Correlation of mandibular impacted tooth and bone morphology determined by cone beam computed topography on a premise of third molar operation
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- Momin, M.A., Matsumoto, K., Ejima, K. et al. Surg Radiol Anat (2013) 35: 311. doi:10.1007/s00276-012-1031-y
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To determine the width and morphology of the mandible in the impacted third molar region, and to identify the location of the mandibular canal prior to planning impacted third molar operations.
Cone beam computed tomography (CBCT) data of 87 mandibular third molars from 62 Japanese patients were analyzed in this study. The width of the lingual cortical bone and apex-canal distance were measured from cross-sectional images in which the cortical bone was thinnest at the lingual side in the third molar region. Images were used for measuring the space (distance between the inner border of the lingual cortical bone and outer surface of the third molar root), apex-canal distance (distance from the root of the third molar tooth to the superior border of the inferior alveolar canal) and the cortical bone (width between the inner and outer borders of the lingual cortical bone).
The means of the space, apex-canal distance and lingual cortical width were 0.31, 1.99, and 0.68 mm, respectively. Impacted third molar teeth (types A–C) were observed at the following frequencies: type A (angular) 37 %; type B (horizontal), 42 %; type C (vertical), 21 %. The morphology of the mandible at the third molar region (types D–F) was observed as: type D (round), 49 %; type E (lingual extended), 18 %; and type F (lingual concave), 32 %.
The width and morphology of the mandible with impacted teeth and the location of the mandibular canal at the third molar region could be clearly determined using cross-sectional CBCT images.
KeywordsCone-beam computed tomographyMolarThirdMandibleInferior alveolar nerve
The extraction of a lower impacted third molar tooth is one of the most common oral surgical operations , yet many postoperative complications cause dysaesthesia as a result of damage to the inferior alveolar nerve (IAN) [11, 19] or lingual fracture of the mandible . Previous paper reported that the injury of the IAN and the lingual nerve are typical risks and the most frequent severe complications in the lower third molar extraction [3, 4, 7, 9, 12]. Indeed, previous studies reported the incidence of IAN damage to be 0.4–20.3 % [5, 21, 29], the risk of permanent IAN injury has been less than 1 % [5, 32, 33], and prolonged hemorrhage in 0.5–5.0 %  of patients during lower third molar extraction. Several factors increase the likelihood of developing these complications, including deeply impacted teeth, less experienced surgeons, the use of burs to remove bone, and the relationship between tooth roots and the mandibular canal [6, 30, 33]. Watanabe et al.  evaluated the bone size and morphology of the mandible and the location of the mandibular canal using CT image and conclude that cross sectional CT image is integral to avoid injury of the IAN.
As well as determining the morphology of the mandible, it is necessary to preoperatively evaluate the radiographic relationship between the mandibular canal and impacted third molars. Panoramic radiography is widely available for these purposes, but has several disadvantages such as image distortion and magnifications, leading to inaccuracies . Conventional computed tomography (CT) preoperatively enables the relationship between the mandibular canal and the lower third molars to be precisely determined [10, 24, 26], but necessitates radiation exposure and has high costs. Recently, cone beam CT (CBCT) has emerged as a new diagnostic modality for the use in examination of the maxillofacial region in endodontics , oral carcinoma , preoperative third molar , and temporomandibular joint (TMJ) disorders . It offers the advantages of limited X-ray radiation exposure, lower cost, high spatial resolution, and a smaller space requirement than conventional CT [1, 15, 16, 22, 29]. Moreover, Tantanapornkul et al.  reported that it is superior to panoramic images in predicting neurovascular bundle exposure during extraction of impacted third molar teeth. Previous studies reported that CBCT examination is important for preoperative radiographic evaluation of complicated impacted lower third molar [2, 23]. Ghaeminia et al.  investigated the position of impacted third molar in relation to the mandibular canal, and concluded that CBCT is a more accurate imaging modality for determining the relationship of the third molar to the mandibular canal.
In this study, the bone width and the shape of the mandible at the impacted third molar region and the location of the mandibular canal were determined precisely from CBCT cross-sectional images of 62 Japanese patients. Such precise knowledge will be helpful in optimizing the planning of an extraction of the impacted third molar.
