Abstract
This prospective study evaluated the role of cone beam computed tomography (CBCT) in the treatment of patients with impacted mandibular third molars at increased risk of inferior alveolar nerve (IAN) injury. Subjects with an increased risk of IAN injury, as diagnosed on panoramic radiographs, were enrolled in this study and underwent additional CBCT imaging. Two oral maxillofacial surgeons independently planned the surgical technique and estimated the risk of IAN injury on panoramic radiographs and on CBCT images. A test of symmetry and the McNemar test were executed to calculate the differences between the two imaging modalities. The study sample comprised 40 patients (mean age 27.6 years) presenting 53 mandibular third molars. Risk assessment for IAN injury based on panoramic radiography compared with CBCT imaging differed significantly ( P < 0.005). After reviewing the CBCT images, significantly more subjects were reclassified to a lower risk for IAN injury compared with the panoramic radiograph assessments. This change in risk assessment also resulted in a significantly different surgical approach ( P < 0.03). The results of this study show that CBCT contributes to optimal risk assessment and, as a consequence, to more adequate surgical planning, compared with panoramic radiography.
The removal of third molars is one of the most common surgical procedures performed by oral and maxillofacial surgeons, often followed by an uneventful convalescence. As with any surgical procedure, the removal of third molars is associated with complications, including damage to the inferior alveolar nerve (IAN) with subsequent neurosensory impairment in the lower lip and chin. Removal of impacted mandibular third molars is the main cause of permanent IAN injury, outweighing implant and orthognathic surgery as an etiological factor .
Many studies have investigated the risk factors influencing the prevalence of IAN injury after the removal of mandibular third molars . In those most at risk of IAN injury, the third molar root lies close to the mandibular canal . In these cases, the IAN may be injured during third molar removal, directly by surgical instruments , or indirectly by the manipulation of the IAN during unfavourable movements of the third molar roots . This risk can be reduced by preoperative radiographic assessment of the anatomical relationship of the third molar root and the mandibular canal. This information should help the surgeon to determine the risk of postoperative sensory impairment and to modify the operative technique to minimize the risk of IAN injury.
Whilst different imaging techniques are available for this purpose , in many cases a panoramic radiograph suffices in the preoperative assessment of the impacted third molar. If the mandibular canal and the third molar are close it is more difficult to evaluate the anatomical relationship because there is insufficient information on the buccolingual view . In such cases, three-dimensional (3D) imaging, such as computed tomography (CT) or cone beam CT (CBCT) may be valuable . CBCT has been reported to be a reliable imaging modality in the assessment of the buccolingual position of the mandibular canal in relation to the third molar . The usefulness of the additional information provided by 3D imaging in the presurgical workup and management of impacted third molars is a topic for discussion .
The aim of the present study was to evaluate if the additional information provided by CBCT images influences the surgical strategy in the treatment of patients with impacted mandibular third molars in high risk cases. More specifically, it considered whether there is any difference in risk assessment for IAN injury when using CBCT and panoramic radiography and whether this results in a different surgical approach.
Materials and methods
Consecutive patients who consulted for mandibular third molar extraction, were potential candidates for this prospective study. Only subjects with one or both impacted third molars at risk of IAN injury, as diagnosed on digital panoramic radiographs, underwent additional CBCT imaging and were enrolled in this study. Increased risk of IAN injury was defined as the presence of one or more radiographic signs, which have been reported to be suggestive of a close relationship between the mandibular canal and third molar, such as: interruption of the white line of the mandibular canal wall; darkening of the root; diversion of the mandibular canal; narrowing of the mandibular canal; narrowing of the roots; and deflection of the roots . All patients were informed of possible complications following the removal of the third molar and written informed consent was obtained from all subjects.
Digital panoramic radiographs were made with a Soredex Cranex Tome device (Soredex, Helsinki, Finland), operated at 81 kV and 10 mA using a photostimulable phosphor plate. The CBCT mandibular scan was acquired using the i-CAT™ 3D imaging system (Imaging Sciences International Inc, Hatfield, PA, USA). The scanner specifications are listed in Table 1 .
| X-ray source | High frequency, constant potential, fixed anode 120 kVp, 3–8 mA (pulse mode) |
| X-ray beam | Cone-beam |
| Focal spot | 0.5 mm |
| Field of view | 6 cm |
| Image detector | Amorphous silicon flat panel 20 cm × 25 cm |
| Voxel size | 0.25 mm |
| Grey scale | 14 bit |
| Scan time | 20 s |
| Effective dose | 47 μSv |
In a darkened room, the panoramic radiographs of all patients were demonstrated in randomized order followed by the CBCT images on a 17 in. PC monitor. Evaluation was carried out by two trained oral and maxillofacial surgeons independently. Both surgeons were experienced in diagnosing maxillofacial structures from both imaging modalities and were blinded for the clinical outcome.
