Is cone beam computed tomography accurate in predicting inferior alveolar nerve exposure during mandibular third molar extraction?

doi:10.21203/rs.3.rs-4978551/v1

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Research Article

Is cone beam computed tomography accurate in predicting inferior alveolar nerve exposure during mandibular third molar extraction?

https://doi.org/10.21203/rs.3.rs-4978551/v1

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Objectives

This study aims to evaluate the accuracy of cone beam computed tomography (CBCT) in predicting the exposure of inferior alveolar nerve (IAN) during complicated mandibular third molars (M3M) extraction.

Methods

115 M3Ms with canal cortical defect signs on preoperative CBCT were extracted. Candidate variables included sex, age, types of CBCT machine, the Winter classification of M3Ms, the direct contact size between IAN and tooth root on CBCT, the size of cortical defect on CBCT. The primary outcome was the exposure of IAN and the exposed neurovascular bundle size which was recorded measured under endoscope. Bland-Altman analysis was performed to assess the agreement between the CBCT and endoscopic measurements.

Results

85/115 (73.9%) M3Ms with canal cortical defect signs on preoperative CBCT had intraoperative exposure of IAN. The average length and width of the exposed IAN were 5.89 ± 1.72mm and 2.48 ± 0.79 mm, which were significantly smaller than the direct contact size between IAN and tooth root on CBCT (9.69 ± 3.05mm and 3.26 ± 0.87 mm, P < 0.001) but larger than the cortical defect size (5.06 ± 2.05mm and 2.10 ± 0.54 mm, P < 0.05). The probability of intraoperative IAN exposure was statistically different among different Winter classifications of M3M and the probability of IAN exposure was higher in non-horizontal impacted type.

Conclusions

Not all M3Ms with tooth-IAN contact signs on preoperative CBCT indicated intraoperative IAN exposure. The contact sizes on CBCT were always larger than the intraoperative endoscopic measurements. IAN exposure can be accurately predicted by the length of cortical defect on CBCT. Non-horizontal impaction predisposed the M3M to a higher risk of intraoperative IAN exposure.

Clinical Relevance:

Endoscope provides the possibility to observe and record the IAN exposure directly. IAN exposure can be accurately predicted by the length of cortical defect on CBCT instead of the direct contact size between IAN and tooth root. Non-horizontal impaction predisposed the M3M to a higher risk of intraoperative IAN exposure.

CBCT

Endoscopy

Impacted lower third molars

Inferior alveolar nerve

Extraction of mandibular third molar (M3M) is the most commonly performed operation in oral and maxillofacial surgery. Inferior alveolar nerve (IAN) exposure may occur during extraction, thus increasing the risk of neurosensory deficit. Study by Gennaro et al. had shown that there was a high incidence of neurosensory disturbance in the lower lip and chin after sagittal split osteotomy surgery and intraoperative quantity of IAN exposure[1]. A prospective study of impacted tooth extraction also showed intraoperative exposure of IAN bundle carried a 20% risk of paresthesia and 70% chance of recovery by 1 year[2]. Exposure of IAN if not treated carefully such as scraping the alveolar fossa may lead to neurosensory deficit[3].

Cone beam computed tomography (CBCT) provides clear high-contrast images with high isotropic spatial resolution[4]. IAN is located in the mandibular canal and the presence of cortical bone of canal is considered to be an important barrier between tooth root and IAN. Therefore, IAN exposure is likely to occur in cases of close relationship of tooth root and the IAN[5]. Preoperative CBCT has been widely used to evaluate the relationship between M3M tooth roots and the mandibular canal[6–14], and predict the risk of IAN exposure.

