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 Table of Contents  
ORIGINAL ARTICLE
Year : 2021  |  Volume : 9  |  Issue : 1  |  Page : 6-16

Can a panoramic radiograph predict the inferior alveolar nerve canal's position, or canal compression, near third molar root apices?


1 Department of Oral and Maxillofacial Surgery, Walter Reed National Military Medical Center, Bethesda, MD, North Chicago, IL, USA
2 Captain James A. Lovell Federal Health Care Center, North Chicago, IL, USA

Date of Submission27-Feb-2021
Date of Decision20-Mar-2021
Date of Acceptance23-Mar-2021
Date of Web Publication20-May-2021

Correspondence Address:
Peter D Cervenka
Naval Medical Center Portsmouth, Naval Dental Clinic Norfolk, 1647 Admiral Taussig Blvd., BLDG. CD3, 2nd Flr, Room 260, Norfolk, VA 23511
USA
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/jomr.jomr_4_21

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  Abstract 


Background: Injury to the inferior alveolar nerve can occur during surgical removal of the mandibular third molars (M3Ms), resulting in numbness of the mandibular teeth, chin, and lower lip. This occurs when the roots of M3M compress the inferior alveolar canal (IAC) against the lingual cortical plate. Cone-beam computed tomography (CBCT) reveals the buccolingual relationship of the IAC and M3M, but the panoramic radiograph (PR) has not been evaluated to determine whether the plane film can reveal this relationship. Aims: The aim of the study is to determine whether PR could predict buccal or lingual IAC position or compression near M3M. Design: This retrospective study evaluated 200 M3M sites in 42 women and 67 men, aged 17–28 years. Ninety-one bilateral measurements were taken; 98 left and 102 right halves were analyzed. Methods: IAC position and degree of compression were interpreted from the CBCT. Utilizing imaging software, 13 different measurements were obtained. Statistical Analyses: Fisher's exact test, t-test, principal component analysis, and multivariate analysis were utilized. Results: Male and female canal positions significantly differed. Canal position was significantly different comparing partially erupted to erupted and fully bony impacted molars. Lingual canal position, versus buccal, was more frequently associated with moderate-to-severe IAC compression. “Anterior-posterior ramus at the occlusal plane” and “anterior ramus to anterior IAC at the occlusal plane” contributed most to the severity of IAC compression. Conclusion: The variables evaluated did not identify a significant relationship. A larger data set is needed to evaluate any role the dimension of the ramus has on IAC compression.

Keywords: Anatomy, computed tomography, digital imaging/radiology, imaging, oral and maxillofacial surgery, radiography


How to cite this article:
Cervenka PD, Burkes JN, Steffy DD. Can a panoramic radiograph predict the inferior alveolar nerve canal's position, or canal compression, near third molar root apices?. J Oral Maxillofac Radiol 2021;9:6-16

How to cite this URL:
Cervenka PD, Burkes JN, Steffy DD. Can a panoramic radiograph predict the inferior alveolar nerve canal's position, or canal compression, near third molar root apices?. J Oral Maxillofac Radiol [serial online] 2021 [cited 2021 Jun 19];9:6-16. Available from: https://www.joomr.org/text.asp?2021/9/1/6/316484




  Introduction Top


Appreciation of the inferior alveolar nerve canal (IAC) location and cortication status near third molar root apices is paramount in anticipation of increased risk for nerve injury.[1],[2],[3],[4] Panoramic radiography (PR) is the standard diagnostic screening modality for routine third molar surgery[5] but cannot distinguish the IAC buccal–lingual relationship to the mandibular third molar (M3Ms). The vertical tube shift[6] addresses this issue though not without limitations. The radiographic signs studied by Howe and Poynton[7] and popularized by Rood and Shehab[8] have increased our ability to identify cases with increased risk for nerve injury.[9],[10],[11],[12],[13],[14] Arguing questionable reliability of these indicators[15] prompts others to seek reassurance with three-dimensional visualization.[16] Cone-beam computed tomography (CBCT) reveals the IAC buccolingual relationship to the M3M[17],[18],[19] though its use is not associated with a decreased incidence of nerve injury, its value lies in predicting nerve exposure and the increased potential for nerve injury following removal of M3M.[9],[18],[20],[21],[22],[23],[24],[25] However, CBCT utilization is burdened by increased radiation exposure, added cost, and availability.

