الفهرس 
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Abstract
A wisdom tooth (or third molar), refers to one of the molars in humans. Wisdom teeth generally appear between the ages of 17 and 25. Among all the teeth, mandibular third molars are the most commonly impacted teeth.
The extraction of impacted mandibular third molars is the most common minor dentoalveolar surgery in a dental office. Some complications may be encountered during the surgery. The most important complication is postsurgical sensory impairment.
Before planning extraction of an impacted mandibular third molar, the proximity between the mandibular canal and the impacted molar should be assessed to minimize the risk of inferior alveolar nerve damage.
CBCT systems have been designed for imaging hard tissues of the maxillofacial region. CBCT is capable of providing sub-millimeter resolution in images of high diagnostic quality, with short scanning times. CBCT is well suited for imaging the craniofacial area. It provides clear images of highly contrasted structures and is extremely useful for evaluating bone.
This study is aiming to study the validity of CBCT in preoperative assessment of mandibular third molar to avoid nerve injury, in cases where there is radiographic evidence of close proximity between the inferior dental canal and mandibular third molar.
Thirty patients with 40 impacted third molar were selected from outpatients of dental clinics of Minia University Dental Hospital. The patients were of both sexes and varying ages above 18 with the mean age 28 years , All cases were subjected to history taking, clinical examination and CBCT examination.
The both the intra examiner agreements and inter examiner agreements of radiographic signs CBCT and presence of contact in CBCT were almost perfect between observers (> .90).
Most teeth in this study were partially impacted and had two roots, important factors in surgery, Additionally, the mandibular canal was primarily located inferior to the mandibular third molar in (45%) of cases, lingual to mandibular third molar in (30%) of cases, buccal to mandibular third molar in (20%) of cases, and between the roots in (5%) of cases.
In this study the incidence of root contact with the canal in CBCT was found in 65% of the third molars.
The results of this study showed that narrowing of the mandibular canal in panorama had a predictive value of contact status (63%).
According to our findings, it seems that the probability of tooth contact with the mandibular canal increases in the presence of interruption of white line, deviation of the canal or darkening of the roots and espically in the cases in which more than one of the studied panoramic findings exist.
The study showed that tooth contact with the mandibular canal was observed in (81%) of the cases that showed the presence of more than one of the panoramic signs studied.
CBCT provide a valid method in the preoperative assessment of impacted mandibular third molar.