Classification of impacted mandibular third molars using cone beam computed tomography based on neurological risks: N.R.C.

Introduction: Considering the close proximity of the third molar roots and the mandibular canal, also the location of the lingual nerve and its anatomic variations, both nerves may be injured during extraction. The aim of this observational study was to optimize a classification of the relationship between the third molar and its neuroanatomical environment, mandibular canal and lingual bone plate, which is inspired from the neurological risks involving cone beam CT (CBCT) images, and to apply this classification to our sample. Materials and methods: A retrospective, observational study was performed involving 100 CBCT (171 mandibular third molars). Two medical doctors, qualified in human anatomy, independently evaluated all CBCT using coronal oblique sections. The classification was approved by agreement between both observers. After that, it was applied to our sample by one of them. The SPSS software, version 21.0 (SPSS® Inc., Chicago, Illinois, USA) was used for the statistical analysis. Results: The final consensual classification presented three Classes (A, B, L) with their four subclasses (from 1 to 4). The rare cases found in our sample were grouped separately under the name of “uncommon”, and were divided into four specific subclasses. The status of the lingual bone plate was assessed by the presence or the absence of a fenestration or a cortical thinning. In the current study, when the mandibular canal presented a direct contact with the third molar with a reduced calibre, lingual course was the most frequent anatomical situation in both genders (males 66.7%/females 70.5%). There is not a significant correlation between the proximity of the third molar to the mandibular canal neither with age nor with gender. In contrast, the proximity to the third molar is significantly associated with the course of the canal. The highest significance was found with the lingual course. Statistical analysis showed a nonsignificant correlation between the status of the lingual bone plate and the age then the gender. Discussion and conclusion: The use of the proposed classification (NRC) could be a support for clinical practice. We suggested a common language among operators and during collaboration with radiologists in order to facilitate the clinical discussion and to correlate the surgery outcomes to the CBCT classes for more adjustment of the surgical procedures