Background

Previous studies on IAC anatomy have evaluated the canal's vertical position,[26],[27] the relationship to nonthird molar root apices,[28] bi- and tri-furcated variations,[29],[30] buccal-lingual position variation with age, gender, and race,[31],[32] buccal-lingual position in relation to the ramus,[33] horizontal canal location,[17],[34],[35] and the canal's anterior-posterior ascent[36] and diameter[37],[38] along with characteristic variations of the mental foramen.[39],[40] In addition, the inferior alveolar neurovascular bundle anatomy has been studied extensively.[38],[41],[42]

Previous anatomical CBCT studies have characterized the buccal-lingual course of the mandibular canal as it traverses the ramus and mandibular body,[33] compared caliper determined canal measurements as they relate to CBCT derived measurements,[43] studied the canals relationship to mandibular tooth root apices, adjacent bone margins, the mental foramen, and the anterior loop of the canal,[44] and have cross compared cadaver and CBCT determined canal diameters for accuracy.[45] CBCT studies have also used 3D statistical models to deduce the canal's course,[46] the canal's course within the ramus of class III prognathism,[47] and the canal's relationship to premolars and molars.[48]

Furthermore, multiple studies have cross compared third molar IAC PR findings with CBCT analysis evaluating the canals position in relation to root darkening and loss of the IAC white line,[49] have identified loss of IAC cortication as cause for darkening of the roots,[50],[51]investigated radiographic findings associated with IAC cortical absence,[13],[52],[53] have determined the anatomic reliability of reconstructed volumetric computed tomographic panoramic images compared to conventional PR,[54] have evaluated the accuracy in determining the anatomical position of the M3M in relation to the IAC[9] and have identified risk factors associated with nerve injury.[9],[18],[24],[25],[53]

When the IAC is positioned lingual to the M3M with a change in canal shape (compression) and loss of the cortical canal margin, there is an increased risk for nerve exposure/transient or permanent nerve injury.[9],[18],[24],[25] The CBCT evaluation reveals the significance of PR findings perceived as IAC M3M root intimacy and has refined interpretation of the plane film.

To our knowledge, there has not been a study to determine whether mandibular dimensions could predict IAC position and shape. The purpose of this study was to investigate whether mandibular dimensions (distances, angles, and ratios) from a PR can predict IAC positioning and/or canal compression near third molar root apices.


  Methods Top


This is a retrospective radiographic study of imaging from routine third molar (wisdom) teeth evaluations between July 2018 and March 2019. Due to the retrospective nature of this study, it was granted an exemption in writing by the institutional review boards at Hines VA (18-013) and the National Capital Consortium (896,402). PRs were obtained on a Planmeca Promax Panoramic machine; radiographs were reviewed using X-ray DCV viewer (Apteryx Imaging Inc., Akron, OH, USA). Patients whose radiographs displayed IAC and M3M intimacy [Figure 1] as evidenced by IAC superimposition in addition to darkening or deflection of the roots, deviation of the IAC, or loss of the canal's cortical margin underwent CBCT imaging (Kodak Carestream 9300C). Imaging that revealed paracoronal, or intrabony, pathosis or impacted, or missing, adjacent second molars was not included in the study. CBCT images were interpreted by an oral radiologist who, in conjunction with clinical findings, recorded third molar eruption status (erupted, partially erupted [PE], or full bony impaction), angulation (vertical, mesioangular, horizontal, or distoangular), IAC canal position (buccal or lingual to the M3M), and degree of compression (none, mild, moderate, or severe) [Figure 2]. Utilizing mandibular halves as study sites, at least 100 lingually and 100 buccally IAC positioned sites were collected. Only corresponding diagnostic radiographs were considered (visualization of the entire mandible, neutral posture, and aligned focal trough) for analysis. From the radiograph and dental record, the patient's age and gender were recorded, and images were then deidentified and uploaded into Dolphin Imaging software (Dolphin Imaging, Chatsworth, CA, USA). Radiographs were divided into two equal groups and then analyzed by separate investigators (PC and JB). Utilizing Dolphin's image measurement interface, calibration was performed based on the manufacture's panoramic digital sensor size (14 cm × 20 cm), and dimensional analysis was performed on the mandibular half of the respective M3M site.
Figure 1: A representative panoramic radiograph exhibiting bilateral inferior alveolar canal-third molar root intimacy

