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.
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.
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
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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 email@example.com