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Table of Contents   
ORIGINAL RESEARCH  
Year : 2020  |  Volume : 31  |  Issue : 1  |  Page : 91-102
Comparison of panoramic radiograph and cone beam computed tomography findings for impacted mandibular third molar root and inferior alveolar nerve canal relation


1 Department of Oral Medicine and Radiology, Ahmedabad Dental College and Hospital, Ahmedabad, Gujarat, India
2 Department of Oral Medicine and Radiology, Government Dental College and Hospital, Ahmedabad, Gujarat, India
3 Department of Pedodontics and Preventive Dentistry, Ahmedabad Dental College and Hospital, Ahmedabad, Gujarat, India

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Date of Submission05-Jul-2018
Date of Acceptance08-Oct-2018
Date of Web Publication02-Apr-2020
 

   Abstract 


Objectives: This study was aimed at determining the specific signs of close relationship between impacted mandibular third molar (IMTM) root and inferior alveolar nerve (IAN) canal as visualised on digital panoramic radiograph or orthopantomograph (OPG) which should indicate as well as contraindicate the need for cone beam computed tomography (CBCT) imaging based on hierarchical model of Fryback and Thornbury. Materials and Methods: The sample consisted of 120 individuals (200 IMTM) who had undergone OPG as well as CBCT imaging as preoperative radiographic evaluation before surgical extraction. On panoramic radiographs; Rood and Sheehab's radiographic signs of IMTM root and IAN canal, impacted third molar position based on Winter's classification and depth of impaction based on Pell and Gregory's classification were evaluated. On CBCT; presence/absence of corticalization and the status of the buccal and lingual cortices (thinning/perforation) were evaluated. These findings of OPG were compared to CBCT findings to determine the better modality based on hierarchical model of Fryback and Thornbury. Results: Statistically significant association was found between radiographic signs of Rood and Sheehab such as 'no relation' and 'superimposition' on OPG and presence of corticalization between IMTM root and IAN canal on CBCT. Statistically significant association was also found between 'mesioangular' and 'vertical' positions of Winter's classification as well as 'class 1A' of Pell and Gregory's classification of IMTM on OPG and presence of corticalization on CBCT. CBCT was found to be having lesser variance and greater F value as compared to OPG for evaluation of IMTM. Conclusions: On OPG; Rood and Sheehab's radiographic signs such as 'no relation' and 'superimposition', Winter's 'mesioangular' and 'vertical' and Pell and Gregory's 'Class 1A' are not indicative for CBCT examination before surgical extraction is attempted. On OPG; Rood and Sheehab's radiographic signs such as interruption of white line, darkening of root, darkening of canal, deflection of root, narrowing of canal as well as diversion of canal are all frequently associated with absence of corticalization between IMTM root and IAN canal and hence require CBCT examination before surgical extraction is attempted so that post-operative neurological complications can be minimised. Also, Winter's horizontal, distoangular and others as well Pell and Gregory's classes 1B,1C,2A,2B,2C,3A,3B,3C are indicative for CBCT examination before surgical extraction is attempted. Fryback and Thornbury model-based comparison proves that CBCT is a better radiographic modality as compared to OPG for evaluation of IMTM relation with IAN canal.

Keywords: Cone beam computed tomography, dental panoramic radiography, impacted mandibular third molar, inferior alveolar nerve canal

How to cite this article:
Patel PS, Shah JS, Dudhia BB, Butala PB, Jani YV, Macwan RS. Comparison of panoramic radiograph and cone beam computed tomography findings for impacted mandibular third molar root and inferior alveolar nerve canal relation. Indian J Dent Res 2020;31:91-102

How to cite this URL:
Patel PS, Shah JS, Dudhia BB, Butala PB, Jani YV, Macwan RS. Comparison of panoramic radiograph and cone beam computed tomography findings for impacted mandibular third molar root and inferior alveolar nerve canal relation. Indian J Dent Res [serial online] 2020 [cited 2021 May 10];31:91-102. Available from: https://www.ijdr.in/text.asp?2020/31/1/91/281819