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Figure 2: Cone-beam computed tomographic coronal sections through M3M sites demonstrating mild (a), moderate (b), and severe (c) compression of the inferior alveolar canal. M3M: Mandibular third molar

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The measurements obtained were as follows [Figure 3]: (1) “anterior-posterior ramus at occlusal plane,” the distance, in millimeters (all measurements in millimeters unless otherwise noted), from anterior to posterior ramal cortex along a line extending posteriorly from the cusp tip of the mandibular second premolar/bicuspid to the distobuccal cusp tip of the mandibular second molar (hereafter referred to as occlusal plane.; (2) “anterior ramus to lingula” (in figures as AR to Ant IAC at Occ plane), the distance from the anterior ramal cortex to the anterior cortical margin of the lingula along the occlusal plane; (3) “posterior ramus to lingula” (in figures as PR to Post IAC at Occ Lane), the distance from the posterior ramal cortex to the posterior cortical margin of the lingula along the occlusal plane; (4) “sigmoid notch to lingula,” the distance from the inferior most portion of the sigmoid notch to the superior most cortical margin of the lingula; (5) “ramus to inferior border angle,” the angle, in degrees, formed by the intersection of a line drawn from the posterior most cortical margin of the condyle to the most inferior and posterior cortical margin of the ramus intersecting a line from the inferior most cortical margin of the mandibular body at the mental foramen to the most posterior and inferior cortical margin of the mandibular body at the angle; (6) “condyle to inferior border,” the distance from the most superior and posterior cortical margin of the condyle to the most inferior and posterior cortical margin of the mandibular angle; (7) “tooth height to mandibular body height,” the ratio, in decimal value, of the distance in millimeters from the distobuccal cusp tip, to the inferior most point of the distal root apex, of the second molar to the distance from the alveolar crest cortical margin of the interproximal bone between the first and second molar to the inferior most cortical margin of the mandibular body; (8) “IAC width at M3M” (in figures as Canal Width) the distance from the superior to the inferior cortical margins of the IAC along a line drawn from the distobuccal cusp tip, to the distal root apex, of the M3M; (9) “IAC height at the lingula,” the greatest vertical distance between the superior and inferior cortical margins of the IAC at the lingula; (10) “IAC to inferior border at second molar,” the distance from the inferior cortical margin of the IAC to the inferior-most cortical margin of the inferior border of the mandible, along a line drawn from the distobuccal cusp tip to the distal root apex of the second molar; (11) “IAC to inferior border at first molar,” the distance from the inferior cortical margin of the IAC to the inferior-most cortical margin of the inferior border of the mandible, along a line drawn from the distobuccal cusp tip to the distal root apex of the first molar; (12) “alveolar crest to superior mental foramen,” the distance from the cortical margin of the alveolar crest to the superior most cortical margin of the mental foramen along a line drawn perpendicular from the alveolar crest to the mental foramen; (13) “inferior border to inferior mental foramen,” the distance from the inferior most cortical margin of the inferior border of the mandible to the inferior most cortical margin of the mental foramen along a line drawn perpendicular from the inferior border of the mandible. These 13 measurements, along with patient and third molar demographics, were used as variables to determine if their values, or a combination thereof, related to IAC position and/or compression.
Figure 3: The 13 measurements obtained: (1) Anterior-posterior ramus at occlusal plane; (2) Anterior ramus to lingual; (3) Posterior ramus to lingula; (4) Sigmoid notch to lingula; (5) Ramus to inferior border angle; (6) Condyle to inferior border; (7) Tooth height to mandibular body height; (8) inferior alveolar canal width at M3M; (9) inferior alveolar canal height at the lingula; (10) inferior alveolar canal to inferior border at second molar; (11) inferior alveolar canal to inferior border at first molar; (12) Alveolar crest to superior mental foramen; (13) Inferior border to inferior mental foramen. M3M: Mandibular third molar

