Relationship of the Inferior Alveolar Canal to Impacted Third Molars As Evaluated by Cone Beam Computed Tomography

Relationship of the Inferior Alveolar Canal to Impacted Third Molars As Evaluated by Cone Beam Computed Tomography

Mohammed Barayan, B.D.S.,* and Mansur Ahmad, B.D.S., Ph.D.**:

Introduction

Third molars are among the most frequently extracted teeth in the young adults. Presence of pain,   pericoronitis, carious lesions, cysts or tumors, orthognathic surgical planning, and root resorption of  the adjacent teeth are some of the indications for third molar extraction. Extraction of the third molars  may traumatize the inferior alveolar neurovascular bundle. The current literature indicates that temporary loss of sensation related to third molar extraction ranges from 0.4% to 22%.1 Permanent  damage to the neurovascular bundle — i.e., loss of sensation for at least six months — occurs in  about one percent of the third molar extractions. The risk of inferior alveolar canal (IAC) damage  during third molar surgery is dependent upon several factors: location and contact of the canal to the  tooth, degree and orientation of impaction, bone mass and density, age of the patient, and skill of the  surgeon.2,3   Complications from third molar surgery, including nerve damage, may be as high as 30.9%.4 Apart from neurovascular damage, other complications from third molar extraction include  infection and jaw fracture. In addition to these issues, complications arising from third molar  extractions and nerve damage lead to the highest number of malpractice suits against oral  surgeons.5

To reduce the morbidity related to third molar extractions, reliable radiographic examination is  essential to evaluate the degree and orientation of impaction, deflection of the root, location of the  canal, relationship of the canal to the roots, and thickness of the cortical plates. Panoramic  radiography is one of the most common imaging methods for preoperative planning of third molar extractions. Based on two-dimensional radiographs, Rood and Shebab had developed a diagnostic  criteria to describe the location of the IAC and relationship of the IAC to the teeth.6 Traditional images  such as periapical and panoramic radiographs can only provide limited information about the status  of the third molars and their relationship to the canals.7 The IAC often follows a tortuous course in the third molar area.8 Therefore, using an image-shift principle on multiple intraoral radiographs may not  provide adequate information about the path of the IAC.

Cone beam computed tomography (CBCT) scans allow cross-sectional imaging of the maxillofacial  structures. Several studies have shown that CBCT is a useful imaging method to locate IAC.9,10 Since  the introduction of CBCT to dentistry, oral surgeons in private practice in Minnesota have used this  imaging technology for third molar pre-operative planning. Some of these scans are referred for  interpretation to the oral and maxillofacial radiology clinic at the University of Minnesota. The  purpose of this retrospective study is to evaluate the relationship of the inferior alveolar canal to the  third molars that required surgical removal in private oral surgery offices.

Materials and Methods

Study population. For this retrospective study, 50 CBCT scans (87 third molars), acquired from  January to December 2010, were evaluated. All the scans were requested by oral and maxillofacial surgeons in private practice considering surgical extraction of the third molars. The evaluation of the  existing data was approved by the Institutional Review Board of the University of Minnesota.  Panoramic or periapical radiographs were exposed on all the patients and were interpreted by the oral surgeons. Based on the findings from these two-dimensional images, the referring oral surgeons  decided the need for CBCT imaging.

Scan parameters. All the scans were done using Next Generation i-CAT® (Imaging Sciences, Hatfield, PA) CBCT units. Voxel size in the study population was 0.40 mm. All the scans were small  field of view (9 cm) for mandible only.

Observers. Two observers (a radiology demonstrator and a board certified oral and maxillofacial   radiologist) reviewed all the cases using iCATVison software (Imaging Sciences). If there was a  disagreement on the diagnosis, a consensus was reached after a discussion between the two observers.

Radiographic findings. Following are the radiographic features recorded: location of the canal in   relation to the roots, margin of the canal, and position of the roots in the alveolar bone.

Results

The study contained 87 third molars (45 left and 42 right) from 50 patients. In the majority of the third   molar areas (72%) in our population, the IAC was lingual to the root structures (Table I). In this study  population, scanned for complicated third molar extractions, only seven (8.1%) canals were located  inferior to the apices. In 16% (n =14) of the cases, the canal was between the root structures (inter- radicular). In about six percent (n = 5) of the third molar areas, the IAC was bifurcated. About 62% (n  = 54) of the canals had thinned or perforated the lingual cortical plate. In other words, in these (62%)  instances, the lingual cortical plate of the alveolar bone was not visible in the region of the canal.

In approximately 91% of the third molar areas, no cortical margin was present between the IAC and  the tooth structure (Table II). A cortical margin of the canal is defined as a thin, fairly uniform,   radiopaque line of the wall of the canal.