   Introduction Top


Impacted tooth is a tooth that is not allowed to erupt into dental arch within the estimated time because of malposition, lack of space or a physical barrier within the pathway of eruption.[1],[2] Impacted mandibular third molars (IMTM) cause various problems, ranging from pericoronitis, detrimental effects on mandibular second molars and periodontal problems to certain types of cysts or odontogenic tumours, primary or secondary dental crowding.[2],[3],[4],[5],[6] Surgical extraction of impacted mandibular third molars is the most common surgical procedure performed by oral and maxillofacial surgeons.[2],[4],[5],[6],[7],[8],[9],[10],[11],[12],[13],[14],[15],[16],[17],[18]

Several complications are associated with extraction of impacted mandibular third molars.[4] These can be pain, swelling, excessive bleeding, infection and reduced mouth opening; however, neurological complications involving the inferior alveolar nerve (IAN), buccal nerve and the lingual nerve are seen as the most severe postoperative complications after removal of a mandibular third molar.[2],[6],[7],[8],[9],[10],[11],[13],[14],[17],[19],[20],[21],[22],[23],[24],[25] According to various surveys, the rate of these neurological complications has varied from 0.5-1% for cases involving permanent damage and 5-7% for cases involving temporary damage.[6],[11],[14],[17],[20],[26],[27] The risk increases dramatically when there is an intimate relationship between IMTM and IAN canal.[6],[7],[8],[9],[10],[16],[20],[21],[22],[24] In these cases, the incidence of temporary damage to the inferior alveolar nerve rises to about 30% of extractions.[10],[16],[20],[27] In order to minimise number of complications during mandibular third molar extraction, several classifications have been developed that are assessing the difficulty of surgical procedure and helping to create an optimal treatment plan. The most popular are Winter's and Pell and Gregory's systems who are classifying the inclinations and positions of the third molars based on the relation among the dental longitudinal axis, occlusal plane and ascending mandibular ramus. These systems have been extensively adopted and applied in clinical practice.[1],[2],[21] An accurate radiographic topographic diagnosis is necessary to evaluate all possible problems related to a third molar extraction.[5],[18],[20],[21],[24] A preoperative radiograph 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.[1],[5],[8],[9],[13],[15],[16],[19],[25],[28],[29]

Periapical radiographs have been used for many years to assess the jaws during impacted teeth surgery.[1],[11] The dental panoramic radiograph or orthopantomograph (OPG) is the most commonly used radiograph by oral and maxillofacial surgeons to view impacted mandibular third molars and estimate the risk of damage to the inferior alveolar nerve.[6],[9],[10],[11],[14],[16],[17],[18],[19],[21],[22],[23],[24],[27],[29] The major advantages of OPG are the broad coverage of oral structures, relatively low radiation exposure, and relatively inexpensive of the equipment. The major drawbacks of OPG are: lower image resolution, high distortion, and presence of phantom images.[1] Several investigators have described the anatomic relationship of the IMTM root and IAN canal and have identified several radiologic signs.[13] Studies suggest that seven specific signs observed on OPG (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 IMTM and IAN canal. However, the presence or absence of these radiographic signs does not always determine the possibility of injury to the inferior alveolar nerve (IAN).[5],[6],[29] The OPG image does not always provide reliable data on the precise relationship between the anatomical structures as it is two-dimensional and presents distortions.[6],[7],[9],[17],[18],[23],[29]

Cone beam computed tomography (CBCT) has revolutionized diagnostic imaging in dentistry. CBCT imaging provides three-dimensional volumetric data reconstruction of dental and associated maxillofacial structures with isotropic resolution and high dimensional accuracy.[10],[30],[31],[32] Over the last few years, CBCT is becoming more common in clinical practice thanks to its spatial resolution and the lower radiation dose as compared to conventional CT.[6],[7],[21],[24],[28] Increasingly, CBCT is replacing medical CT because it provides adequate image quality associated with a lower exposure dose. 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.[5],[24] Applications in implantology, endodontics, orthodontics and oral and maxillofacial surgery have been reported.[6],[7],[21],[24],[28] Cone Beam Computed Tomography (CBCT) have been specifically advocated as method of choice when there is need to have a three-dimensional view of the impacted mandibular third molar and adjacent anatomical structures such as inferior alveolar nerve canal and lingual cortex.[1],[6],[13],[15],[26],[33]