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Statistical analyses were carried out using Fisher's exact test and t-test. Individual measurements from the 13 variables were analyzed in RStudio (Boston, MA, USA), utilizing the packages readxl and psych.[55] The packages factoextra[56] and ropls[57] were utilized for principal component analysis (PCA) and multivariate analyses. The multivariate analyses were carried out with scaled numerical data excluding age, adding categorical variables as classifiers. Ggplot2[58] in RStudio and Graphpad Prism[59] were used for visualization of the plots.


  Results Top


Radiographic images from 109 patients, 42 women and 67 men, aged 17–28 years were included in the study (mean age = 18.77); 70 patients were aged 18 years old. Ninety-two out of 109 patients had bilateral mandible measurements taken; 98 left (site #17) and 103 right (site #32) halves were analyzed, creating a matrix of 201 mandible measurements from 109 patients. CBCT analysis confirmed that 100 sites had buccal and 101 sites had lingual IAC position. Of these, female IAC positions were 34.32% buccal (n = 23) and 65.67% lingual (n = 44). In contrast, male IAC positions were 57.89% buccal (n = 77) and 42.11% lingual (n = 56) [Figure 4]. This difference in IAC position between the two genders was found to be significant (P = 0.0026, Fisher's exact test).
Figure 4: Gender and inferior alveolar canal position differences, as a percentage

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Of the 201 analysis sites, 45 molars were erupted (E), 149 were PE, and 7 were unerupted/full bony impacted. A total of 52 vertical, 88 mesio-angular, 15 disto-angular, and 46 horizontally positioned teeth were observed. There was no significant difference in eruption status based on gender (P = 0.7295, Fisher's exact test), age (P = 0.07781, Fisher's test), or tooth angulation (P = 0.09401). However, there was a significant difference in IAC position and tooth eruption status (P = 0.0001478, Fisher's exact test). Erupted and fully bony erupted molars were more frequent in the mandible measurements with a buccal IAC position (n = 38) compared to lingual (n = 14). Partially bony erupted molars were more frequent in the mandibles with a lingual IAC position (n = 86) than buccal (n = 62) [Figure 5]. Lingual IAC position was found to be more frequently associated with moderate-to-severe compression (n = 69) compared to a buccal IAC position (n = 19). This is in contrast to the molars with buccal IAC position (n = 81) having none-to-mild IAC compression than those with a lingual IAC position (n = 31) [Figure 6]. This association is statistically significant (P ≤ 2.2e-16, Fisher's exact test). The measurements obtained from the panoramic radiographs are summarized in [Table 1] and [Table 2].
Figure 5: Inferior alveolar canal position and tooth eruption status. Erupted (E), full bony impacted, and partially erupted

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Figure 6: Degree of inferior alveolar canal compression and buccal-lingual position

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Table 1: Summary of measurements obtained from panoramic radiographs having inferior alveolar canals positioned buccal to the third molar

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Table 2: Summary of measurements obtained from panoramic radiographs having inferior alveolar canals positioned lingual to the third molar

