Abstract: BACKGROUND: The root morphology of the mandibular second molar is complex and diverse, making root canal treatment challenging. In addition to the mandibular third molar, the root apex of the mandibular second molar is the closest to the mandibular canal among the mandibular posterior teeth. During root canal treatment and apical surgery on the mandibular second molar, the inferior alveolar nerve is at risk of accidental damage, which can lead to changes in temperature sensation, pain, and mechanical sensitivity in the affected area, ultimately impacting the oral physiological functions of patients.
OBJECTIVE: To investigate the distance and positional relationship between the root apex of the mandibular second molar and the mandibular canal across various anatomical types, and to evaluate the influence of sex, side, and age on this relationship with the purpose of enhancing the safety of mandibular second molar treatments.
METHODS: The cone beam computed tomography data of 330 eligible patients were included in the final analysis. A total of 660 mandibular second molars from 164 males and 166 females were analyzed. The cohort included 200 cases in the youth group, 103 cases in the middle-aged group, and 27 cases in the older adult group. The distance and positional relationship between the root apex of the mandibular second molar and the mandibular canal were measured across different subtypes, including conical fused root type, double-rooted type, and C-shaped root type. SPSS 24.0 software was used to compare and analyze the differences across genders, sides, and ages.
RESULTS AND CONCLUSION: (1) The most common type of double-rooted type was often found in males. This root morphology in female and young individuals was more complex, and the detection rates for conical fused roots and C-shaped roots were higher. (2) The apex of the conical fused root was closest to the mandibular canal, followed by the C-shaped root and then the double-rooted type. The distal root of the double-rooted type was closer to the mandibular canal than the mesial root. In males, the distance from the mandibular canal to the proximal and distal roots of the double-rooted type was greater than in females. Additionally, the distance between the mandibular canal and the root apex of the mandibular second molar increased with age. (3) The positional relationship between the mandibular canal and the mandibular second molar can be classified as buccal lateral, directly below the root apex, lingual lateral, close to the root apex, or through the root tip. Conical fused roots and C-shaped roots in females are more likely to be located directly above the mandibular canal than in males. (4) The results indicated that conical fused roots and C-shaped roots were more common in female and young individuals, with a shorter distance between the root apex and the mandibular canal. Furthermore, the positions of these two root types were closely related, which increases the risk of damage to the inferior alveolar nerve during root canal treatment.

Key words: mandibular second molar, root apex, mandibular neural tube, distance, location, CBCT, inferior alveolar nerve, root canal therapy