Monaco et al.,[20] Tantanapornkul et al.,[22],[24] Neves et al.,[5] Jung et al.,[12] Harada et al.,[34] AhmetandYildiray[10] and Vinay Kumar et al.[6] have conducted studies comparing the radiographic signs seen on OPG with cone beam CT findings for investigating the relationship of IMTM with IAN canal; but none of them have included all the nine radiographic signs of Rood and Sheehab. Moreover, none have evaluated/validated CBCT imaging for the same purpose with the help of any study model. Hence, this study attempts to establish a guideline for the use of CBCT imaging for impacted mandibular third molars based on hierarchical model of Fryback and Thornbury. This study has also modified the language content of the hierarchical assessment levels of the Fryback and Thornbury with respect to the impacted mandibular third molar evaluation; which makes it easier to use this model for further research.


   Materials and Methods Top


The present retrospective study was conducted following approval of the institutional ethical committee. The sample consisted of 120 individuals (200 impacted mandibular third molars) who previously underwent a preoperative radiographic evaluation before extraction of the impacted mandibular third molar. Only those individuals which were advised both OPG as well as CBCT for the purpose of impacted mandibular third molar were included for the study. Digital panoramic radiographs (OPG) were obtained using PaxI3D Smart machine (Vatech Co Ltd, Hwaseong-si, Gyeonggi-do, Korea) operating at 74 kVp/12 mA and an exposure time of 10 seconds. Cone beam computed tomographs (CBCT) were obtained with the same machine operating at 94kVp/8.7 mA and an exposure time of 18 seconds for CBCT with a voxel size of 0.1 mm and a field of view of 10 × 8 cm2.

All the OPGs and CBCTs were evaluated independently by two radiologists with at least 2 years of experience in CBCT diagnosis. There was a gap of 15 days between the OPG and CBCT evaluation of any patient by a radiologist in order to avoid memory bias. Any disagreements amongst the radiologists were subsequently resolved after a discussion till both of them reached a mutually agreed conclusion. After a period of 30 days; 50 panoramic radiographs and CBCT images were re-evaluated to analyse the intra-observer agreement.

The images were evaluated on a computer monitor (16 inch LED screen with 1920 × 1080 resolution) under dim lighting conditions after adjustment of brightness and contrast and on a full screen view of the region of interest in order to ensure accuracy in diagnosis.

On the OPG, IMTM was evaluated to determine its relationship with IAN canal according to the criteria established by Rood and Sheehab [Figure 1]. Also, all IMTM positions were classified into one of the classes of Winter's classification as well as Pell and Gregory's classification [Figure 2] and [Figure 3].
Figure 1: Schematic diagrams with cropped panoramic images showing radiographic signs (a) Darkening of the root. (b) Narrowing of the root. (c) Dark and bifid apex. (d) Interruption of the white line of the canal. (e) Narrowing of the canal. (f) Deflection of the root. (g) Diversion of the canal. (h) Superimposition. (i) No relation

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Figure 2: Schematic diagrams with cropped panoramic images showing impacted mandibular third molars by Winteræs classification. (a) Mesioangular. (b) Distoangular. (c) Vertical. (d) Horizontal. (e) Others

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Figure 3: Schematic diagrams with cropped panoramic images showing impacted mandibular third molars by Pell and Gregoryæs classification (Class 1A, Class 2B and Class 3C)