In clinical practice, CBCT is used to evaluate the direct contact size between the tooth root and IAN by measuring the length of the curve along the long axis of the root in cases without the bone barrier. Susarla et al. found that loss of cortical integrity had a high sensitivity but low specificity for the prediction of IAN exposure[15]. Our previous study also showed that direct contact on CBCT between tooth root and IAN did not predict the actual IAN exposure[16]. The accuracy of CBCT in predicting IAN exposure is still controversial in previous studies due to a lack of methods for direct intraoperative measurement and documentation. Moreover, intraoperative IAN exposure is due to incomplete mandibular cortical bone and it is unknown whether the canal cortical bone defect size is consistent with the direct contact size between IAN and tooth root on CBCT. Therefore, the relationship between the root-IAN contact size, the cortical defect size on CBCT and the actual IAN exposure size also remains elusive. The purpose of this retrospective study is to evaluate the accuracy of CBCT in predicting the exposure of IAN and the size of exposed IAN during M3Ms extraction. The specific aims are 1) to observe IAN exposure probability in cases without bone barrier between tooth root and IAN after tooth extraction; 2) to measure the tooth-IAN contact size and the canal cortical defect size on CBCT and the size of intraoperatively-exposed IAN under endoscope; 3) to identify the relationship between IAN exposure and other variables.

Patients

This retrospective study consisted of consecutive patients who underwent M3M extraction at the Department of Oral and Maxillofacial Surgery from Nov 2020 to Jan 2022. Inclusion criteria were: 1) patients over 18 years of age; 2) completely or partially impacted M3M; 3) preoperative CBCT showed the cortical bone of the mandibular canal was discontinuous, or the M3M root compressed or encircled the mandibular canal. Patients or the M3M had surgery contraindications were excluded, e.g. pregnant females, M3M with acute inflammation, or with pulp, periodontal and periapical diseases. This study was conducted in accordance with the principles of the Declaration of Helsinki and approved by the institutional biomedical ethics committee. Informed consent was obtained from all participants.

Surgical procedure

Extraction of M3M was performed under local anesthesia by one senior surgeon. Briefly, an incision on the buccal side of the lower second molar at the distal or proximal mesial axial angle and from distal to the lower second molar were made. The mucoperiosteum flap was elevated. The crown of M3M was segmented using a contra-angle high-speed rotary handpiece. During the crown segmentation, a 4.0 mm Storz Tricam SL II SCB endoscope (Karl Storz, Tuttlingen, Germany; Cat. No. 20223020-1) with 30º or 70º view angle was used to check the medial part of the crown and the distal root surface of lower second molar. M3M roots were segmented and adjacent bone was partially removed by wavy blade piezosurgery under endoscope. Minimally invasive elevator was used to remove the roots. Following copious irrigation, the socket was endoscopically examined. Any IAN exposure was recorded and the size of exposed IAN was measured. The incision was closed and all patients received amoxicillin 500 mg 3 times daily for 3 days.

Variables

Candidate variables included sex, age, manufacturer of CBCT machine and scan parameters, Winter classification of M3Ms, direct contact size between IAN and tooth root on CBCT, and cortical defect size on CBCT. Winter classification was based on the position of the impacted M3M to the long axis of the second molar as shown in Table 1. We also took Quek’s classification system[17] into consideration, using the angle formed between the intersected long axis of second and third molars, and defined vertical as 0°±10°, horizontal as 90°±10°.

Table 1

Winter Classification for Impacted M3M
Impaction Class	Definition
Horizontal	Long axis of the third molar is horizontal (90°±10°)
Mesioangular	Impacted tooth is tilted toward the second molar in a mesial direction
Vertical	Long axis of the third molar is parallel to the long axis of the second molar (0°±10°)
Distoangular	Long axis of the third molar is angled distally or posteriorly away from the second molar
Inverse	The tooth is reversed and positioned upside down
Buccal/lingual obliquity	The tooth is buccally (tilted toward the cheek) or lingually (tilted toward the tongue) impacted
Transverse	The tooth is horizontally impacted in a cheek–tongue direction

