Objective: To explore the prevalence of bone defect and alveolar bone thickness changes in the mandibular incisors of untreated adults and post-orthodontic treatment adults, with the aim of providing strategies for preventing and managing alveolar bone defects during orthodontic treatment. Methods: This study was reviewed and approved by the Medical Ethics Committee. Clinical records, panoramic radiographs, cephalometric radiographs, and cone beam computed tomography (CBCT) images and informed consent were obtained for 150 untreated adults and 150 post-orthodontic adults. The untreated adults and post-orthodontic adults were respectively divided into three subgroups: skeletal ClassⅠ, Class Ⅱ and Class Ⅲ, with 50 cases per subgroup. Meanwhile, 60 cases with completeness of pre- and post-orthodontic data were enrolled from 150 post-orthodontic adults, including 20 cases each of skeletal ClassⅠ, Class Ⅱ, and Class Ⅲ. Cephalometric radiographs were imported into Dolphin software to measure skeletal parameters. CBCT images were imported into Mimics software to assess alveolar bone defects and to measure alveolar bone thickness of mandibular incisors among three groups: 150 untreated adult groups, 150 post-orthodontic groups and the pre- and post-treatment status of 60 patients selected from the latter group. Results: Untreated adult patients: the prevalence of labial dehiscence and fenestration in the mandibular incisors was higher than that on the lingual side among skeletal ClassⅠ, Ⅱ, and Ⅲ malocclusion patients, and there was a statistically significant difference in the alveolar bone thickness of the mandibular incisors among the three classes. Post-orthodontic treatment adults: for skeletal ClassⅠ and Ⅱ patients, the prevalence of lingual bone dehiscence in the mandibular incisors was significantly higher in the extraction groups than in the non-extraction groups; correspondingly, the lingual alveolar bone was also thinner in the extraction groups; Class Ⅱ non-extraction patients showed a higher prevalence of labial bone fenestration but a lower prevalence of lingual bone fenestration in mandibular incisors compared to Class Ⅱ extraction patients; the orthodontic-orthognathic combined treatment group showed significantly higher prevalence of labial/lingual bone dehiscence and thinner alveolar bone at multiple sites in the mandibular incisors compared to the camouflage group in skeletal Class Ⅲ patients. Comparison of mandibular incisor bone defects and thickness before and after orthodontic treatment in adult patients: in skeletal ClassⅠ and Ⅱ patients treated with premolar extraction and Class Ⅲ patients treated with orthodontic-orthognathic combined treatment, the lingual alveolar bone of mandibular incisors exhibited significant resorption and thinned after treatment, and this was accompanied by an increased prevalence of dehiscence; in non-extraction patients, ClassⅠ non-extraction patients showed thinning of the crestal-labial bone and apical-lingual bone, Class Ⅱ patients showed thinning of the crestal-labial bone and middle-labial bone of the mandibular incisors, along with an increased prevalence of dehiscence Conclusion In malocclusion adults, alveolar bone defects were already present in the mandibular incisors before orthodontic treatment. The alveolar bone defects and thickness in mandibular incisors among post-orthodontic adults were influenced by the treatment plan and Class of skeletal malocclusion.