Dentomaxillofacial Radiology (2009) 38, 11-16
© 2009 British Institute of Radiology
doi: 10.1259/dmfr/83819416
Correlation of darkening of impacted mandibular third molar root on digital panoramic images with cone beam computed tomography findings
W Tantanapornkul*,1,2,
K Okochi1,
A Bhakdinaronk2,
N Ohbayashi1 and
T Kurabayashi1
1Oral and Maxillofacial Radiology, Graduate School, Tokyo Medical and Dental University, Tokyo, Japan; 2Department of Oral Diagnosis, Faculty of Dentistry, Naresuan University, Phitsanulok, Thailand
*Correspondence to: Weeraya Tantanapornkul, Oral and Maxillofacial Radiology, Graduate School, Tokyo Medical and Dental University, Yushima 1-5-45, Bunkyo-ku, Tokyo 113-8549, Japan. E-mail: weeray_t{at}yahoo.com
Received 8 November 2007;
revised 11 December 2007;
accepted 27 December 2007
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Abstract
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Objectives: Darkening of the lower third molar root on panoramic images is known to indicate an intimate relationship between the root and mandibular canal. The objective of this study was to investigate the anatomical relationship between the third molar root and its surrounding structures that leads to this panoramic finding.
Methods: Imaging findings of 253 impacted lower third molars examined by both digital panoramic radiography and cone beam CT were reviewed. Panoramic images were evaluated to detect the presence or absence of darkening of the root where the mandibular canal was superimposed. Cone beam CT images were evaluated for the presence or absence of the following two findings: (1) grooving of the root and (2) thinning or perforation of the cortical plate by the root. The correlation between the panoramic and cone beam CT findings was examined using logistic regression analysis.
Results: 80 (32%) third molars showed a panoramic finding of darkening of the root. Between cone beam CT findings, cortical thinning or perforation alone was significantly correlated with this panoramic finding (80%, P < 0.001).
Conclusions: The panoramic finding of mandibular third molar root darkening was considered to reflect cortical thinning or perforation rather than grooving of the root.
Keywords: cone beam computed tomography; mandibular canal; panoramic radiography; third molar
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Introduction
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The extraction of impacted mandibular third molars may cause dysaesthesia due to damage to the inferior alveolar nerve. Several factors are considered to be associated with nerve dysaesthesia.1–3 It is known that the risk markedly increases when there is direct contact between the nerve and tooth root.3, 4 Panoramic radiographs are most commonly used for assessing the relationship between these two structures preoperatively.
Many studies analysing panoramic imaging features reported that the darkening of the third molar root where the mandibular canal was superimposed was strongly suggestive of an intimate relationship between the root and nerve, or nerve injury following third molar extraction.2–10 Such a dark band across the root is considered to reflect grooving of the root carved by the canal.6, 11, 12 It has been widely accepted that a loss of root dentin caused by the canal reduces the radiopacity on X-ray images. On the other hand, Mahasantipiya et al13 recently reported that darkening of the root may represent thinning of the cortical plate rather than grooving of the root, although they did not show any evidence to support their view.
Although the presence of darkening of the mandibular third molar root is an important panoramic finding for dentists and oral surgeons, it is still uncertain what anatomical features this finding reflects. To answer this clinical question, we correlated the imaging features of panoramic radiographs with those of cone beam CT, which could provide information on the three-dimensional anatomical relationship between the third molar root and adjacent structures.
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Materials and methods
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The study was approved by our institutional review board and written informed consent was obtained from all patients.
Patients and data collection
All patients who consulted the Oral Surgery Department, Dental Hospital, Tokyo Medical and Dental University, for extraction of impacted mandibular third molars between October 2004 and September 2006 underwent pre-operative panoramic radiography. Among them, 196 patients (253 impacted teeth) who underwent additional examination by cone beam CT because of panoramic features of superimposition of the tooth root on the canal were included in this prospective study. The patients comprised 71 males and 125 females, with a mean age of 32 years (range, 18–69 years).
