Correlation of panoramic radiography and cone beam CT findings in the assessment of the relationship between impacted mandibular third molars and the mandibular canal

Abstract

Objectives

The aim of this study was to assess the reliability of four panoramic radiographic findings, both individually and in association, in predicting the absence of corticalization between the mandibular canal and the third molar on cone beam CT (CBCT) images.

Methods

The sample consisted of 72 individuals (142 mandibular third molars) who underwent pre-operative radiographic evaluation before extraction of impacted mandibular third molars. On panoramic radiographs, the most common signs of corticalization (darkening of roots, diversion of mandibular canal, narrowing of mandibular canal and interruption of white line) and the presence or absence of corticalization between the mandibular third molar and the mandibular canal on CBCT images were evaluated.

Results

Darkening of roots and interruption of white line associated with the absence of corticalization between the mandibular third molar and the mandibular canal on CBCT images were statistically significant, both as isolated findings (p = 0.0001 and p = 0.0006, respectively) and in association (p = 0.002). No statistically significant association was observed for the other panoramic radiographic findings, either individually or in association (p > 0.05).

Conclusion

Darkening of roots and interruption of white line observed on panoramic radiographs, both as isolated findings and in association, were effective in determining the risk relationship between the tooth roots and the mandibular canal, requiring three-dimensional evaluation of the case.

Introduction

Extraction of third molars is a routine procedure in maxillofacial surgery, whether for prophylactic or for orthodontic reasons. Usually there is little risk to adjacent structures, although in some cases there may be complications owing to the intimate relationship between the roots of the third molars and the mandibular canal and/or the mandibular lingual cortex.

Third molars have a high incidence of impaction associated with many conditions such as pericoronitis, caries on the distal surface of the second molar, pain, external root resorption and odontogenic cysts or tumours.^1,2 Difficulties in the eruption of the third molars, particularly in the mandible, are attributed to a lack of space caused by late formation and the phylogeny of the mandible.¹

An imaging exam is undoubtedly an essential tool for diagnosis and surgical management because it provides valuable information about the tooth position, the number/morphology of the roots and, especially, the relationship of the tooth to adjacent structures. Studies suggest that seven specific signs observed on panoramic radiograph (darkening of roots; deflection of roots; narrowing of roots; bifid root apex; diversion of canal; narrowing of canal; interruption in the white line of the canal) are reliable ways to assess the relationship between the third molar and the mandibular canal.^3,4 However, the presence or absence of these radiographic signs does not always determine the possibility of injury to the inferior alveolar nerve (IAN). This indicates that panoramic radiograph does not have a high diagnostic accuracy when used in the assessment of risk in surgical extractions of lower third molars.⁵

Cone beam CT (CBCT) is a radiographic method that has been used in several areas of dentistry because it shows three-dimensional (3D) images of dental structures in addition to providing clear structural images with high contrast. It is being used increasingly in many dental specialties, including orthodontics,^6,7 orthognathic surgery^8,9 and implant dentistry.¹⁰ Increasingly, CBCT is replacing medical CT because it provides adequate image quality associated with a lower exposure dose.^11,12 Other advantages of CBCT are the low cost of the examination compared with CT, fast scanning time, lower number of artefacts and real-time image analysis.^13,14

The present study may provide information for maxillofacial surgeons who need to decide whether CBCT is required in addition to pre-operative panoramic radiography in mandibular third molar extractions. Therefore, the aim of this study was to assess the reliability of four panoramic radiographic signs, as both isolated findings and in association with each other, in predicting the absence of corticalization between the mandibular canal and the third molar on CBCT images.

Materials and methods

The present retrospective case-control study was carried out following approval of the FOP/UNICAMP Ethical Committee (057/2010), and informed consent was obtained from all volunteers. The sample consisted of 72 individuals (142 mandibular third molars) who underwent pre-operative radiographic evaluation before extraction of impacted mandibular third molars. Digital panoramic radiographs were obtained using an Orthopantomograph® OP100 D unit (Instrumentarium Corp., Imaging Division, Tuusula, Finland), operating at 66 kVp/2.5 mA and an exposure time of 17.6 s. 3D images were obtained using a Classic i-CAT CBCT unit (Imaging Sciences International, Inc., Hatfield, PA), operating at 120 kVp/8 mA, with a 0.25 mm voxel size and a field of view of 8 cm. Individuals with radiological evidence of intra-osseous pathologies (e.g. cysts or tumours) associated with the third molars were excluded from the study.