Preoperative CBCT scans were taken using one of the following machines and parameters: 1) NewTom VGi (Quantitative Radiology, Verona, Italy) in 110 kV and 3.65mA (Scanning time, 36 s; field of view, 160mm×160mm; slice, 0.3 mm); 2) 3D Accuitomo 170 (J Morita Mfg., Corp., Kyoto, Japan) in 90 kV and 10mA (Scanning time, 15s; field of view, 60mm×60mm; slice, 0.375 mm); 3) iCAT FLX (Imaging Sciences International Inc., Hatfield, PA, USA) in 120 kV and 5mA (Scanning time, 26.9 s; field of view, 160mm×130mm; slice, 0.3 mm). The direct contact size and the canal cortical defect size were measured twice on CBCT including length and width by the same observer as demonstrated in Fig. 1.

The co-primary outcomes were the intraoperative exposure of IAN (yes or no) and the exposed IAN size including length and width. The IAN exposure was defined as direct visualization of the IAN via endoscope. The length and width of any exposed IAN were measured using a prebent periodontal probe as a reference (Fig. 2).

Statistical analysis

The statistical analysis was performed using Stata 15.0 (StataCorp, College Station, Texas, USA). Continuous variables (age, length and width of contact section on CBCT and the exposed IAN) were expressed as the mean standard deviation, or median and range if they were not normally distributed. Categorical (types of CBCT machines, Winter classification) and binary variables (sex, IAN exposure) were reported as proportions or percentages. Bland-Altman analysis was performed to assess the agreement between the CBCT and endoscopic measurements. A P-value of < 0.05 was considered statistically significant. This study is reported in accordance with the Strengthening the Reporting of Observational Studies in Epidemiology (STROBE) guidelines.

Descriptive statistics and comparison between the exposed and unexposed groups

A total of 105 patients (40 males and 65 females) with an average age of 29 years (from 18 to 55 years) and 115 M3M were included. 85 (73.9%) M3Ms had intraoperative exposure of IAN under endoscope. The descriptive statistics of the 115 M3M and comparison between the exposed and unexposed groups were listed in Table 2. There was a statistical difference in nerve exposure among different impacted types as shown in Fig. 3.

Table 2

The descriptive statistics and comparison between IAN exposed and unexposed groups
Characteristic	Total (n = 115)	Exposed (n = 85)	Unexposed (n = 30)	P value
Types of CBCT machine
i-CAT FLX	50	40	10	0.174
3D Accuitomo 170	25	15	10
NewTom VGi	40	30	10
Winter classification
Horizontal	50	30	20	0.026
Mesioangular	52	44	8
Vertical	10	8	2
Inverted	3	3	0
IAN in contact with tooth root on CBCT (mm)
Length	9.69 ± 3.05	9.92 ± 3.04	9.03 ± 3.03	0.170
Width	3.26 ± 0.87	3.28 ± 0.86	3.21 ± 0.91	0.724
Cortical defect on CBCT (mm)
Length	5.06 ± 2.05	5.90 ± 1.61	2.66 ± 0.93	< 0.001
Width	2.10 ± 0.54	2.26 ± 0.48	1.65 ± 0.42	< 0.001
Exposed IAN (mm)
Length		5.89 ± 1.72	N/A
Width		1.96 ± 0.84	N/A

There were no significant differences in tooth-IAN contact size on preoperative CBCT between intraoperative IAN exposed and unexposed group. However, the independent sample t-test showed the length and width of exposed IAN were significantly smaller than those of IAN in contact with tooth root on CBCT in IAN exposed group (P < 0.001 for both). The Bland-Altman plots showed inconsistency between the tooth-IAN contact size on CBCT and endoscopic IAN-exposure size (Fig. 4).

