Abstract: Objective: To explore the incidence and influencing factors of black triangle in patients with single mandibular incisor extractions after orthodontic treatment. Methods: Eighty fixed orthodontic treatment patients who had one mandibular incisor extracted were enrolled in this retrospective study, and were divided retrospectively into the normal group and the black triangle group based on whether there was a black triangle between the adjacent teeth of the extracted tooth in the posttreatment intraoral frontal photos. According to the intraoral photos, digital models, panoramic radiographs, and other clinical data, the position of interdental contact point, root angle, distance from interdental contact point to alveolar bone crest, width and area of embrasure space, and other clinical parameters were measured and analyzed. Results: After extraction of one mandibular incisor, the incidence of black triangle was 72.5%, with mild incidence of 57.5%, moderate incidence of 15%, and no severe black triangle occurred. There were statistically significant differences between the normal group and the black triangle group in age, position of interdental contact point, root angle, distance from interdental contact point to alveolar bone crest, and width and area of embrasure space. Logistic regression analysis showed that the adjacent teeth root angle and the distance from interdental contact point to alveolar bone crest were the main risk factors for the black triangle after fixed orthodontic treatment in patients with mandibular incisor extraction. Conclusion: The incidence of black triangle was higher in patients who had one mandibular incisor extracted. Orthodontic treatment needs to control the root angle of the adjacent teeth and maintain the health of periodontal tissue so as to reduce the occurrence of black triangle.

Key words: fixed orthodontics, black triangle, tooth extraction treatments