Panoramic radiography and CBCT images were evaluated independently by two radiologists with at least 2 years of experience in CBCT diagnosis, on a computer monitor (21 inch LCD monitor with 1280 × 1024 resolution), under dim lighting conditions. The images were evaluated with the use of the “zoom” tool and manipulation of brightness and contrast. Any disagreements were resolved subsequent to the initial observation, when each observer discussed his findings with the other in order to reach a final evaluation. After 30 days, 40 panoramic radiographs and CBCT images had been re-evaluated.

On the panoramic radiographs, the relationship between the mandibular third molar and the mandibular canal was evaluated according to the criteria established by Rood and Shehab.³ In the present study, we used the most common signs (darkening of roots, diversion of mandibular canal, narrowing of mandibular canal and interruption in white line) associated with a higher risk of IAN injury^15-18 (Figure 1). On the CBCT images, the presence or absence of corticalization between the mandibular third molar and the mandibular canal was evaluated using cross-sectional images. Absence of corticalization was defined as a loss of cortical lining between the tooth root and the mandibular canal (Figure 2).

Figure 1.

Cropped panoramic radiographs showing (a) darkening of roots, (b) diversion of mandibular canal, (c) narrowing of mandibular canal and (d) interruption in white line

Figure 2.

Cone beam CT cross-sectional images showing (a) presence and (b) absence of corticalization between the mandibular third molar and the mandibular canal

Data were analysed using BioEstat software v. 5.0 (Ayres Company, Pará, Brazil). The kappa statistic was used to calculate intra- and interobserver agreement (less than or equal to 0.40, poor agreement; 0.40–0.59, moderate agreement; 0.60–0.74, good agreement; 0.75–1.00, excellent agreement). Comparison between panoramic and CBCT findings was performed using Fisher's exact test, with a significance level of p < 0.05.

Results

The sample was composed of 28 males (38.9%) and 44 females (61.1%); ages ranged between 18 and 39 years (mean 27.2 years). Statistically significant differences were not observed for gender or age (p > 0.05). The intra- and interobserver agreement kappa values for the panoramic radiographs were 0.77 (Observer 1: first vs second evaluation), 0.84 (Observer 2: first vs second evaluation) and 0.76 (first evaluation: Observer 1 vs Observer 2). For the CBCT images, the kappa values were 0.79 (Observer 1: first vs second evaluation), 0.86 (Observer 2: first vs second evaluation) and 0.82 (first evaluation: Observer 1 vs Observer 2). For both image modalities, the agreement was excellent.

Table 1 summarizes the panoramic and CBCT findings according to the relationship between the roots of the third molars and the mandibular canal. Only darkening of roots and interruption in white line, as both isolated findings (p = 0.0001 and p = 0.0006, respectively) and in association with each other (p = 0.002), demonstrated a statistically significant association with the absence of corticalization between the mandibular third molar and the mandibular canal in CBCT images. No statistically significant association was observed for the other panoramic radiographic signs, either as isolated findings or in association with each other (p > 0.05).

Table 1. Correlation between panoramic and CBCT findings.

Radiographic findings	Cases in panoramic radiography (n = 142)	Presence of corticalization in CBCT images (n = 142)	Absence of corticalization in CBCT images (n = 142)	p-value
DR	25 (17.6)	6 (4.2)	19 (13.4)	0.0001
DMC	6 (4.2)	2 (1.4)	4 (2.8)	>0.05
NMC	5 (3.5)	4 (2.8)	1 (0.7)	>0.05
IWL	29 (20.4)	12 (8.5)	17 (12.0)	0.0006
DR + DMC	2 (1.4)	0 (0.0)	2 (1.4)	>0.05
DR + NMC	0 (0.0)	0 (0.0)	0 (0.0)	>0.05
DR + IWL	5 (3.5)	0 (0.0)	5 (3.5)	0.002
DMC + NMC	0 (0.0)	0 (0.0)	0 (0.0)	>0.05
DMC + IWL	2 (1.4)	0 (0.0)	2 (1.4)	>0.05
NMC + IWL	1 (0.7)	0 (0.0)	1 (0.7)	>0.05

CBCT, cone beam CT; DMC, diversion of mandibular canal; DR, darkening of roots; NMC, narrowing of mandibular canal; IWL, interruption in white line.