Imaging
Digital panoramic radiographs were taken using Super Veraviewepocs (Morita Corp., Kyoto, Japan) operated at 60–80 kVp and 5–10 mA, with a photostimulable phosphor plate (ST III; Fuji Film Medical Co., Ltd, Tokyo, Japan), and the plates were processed with an FCR 7000 system (Fuji Film Medical Co., Ltd.) The panoramic images were evaluated on a cathode ray tube (CRT) monitor (Dell E193FP; Dell Japan, Kanagawa, Japan).
For the cone beam CT apparatus, the 3DX multi-image micro CT (3DX; Morita Corp., Kyoto, Japan) developed by Arai et al14 was used. The impacted third molars were imaged at a tube voltage of 80 kV, a tube current of 2 mA and exposure time of 17 s. After scanning, contiguous sectional images in three directions (parallel section (parallel to the dental arch), cross section (perpendicular to the dental arch) and horizontal section images) were reconstructed from the projection data with a slice width of 1 mm. Scrolling the contiguous sectional images using dedicated 3DX software (Morita Corp., Kyoto, Japan), the observers evaluated the images on a CRT monitor. When necessary, the window setting was adjusted to optimize images for evaluation.
Evaluation of images
Two observers (WT and KO) independently evaluated the images. Panoramic images were evaluated for the presence of the darkening of the impacted third molar root where the mandibular canal was superimposed. For cone beam CT images, the presence of each of the following two features was evaluated:
- grooving of the root caused by the mandibular canal
- thinning or perforation of the cortex by the root and/or the mandibular canal
The coronal sections were used for cone beam CT evaluation because they showed the relationship between the tooth root and adjacent structures most clearly.
When disagreement existed between the two observers, consensus was reached by discussion. Interobserver agreement was evaluated using kappa statistics.
Statistical analysis
Cone beam CT features that correlated with the darkening of the root on panoramic images were defined by univariate logistic regression analysis. The sensitivity, specificity and accuracy of each cone beam CT feature in depicting the darkening of the root on panoramic images were calculated. P < 0.05 was considered significant.
The kappa values were calculated to assess interobserver agreement. A kappa value of less than 0.40 was considered to show poor agreement; that of 0.40–0.59, fair agreement; that of 0.60–0.74, good agreement; and that of 0.75–1.00, excellent agreement.15 SPSS 13.0 for Windows (Northwestern University Information Technology, Evanston, IL) was used for all calculations.
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Results
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Interobserver agreement was excellent for the assessment of panoramic images, showing a kappa value of 0.75. In the assessment of cone beam CT images, agreement was excellent: 0.79 for the grooving of the root, and 0.91 for thinning or perforation of the cortex.
80 (60 patients) of the 253 impacted teeth showed a panoramic feature of darkening of the root and the remaining 173 teeth showed no such feature. The position of the 80 impacted teeth and their buccolingual relationship to the mandibular canal based on imaging findings are shown in 
Tables 1 and 2. On cone beam CT, the grooving of the root caused by the canal and thinning or perforation of the cortex were found in 66 and 120 cases, respectively. As shown in Table 3, logistic regression analysis showed that thinning or perforation of the cortex alone was significantly correlated with darkening of the root on panoramic images.
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Table 1 Position in bone of 80 impacted mandibular third molars showing panoramic feature of darkening of the root
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Table 2 Buccolingual position of the mandibular canal in relation to the tooth root in 80 impacted third molars as determined by cone beam CT
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When it was used as a diagnostic criterion to predict the appearance of root darkening, the sensitivity, specificity and accuracy were 80%, 68% and 72%, respectively (Table 4). Representative cases are shown in Figures 1–3.
