The impact of fixed orthodontic appliances and clear aligners on the oral microbiome and the association with clinical parameters: A longitudinal comparative study

Introduction: Orthodontic treatment interferes with oral hygiene and promotes plaque retention, which leads to gingival inflammation and enamel demineralization. Although removable clear aligners (CAs) are designed to improve oral hygiene compared with fixed appliances (FAs), comprehensive studies comparing their respective effects on the oral microbiome are limited. This longitudinal study investigated the microbial changes during orthodontic treatment with FA and CA in correlation with clinical parameters. Methods: Clinical parameters and supragingival plaque were collected from 12 study participants for the FA or CA treatment groups at baseline and at least twice at the 1, 3, 6, and 12-month follow-up appointments. The plaque was also harvested from the aligner tray for the CA group. Microbiome composition was determined via 16S rRNA gene sequencing, compared between groups, and correlated with clinical parameters. Results: Plaque (PI) and gingival indexes (GI) increased significantly in the FA but not the CA group. Beta but not alpha diversities of the microbial communities were distinct between the 2 treatment groups, even though genus-level differences were not significant except for Leptotrichia. The CA tray harbors a unique plaque community. Elevated PI and GI in the FA group correlated with a higher abundance of disease-related genera. Conclusions: Orthodontic treatments trigger appliance-related plaque community shifts from baseline, and the CA tray environment attracts distinct microbial communities. In comparison with FA, the use of CA resulted in better oral health index outcomes, which is reflected by the corresponding PI and GI-associated oral microbial communities.