As there were significant differences in cortical defect size on preoperative CBCT between IAN exposed and unexposed group in Table 2, we also analyzed the cortical defect size and the exposed IAN size in Fig. 5. Moreover, logistic regression showed that IAN exposure was significantly associated with the cortical defect length (0.1mm) on CBCT (OR = 1.38, 95% CI 1.14 to 1.67, P = 0.001). Linear regression analyses showed that the length and width of exposed IAN were significantly associated with the cortical defect length (P<0.001, P = 0.013) and width (P = 0.004, P<0.001).

There was a significant difference between the horizontal and mesioangular impacted type (p = 0.005) when Chi-square test was used to evaluate whether the proportion of IAN exposure in different groups of Winter classification was consistent. We also used ANOVA to test the correlation between the actual size of exposed IAN and (1) Winter classification types; (2) types of CBCT machine. A statistical difference was found between exposed IAN length and Winter classification types (p = 0.001), but no difference was found in width (p = 0.679). There were no statistical differences in correlation analysis between the length and width of exposed IAN and those of IAN in contact with root on CBCT (p = 0.549 and 0.199).

Major findings

This study found that (1) not all M3Ms with root-IAN directly contact signs on preoperative CBCT appeared IAN exposure intraoperation; (2) the size of IAN in contact with tooth on CBCT were always larger than the size under endoscope, (3) IAN exposure can be predicted by the size of cortical defect on CBCT, (4) the IAN exposure ratio was different among tooth in different types of Winter classification.

Evaluation of the relationship between the M3M and the mandibular canal before tooth removal is critical to the alveolar surgeon, because the IAN is located in the mandibular canal and IAN injury is one of the most serious complications of M3M removal. Studies have proposed risk signs of a close relationship between the tooth roots and the IAN: darkening of the root; deflection of the root; narrowing of the root; superimposition of the root; bifurcation of the root over the inferior alveolar canal; diversion of the inferior alveolar canal; interruption of the cortex of the IAC[18]. CT was used as a gold standard for the relationship between the tooth roots and the IAN[3, 10, 12, 13]. The benefits of CT are the visualization of the anatomic relationship in multiple planes (axial, coronal, and sagittal) and the generation of 3D reformations. There were few standard parameters of CBCT for predicting the nerve exposure. Nakayama et al have shown an increased risk for IAN exposure when the third molar contacts the IAN canal[8]. However, CBCT may be not a perfect tool to predict IAN exposure. Susarla et al. have questioned the accuracy of CBCT in predicting IAN exposure and studied on 80 M3Ms. They found that loss of cortical integrity had a high sensitivity but low specificity as a diagnostic test for IAN visualization, but a cortical defect size ≥ 3 mm was associated with an increased risk for intraoperative IAN visualization with a high sensitivity and specificity (≥ 0.82)[19].

There were some cases with actual-exposed IAN that could be observed under direct vision by naked eyes or under 2-power loupe magnification[2, 8, 20–22] or IAN exposure observation under endoscope[23]. However, these studies were not ideal for nerve observation, possibly for the following reasons: 1. IAN was located deep in the socket; 2. blood leakage from the socket made it difficult to identify IAN; 3. lack of visual records such as photographs and videos resulting in poor repeatability of observations and measurements. The endoscope can reach deep into the mouth, with light source and amplification effect, which makes it possible to observe the previously unseen structures. Endoscopy can be used to investigate a larger number of patients and provide a more powerful basis for the prediction of preoperative CBCT. The endoscope can also observe and record the conditions that could not be seen before during and after tooth extraction. IAN injury is a complication of M3Ms extraction which needs to be reduced in clinical practice. Although IAN exposure did not necessarily predict IAN injury[8, 20–22], IAN exposure was associated with increased risk of IANI[24], reduced probability of nerve recovery, and longer recovery times[1, 2] in previous studies. The process of tooth dislocation would exert force on the surrounding tissues. Compared with hard tissue such as bone, IAN were soft tissue with relatively poor tolerance, and forces in different directions may cause damage to nerves[3]. Therefore, when the tooth was in direct contact with IAN, the nerve was more likely to be affected by the force during tooth extraction process, and the possibility of IANI was also higher. The IAN exposure indicated the direct contact relationship between IAN and tooth, which had important clinical significance. We chose IAN exposure and the size of IAN as clinical outcomes for the following reasons: first, we were interested in the accuracy of the actual anatomical structure that can be predicted by imaging signs; second, the introduction of endoscopy made it possible to directly observe IAN exposure and measured the size as objective information. The focus of this study was to analyze the accuracy of CBCT in predicting IAN exposure and there was no further discussion on IAN injury, although we were also concerned about the relationship between IAN exposure and IAN injury as well as symptoms and prognosis of IAN injury. It is easy to understand that the nerve discontinuity leads to numbness, but the reason for numbness in continuous nerves is not clear. It has been proposed that if the tooth dislocation cannot be performed along the long axis of the tooth may cause IAN injury[25]. The endoscopy provides a way to directly observe (1) the anatomical structure, tooth and root orientation during tooth extraction to guide the operation and the dislocation direction; (2) IAN after extraction, through which it may be possible to identify normal and abnormal nerve states for early intervention and the restoration of nerve function. Studies on IAN under endoscopy are still in progress and more issues will be explored in the future.