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Figure 1 (a,b) 27-year-old woman with impacted third molar of the left mandible. (a) Darkening of mesial root was found on panoramic image. (b) On cross-sectional cone beam CT image, the thinning of the lingual cortex by the root and the mandibular canal (arrow) were clearly seen. (c,d) 51-year-old man with impacted third molar of the left mandible. (c) Root darkening was present on the panoramic image. (d) Perforation of the lingual cortex by the roots was found on cone beam CT image (arrow). In both cases, there was no grooving of the root
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Figure 2 (a,b) 30-year-old woman with impacted third molar of the right mandible. Even though darkening of the third molar root was not found on (a) panoramic image, (b) cone beam CT image showed grooving of the roots (arrow)
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Figure 3 (a,b) 26-year-old man with impacted third molar of the right mandible. Although (a) the panoramic finding that indicates the close relationship between the third molar root and the mandibular canal was found, only (b) lingual cortex perforation (arrow) was found on cone beam CT image without direct contact between the root and the mandibular canal
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Discussion
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Numerous studies2–10 have been performed to determine panoramic imaging features and have reported that the darkening of the third molar root where the mandibular canal was superimposed was strongly suggestive of an intimate relationship between the root and canal, or nerve injury after extraction. This panoramic finding is considered to reflect grooving of the root caused by the canal. The results of the present study were not consistent with those of previously published findings:6, 11, 12, 14 a significant correlation between grooving of the third molar root by the canal on cone beam CT images and darkening of the third molar root on panoramic images was not found in our study. However, our study results were in accordance with the report by Mahasantipiya et al,13 who stated that darkening of the root may represent thinning of the cortical plate rather than grooving of the root (Table 3
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We evaluated digital panoramic radiography and found the darkening of the third molar root where the mandibular canal was superimposed in 80 (32%) of 253 teeth. The frequency of such a panoramic finding varies widely depending on the researchers (6–51%).3–5, 7, 8, 11, 13 On comparison with those studies, the frequency of the darkening of the third molar root in our study was not beyond this range.
Although the sensitivity (the probability that cases in which panoramic images showing darkening of the root would show thinning or perforation of the cortex on cone beam CT images) was 80% (64 of 80), the specificity (the proportion of cases in which darkening of the root was not seen on panoramic images who did not show thinning or perforation of the cortex on cone beam CT images) showed a relatively low value (68%; 117 of 173) (Table 4). This suggests difficulty in indicating whether the cortical plate was thinned or perforated when darkening of the root was not seen on panoramic images.
In the present study, we focused on factors that affect the presence of third molar root darkening on panoramic images through the use of cone beam CT images as the standard. This technique can provide three-dimensional images with a high resolution and low radiation dose.16 Applications in such general clinical fields as dental implant, endodontics, periodontics and oral surgical procedures have also been reported.16–20
Such cortical thinning or perforation found in 80% of the cases with root darkening on panoramic images is highly relevant to the surgeon and might serve to lower the risk of damage to the lingual nerve, fracturing of the lingual cortex and dislocations of bone or tooth fragments to the adjacent structures such as the floor of the mouth or sublingual space. In addition, the patient can be given more information about the procedure and the associated risk before and after the extraction. The fracture of the lingual cortical plate, the accidental displacement of the third molar or its root fragments or traumatic injuries to the related structures are not common, but are serious complications of third molar extraction. Stacy21 reported that the presence of a perforation of the lingual alveolar plate associated with the lower third molar is serious, as it creates direct access to the submandibular space. Such complications usually occurred when the tooth was located lingually, where there was fenestration of the lingual cortical plate with root exposure, and where the surgical technique may have been inadequate.22, 23 Idiopathic perforation of the lingual bone plate in the third molar region is another important concern. The socket of the lower third molar lies in the lingual prominence of the mandible, so that the lingual bone plate is much thinner than the buccal plate. Occasionally, the lingual bone is very thin and the root apices may actually perforate it. Perforation of the lingual bone plate in the third molar region may be caused by two factors: depression of the tooth into the bone and excessive periosteal resorption of the bone.24 Displacement of a tooth or root into the subperiosteal space through lingual cortical bone loss can occur during third molar removal, resulting in lingual anaesthesia caused by laceration of the lingual nerve.25
Interestingly, we found one case in which, in spite of the panoramic finding indicating the close relationship between the third molar root and mandibular canal, the finding of lingual cortex perforation was found only on the cone beam CT image and not direct contact between the root and mandibular canal (Figure 3). It may be said that, other than the close relationship between these two structures, root darkening on panoramic radiographs can also reflect perforation of the lingual cortex. However, further investigations are necessary to validate its clinical usefulness.
In conclusion, the panoramic finding of impacted mandibular third molar root darkening was considered to reflect thinning or perforation of the cortical plate rather than grooving of the root. Cortical thinning or perforation was found in 80% of the cases with this panoramic finding. Such information will be important for surgeons to avoid the risk of lingual nerve injury at the time of extraction.
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Abstract
Introduction