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On the CBCT images, each impacted mandibular third molar was evaluated on axial, sagittal, coronal sections as well as 3D reconstruction to evaluate presence of corticalization (presence of sound cortical bone) or absence of corticalization (absence of sound cortical bone) between the IMTM root and the IAN canal [Figure 4]. Also, evaluated were the position of the IAN canal in relation with the IMTM root whether the buccal and lingual cortices are intact, thinned or perforated in the region of IMTM.
Figure 4: Schematic diagrams with cropped cone beam computed tomography coronal sections showing presence/absence of corticalization between root and inferior alveolar nerve canal. (a) Canal located apical to root with presence of corticalization. (b) Canal located apical to root with absence of corticalization

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The hierarchical model of Fryback and Thornbury was thoroughly assessed and evaluated for each IMTM by each of the two radiologists separately. All the parameters at each of the six levels of the model were carefully evaluated for each IMTM on OPG and CBCT separately. When the modality under study seemed to satisfy the parameters for a particular level of the model; then it was addressed as score 1. Being a hierarchical model; when a radiographic modality failed at a particular level, score 0 was given and the next level evaluation was not performed. Hence, the minimum score possible for a modality for one particular impacted mandibular third molar was 0 and maximum possible score was 6. Typical measures of the levels described in the model are as mentioned below shown in [Table 1].[19],[30],[32],[35]
Table 1: Fryback and thornbury hierarchical model of efficacy (modified to evaluate impacted mandibular third molars)

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Data were analysed using the SPSS software. The kappa statistic was used to calculate intra and inter observer agreement (less than or equal to 0.40 is poor agreement; 0.40-0.59 is moderate agreement; 0.60-0.74 is good agreement and 0.75-1.00 is excellent agreement). Comparison between the OPG and CBCT findings were performed using the Fisher's exact test, with a significance level of P < 0.05. The evaluation of OPG against CBCT by Fryback and Thornbury model was done by one-way ANOVA test. The simple frequency distribution statistic was employed to determine the most common occurrence amongst the various possibilities of the status of buccal and lingual cortices.


   Results Top


The sample consisted of 120 individuals (200 impacted mandibular third molars) composed of 108 males (54%) and 92 females (46%) with age ranged between 19 and 72 years (mean age 45.5 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 0.89 (observer 1: first vs second evaluation), 0.80 (observer 2: first vs. second evaluation) and 0.77 ( first evaluation: observer 1 vs. observer 2). For the CBCT images, the kappa images were 0.80 (observer 1: first vs. second evaluation), 0.86 (observer 2: first vs. second evaluation) and 0.90 ( first evaluation: observer 1 vs. observer 2). For both imaging modalities, the agreement was excellent.

[Table 2] summarizes the correlation of the radiographic signs seen on OPG with presence of corticalization between impacted third molar root and inferior alveolar nerve canal as seen on CBCT. Only radiographic signs like superimposition (P = 0.003) and no relation (P = 0.0001) demonstrated a statistically significant association with presence of corticalization between IMTM and IAN canal. No statistically significant association was demonstrated between all other signs such as interruption of white line, darkening of root, darkening of canal, diversion of canal, deflection of root and narrowing of canal (as isolated findings as well as in combination) with presence of corticalization between the IMTM root and IAN canal (P > 0.05).
Table 2: Correlation between radiographic signs on orthopantomograph with presence/absence of corticalization on cone beam computed tomography

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[Table 3] summarizes the correlation of the Winter's classes of IMTM seen on OPG with presence of corticalization between IMTM root and IAN canal as seen on CBCT. The table denotes that vertical (P = 0.007) and mesioangular (P = 0.001) IMTM demonstrated a statistically significant association with presence of corticalization between impacted third molar root and inferior alveolar nerve canal. No statistically significant association was demonstrated between horizontal, distoangular and other impaction classes of Winter's classification and presence of corticalization between the impacted third molar root and inferior alveolar nerve canal on CBCT (P > 0.05).
Table 3: Correlation of winter's class of impacted third molar on orthopantomograph with presence/absence of corticalization on cone beam computed tomography