Only about 15% of the roots were centrally located in the alveolar bone (Table III). In addition to the  canals being on the lingual aspect, a majority of the root structures (76%) were also closer to the  lingual cortex.

Discussion

The patients in our study were deemed difficult third molar extraction cases based on findings from  periapical or panoramic radiographs. In this selected group of patients, approximately 72% of the  canals were lingual to the tooth structures. About 90% of the canals were in contact with the tooth  structures in the worst case scenario. These numbers are higher than other studies, probably  because the patients in our study were considered difficult extraction cases by the oral surgeons. In  most of the cases, the surgeons ordered CBCT work when canals were superimposed over the root  structures. In this population, only eight percent of the cases had IAC below the root apices.  Therefore, it appears that surgeons were not frequently requesting CBCT scans in cases where the  canal was not superimposed, and probability of damage to the neurovascular bundle was low. In  addition, in most of the cases where one IAC was below. the root structure had the contralateral  canal either on the buccal or lingual of the roots. Therefore, we assume that the CBCT order was  primarily for the tooth with superimposed canal on a two-dimensional image.

Previous studies have shown that 17-51% of the IACs are located buccal to the third molars.10-14  These studies also showed that 19-49% of the canals are located lingual to the third molars. In 7- 51% cases, the canals were located inferior to the apices of the third molars. In our study, about  72% of the cases were lingual to the third molars. It is not surprising that IAC in the region of the  third molars are more likely to be on the lingual aspect, since the orifice of the canal is on the lingual  aspect of the ramus. Nerve damage following third molar extraction is more likely to occur when the  IAC is lingual to the tooth.10 In 62% of our cases, the IACs had perforated or thinned the lingual cortex. The displacement of the canal toward the cortical plate and subsequent perforation may have  happened during the growth and development of the third molar.

All the third molars in our study were partially or fully impacted. Previous studies have indicated that  unerupted molars are more likely to be closer to the canal compared to erupted molars.15 In our study of impacted molars, 91% of the IACs were in contact with the root structures. Several reports  indicate that the damage to the inferior alveolar canal from third molar extraction ranges from 0.4-5.5%. However, in cases where the canal is closely in contact with the roots, the damage to the  neurovascular bundle ranges as high as 23-35%.11,16 Therefore, we conclude that extraction of  impacted third molars may result in higher incidence of neurovascular damage compared to  extraction of erupted third molars.

Previous studies have shown that 4-15% of the canals are in inter-radicular position.10-13 In our study, only eight percent of the IACs were inferior to the roots, but 16% of IACs were inter-radicular. Many  of the third molars in our study were mesioangularly or horizontally impacted. In such cases, the  canal has the chance to remain close to the canal for a substantial length of the root. In such  situations, in a few slices the canal may be either lingually or buccally positioned, but in a few other  slices may curve into interradicular space.

Compared to other radiographic examinations, CBCT demonstrates a greater reliability to identify  the relationship of the IAC to mandibular third molars.17 Panoramic radiographs have a limited role in  predicting physical contact between IAC and third molars.7 Several studies have recommended  cross-sectional imaging, such as CBCT, to reduce the chance of damage to the canal during  surgical extraction of impacted third molars.13,16-19 One study showed CBCT has a low predictive  ability of IAC damage in cases where the canal is closely in contact with the tooth structure, and is  not superior to panoramic radiography.10 However, the same group of authors has recently reported

that CBCT findings alter surgical planning.9 One study has indicated that Rood’s diagnostic criteria6  based on two-dimensional images is not reliable in predicting contact of the canal to the tooth when  the canal is superimposed over the roots.20

In previous reports,8 and in many of the authors’ cases, the IAC traveled from near the buccal cortical  plate to the lingual cortical plate within the dimensions of the third molar. An intraoral or a panoramic  radiograph does not have the ability to display bucco-lingual relationship of such a tortuous IAC path  in a short span of bone. Cross-sectional imaging, such as CBCT scan, may be an appropriate choice  of examination when the canal is seen superimposed over the roots on panoramic or periapical  radiographs. In cases where the canal is not superimposed on the roots, panoramic or periapical  radiographs maybe adequate for surgical planning.17 Routine use of high radiation medical CT in  surgical planning of third molar extractions is not recommended.21

References

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*Dr. Barayan is from King Abdulaziz University, Jeddah, Saudi Arabia.

**Dr. Ahmad is from Department of Diagnostic and Biological Sciences, University of Minnesota School of Dentistry, Minneapolis, Minnesota. Email is ahmad005@umn.edu