Anatomical differences in lower third molars visualized by 2D and 3D X-ray imaging: clinical outcomes after extraction


The purpose of this study was to evaluate the relationship between third molars and the inferior alveolar canal using panoramic radiographs and cone beam computed tomography (CBCT) scans and to assess clinical outcomes after third molar removal retrospectively. The degree of superimposition, buccolingual position (buccal, central, and lingual) and physical relationship (separation, contact, and involved) were measured using CBCT scanning. Post-extraction complications were recorded. Based on radiographic evaluation, 45.9% of third molar roots were inside the inferior alveolar canal, 21.3% were in contact with the inferior alveolar canal, and 32.8% were separated from the canal. The frequency at which the mandibular canal was separated from the root apex was significantly higher when the canal was in the buccal position (80.0%) than in the central (20.0%) and lingual positions (0.0%). Although on panoramic radiographs all third molars were directly superimposed on the inferior alveolar canal, CBCT showed direct contact or canal involvement in 67.2% and separation of the canal from the root apex in 32.8%. Complications occurred in nine patients: eight had third molar root apices inside or in contact with the inferior alveolar canal. The prevalence of post-extraction complications correlated with the absence of cortication around the inferior alveolar canal.

The incidence of inferior alveolar nerve (IAN) injury after surgical removal of mandibular third molars (M3) ranges from 0.5% to 8%, but the risk of permanent injury, in which sensory impairment lasts longer than 6 months, is less than 1%. Although the risk of permanent injury during M3 removal is low, it can be distressing for patients when it happens. An accurate preoperative assessment of M3 proximity to the inferior alveolar canal (IAC) before extraction may decrease the risk of IAN injury.

Panoramic radiography is the standard diagnostic tool for preoperative assessment of the relationship of the IAC to the M3 root apices. There are several panoramic radiographic findings that can help clinicians evaluate the proximity of the IAC to the M3. Sedaghatfar et al. identified five radiographic findings (darkening of the tooth root, narrowing of the tooth root, interruption of the white lines, diversion of the canal, and narrowing of the canal) associated with IAN exposure during M3 extraction. Although panoramic radiographs are helpful they do not allow precise visualization of the relationship between M3 and the IAC. Maegawa et al. found that in six of seven patients where the IAN was visible after M3 removal, cortication of the cranial or caudal border of the M3 and the periodontal membrane space of the M3 were not clearly visualized by panoramic radiography.

Computed tomography (CT) provides surgeons with high resolution three dimensional (3D) images, but CT is not the standard imaging technique used for evaluating impacted M3s due to the high radiation exposure and expense. Cone beam CT (CBCT) has the advantages of high resolution and low radiation dose and is more affordable.

The close proximity of the mandibular M3 roots to the IAC is often seen on panoramic radiographs. 3D imaging with CBCT scans provides a better view of the relationship of these structures. Although it often appears on Panorex that there is close proximity of the IAC to the M3 roots, the low incidence of IAN injury/exposure during extraction suggests that this relationship may not be as it appears on a two dimensional (2D) radiograph. The aim of this study was to determine whether there is a correlation between 2D panoramic radiographs showing superimposition and CBCT findings. Additionally, the authors estimated the prevalence of post-extraction complications in cases where the IAC was in direct contact with the M3 roots.

Materials and methods

This study was a retrospective study of patients referred to the Department of Oral and Maxillofacial Surgery, Anam Hospital, Korea University, South Korea, for extraction of impacted M3s between July 2010 and December 2010. All patients had panoramic radiographs and CBCT scans were taken only when the M3 roots were superimposed on the IAC on panoramic view. Patients were informed of possible complications following removal of the M3 and written informed consent was obtained from all patients. This study was done under IRB policy (AN12153-001).

Digital panoramic radiographs were taken with an Auto 3E CM (Asahi, Japan) operated at 75 kV and 10 mA using a photo-stimulable phosphor plate. CBCT examinations were performed with an AZ3000CT 3-D Imaging System (Asahi, Japan). The imaging parameters for the AZ3000CT were 6 mA and 85 kV with a 0.5 mm × 0.5 mm fixed focal spot. The field of view (FOV) was 80 mm in height and 75 mm in diameter. Total scanning time was 17 s. The normal voxel size was 0.1 mm.

Evaluation of panoramic radiographs

The vertical relationship between the M3 root and the IAC was classified according to the degree of superimposition on the panoramic radiograph. Class A, root apex superimposed on less than one-third of the canal; Class B, root apex superimposed on one-third to two-thirds of the canal; Class C, root apex superimposed on more than two-thirds of the canal ( Fig. 1 ).

Fig. 1
Vertical relationship between M3 and IAC on panoramic radiographs. (a) Class A <1/3 overlap; (b) Class B 1/3–2/3 overlap; (c) Class C >2/3 overlap.

