IAN injury is a serious complication during extraction of mandibular third molars. Risk factors for injury include surgeon’s experience, age and sex of the patient, operative tissue damage, postoperative edema and surgical procedures . It has been reported that the most important factor for IAN injury is the anatomical relationship between the impacted third molar and the IAC [13, 14]. However, other authors have emphasized that multiple factors, including surgeon’s experience, surgical technique, institutional setting, and anatomical and radiographic factors are associated with an increased risk of IAC damage [8, 15].
Accurate preoperative evaluation is necessary for successful surgery because the oral surgeon must know the angle and/or type of impacted third molar to select a suitable procedure and to prevent IAN injury and perforation and fracture of the mandible . Panoramic radiography is a standard diagnostic tool for initial assessment of the relationship between the IMTM and the IAC. Because this method produces two-dimensional images, it cannot provide information in axial, coronal and sagittal planes . CBCT is a more reliable imaging method in the preoperative assessment of mandibular third molars [10, 15, 17, 19, 21, 24, 27]. In this study, the correlations between preoperative DPR and CBCT findings and intra- and interobserver agreement were investigated.
The number of mandibular third molar roots visible on DPR versus CBCT images has been investigated in relatively few studies [22, 28]. These studies reported that panoramic radiography has limited accuracy in determining the number of roots and that CBCT is more reliable for this purpose [22, 28]. In the present study, intraobserver agreement was excellent for all variables on both DPR and CBCT images, whereas interobserver agreement was moderate for detection of number of roots on DPR images. Also, there was a statistically significant difference in number of roots detected on DPR versus CBCT images. The results demonstrated that DPR images were inadequate to detect multiple roots of IMTM. This result is in agreement with previous studies [22, 28].
Several studies have reported that the risk of IAN injury increases when specific findings are observed on panoramic images taken to determine the relationship between third molars and the IAC [4, 24, 29–33]. These findings include darkening of the roots and interruption of the white line of the IAC [24, 29–35]. Eyrich et al. reported that narrowing of the IAC increased the risk of IAN impairment . The probability of contact between third molar roots and the IAC was higher in cases with the abovementioned signs on panoramic images [24, 29–36]. Gomes et al. reported no statistically significant association between the presence of panoramic radiographic signs and IAN paresthesia after third molar extraction . However, Ghaeminia et al. found that there was a significant association between panoramic radiographic signs and IAN exposure , a finding that has been supported by several authors [24, 27, 35, 36]. These authors agreed that CBCT is useful for the assessment of IMTM in the buccolingual direction. In the present study, contact between the third molar roots and the IAC was most often detected on CBCT images in cases with interruption of the white line of the IAC and darkening of the roots on DPR images, a finding in agreement with several previous studies [12, 24, 27, 29–32, 34].
Oral surgeons must know the type and/or angle of the impacted third molar before surgery to prevent perforation and fracture of the mandible, and to select appropriate operation procedures . Previous studies have classified the IMTM as vertical, horizontal or angular, based on its orientation to the mandible [21, 32]. Tantanapornkul et al. reported that the horizontal type was the most frequent (52%), followed by angular (32%) and vertical (16%) . Momin et al. reported similar results, with 42% horizontal, 37% angular and 21% vertical . Msagati et al. and Syed et al. found that the mesioangular type was the most common (76% in Msagati’s study and 50.75% in Syed’s study) [37, 38]. Lübbers et al. reported that mesially angulated (40.2%) and vertical (29%) were the most common types . In the present study, the most frequent type was found to be angular (42.28%), followed by vertical (34.24%) and horizontal (23.48%). This finding was in agreement with the results of Lübbers et al. . Differences between studies may arise from different study samples.
The shape of the mandible is an important factor in determining the use of elevators during surgery to avoid direct or indirect pressure on the IAN and perforation or fracture of the bone [21, 39]. Two-dimensional images cannot provide information about bone morphology. Preoperative palpation of the related region and 3D imaging are necessary to determine the shape of the mandible [39–41] and to provide important information for the oral surgeon during elevation. The shape of the posterior mandible has been described in the literature by different classifications for various purposes. The posterior mandible has been categorized as convex, parallel or undercut for implant placement [39–41]. Watanabe et al. classified the mandible as round, lingual concave or buccal concave, and reported that the round shape (61%) was the most common . In contrast, Lin et al. found that the least common type was round (21%) in the posterior mandible . Momin et al. classified the mandible as round, lingual extended or lingual concave to assess bone morphology in the third molar region for preoperative planning. They reported that the round shape (49%) was the most common, followed by lingual concave (32%) and lingual extended (18%) . The authors also investigated the correlation between impaction type and mandibular shape and reported that there was no significant association between these variables. In this study, mandibular shape was classified into three types: round, lingual extended and lingual concave. The prevalence of round and lingual concave types was equal (approximately 46%) and the lingual extended was least common (approximately 8%). This finding is in agreement with the results of Momin et al. . Different mandibular shape prevalence in different studies can arise from racial features. There was a significant difference in impaction type according to mandible shape in this study, in contrast to the findings of Momin et al. .
Surgeon’s knowledge about the location of the IAN is very important in the preoperative evaluation of impacted third molars to direct the elevator and luxate the involved tooth. IAN injuries commonly occur during third molar removal because of compression and traction on the nerve through movements of the tooth roots . IAN injury may occur during elevation of mesioangular impacted third molars because the roots may move downward and may compress the nerve . Also, movements of the third molar root in the buccolingual direction can cause compression of the IAN. The surgical approach is generally started on the buccal side of the impacted third molar in cases in which the surgeon lacks information about the buccolingual course of the IAN before surgery. However, the IAN may experience undesirable forces if it is positioned lingually, and IAN injury has been reported in such cases [17, 23]. CBCT images allow the clinician to perform comprehensive treatment planning and surgical method selection during preoperative assessment . Previous studies have reported that the IAC is most frequently positioned on the lingual side of impacted third molars and that contact between the IAC and the impacted teeth was generally observed in those cases [10, 17, 23, 34]. In the present study, the IAC was most frequently located on the lingual side of the IMTM and they were commonly in contact.