Values are given as number (percentage) unless otherwise indicated.

Discussion

An accurate radiographic diagnosis is essential to evaluate and predict the possible outcome related to the extraction of impacted third molars. Periapical radiography (obtained using the parallel technique), panoramic radiography and CT offer an increasing level of image definition and enable the maxillofacial surgeon to make a more accurate diagnosis.

In oral and maxillofacial surgery, panoramic radiography is a supplementary examination initially requested to assess impacted third molars and estimate the risk of IAN damage. However, this image modality has a distortion, so it does not always provide enough information to determine the actual risk level.

Several studies^15,17-24 that evaluated the relationship between the mandibular canal and the third molar suggest pre-operative evaluation by CT when specific signs are observed in panoramic radiography. In the present study, most panoramic radiographic signs were interruption of white line (20.4%) and darkening of roots (17.4%). These findings conform with those in other studies.^{5,16,17,21,25,26}

The anatomical proximity of the roots of the third molar and the mandibular canal can result in IAN injuries during the extraction of the third molars. To avoid this problem, many studies have proposed risk factors based on the findings of panoramic radiography.^15,17-24 However, as a two-dimensional exam, it does not provide specific diagnostic information about the relationship between the third molar and the mandibular canal. Knowledge of this relationship information allows for secure management in cases of severe impactions.

Several studies have demonstrated that the presence of two or more signs on a panoramic radiograph indicates an increased risk of IAN exposure^{15,18,20,24,27} or injury.^5,16,19,24 However, comparison of CBCT findings with two panoramic radiographic signs in association has not been previously investigated. In the present study, darkening of roots and interruption in white line associated with the absence of corticalization between the mandibular third molar and the mandibular canal in CBCT images were statistically significant, as both isolated findings and in association with each other. In the study by Gomes et al,⁵ no statistically significant association was observed between the presence of the seven panoramic radiographic signs and IAN paraesthesia after third molar extractions. However, Ghaeminia et al²⁷ observed that three panoramic radiographic signs (interruption of white line, darkening of roots and diversion of the mandibular canal) were significantly associated with IAN exposure. Clinical studies have demonstrated that the absence of corticalization between the tooth roots and the mandibular canal on spiral CT images¹⁵ and IAN exposure^15,18 are significantly associated only with some panoramic radiographic signs—in particular, darkening of roots and interruption of white line. Szalma et al¹⁶ identified three signs on panoramic radiograph that were significantly associated with IAN paraesthesia (interruption of white line, diversion of the mandibular canal and darkening of roots).

Monaco et al²¹ determined the reliability of panoramic radiograph in evaluating the relationship between the mandibular canal and the roots of the third molar based on multidetector CT images. According to the authors, the 3D exam should be carried out when darkening of roots, narrowing of the mandibular canal and interruption of white line are observed on panoramic radiograph. In our results, we found that the narrowing of the mandibular canal was not associated with higher risk of contact between the tooth roots and the mandibular canal, either as an isolated finding or in association with other panoramic radiographic signs, based on CBCT images.

In a few cases, darkening of the roots could be associated with thinning or perforation of the lingual cortical by the tooth roots, instead of being related to a “true contact” relationship between the mandibular canal and the tooth roots.^22,28 In the present study, this relationship could not be assessed because the thinning or perforation of the lingual cortical by the tooth roots was not evaluated.

The results obtained in the present study corroborate with those of Nakagawa et al,²⁹ in which absence of white line on panoramic radiograph was significantly associated with an increased risk of contact between the third molar and the mandibular canal on CBCT images; however, this relationship was not observed by Nakamori et al.²⁴

It has been observed that only 3D exams, i.e. CT^{15,17,22-24,26} and MRI,³⁰ are effective in determining the true relationship of the tooth roots and the mandibular canal. However, in view of the socioeconomic conditions of some developing countries, the high cost of those exams and the radiation dose (for CT only) makes the use of panoramic radiography clearly justified in the pre-operative evaluation in third molar surgery, despite the fact that its predictability is low with regards to the emergence of nerve lesions during the procedure.

In conclusion, panoramic radiography is an effective method for pre-operative assessment of mandibular third molars. Darkened roots and interruption of white line observed on panoramic radiographs, as both isolated findings and in association with each other, are effective in determining the risk relationship between the tooth roots and the mandibular canal, requiring 3D evaluation of the case.