A comparative study of panoramic radiography and cone-beam computed tomography about the relationship of mandibular third molar an inferior alveolar canal

doi:10.21203/rs.3.rs-1888126/v1

Purpose To evaluate the accuracy of Panoramic radiography（PAN） by using CBCT to compare the anatomical relationship between the mandibular third molar and the inferior alveolar nerve canal (IAN).

Methods The research sample included the imaging information of 953 MTMs and adjacent mandibular canals were studied. Statistical software SPSS25.0 was used, and chi-square test was used to analyze the correlation between demographic factors, type of obstruction, special imaging findings of PAN and IAN canal defect in CBCT. The factors related to IAN canal defect were evaluated by Logistic regression.

Results Among the 953 study subjects, 757 cases of inferior alveolar nerve canal cortical bone were intact and 196 cases of IAN canal are defect. The differences between mandibular duct wall defects in the classification of MTM, the darkening of roots, the interruption of the white line, the deviation diversion of canal, and the narrowing of canal were statistically significant. In logistic regression analysis, P&G classification (mesial-distal), the darkening of roots, and the interruption of the white line were risk factors for mandibular duct wall defects.

Conclusions The interruption of the white line, darkening of the root, and the P&G classification (mesial-distal) suggest an increased rate of IAN canal defects. The special imaging manifestations related to the absence of cortical bone in the mandibular canal wall in univariate analysis, this study still recommends that further detailed investigation of CBCT is recommended.

Mandibular third molar

Inferior alveolar nerve canal

Panoramic radiography

CBCT

There are many postoperative complications after mandibular third molar extraction。 Such as bleeding, opening restriction, and infection, of which inferior alveolar nerve (IAN) injury is one of the most serious complications. The main symptoms of injury to the inferior alveolar nerve were anesthesia or altered sensation of the lower lip and chin [1, 2]. The closer the anatomical relationship between MTM and IAN canal, the greater the possibility of inferior alveolar nerve canal bone wall defect and the higher the risk of postoperative nerve injury. [3–5] Therefore, a detailed preoperative imaging examination is essential. Panoramic radiography (PAN) is the most commonly used imaging modality prior to MTM extraction to observe impacted mandibular third molars and assess the risk of inferior alveolar nerve injury. But PAN images do not always provide reliable data about precise relationships between anatomical structures because it is two-dimensional and presents distortions. [5, 6] Cone beam computed tomography (CBCT) is gradually widely used in stomatology. CBCT images can provide isotropic resolution and high-dimensional accuracy [7], which imaging examinations can meet the most basic diagnostic significance [8], so CBCT has not become the preferred imaging method before MTM extraction. Therefore, in this study, the anatomical relationship between the MTM and the IAN canal was compared between the PAN and CBCT to evaluate the accuracy of PAN in predicting the defect of the IAN canal wall and the spatial relationship between the MTM and IAN canal, and provided a basis for clinical work.

This study was reviewed and approved by the Ethics Committee of the Stomatological Hospital Tianjin Medical University. A total of 953 cases of mandibular third molars and their adjacent mandibular canals were collected from 544 patients who were admitted to the Department of Oral and Maxillofacial Surgery, Tianjin Medical University Stomatological Hospital and underwent CBCT examination, including 478 males and 475 females. CBCT (KaVo 3D eXam) scanning range: 160mm×130mm, voxel size 0.25mm³, scanning voltage 120kV, current 5mA, exposure time 4 seconds, tomographic image layer 0.25mm. Image information was analyzed and evaluated using eXamVision software.

The Pell and Gregory Classification and the Winter classification were used in this study. (a) The Pell and Gregory (vertical) Classification of impacted third molar depth in relation to the cementoenamel junction of the second molar. (b) The Pell and Gregory (mesial-distal) Classification of impacted third molars in relation to the anterior border of the Ramus. (Fig. 1) (c) The Winter classification for impacted third molar angulation. (Fig. 2)

1、 Special imaging findings of MTM and IAN canal in PAN [9]

There are seven kinds of special imaging manifestations in Pan images that are significantly related to mandibular canal wall defects. Among them, there are four categories related to MTM abnormalities: Darkening of the root; Deflected root; Narrowing of the root; Dark and bifid apex of root. There are three categories in the special imaging manifestations of mandibular canal wall defect: Interruption of the white line of canal; Narrowing of canal. Diversion of canal.

2、Observation of integrity of mandibular nerve canal by CBCT

In CBCT images, the positional relationship and integrity of the MTM and IAN canal. Were. observed from three-dimensional angles of coronal plane, horizontal plane and sagittal plane.