In our previous study, we used endoscope assisting removing M3Ms or residual tooth of M3Ms[26] and found that the size under endoscope was smaller than the direct contact size between IAN and tooth root on CBCT, but no further statistical analysis was conducted due to the small sample size[16]. CBCT can provide 3D structural information, which can help doctors predict the difficulty and risk of operation. However, we found that direct contact between IAN and tooth root on CBCT imaging did not indicate the actual IAN exposure during M3Ms extraction operation in clinical practice. Therefore, as an auxiliary examination tool, CBCT could reflect the relationship between tooth and surrounding structures in its way but the imaging of CBCT is affected by many factors, which cannot be considered as real.

In this study, the IAN exposure rate was 73.9% (85/115), which confirmed that CBCT can be used as an auxiliary indicator of intraoperative IAN exposure but it cannot be used as a gold standard because not all contact signs between IAN and tooth root shown on CBCT had intraoperative IAN exposure. Signs of direct contact between IAN and tooth root on CBCT could not predict IAN exposure intraoperation. In some cases, the exposure of the IAN was not observed under endoscopy after tooth extraction, although the contact distance was long. The reason may be that there was a thin layer of bone between IAN and tooth root, which was difficult to be shown on CBCT. Mischkowski et al. studied the geometric accuracy of CBCT and obtained the mean absolute measurement error for linear distances was 0.26 mm[27]. Lascala et al. evaluated the accuracy of the linear measurements obtained in CBCT images using a NewTom compared with caliper measurement on dry skulls and found that the real measurements were always larger than those for the CBCT images[28]. The images presented by different machines may also play a part in measuring in theory, but this study did not show statistical differences between CBCT machines.

The inconsistency between preoperative CBCT and endoscopic measurements may be explained by the measurement methods and different state of IAN in pre- and post-operation. In clinical practice, the mandibular canal image was usually used to replace IAN which cannot be directly observed on CBCT. The clinical measurement of the direct contact length was the longest distance of IAN in contact with tooth root along the long axis of IAN. The state of the nerve before tooth extraction cannot be directly observed. When no bone image can be observed between the mandibular canal and the tooth root, it is considered that the root is in direct contact with IAN. Due to the presence of tooth roots, it is often considered that IAN would adapt to the shape of tooth roots which are often curved rather than straight. So far, there are no articles describing the state of observation of IAN after tooth extraction due to lack of contact and pressure from the root after tooth extraction. In this study, the postoperative state of the IANs were directly observed and the size of exposed IANs were measured by endoscope. IANs were straight in cases with IAN exposure under endoscope, which may indicated that after tooth root removal, in other words, the compression by natural development was lifted, the IAN became “non-adaptive state” to hard tissue forms such as the root. We also need to recognize that there are limitations using endoscopic measurements. The defect window under endoscope may not be parallel to the direction of the long axis of tooth root as we measured on CBCT.