Materials and methods
Patient characteristics and measurements
CBCT examinations were performed with 3D Accuitomo, FPD type (Morita Corp., Kyoto, Japan). The impacted third molars were imaged at a tube voltage of 80 kV, a tube current of 8 mA, and an exposure time of 17 s. After scanning the contiguous sectional images in three directions: parallel section (parallel to the dental arch), cross-section (perpendicular to the dental arch), and horizontal (parallel to the occlusal plan) section, images were reconstructed from the projection data with a slice width of 1 mm, which was perpendicular to the crown to the apex of distal root of the third molar.
All statistical analysis was performed with IBM SPSS Statistics 19 (Japan IBM, Tokyo, Japan). Gender differences were examined, in measurements of space, apex-canal distance and cortex, by the Mann–Whitney U test. The Kruskal–Wallis test was used to compare each measurement of the mandible shape and impaction type. When the statistically significant difference was observed by the Kruskal–Wallis test, the Mann–Whitney U test with the Bonferroni correction was used to evaluate between-group differences. The Spearman’s correlation coefficient by rank was used to compare measurements from two observers. A p value <0.05 was considered to be statistically significant.
Comparison between types of impacted teeth and mandible shape
Relationship between shape of mandibular bone and type of tooth impaction
Measurements of space, apex-mandibular canal distance and lingual cortical thickness
Measurement of variables according to gender
0.32 ± 0.22
2.31 ± 1.87
0.66 ± 0.35
0.29 ± 0.26
1.79 ± 1.39
0.70 ± 0.41
0.31 ± 0.25
1.99 ± 1.60
0.68 ± 0.39
Comparison of variables according to type of tooth impaction
0.31 ± 0.24
1.88 ± 1.54
0.71 ± 0.38
0.32 ± 0.28
2.34 ± 1.71
0.66 ± 0.38
0.27 ± 0.19
1.45 ± 1.35
0.70 ± 0.42
The Spearman’s correlation coefficients by rank of the space, apex-canal distance and lingual cortical thickness between two observers (M.O, M.A.) were 0.51, 0.78, and 0.58, respectively. Kappa coefficients in evaluating the shape of the mandible and lingual cortical thickness were 0.50 and 0.71, respectively.
The most important findings of the present study were that there are no differences between the type of impaction of the mandibular third molar that result from differences in the shape of the mandible, and that the lingual extended type is an anatomical entity compared to the round and lingual concave type.
For successful operations, a dental surgeon must know the angle and/or type of third molar tooth to optimize the selection of the operation procedure, and to prevent the perforation and fracture of the lingual cortical bone. Predicting neurological complications prior to surgical intervention is an obvious wish for all surgeons. The type of tooth impaction and the shape of the mandible bone may influence the third molar operation procedure. IAN injury can result from a number of actions including the use of elevators, which put direct or indirect pressure on the nerve during extraction. Although radiographic assessment is necessary for evaluating the topographic relationship between the mandibular canal and impacted third molar, its two-dimensional image limitations are well known and are unreliable in predicting nerve position .
The present study classified the impacted teeth into three types. Tantanapornkul et al.  previously reported that the horizontal type is the most frequent (52 %), and that the vertical type is rare (16 %). This was largely confirmed in our study (horizontal type, 42 %; vertical type, 21 %), although we also found that the angular type occurs in 37 % of cases. We speculate that the differences between our present data and those of Tantanapornkul et al. reflect the different number of patients in the two studies.
The shape of the mandible is of key importance for appropriate third molar operational procedures, and to prevent any occasional accidents including perforation or fracture of the bone. We classified the shapes of the mandible into three types: type D (round), type E (lingual extended), and type F (lingual concave). Watanabe et al.  evaluated the mandible size and morphology using CT, and reported that a round shape was most frequently observed (59–64 %) and the lingual concavity was present in 18–39 % of cases. We also found the round shape to be the most common (49 %), while lingual concavity was present in 32 % of cases, and the lingual extended type was observed in 18 % of cases. However, no significant differences were shown between the three types of impacted teeth. The impaction type can be determined by panoramic tomography and intraoral projection, and is not associated with mandibular shape which is not diagnosed using these modalities. Therefore, three dimensional imaging using CT and CBCT, or preoperative palpation of the region is necessary to confirm the shape of the mandible. To protect against lingual perforation or IAN damage during elevation, the elevator should be placed according to the angle of the impacted tooth.