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[Table 4] summarizes the correlation of the Pell and Gregory's classes of IMTM seen on OPG with presence of corticalization between IMTM root and IAN canal as seen on CBCT. The table denotes that only Class 1A (P = 0.0002) demonstrated a statistically significant association with presence of corticalization between IMTM root and IAN canal. No statistically significant association was demonstrated between all other classes of Pell and Gregory's classification and presence of corticalization between the impacted third molar root and inferior alveolar nerve canal on CBCT (P > 0.05).
Table 4: Correlation of Pell and Gregory's class of impacted third molar on orthopantomograph with absence of corticalization on cone beam computed tomography

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[Table 5] demonstrates the diagnostic accuracy of panoramic radiograph (while keeping CBCT findings as gold standard) in predicting close relation between the impacted third molar root and inferior alveolar nerve canal. When radiographic signs of close relation (all radiographic signs except no relation) were analysed against no relation; panoramic radiograph showed 98.55% sensitivity and 48.39% specificity.
Table 5: Diagnostic accuracy of orthopantomograph for radiographic signs of close relation between impacted mandibular third molar and inferior alveolar nerve canal

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[Table 6] demonstrates the comparison of OPG versus CBCT scores based on Fryback and Thornbury model. The total scores and mean of both radiographic modalities were evaluated. The overall average of OPG was 2.58 and CBCT was 5.63; variance of OPG was 1.16 and CBCT was 1.08; F value of OPG was 5.80 and CBCT was 6.68; and the P value of OPG was 0.021 and CBCT was 0.01.
Table 6: Comparison of orthopantomograph versus cone beam computed tomography scores based on fryback and thornbury model

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[Table 7] demonstrates the frequency distribution of the patients with various radiographic signs on OPG and the corresponding status of the buccal and lingual cortices seen on CBCT. The table demonstrates that the most number of third molars were associated with lingual cortical thinning (72 third molars) as well as lingual cortical perforation (72 third molars). The next most frequent in occurrence was complete buccal and lingual cortex (27 third molars) and buccal thinning with lingual cortical perforation (16 third molars).
Table 7: Distribution of patients with radiographic signs on orthopantomograph versus status of buccal and lingual cortices on cone beam computed tomography

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[Chart 1] is a three-dimensional linear graphical representation of OPG versus CBCT scores for various radiographic signs based on Fryback and Thornbury model. The OPG score is higher for superimposition and no relation; almost similar for interruption of white line; and higher for CBCT for all other radiographic signs as compared to OPG.



[Chart 2] is a ring pie chart showing distribution of the status of buccal and lingual cortices amongst the study sample. The lingual cortical thinning and perforation are most commonly seen amongst the impacted mandibular third molars.




   Discussion Top


A preoperative radiographic evaluation of the exact relationship between the roots of the impacted mandibular third molar and the inferior alveolar nerve canal would help in predicting, and possibly avoiding postoperative sensory impairment.[6],[11],[21],[24] The panoramic radiograph is a useful screening tool for assessing the anatomical relationship between IMTM root and IAN canal. One of the radiographic interpretation disadvantages of both intraoral and panoramic radiography is that they provide information in only two dimensions.[23] The absence of the cortical bone of the mandibular canal may not be clearly evident with the two dimensional radiographic methods, and it is impossible to determine whether its course is buccal or lingual to the roots or between the roots.[5],[21],[23],[27] Experienced oral and maxillofacial surgeons reported that a dependence on a panoramic radiograph alone was unreliable. Dental cone beam CT has been accepted for its high sensitivity and as significantly superior to panoramic radiographs in predicting neurovascular bundle exposure. It is an excellent method for localizing the canal, and its relationship to the lower third molar roots as reformatted images can be generated through the mandibular body in any plane.[5],[23] Owing to the cost, limited availability and radiation dose, CBCT is usually not the radiographic technique of choice for a preoperative radiographic evaluation of lower third molars.[5],[23],[27],[29]