CBCT images were assessed using the i-CAT Vision ® software program (Imaging Sciences International Inc, Hatfield, USA). The implant planning screen and the multiplanar reconstruction screen were employed to scroll through the axial, sagittal and coronal planes. The slice thickness was 1 mm. The panoramic radiographs were assessed through the Digora ® software program (Digora for Windows 2.5 Rev 1 © , Soredex, Tuusula, Finland). The surgeons could use the toolbar to adjust the contrast or use the zoom function.
The vertical relationship between the third molar root and the mandibular canal, as seen on panoramic radiographs, was assessed into the following categories: 1, the root apex just touching the upper wall of the mandibular canal; 2, the root apex halfway along the mandibular canal; or 3, the root apex under the inferior wall of the mandibular canal ( Fig. 1 ).
The buccolingual relationships of the mandibular canal in relation to the roots of the third molar, as observed on CBCT images in sagittal, transversal and coronal dimensions, were classified as lingual, buccal, interradicular or inferior.
In the light of an overall risk of approximately 1% , the risk of permanent IAN injury was evaluated from both imaging modalities independently, based on a 5 degree scale (no risk, small risk, moderate risk, high risk, and very high risk).
The surgeons planned their surgical technique based on information provided by panoramic radiographs and CBCT images. The surgical planning was divided in subgroups as listed in Table 2 .
| Subgroup | Question | Answer | |
|---|---|---|---|
| A | Removal of buccal bone | I can remove buccal bone until the bifurcation safely without damaging the IAN | Yes/No |
| B | Elevator placement | I can place the elevator safely without crushing the IAN | Yes/No |
| C | Tooth sectioning | (1) I will decapitate the crown/roots to prevent IAN injury | Yes/No |
| (2) After sectioning of the crown or fracture of the root, I can remove extra buccal and distal bone safely without damaging the IAN | Yes/No | ||
| D | Direction of tooth removal | To prevent injury to the IAN, I attempt to remove the root/crown in a…. | 1. Lingual direction |
| 2. Buccal direction | |||
| 3. Buccal and lingual direction | |||
| 4. Undeterminable |
Statistical analyses
To test the difference between CBCT and panoramic radiography in risk assessment for IAN injury, a test of symmetry was carried out. The McNemar test was executed to calculate the difference between CBCT and panoramic radiography in surgical planning. Probability values of less than 0.05 were considered statistically significant. All statistical analyses were performed by using the SAS program (SAS Institute Inc., Cary, NC, USA), version 9.1.
Results
The study sample consisted of 53 impacted third molars from 40 patients (20 women and 20 men) with an average age of 27.6 years (range 20–62 years). A statistically significant difference was present between risk assessment for IAN injury based on panoramic radiography and CBCT imaging. After reviewing the CBCT images, significantly more third molars were reclassified as having a lower risk for IAN injury than was assessed by panoramic radiographs ( Table 3 ).
| Lower risk assessment | Equal risk assessment | Higher risk assessment | P -value | |
|---|---|---|---|---|
| Surgeon 1 | 33 (62.3%) | 15 (28.3%) | 5 (9.4%) | 0.0008 |
| Surgeon 2 | 22 (41.5%) | 27 (50.9%) | 4 (7.6%) | 0.005 |
Significant correlation was also seen for the vertical relationship of the third molar with the mandibular canal and the difference in risk assessment between CBCT images and panoramic radiographs. In cases where the root apex was positioned more caudal to the inferior border of the mandibular canal, the risk of injuring the IAN was assessed significantly lower after reviewing the CBCT images ( Table 3 ; P < 0.0001).
Observing the CBCT images gave rise to a different surgical approach from that based on information provided by panoramic radiographs ( Table 4 ). After analyzing the CBCT images, both surgeons decided to remove buccal bone, to place elevators and to remove extra bone after root sectioning in significantly more cases than when using panoramic radiographs. In 87% of cases in which assessment was based solely on observing panoramic radiographs, both the surgeons were unable to determine whether they should luxate the third molar to the buccal or lingual side After observing the CBCT images, the surgeons were confident in their planned direction of third molar removal in all cases ( Table 5 ).
| Surgeon 1 | Surgeon 2 | ||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
| Panoramic Radiograph | Cone beam CT | P -value | Panoramic Radiograph | Cone beam CT | P -value | ||||||
| Yes | No | Yes | No | Yes | No | Yes | No | ||||
| A | Removal of buccal bone * | 37 | 16 | 47 | 6 | 0.008 | 24 | 29 | 50 | 3 | <0.0001 |
| B | Elevator placement * | 4 | 49 | 13 | 40 | 0.029 | 16 | 37 | 31 | 22 | 0.002 |
| C1 | Tooth sectioning | 52 | 1 | 51 | 2 | NS | 50 | 3 | 48 | 5 | NS |
| C2 | Removal of extra bone * | 28 | 25 | 41 | 12 | 0.003 | 22 | 31 | 41 | 12 | <0.0001 |
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