We used the CBCT image tool to measure space, apex-canal and lingual cortex as three variables. The measuring precision of this image tool modality has previously been verified, and it is used to measure small subjects such as the thickness of the mandible and the roof of the glenoid fossa . We considered that both space and lingual cortex can affect the risk of lingual cortical fracture during tooth extraction, so measuring the apex-canal distance indicates the risk of damage to the mandibular canal and IAN. The SDs in both space and apex-canal were considerably large showing great variation, and indicating that they are difficult to predict without three dimensional diagnoses. No significant difference was observed in this study between males and females for all three variables, suggesting no gender-specific difference in anatomical risk of lingual cortical fractures or damage of the mandibular canal and IAN. However, the bone quality of the extraction region, which affects the success of the procedure, was not assessed and this should be undertaken in future studies.
Statistical analysis revealed no significant relationship between the type of impaction and space, apex-canal or lingual cortex. Although we predicted that the horizontal and vertical types would have a shorter apex-canal than angular types, this was not proven. The lingobuccal position between the apex and mandibular canal, the curve of the mandibular canal, and cross sectional image evaluation may have affected our results. Sectional sliced imaging (using sagittal or horizontal slices) might produce a different set of results.
Comparison of mandibular shapes demonstrated that space and cortex, predicting factors of lingual cortical fractures, of both round and lingual concave shapes were greater than those of lingual extended. Meanwhile, the apex-canals of lingual extended types were significantly larger than those of round types, and tended to be thicker than those of lingual concave. It should be noted that thinner apex-canals are considered to be associated with a higher risk of IAN mandibular canal damage. Thus, the lingual extended type is a low risk group regarding mandibular canal damage, but a high risk group for lingual cortical fracture; moreover, the operation risk depends on the experience of the surgeon. Furthermore, this type cannot be differentiated from other shapes by intraoral projection, but only using three dimensional image diagnoses or palpation of the region.
In the third molar region, extraction might interfere with the neurovascular bundle in the canal. The location of the mandibular canal was determined in the present study by measuring the apex-canal distance using CBCT images. Values were lowest in the round shape (1.70 ± 1.66 mm) and highest in the lingual extended (2.85 ± 1.60 mm), indicating that the apex-canal distance tends to decrease with the round shaped type. We also observed that apex-canal SDs were greater than those of other variables. It is highly likely that this evaluation point might have affected the length of impacted teeth and the curve of the mandibular canal. The space ranged from 0.11 to 0.36 mm and the lingual cortical bone from 0.44 to 0.74 mm in the third molar region. As the lingual cortical bone has a higher risk of perforation or fracture, the elevator should be carefully elevated in the lingual side. The risk of lingual cortical bone perforation is also greater with the vertical type of impacted teeth, although the lingual cortical bone was almost identical in all types. The possibility of perforation seems to be lowest with the angular type of impacted teeth because of sufficient lingual cortical bone.
In this study, the number of patients of this study population is quite limited. However, the age of the patients is from second decade to eight decades; therefore, we justified all the ages for male and female. Furthermore, the patients were referred from the different dental clinics to our dental hospital for CBCT analysis. Therefore, these patients were clinically categorized as a risk group by screening method.
The panorama radiography has a reasonable diagnostic accuracy in the preoperative evaluation of the relationship between third molars and inferior alveolar canal, including position of the IAC in the vertical plane with variable magnification . The panoramic radiological sign (the darkening of the root, deflection of the root, narrowing of the canal, bifid root apex etc.) could be assisted to operate the third molar extraction ; however, these sign does not show including the buccolingual information and mandibular morphological shape (lingual extended, lingual concave, round shape). Although we understood that the panorama radiography would be used first as a screening modality, our study did not compare panoramic radiography and CBCT findings. In the future study, we will try to indicate the necessity of CBCT examination based on panoramic radiography findings.
In conclusion, we used CBCT cross-sectional images in the present study to precisely determine the bone width, mandible shape, and type of impacted teeth and the location of the mandibular canal, and such knowledge is extremely helpful for oral surgery.
I would like to thank Dr. Koji Hashimoto, who gave the valuable advices for this research work.
Conflict of interest
The authors declare that they have no conflict of interest.