-buffered saline. The toothbrush and ultrasonic groups were cleaned with toothbrushes and ultrasound on D0, 7, 14, 21, 28, 35, and 42 and subjected to pH cycling in vitro for 1 week and 6 weeks. The optical density values of crystal violet in the brackets and enamel biofilm of the three groups were compared on D7 and D42, and the hardness and microstructure of enamel were compared on D42. Results The in vivo biofilm mostly adhered to the slot area of the bracket labial surface, followed by the gingival, mesial, and distal surfaces. After 42 days of in vitro culture, the S. mutans biofilm mostly adhered to the slot area of the bracket labial surface, and little adhered to other surfaces or areas. On D42, the ultrasonic group had the least enamel biofilm, an enamel hardness significantly higher than that of the blank group ( P  < 0.05), and a relatively intact enamel structure. Conclusions The biofilm of orthodontic brackets is most highly distributed in the slot area. In carious environments, ultrasonically removing the biofilm on the bracket can reduce enamel biofilm accumulation and decrease enamel demineralization.