Analysis of External Root Resorption of the Second Molar Associated with an Impaction of the Third Molar by the Application of CBCT

External root resorption (ERR) of a tooth represents a process in which irreversible damage occurs to solid dental tissues, more precisely of the cement and dentin. External resorption occurs on the root surface or periodontal ligament, and it is manifested by the loss of cement and/or dentine. The prevalence of ERR of the second molar associated with an impacted third molar, based on retro-alveolar radiography and orthopantomography, ranges from 0.3% to 24.2%. The aim of this research was to analyze ERR of the upper and lower jaw associated with the impacted third molar by the application of the Come Beam Computed Tomography. This study included 96 third molars in the upper and lower jaw of a total of 46 analyzed patients’ images. Total prevalence of the second molar ERR associated with the impacted third molar was 8.82%. The second molar ERR was observed in 8 patients (7.84%). Frequency of the second molar ERR in the lower jaw is higher, and it is 8.2%, while in the upper jaw it is only 1% (r = 0.032). Impacted third molars associated with the second molar ERR were most frequently in horizontal (5.1%) and mesioangular (4.1%) position (p = 0.000). The second molar ERR could be avoided by the preventive extraction of mesioangular or horizontal third molars. An adequate decision on the extraction of impacted third molars must be made by the careful assessment of risks and benefits of this surgical intervention. Before the surgical intervention, an adequate care of the second molar, adequate restoration, endodontic treatment or root resection should be done if necessary.