Many studies have suggested risk factors for the close relationship between the tooth and the mandibular canal or IAN injury based on the findings from panoramic images.[12],[14],[24],[27] Rood and Sheehab distinguished four radiographic indicators observed in the tooth root (darkening, deflection and narrowing of the root, and a bifid root apex), and the other three in the canal (diversion, narrowing, and interruption in the white line of the canal).[1],[16] The absence of these signs could not fully ensure lack of close contact. This means that when the roots overproject the mandibular canal in the panoramic image and, in particular, if one or more of the signs are present, further radiographic examination to clarify the three-dimensional relationship between the two structures may be suggested.[10],[19] Hence; when IMTM root is touching the IAN canal with absence of any of the above-mentioned signs, it is considered to be 'superimposition' of the IMTM root over the IAN canal. Also, the relationship can be called 'no relation' when presence of healthy and intact bone is visible on OPG between the IMTM root and IAN canal. As there are no studies which have taken the presence of 'superimposition' and 'no relation' signs into consideration; this study was aimed at encompassing all possibilities of the relationship by adding two more signs to already described seven radiographic signs.

When OPG shows an anatomic intimacy between IMTM and IAN canal or when these specific radiographic signs are detected on the radiograph, CBCT examination has to be performed to confirm the real presence and eventually the type of the relationship on a buccal/lingual section.[5],[19],[21],[23],[24] This is in accordance with the current study, which also demonstrates that except for 'no relation' and 'superimposition', all other radiographic signs (whether found as isolated findings or in combinations) are indicative of an absence of corticalization between IMTM root and IAN canal on the CBCT images [Table 2]. This is in accordance with studies[5],[12],[14] which state that radiographic signs such as the darkening of the roots, the interruption of the white lines of the canal, the diversion of the canal, and the narrowing of the canal are associated with neurosensory disturbances or with a close relationship between the lower third molar and the mandibular canal. If no signs of close relationship are observed, the anatomic information obtained with panoramic radiograph is sufficient to plan the surgery.[1],[12],[22]

The impaction degree of mandibular third molar in the ramus of mandible is associated with extraction operation difficulty score and postoperative complications manifestation. Distoangular or horizontal impacted tooth position in combination with deep impaction in the mandibular ramus, can be complicated case even for experienced clinician.[1],[16],[20] This study evaluated the position of the impacted mandibular third molar based on Winter's classification (relation with mandibular second molar) and Pell and Gregory's classification (depth of impaction in relation to the mandibular ramus). The findings of this study are in accordance with the literature[1],[16],[20] that except vertical and mesioangular impacted mandibular third molars, all other class as per Winter's classification were associated with absence of corticalization between IMTM root and IAN canal [Table 3]. This study also demonstrated that except Class 1A of Pell and Gregory's classification, all other classes were associated with absence of corticalization between IMTM root and IAN canal [Table 4].

Literature has showed the negativity of sensitivity and specificity of various panoramic radiographic signs that predict IAN injury during mandibular third molar extraction; their results did not possess predictive ability to determine the relation between IAN and mandibular third molar.[6] This study demonstrated that OPG has 98.55% sensitivity and 48.39% specificity [Table 5]. These findings indicated that OPG might be an excellent screening tool for detecting close relations between IMTM root and IAN canal; but confirmation of these close relations would require three-dimensional imaging. Thus, additional and more advanced 3D imaging techniques such as CBCT scans, are required to clarify these anatomical relationships prior to surgical extraction and there by prevent IAN injury.[6]