3、Statistical analysis

The data were analyzed using the statistical software SPSS25.0, and the Pearson chi-square test was used to explore the correlation between the MTM obstruction type and the special image performance of PAN and the IAN canal defect of CBCT image; the above observational indicators were included in the Logistic regression analysis to observe the accuracy of the surface tomography to assess the mandibular wall defect. The confidence intervals for the statistical analysis were all 95%.

The sample consisted of 544 individuals (953 impacted mandibular third molars) composed of 478 males and 475 females with age ranged between 18 and 78 years (mean age 39.03years). There were 196 cases of IAN canal wall absence and 757 cases of IAN canal wall presence in CBCT images.

1 Statistical study on the types of MTM impacted and IAN canal wall defect

Table 1 Statistical study of P&G (vertical) classification of MTM and IAN
	Total	IAN canal presence	IAN canal absence	P
Level A	537	471_a（87.7%）	66_a（12.3%）	＜0.001
Level B	268	186_b（69.4%）	82_b（30.6%）
Level C	148	100_b（67.6%）	48_b（32.4%）
The same letters represent no significant difference between groups, and different letters represent statistically significant differences between groups

In the statistical study of P & G (vertical) classification and IAN canal absence, the difference between Level A and Level B and C was statistically significant (P<0.001). There was no significant difference between Level B and Level C

Table 2 Statistical study of P&G (mesial-distal) classification of MTM and IAN
	Total	IAN canal presence	IAN canal absence	P
Class Ⅰ	395	374_a（94.7%）	21_a（5.3%）	＜0.001
Class Ⅱ	324	244_b（75.3%）	80_b（24.7%）
Class Ⅲ	234	139_c（59.4%）	95_c（40.6%）
The same letters represent no significant difference between groups, and different letters represent statistically significant differences between groups.

Statistical study of P&G (mesial-distal) classification and IAN canal wall defect, the difference between any two groups was statistically significant, P<0.001.

Table 3 Statistical study of Winter classification of MTM and IAN
	Total	IAN canal presence	IAN canal absence	P
Vertical	344	302_a（87.8%）	42_a（12.2%）	＜0.001
Horizonal	176	126_b（71.6%）	50_b（28.4%）
Mesioangular	315	239_b（75.9%）	76_b（24.1%）
Distoangular	20	13_a,b（65.0%）	7_a,b（35.0%）
Buccolingual	46	40_a,b（87.0%）	6_a,b（13.0%）
Inverted	52	37_b（71.2%）	15_b（28.8%）
The same letters represent no significant difference between groups, and different letters represent statistically significant differences between groups.

In the comparison between groups, the difference between the vertical group and the Mesioangular， horizontal and inverted group was statistically significant, P<0.001.

2 Comparative study of special imaging manifestations in PAN images and CBCT images

Seven specific signs related to MTM or IAN canal abnormalities were observed in PAN images, and the integrity of the mandibular neural canal was compared through CBCT three-dimensional imaging observation. (Fig. 3-9)

As shown in Fig.3, when the darkening of roots was shown on the PAN, CBCT was used to observe it. The coronal and horizontal planes showed that the image of the IAN canal was intact, and the sagittal plane showed the image defect of the IAN canal.

As shown in Fig4, when the deflection of roots was shown on the PAN，the CBCT was used to observe it. Coronal, sagittal, and horizontal planes all showed image defect of the IAN canal.

As shown in Fig.5, when the narrowing of roots was shown on the PAN，the CBCT was used to observe it. Coronal, sagittal, and horizontal planes all showed image defect of the IAN canal.

As shown in Fig.6, when the narrowing of roots was shown on the PAN，the CBCT was used to observe it. Coronal, sagittal, and horizontal planes all showed no defect of the IAN canal.

As shown in Fig.7, when the interruption in the white line of the canal was shown on the PAN，the CBCT was used to observe it. Coronal, sagittal, and horizontal planes all showed image defect of the IAN canal.

As shown in Fig.8, when the diversion of canal was shown on the PAN，the CBCT was used to observe it. Coronal, sagittal, and horizontal planes all showed no defect of the IAN canal.

As shown in Fig.9, when the narrowing of canal was shown on the PAN，the CBCT was used to observe it. Coronal, sagittal, and horizontal planes all showed image defect of the IAN canal.