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To address whether IAC position and degree of compression could be explained by the numerical measurements, PCA was carried out. Principal components (PCs) divide the multidimensional space into two vectors that best capture variance within the data. Unsupervised PCA was able to explain only 28.6% (PC1/dimension 1 [Dim1]) and 16.2% (PC2/dimension 2 [Dim2]) variance of the data [Figure 7]. For a dataset to show some degree of variability between different groups, you would expect either PC1 or PC2 to be above 40% and explain together up to 90% of dataset variability.
Figure 7: Principal component analysis. Dim1: Dimension 1, Dim2: Dimension 2, A. P.: Anterior: posterior, Occ: Occlusal, AR: Anterior ramus, Ant: Anterior, IAC: Inferior alveolar canal, PR: Posterior ramus, Post: Posterior, mandib: Mandibular, MF: Mental foramen, contrib: Contribution

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Most variables had a low contribution to either PC1/Dim1 or PC2/Dim2 components [Figure 8], with eight of the variables having 10%–30% contribution. Therefore, most numerical measurements could only explain a small portion of variance within the data.
Figure 8: A summary of the measurements with the greatest contribution to vectors (Dim: 1 and Dim: 2) capturing variance within the data. A.P.: Anterior: posterior, Occ: Occlusal, AR: anterior ramus, Ant: Anterior, IAC: Inferior alveolar canal, PR: Posterior ramus, Post: Posterior, mandib: Mandibular, MF: Mental foramen

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To rule out the contribution of individual variables that could explain IAC position or degree of compression, a supervised partial least squares discriminate analysis (PLS-DA) was performed. This assigned coordinates and divided the data set along the most promising vectors. However, no clear separation between buccal or lingual IAC position was observed and the PCs could only explain 24% of the variance combined [Figure 9]. Similarly, PLS-DA aimed at explaining IAC degree of compression did not find conclusive results and could only explain 39% variance (PC1 = 19%, PC2 = 20%) [Figure 10].
Figure 9: Inferior alveolar canal position partial least squares discriminate analysis: supervised partial least squares discriminate analysis forcing separation into buccal (B) and lingual (I) directions

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Figure 10: Inferior alveolar canal degree of compression supervised partial least squares discriminate analysis forcing separation into none, mild, moderate, and severe

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The measurements “anterior-posterior ramus at occlusal plane” and “anterior ramus to lingula” were found to be the most promising variables explaining IAC degree of compression, contributing 59.13% [Figure 11] and 66.74% to PC1, respectively [Figure 12].
Figure 11: Inferior alveolar canal degree of compression and the measurement “AP ramus Occ plane,” anterior-posterior ramus at the occlusal plane

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Figure 12: Inferior alveolar canal degree of compression and the measurement “AR-Ant inferior alveolar canal Occ Plane,” anterior ramus to the anterior margin inferior alveolar canal at the occlusal plane

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Plotting the values of “anterior-posterior ramus at occlusal plane” and “anterior ramus to lingula” revealed potential differences between none and severe IAC compression groups. T-test between buccal positioned IAC–none and lingual positioned IAC–severe compression groups did not reach significance value but does suggest a potential difference between the groups (P = 0.0548). T-test between the severe/none IAC compression groups was significant for the “anterior ramus to lingula” (P = 0.0029); however, the sample size was too small to establish a potential relationship (severe n = 29, none n = 30). In addition, no difference was found between other IAC compression groups (mild and moderate) and there seems to be no clear relationship between “anterior-posterior ramus at occlusal plane” and “anterior ramus to lingula” and IAC degree of compression.