Evaluation of CBCT

The CBCT images were assessed with On Demand ® software ( Fig. 2 ). A 3D reconstruction screen was used to scroll through the axial, sagittal and coronal planes with a 0.2 mm slice thickness. The relative position of the canal to the M3 root on CBCT was classified by buccolingual position as buccal, central, or lingual ( Fig. 3 ). The proximity between M3 and the IAC was classified as (Type I) separation, (Type II) contact, or (Type III) involved. The presence of cortication around the IAC was clearly seen in the separation type on the axial and coronal CBCT. There was loss of the cortical lining of the IAC in the contact type, but the round shape of the IAC was not distorted. The round shape of the IAC was flattened by the root in the involved type, and there was loss of the cortical lining ( Fig. 4 ).

Fig. 2
On Demand ® software program. (a) Coronal view, (b) sagittal view, (c) axial view, and (d) 3D reconstructed image.

Fig. 3
Buccolingual position of IAC to M3 on CBCT. (a) Buccal, (b) central, and (c) lingual.

Fig. 4
Relationship between M3 and IAC. (a) Separation, (b) contact, and (c) involved.

Clinical evaluation

The M3s were extracted under local anaesthesia. When the IAC was seen after extraction, it was recorded in the operative report. A week later, patients were seen for follow up examinations, and complications were documented.

Statistical analyses

The χ 2 test was used to determine the statistical differences among the categories on panoramic radiographs and CBCT. Probability values of less than 0.05 were considered statistically significant. All statistical analyses were performed using PASW Statistics 18.


122 M3s from 105 patients (54 women and 51 men) were included in the study. The patients had a mean age of 23.9 years (range 17–43 years) ( Table 1 ).

Table 1
Patients age and sex.
Age (years) Male Female Total
10–19 11 11 22
20–29 34 34 68
30–39 5 8 13
40–49 1 1 2
Total 51 54 105

The vertical relationship between the M3 root and the IAC evaluated on panoramic radiographs showed that there were 29 (23.8%) Class A, 51 (41.8%) Class B, and 42 (34.4%) Class C relationships. Although all of the M3 roots included in this study were involved in the IAC on panoramic 2D images, the CBCT data (3D X-ray image) demonstrated that the majority of M3 (82/122 teeth, 67.2%) were Types II and III and had loss of cortication, while nearly one-third (40/122 teeth, 32.8%) of the roots were separated Type I. The relative position of the IAC to the M3 roots when there was loss of cortication around the IAC on CBCT was: buccal (37/82 teeth, 45.1%); central (24/82 teeth, 29.3%); and lingual (21/82 teeth, 25.6%). The frequency at which the mandibular canal was separated from the M3 apices was significantly higher in the buccal position (80.0%) than in the central (20.0%) or lingual positions (0.0%) ( χ 2 = 26.282, std. residual = 2.0, p < 0.001). Loss of cortication around the IAC was observed in all cases in which the IAC was in the lingual position ( Table 2 ).

Table 2
Relationship between location of IAC relative to M3 roots and proximity of roots to IAC on CBCT.
Buccolingual relationship Proximity of the roots to the canal
Separation (%) Contact (%) Involved (%) Total
Buccal 32 18 19 69 (56.9%)
Central 8 3 21 32 (26.0%)
Lingual 0 5 16 21 (17.1%)
Total 40 (32.8%) 26 (21.3%) 56 (45.9%) 122 (100%)

There was good correlation in the proximity of the M3 roots to IAC between the panoramic radiograph (2D) and CBCT (3D) ( Table 3 ). For Class A, 20 M3 (68.9%) were Type I, 6 (20.7%) were Type II, and 3 (10.3%) were Type III. In the Class A group, the IAC was most commonly observed to be separated from the M3 root and in the buccal position on CBCT images (41.4%). In the Class B group, 12 M3 (23.5%) were Type I, 9 (17.6%) were Type II, and 30 (58.8%) were Type III. In the Class C group, 8 M3 (19.0%) were Type I, 11 (26.2%) were Type II, and 23 (54.8%) were Type III.

Table 3
Relationship between IAC and M3 on panoramic radiograph and CBCT.
CBCT Degree of superimposition on Panorex Total
Class A Class B Class C
Separation (Type I) Buccal 12 12 8 32
Central 8 0 0 8
Lingual 0 0 0 0
Subtotal 20 12 8 40 (32.8%)
Contact (Type II) Buccal 4 4 10 18
Central 2 1 0 3
Lingual 0 4 1 5
Subtotal 6 9 11 26 (21.3%)
Involved (Type III) Buccal 0 9 10 19
Central 2 14 5 21
Lingual 1 7 8 16
Subtotal 3 30 23 56 (45.9%)
Total 29 (23.8%) 51 (41.8%) 42 (34.4%) 122 (100%)
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Jan 24, 2018 | Posted by in Oral and Maxillofacial Surgery | Comments Off on Anatomical differences in lower third molars visualized by 2D and 3D X-ray imaging: clinical outcomes after extraction
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