[Cone beam CT image analysis of mandibular second and third molar association area]

Objective: To explore the imaging characterization of mandibular third molar (M3) and mandibular second molar (M2) and their associated areas using cone beam CT (CBCT). Methods: A total of 226 images of patients with concomitant mandibular second and third molars in the Department of Oral and Maxillofacial Medical Imaging of Nanjing Stomatology Hospital were selected to retrospectively analyze their clinical information and imaging manifestations from January 2020 to January 2024, and their clinical information and imaging performances were analyzed. Clinical information included patient's age, gender, and chief complaint; dental tissues (crowns, roots) and periodontal tissues (periodontium, alveolar bone) of the mandibular second molar were evaluated based on their CBCT, and the position (high, medium, low), type of obstruction, dental tissues, periodontal tissues and bone margins of the alveolar bone between them, extent (concave, oblique, flat) and depth of bone defects, and periodontal membrane imaging changes of the mandibular third molars etc. Results: There were 104 cases of mandibular M2 and M3 as the main complaints and 122 cases of non-main complaints in this study. According to the imaging aspects, the mandibular M2 and M3 associated regions with abnormal CBCT imaging manifestations were mainly categorized into six sagittal representation phenotypes (types Ⅰ-Ⅵ); among them, middle-aged and elderly patients≥45 years of age more often showed abnormal imaging phenotypes (75%), and the peripheral bone defects were more severe. The correlation between the type and height of the mandibular M3 obstruction (high:medium:low=42∶110∶74) and the imaging typology of the mandibular M2 and M3 association regions was statistically significant (P<0.001). The bone defects between mandibular M2 and M3 of the young patients≤25 years of age were predominantly of the oblique type of resorption (41/69 cases), and the least of the flat type (1/69 cases), whereas flat type could reached 27.8% in the middle-aged and elderly patients≥45 years of age, and the circumferential angle of the peripheral bone defects in the mandibular M2 and M3 regions showed strong correlation with age (P<0.001), typology (P<0.001), and the height of the obstruction (P=0.004). The correlation between defect depth and obstruction height for the same fractal type was statistically significant in the age groups≤25 years and 26-44 years (P_{≤25 years}=0.0385, P_{26~44 years}=0.032). There was a significant correlation between mandibular M3 with different types of obstruction, especially proximal-medial (34/43 cases) and horizontal obstruction (8/43 cases), and neighboring M2 suffering from caries (P=0.004). The correlation between obstruction height (median 46.3%, low 50.0%) and M2 extra-root resorption (80 cases) was statistically significant (P<0.001). Conclusions: In this study, we explored the imaging characteristics of the mandibular M2 and M3 association region using CBCT, and established a six-type sagittal classification system for the M2 and M3 association area, providing a reproducible framework for the systematic assessment of the overall status of this region. Further analysis revealed that the incidence of radiographic abnormalities in the bone tissue of this area significantly increased with age. Concurrently, the study confirmed that the impaction status of mandibular M3 is associated with an elevated risk of dental and periodontal diseases affecting M2. These specific findings provide important insights into the pathological relationships within the mandibular M2\M3 region and support clinical decision-making.