To fully rule out any potential variables able to separate none/severe IAC compression groups, a cluster dendrogram (k = 2) was carried out using all numerical measurements [Figure 13] but was unable to classify them.
Figure 13: Cluster dendogram evaluating inferior alveolar canal compression groups: “None” and “severe” using all 13 measured variables

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Finally, we compared the measurements between left and right mandibular halves to establish potential anatomical differences between the sides. Measurements from 92 patients were used and revealed no significant linear differences between the anatomical sites [Figure 14].
Figure 14: Comparing right (y) and left (x) sites (n = 92) across the 13 variables. (1) anterior-posterior ramus at occlusal plane; (2) anterior ramus to lingula; (3) posterior ramus to lingula; (4) sigmoid notch to lingula; (5) ramus to inferior border angle; (6) condyle to inferior border; (7) tooth height to mandibular body height; (8) inferior alveolar canal width at the M3M; (10) inferior alveolar canal to inferior border at second molar; (11) inferior alveolar canal to inferior border at first molar; (12) Alveolar crest to superior mental foramen; (13) Inferior border to inferior mental foramen. M3M: Mandibular third molar

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  Discussion Top


The inherent distortion of PR imaging[60],[61] was deliberately accepted as a consistent and unavoidable variable in the study design. At times, our designated landmarks were not easily appreciated, and precision approximation had to be utilized. Our results show a significant difference in IAC positions between men and women which is supported more by previous findings[32],[62],[63],[64] than those opposing.[65] In contrast to males, females are more likely to have lingual, versus buccally positioned IACs in cases of M3M intimacy. The age group,[16],[17],[18],[19],[20],[21],[22],[23],[24],[25],[26],[27] though coinciding with average ages for M3M removal, might be viewed as a limiting for three reasons; first, vertical impactions (52 in this study) may migrate with age, changing the IAC relationship from lingual, or buccal, to inferior to the apices. Second, mesial impacted, buccally inclined, M3Ms could increase their inclination with age, changing the degree of IAC compression. Finally, if a M3M is retained until elderliness bone loss from disuse atrophy and age-related resorption could alter the IAC M3M relationship.[48]

Our study design prevented random evaluation of third molar sites regarding eruption status. Furthermore, of those erupted third molars, only those with insufficient arch length were included. We purposefully collected CBCT verified, near equal cases of lingual and buccal canal positioning with a disregard for other parameters. Had we collected equal numbers of erupted and unerupted M3M, in addition to buccal and lingual canal positions, interpretation of the relationship between eruption status and canal position would have been possible. Previous studies have shown that the IAC is more frequently found buccal to the M3M. Our results indicate that erupted teeth and buccal IAC position are significantly associated, but this may be a finding of disproportion resulting from clinical indicators (symptoms, decay, and insufficient arch length) prompting removal.

In support of previous studies, there were no differences between left and right anatomical analysis sites.[28] The distances “anterior-posterior ramus at occlusal plane” and “anterior ramus to lingula” between the IAC positioning groups – buccal, having no compression (n = 30) and lingually with severe compression (n = 27) – were 1.7 mm and 1.4 mm less, respectively. These contributed most to the severity of IAC compression; however, due to small sample sizes, further studies are needed to establish this significance. Interestingly, both measurements focus on the dimension of the ramus and encourage further investigation about this region.

An additional limitation of our findings includes the absence of interobserver agreement. Future studies should have a larger sample size, equally representing gender and third molar eruption status. Furthermore, to improve dimensional accuracy, CBCT recreated panoramic images should be evaluated.

This study was unable to identify a significant relationship in the measured mandibular variables that could predict the IAC position or degree of compression. The measurements “anterior-posterior ramus at occlusal plane” and “anterior ramus to lingula” were found to contribute most, but larger sample sizes are needed to establish this significance.


  Conclusion Top


The variables evaluated did not reveal a relationship that could predict canal location. A larger data set is needed to evaluate any role the anterior-posterior ramus length has on the degree of IAC compression.

Disclaimer

The identification of specific products or scientific instrumentation is considered an integral part of the scientific endeavor and does not constitute endorsement or implied endorsement on the part of the author, DoD, or any component agency. The views expressed in this article are those of the author and do not reflect the official policy of the Department of Army/Navy/Air Force, Department of Defense, or U. S. Government.

Acknowledgments

The authors would like to thank Silvia Hnatova for her statistical analysis and interpretation of the data.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
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