It is reasonable that the linear length of exposed IAN was closer to cortical defect length than the contact length. For the relationship of cortical defect size and IAN exposed size, the logistic regression analyses showed that the length of cortical defect on CBCT was the only factor that influences IAN exposure. The cortical defect length was consistent with the IAN exposure length, which may be due to the morphological changes of IAN after tooth extraction: IAN deformed toward the direction of fossa. Therefore, it is necessary to operate gently, especially pay attention not to scratch the site without cortical bone to protect IAN. However, based on the results of this study that the proportion of IAN exposed during M3M extraction was very low (4%) when the cortical defect was less than 3.4mm on CBCT. Therefore, the risk of IAN injury could be underestimated when very small cortical defects (less than 3.4mm) were observed on CBCT. For cases of IAN exposure, linear regression analyses indicated that the larger size of cortical defect on CBCT, the larger size of IAN exposure may be observed although the length of the defect on CBCT is different from that of the actual size of IAN exposure.

In addition, this study found significant differences in the probability of IAN exposure among different impacted types of Winter classifications. Impacted tooth classification is necessary for diagnosis and clinical operation, and Winter classification is a widely used clinical classification. Winter classification has high repeatability[29], and mesioangular impactions (49.2%) were the most common type of impaction[30]. In this study, mesioangular impacted M3Ms (43.5%) were also the most common type. In the case of direct contact between the root and IAN, we need to be more vigilant about the non-horizontal impacted type as they may have a higher probability of intraoperative IAN exposure, and the mesioangular impacted type given that this type is more prevalent in the clinic.

This retrospective study demonstrated that not all M3Ms with IAN and root contacting signs on preoperative CBCT showed IAN exposure intraoperation. The size of IAN in contact with tooth root on CBCT is always larger than the actual size of exposed IAN during M3M extraction measured by endoscope directly. IAN exposure can be accurately predicted by the length of cortical defect on CBCT. The non-horizontal impacted type may have a higher probability of intraoperative IAN exposure.

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Ethics and consent to participate

This study was approved by the biomedical ethics committee of Peking University Hospital of Stomatology (PKUSSIRB-2024101135). Informed consent was obtained from all individual participants included in the study.

Funding

This work was supported by 1) Clinical Research Foundation of Peking University School and Hospital of Stomatology (PKUSS-2023CRF205), 2) Program for New Clinical Techniques and Therapies of Peking University School and Hospital of Stomatology (PKUSSNCT-19B12), 3) National Program for Multidisciplinary Cooperative Treatment on Major Diseases (PKUSSNMP-201903).

Author Contributions

Kenan. Chen contributes to data acquisition, analysis and manuscript writing;

Youbai. Chen contributes to data analysis and manuscript writing/revision;

Peng. Chen contributes to data analysis and manuscript revision;

Junqi. Jiang contributes to data acquisition;

Enbo. Wang contributes to equipment support;

Chuanbin. Guo contributes to funding support；

Xiangliang. Xu contributes to conception, design, data acquisition and revised the manuscript for important intellectual content and gave final approval.

Acknowledgements

This work was supported by the Peking University School and Hospital of Stomatology. Open access funding provided by program for New Clinical Techniques and Therapies of Peking University School and Hospital of Stomatology (PKUSSNCT-19B12).

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Is cone beam computed tomography accurate in predicting inferior alveolar nerve exposure during mandibular third molar extraction?

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Version 1

Abstract

Objectives

Methods

Results

Conclusions

Clinical Relevance:

Figures

Introduction

Methods

Patients

Surgical procedure

Variables

Statistical analysis

Results

Descriptive statistics and comparison between the exposed and unexposed groups

Discussion

Major findings

Conclusion

Declarations

References

Additional Declarations

Status:

Version 1