3 Special imaging performance in PAN images and statistical study of CBCT images

Table 4 Statistical study of special imaging manifestations and IAN canal defects

	Total	IAN canal presence	IAN canal absence	P
Darkening of roots Y	171	119（69.6%）	52（35.2%）	＜0.001
N	782	638（81.6%）	144（18.4%）	＜0.001
Deflection of roots Y	73	52（71.2%）	21（28.8%）	0.071
N	880	705（80.1%）	195（19.9%）	0.071
Narrowing of roots Y	51	38（74.5%）	13（25.5%）	0.371
N	902	719（79.7%）	183（20.3%）	0.371
Bifid root apex Y	38	26（68.4%）	12（31.6%）	0.087
N	915	731（79.9%）	184（20.1%）	0.087
Interruption in the white line of the canal Y	185	118（63.8%）	67（36.2%）	＜0.001
N	768	639（83.2%）	129（16.8%）	＜0.001
Diversion of canal Y	58	38（65.5%）	20（34.5%）	0.007
N	895	719（80.3%）	176（19.7%）	0.007
Narrowing of canal Y	120	83（69.2%）	37（30.8%）	0.003
N	833	674（80.9%）	159（19.1%）	0.003
*Data type is N (%)

As shown in Table 4 ，among the seven types of special imaging findings related to the integrity of the IAN canal, there were statistically significant in darkening of roots, interruption in the white line of the canal, diversion of canal, and narrowing of the canal.

In order to explore the relationship between different numbers of special imaging in PAN and IAN canal defects, this study conducted a correlation study between the two.

Table5 Statistical study of the number of special image and IAN canal defects in PAN
	Total	IAN canal presence	IAN canal absence	P
0	406	362_a（89.2%）	44_a（10.8%）	＜0.001
1	415	323_b（77.8%）	92_b（22.2%）
＞1	132	72_c（54.5%）	60_c（45.5%）
Data type is N (%) The same letter represents no statistical significance for the difference between groups, and the different letters represents group difference is statistically significant.

Table 5 describes a statistical study of the number of specific imaging features and IAN canal defects in PAN imaging. When there was no special imaging in PAN, there were 44 cases of IAN canal defects in CBCT; when the number of special imaging in PAN was 1 case, there were 92 cases of IAN canal defects; when there were more than 1 case of special imaging in PAN, 60 cases of IAN canal defects were observed. The difference between the IAN canal defects and the number of specific images in PAN were statistically significant. In the inter-group comparison, the difference between any two groups is statistically significant.

4 Logistic regression analysis of mandibular canal wall defect

In order to demonstrate clarify the correlation between the imaging performance in PAN and the IAN canal defect, demographic factors, MTM impaction types, and specific imaging performance in PAN were included in Logistic regression.

Table 6 Logistic regression analysis of mandibular canal wall defect
	OR（95%CI）	P
P&G classification(mesial-distal)		＜0.001
Ⅰ	1.00
Ⅱ	4.33（2.40-7.81）
Ⅲ	7.93（4.06-15.48）
Darkening of roots	0.021
N	1.00
Y	1.70（1.09-2.69）
Interruption in the white line of the canal	＜0.001
N	1.00
Y	1.80（1.18-2.74）
Age		0.078
Gender		0.072
P&G classification（Vertical）		0.275
Winter classification		0.507
Deflection of roots		0.059
Narrowing of roots		0.371
Bifid root apex		0.080
Diversion of canal		0.053
Narrowing of the canal		0.168

The logistic regression model was statistically significant and the model fit was good. As shown in Table 6, among the observation indicators included in the model, P&G classification (mesial and distal), darkening of roots, and interruption in the white line of the canal were statistically significant. When MTM was Class II impacted, the probability of mandibular canal wall defect was 4.33 times that of Class I impacted, and when MTM was Class III impacted, the probability of mandibular canal wall defect was 7.93 times that of Class I impacted. When interruption in the white line of the canal appeared, the probability of mandibular canal wall defects on CBCT images increased by 80%; when MTM darkening of roots appeared, the probability of mandibular canal wall defects increased by 70%.

Mandibular impacted third molar extraction with inferior alveolar nerve injury is the most serious postoperative complication. Inferior alveolar nerve injury can be divided into temporary nerve injury and permanent nerve injury [10]. Temporary nerve damage can recover in about a week, while permanent nerve damage can last more than six months or even lifelong [11, 12]. Therefore, preoperative imaging assessment of risk factors for inferior alveolar nerve canal defects is crucial. The age of the patient, the type of MTM impaction, the positional relationship with the MC, the method of anesthesia, intraoperative trauma, and postoperative edema are all related to the possible injury [11].

