| Objective Mandibular interforaminal region was no longer safe for dental implant and associated surgery because of the frequent reports on unexpected severe blooding and pain during and after the surgery. These complications was described as a possible results of bony canal and foramen injury in this region. The purpose of this study was to size and locate the mandibular incisive canal, mental foramen and anterior loop on cone beam computed tomography(CBCT) images, and also compare these images with another usual method—anoramic radiograph(OPG). According to the statistic data and image description, the present study would also recommand a new safety margin for mandibular interforaminal surgery.Materials and Methods The characteristics of the anatomical landmarks in the mandibular anterior region were investigated using CBCT and OPG images obtained from 30 dentulous patients (male13, female 17; average age31.3, age from12to 68) and 3 edentulous. All patients'images were obtained from Stomatology Hospital affiliated to Zhejiang University(Hangzhou, Zhejiang, China), from November,2009 to January, 2010. After image processing and reconstruction, two professional dentists conducted the measurement and observation as follows:①Evaluate the image clarity of incisive canal, mental foramen and anterior loop in mandibular interforaminal region on CBCT and OPG images, by rating them to three grades--- invisible (0 points), visible but not clear (1 point), clear (2 points). And then compared the identification and resolution between both oral radiographies.②Measure the mandibular incisive canal, mental foramen and anterior loop by diameters of three dimensions, distance from upper/lower/lingual/labial mandibular cortex, canal length and other parameters to describe the size, shape, location and directions of these important anatomies. All data was analyzed by statistical package of SPSS 16.0 software.Results①The identification of incisive canal and anterior loop by CBCT was obviously higher than OPG, as well as the image resolution.②On CBCT images:The diameters of incisive canal(vertical x labial-lingual) at different mesial-distal position(mesial of mental foramen, second premolar, first premolar, canine, lateral incisor, middle incisor)were as follows:3.200×3.200mm,2.763×2.800mm,2.146 x 1.984mm,1.840×1.759mm,1.689×1.617mm,1.517×1.483mm,1.150×1.175mm in the left side,2.336×2.467mm,3.500×3.033mm,2.202×2.005mm,1.908×1.837mm,1.755×1.550mm,1.690×1.457mm,1.471×1.333mm in the right side。The vertical position of incisive canal in mandibule was in the lower 1/3, while the labial-lingual position was in the middle 1/3。The average length of incisive canal is 14.255mm in the left side and 15.283mm in the right side. It walked towards the middle of mandibule, and commonly disappears at the incisor apical. It was occasionally observed a terminal branch goes up towards the crest.③On CBCT images:The occurrence rate of mental foramen on both side was 100%(30/30), while the accessory mental foramen was 20%(6/30,left) and 46.67%(14/30, right)。The occurrence rate of anterior loop was 80.56%(29/36, left) and 65.91%(29/44, right). The diameters of mental foramen(vertical×mesial-distal) was 2.315×3.678mm on the left and 2.095 x 3.355mm on the right。Mental foramen was often observed in the apical of second premolar, while the accessory mental foramen was detected between apical of second premolar and first molar or between apical of canine and lateral incisor. The average length of anterior loop was 5.710mm on the left and 5.955 on the right.Conclusion Respecting current study, new safty margins are suggested:The chin bone harvest should be at lease 8mm mesial to bilateral mental foramen and have a maximum harvest depth of 4mm; The premolar implant mesial to mental foramen should be shorter than 10mm.It is even better to make a individual surgery plan according to CBCT images.
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