The use of any ionizing radiation-based imaging modality is associated with a risk of radiation-induced cancer. The basic premise that justifies the use of these imaging modalities is that the benefits provided by the radiographic examination must far outweigh the risks associated with radiation exposure. Such benefits may be in the form of increased diagnostic efficacy, enhanced treatment planning or better therapeutic outcomes. In an era of increasing health care expenses, it is also important to justify higher costs that may be associated with a newer imaging modality.[30] Selection of an imaging modality for a particular diagnostic task must be driven by the fact that the modality provides the most accurate diagnosis. However, the impact of enhanced diagnosis on decision making and therapeutic outcome must also be considered in the choice of the most appropriate imaging modality.[30],[31] To address these principles, Fryback and Thornbury proposed a six-tiered hierarchical model to evaluate applications of new technologies for diagnostic imaging.[19],[30],[32],[35] There is a paucity of research articles aimed at validating the use of CBCT for impacted mandibular third molars by means of any such assessment model; hence this study was conducted with an aim of performing such analysis; and thereby, establishing the guideline for use of CBCT in impacted mandibular third molar cases. Also, this study has attempted to modify the Fryback and Thornbury model to make it more understandable with respect to evaluating the impacted third molars by means of OPG and CBCT. This study demonstrates that as the P value and variance of CBCT is lesser than that of OPG while F value of CBCT is greater than that of OPG; CBCT is proven to be a better radiographic modality than OPG for evaluating the impacted mandibular third molars [Table 6] and [Chart 1].

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. It has been 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. 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.[22] If the mandibular canal is seen to be positioned lingual to the root or between roots on CBCT, a surgical procedure would involve a high risk of exposing the inferior nerve and a high risk of postoperative dysesthesia of the inferior nerve.[9] In accordance with the above-mentioned studies and to overcome the drawbacks of panoramic radiography, a three dimensional (3D) imaging with CBCT is recommended for cases in which IMTM root and IAN canal are in close proximity, so as to evaluate the bucco/lingual direction of IAN.[6],[7],[8] The detailed information hence achieved proves to be helpful when deciding whether to remove, not remove or how to remove a symptomless third molar and when obtaining correct informed consent.[7],[8]

This study demonstrated that thinning and perforation of the lingual cortex was the most common occurrence. Also, the radiographic sign 'interruption of the white line' was most commonly associated with lingual cortical thinning, perforation as well as with intact buccal and lingual cortices [Table 7] and [Chart 2]. Also; signs such as interruption of white line, darkening of root, narrowing of root, darkening of canal, narrowing of canal, deviation of canal and deflection of roots are more associated with thinning and perforation of the lingual cortex (both as isolated findings and in combinations). This is in accordance with literature which states that, 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.[5]


   Conclusions Top


The impacted mandibular third molar as seen on panoramic radiograph provides limited guidance to the oral surgeon about the difficulty that will be encountered in surgical extraction as well as about the possibility of its post-operative neurological complications [Chart 3].



On OPG; Rood and Sheehab's radiographic signs such as 'no relation' and 'superimposition', Winter's IMTM position such as mesioangular and vertical and Pell and Gregory's Class 1A are not indicative for CBCT examination before surgical extraction is attempted.

On OPG; Rood and Sheehab's radiographic signs such as interruption of white line, darkening of root, darkening of canal, deflection of root, narrowing of canal as well as diversion of canal are all frequently associated with absence of corticalization between impacted third molar root and inferior alveolar nerve canal and hence result in postoperative neurological complications. Also, Winter's IMTM position such as horizontal, distoangular and others as well as Pell and Gregory's classes 1B, 1C, 2A, 2B, 2C, 3A, 3B, 3C are associated with absence of corticalization between IMTM root and IAN canal. For such cases, three-dimensional examination like CBCT imaging is must before surgical extraction is attempted.

OPG has been established as a screening tool for oral surgeons to identify the difficult disimpaction cases as well as to prior inform and take consent of patients about the possible complications. The decision- [Chart 2]: Ring pie chart showing the distribution of status of the cortices amongst the patients under studymaking process whether to opt for CBCT imaging or not after OPG can be made easier as well as more accurate with the help of the flowchart devised with the help of this study.

The modified Fryback and Thornbury model proved that CBCT is a better radiographic modality as compared to OPG for evaluation of impacted mandibular third molars. This study warrants further research using such model while keeping actual relation between IMTM root and IAN canal found at the time of surgical extraction as gold standard for validating even the CBCT findings.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
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DOI: 10.4103/ijdr.IJDR_540_18

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