ASSESSMENT OF MANDIBULAR 3RD MOLAR APICES TO THE INFERIOR DENTAL CANAL USING CONVENTIONAL RADIOGRAPHY AND CBCT

Conventional radiographs provide only a 2 dimensional view of complicated three dimensional structures. The proximity, angulations, location of impacted teeth, related to the IAN can be adequately determined using conventional 2-D periodical and panoramic radiographs. Computed tomography necessitates the administration of a high dose radiation, especially when the slice width is shortened to increased cost are the negative aspects of computed tomography compared with conventional imaging. Cone beam computed tomographies are becoming more readily available for use in maxilla facial applications. CBCT provides better image quality of teeth and their surrounding structures, compared with conventional CT and offers high spatial resolution. CBCT seems to be more accurate data about inferior alveolar canal. Hence the present study was undertaken to assess and select roots of third molar with close proximity to inferior dental canal in panoramic radiograph having radiographic signs of darkening of root, disruption of canal cortex and canal deviation and to evaluate the same roots with CBCT using trans axial and coronal sections which is angulated parallel to long axis to tooth, axial and coronal sections for proximity of inferior dental canal with the apices of third molar root.

Extraction of impacted mandibular third molar is a routine procedure in oral surgery due to recurrent pericoronitis, caries and pulpits, orthodontic reasons, cysts and tumours. The most common complications being pain, root tip fracture, paresthesia, alveolar osteitis, temporomandibular joint disorder.
When there is a closer relationship between the tooth and inferior alveolar nerve, the incidence of nerve injuries high as 23-35%. An accurate preoperative radiographic examination is therefore considered indispensable before extraction of third molars. This examination should help the surgeon to evaluate the difficulty of the operation and to choose the most appropriate surgical technique for example, where to remove bone, how to split the tooth and in which direction the roots can be lifted.
Conventional radiographs provides only a 2 dimensional view of complicated three dimensional structures.
The proximity, angulations, location of impacted teeth, related to the IAN can be adequately determine using conventional 2-D periodical and panoramic radiographs. Computed tomography necessitates the administration of a high dose radiation, especially when the slice width is shortened to increased cost are the negative aspects of computed tomography compared with conventional imaging.
Cone beam computed tomographies are becoming more readily available for use in maxilla facial applications. CBCT provides better image quality of teeth and their surrounding structures, compared with conventional CT and offers high spatial resolution. CBCT seems to be more accurate data about inferior alveolar canal.
Hence the present study was undertaken to assess and select roots of third molar with close proximity to inferior dental canal in panoramic radiograph having radiographic signs of darkening of root, disruption of canal cortex and canal deviation and to evaluate the same roots with CBCT using trans axial and coronal sections which is angulated parallel to long axis to tooth, axial and coronal sections for proximity of inferior dental canal with the apices of third molar root.

Methodology
The study was carried out following approval of ethical committee and scientific committee SRM University and informed patients who underwent pre-operative radiographic evaluation before extraction of impacted mandibular third molars were obtained. Radiographs with close proximity of third molar with inferior dental canal were selected based on the following radiographic signs like darkening of the root, disruption of canal cortex, deviation of thee inferior dental canal.
Darkening of the root is considered when increased radiolucency of the root is evident where the canal crosses the root. Disruption of canal cortex is observed when two radiopaque lines that constitute the roof and floor of the inferior alveolar canal. The canal cortex is considered to be disrupted if it disappears on reaching the tooth structure. Either one or both lined may be involved. Deviation of the inferior dental canal is considered to be deviated if, when it crosses the mandibular third molar, it changes its direction.
Intra osseous pathologies like cyst or tutors, third molars with root resorption, third molar with developing roots were exclude from the study.