The impaction degree of mandibular third molar in the ramus of mandible is associated with extraction operation difficulty and postoperative complications. The classification of MTM impaction is often used to assess the difficulty of tooth extraction and the risk of nerve damage [13, 14]. This study evaluated the position of the impacted mandibular third molar based on Winter's classification and Pell and Gregory's classification. Among the P&G classifications (vertical) in this study, Level A was the most common, but its associated IAN canal defect rate was lower than that of the other two groups of classification. There was no significant difference in the rate of IAN canal defect between the Level B and Level C. In the P&G classification (mesial-distal), the differences between the three types of impacted were significant. The narrower the space between the anterior edge of the mandibular ramus and the second molar, the closer the relative distance between the third molar and the mandibular canal, the higher the probability of IAN canal defect. In Winter's classification, vertical impacted MTMs are the most common, Mesioangular and horizontal impacted MTMs are more common, and inverted, buccolingual and distoangular impacted MTMs are less common. The vertical impacted MTM had the lowest rate of IAN canal defect, and there was a statistical difference between the vertical impacted MTM and the horizontal, mesioangular, and inverted impacted types. The reason is that the mandibular canal is closer to the lower edge of the mandibular body than to the alveolar ridge; it is closer to the anterior edge of the mandibular ramus than the posterior edge, the findings of this study are in accordance with literature. [15]

PAN is the imaging test of choice before MTM extraction to assess the difficulty of third molar extraction and predict the risk of postoperative nerve damage. Scholars such as Rood and Shehab [9, 16, 17] confirmed four radiographic indicators observed in the tooth root (darkening of roots, deflection of roots 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) to predict the risk of postoperative nerve injury. However, PAN is the projection of anatomical structures in the largest vertical section of the projection direction, and its resolution is not uniform throughout the mandible; involuntary head movement of the patient during the imaging process may also cause changes in the position and scale of the image, resulting in image distortion [8, 18, 19]. CBCT images can distinguish the spatial position and distance relationship between the MTM and the IAN canal, as well as the mandibular canal wall defect from the coronal, sagittal and horizontal continuous rolling cross-sectional images [20, 21], and predict and evaluate risk of postoperative nerve damage.

The literature has showed that panoramic imaging signs were negative for both sensitivity and specificity in predicting IAN injury during mandibular third molar extraction; their results did not have predictive ability to determine the relationship between IAN and mandibular third molar. [5] A study showed that PAN has a sensitivity of 98.55% and a specificity of 48.39%, and PAN may be an excellent screening tool to detect the close relationship between IMTM roots and IAN canals. [16] Therefore, Therefore, it has important clinical significance to understand the CBCT imaging performance of special imaging in PAN by comparison. A number of scholars' studies have shown [22–24] the darkening of roots and interruption in the white line of the canal in PAN imaging are the most common risk factors for IAN canal defects.

this study found that the probability of mandibular canal wall defects in CBCT was higher when these two types of imaging findings appeared on PAN images. Previous studies have also reported an increased probability of IAN injury when there are two or more specific imaging findings. [25, 26]

In the logistic regression model, the P&G classification of mandibular third molars (mesio-distal direction), darkening of roots and interruption in the white line of the canal were statistically significant. When the PAN shows that the P&G classification is Class II and Class III, the probability of mandibular canal cortical defect is higher than that of Class I. In preoperative imaging examination, the P&G (mesiodistal direction) classification is a more important risk factor than the P&G (vertical direction) classification. In the univariate analysis, the diversion of canal and narrowing of canal with significant relationship with the IAN canal defect were not included in the Logistic regression model. This study still believes that further CBCT examination should be performed to determine whether the bone wall is missing to provide a basis for intraoperative and postoperative evaluation. However, for the special imaging manifestations related to the absence of cortical bone in the mandibular canal wall in univariate analysis, this study still recommends that further CBCT examination should be performed to determine whether the bone wall is missing to provide a basis for intraoperative and postoperative evaluation.

Author Contribution

Jian Zhang Analyzed experimental data and wrote papers

Xiaoyu Gao acquired and interpretation the data.

Wenyi Zhang designed this study

All authors reviewed the manuscript

Conflict of Interest statement:

The authors declare that there was non-financial conflict of interests

Funding

No funding was obtained for this study

Ethics Approval and Consent to Participate

The informed consent obtained from study participants was written

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No competing interests reported.

A comparative study of panoramic radiography and cone-beam computed tomography about the relationship of mandibular third molar an inferior alveolar canal

Status:

Version 1

Abstract

Figures

Introduction

Methods

Results

Discussion

Declarations

References

Additional Declarations

Status:

Version 1