Results:
In our study roots of third molar with close proximity with inferior dental canal in panoramic radiograph having radiographic signs of darkening of root , disruption of canal cortex and canal deviation were trans axial, axial and coronal sections. CBCT images were used to determine proximity and position of inferior dental canal with third molar root apices, grooving of third molar root and canal cortication. 22 roots showed darkening of root in OPG. 23 roots had radiographic finding of disruption of canal cortex and 2 root had radiographic finding of canal deviation. As sample size was small in the category of radiographic finding of canal deviation no statistical analysis was done.
Our study had predominant prevalence of mesioangular impactions of 59.6% followed by 17% of distoangular impactions, 12.8% of horizontal impactions and vertical impactions of 10/6%. 68.2% of mesioangular impactions were present in radiographic feature of disruption of canal cortex and distoangular, vertical and horizontal impactions in disruption of canal cortex were of 30.4%, 8.7%, 13% respectively and in coal deviation, 100% of mesioangular impactions were evident. In position of inferior alveolar canal in relation to third molar, buccally and inferiorly positioned inferior dental canal showed predominance of 44.7% and lingually placed canal of 8.5%. Canal placed interradicularly in 2.1% of roots. 40.9 % of buccal and inferior positioned canals in relation to third molars in teeth with radiographic signs of darkening of root were present. Lingually and intereradicularly placed canal in 13.6% and 4.5% of roots respectively with p-0.026. 2% of buccal and 43.5% of inferiorly positioned inferior dental canal in relation to third molar were present in teeth with radiographic sign of disruption of canal cortex, lingually and interradicularly placed canals in disruption of canal cortex is 4.3% and 100% of inferior canals were present.

Discussion
21 patients, 16 males and 5 females (30 teeth) were selected for close relationship with inferior dental canal. 47 roots which had close relationship with inferior dental canal with panoramic radiographic sign of darkening of root, disruption of canal cortex and canal deviation were selected and 3d imaging with CBCT was done. CBCT images were analysed for position & proximity of inferior dental canal in relation to third molar root, grooving of the roots of third molar and canal corticalisation.
Seven radiological signs had been suggested as indicative of a close relationship between the mandibular third molar root and the inferior alveolar canal. Four signs seen on the root of the tooth are darkening of root, narrowing of roots, deflection of root, bifid apex. Changes in the appearance of the inferior alveolar canal are deviation of canal, narrowing of canal, loss of lamina dura. Previous studies (9,10) showed three signs had increased risk of injury which are darkening of root, disruption of canal cortex and canal deviation.
Panoramic radiograph is frequently used in preoperative evaluation of impacted tooth. But inadequacy of two dimensional images of panoramic radiograph restricts it from assessing complicated tooth to inferior dental canal relations. CT is useful with its cross sectional imaging capability to analyse different tooth relationships with the canal. Cone-beam computed tomography generates threedimensional (3D) data at lower absorbed doses of radiation than conventional computed tomography with high resolution images, is suitable in evaluating relationships of mandibular canal with impacted tooth.
Grooving in our study is only 4.3% which is not in agreement with earlier study (22) which showed grooving in 23%. This difference can be due low prevalence of lingually placed canal in our study of 8.5% in which grooving occurs in preponderance. Even though grooving were present in low prevalence they were present only in the lingual side which is in concurrence with earlier studies (8,11,12) who reported that grooving of the tooth root mainly occurs at the lingual side. Even though our frequency of grooving is less of 4.3%, they were present only in radiographic sign of darkening of root which is in concurrence with Previous studies (16,18) which showed grooving of the root occurred more frequently among darkening of roots. Previous study (12) stated that a dark band on panoramic radiograph was an important indicator of the grooving of the tooth by the canal which is not in concurrence with our study which has 9.1% of grooving in darkening of root.
Canal corticalisation was absent in 55.3% which was higher than earlier study (42) in which 40.3% of absence of corticalisation is evident the difference of which can be attributed to their's large sample size of 695 teeth. In the present study absence of canal corticalisation is evident in radiographic sign of darkening of root in 45.5% which was less when compared to earlier study (14) in Darkening of roots which had 76% of absence of corticalisation. Absence of canal corticalisation in radiographic sign of disruption of canal cortex is 61% in the present study, which is in agreement with earlier study (14) in radiographic sign of Interruption of canal cortex which showed 58.5% of absence of corticalisation.

Conclusion
Though Panoramic radiographs reveal the close proximity of mandibular third molar to inferior dental canal, CBCT images can be used for accurate assessment of proximity and to obtain in depth information about the position of inferior dental canal, grooving of third molar root and canal corticalisation, which are imperative for preoperative planning of surgical removal of impacted tooth. This outweighs the risk of increased radiation and benefits the patient. So CBCT can be used for preoperative planning to patients with impacted tooth having radiographic sign of darkening of root and disruption of canal cortex. Cross sections along long axis of tooth can be used for easy assessment of relationships between impacted tooth root apices and inferior dental canal.