- Continuing Education
Clinicians shall identify potential harm to anatomical structures and should be able to determine the need for a pre-surgical evaluation.
Visualization and mapping of the mandibular canal is one of the advantages of a CBCT study of the mandibular arch. For most patients, a well-defined, corticated radiolucent track will be visualized in cross-sectional images. It typically will be located adjacent to the lingual cortex, originating at the mandibular foramen and extending anteriorly to the mental foramen. The distance from the alveolar crest to the visualized superior border of the mandibular canal can be determined with accuracy using a diagnostic CBCT image. Changes in the path of the mandibular canal and variants of normal may be assessed as well.
In the anterior mandible, it is essential to evaluate the area and identify the anatomical structures of the mandibular incisive canal, the lingual foramen and mental foramen to avoid hemorrhage and neurovascular damage.
In the posterior mandible the submandibular fossa extension is an area to be assessed prior to implant placement. The submandibular fossa is located below the mylohyoid ridge in the area of the mandibular molars. If a pronounced concavity is present, it may not be realized clinically or in a panoramic radiograph, with the potential for implant placement through the lingual cortical plate and high risk of hemorrhage that may be life threatening.
In the anterior maxilla, the incisive foramen is located posterior to central incisors and its diameter may vary. Size, exact localization and topography of such foramen are important because impinging on this anatomical structure can cause neurovascular damage.
In the posterior maxilla, one of the important structures are the maxillary sinuses. When planning for a sinus lift, a CBCT of the area may be requested for evaluation of both sinus health and anatomy. The middle superior alveolar canal will be visualized as a small round radiolucent structure running antero-posteriorly within the lateral wall of the maxillary sinus at the premolar area and should be identified prior to flap procedures in that area. Another anatomical structure in the posterior maxilla is the greater palatine foramen, located distal to the second molar area; its localization is extremely important for flap design. It is important to mention that in CBCT imaging, the location of the greater palatine foramen (osseous opening) will be visualized but the neurovascular bundle (soft tissue) will not be seen. After a sinus lift surgery, a CBCT study can provide information regarding the post-surgical outline of bone grafting as well as how well adapted the bone graft is.
When a block bone graft is planned, information gained from a CBCT scan will assist in donor site evaluation. Autologous bone graft is harvested from the patient’s symphysis or mandibular ramus. In the ascending ramus the distance from the mandibular canal to the buccal cortex should be accurately measured to avoid bleeding or neurological problems. Evaluation for the presence of bifid canals, not uncommon in the ascending ramus, should be determined. If the graft will be harvested from the symphysis, it is important to evaluate the anatomy of the area to avoid damaging apices of teeth or neurovascular bundles like the mandibular incisive canal.
One common treatment consideration for the use of CBCT is the assessment of the unerupted mandibular third molars. The cross-sectional images are extremely helpful in detecting the relationship and proximity of the mandibular canal to the roots of the third molar (Figure 12).
Figure 12. Anatomic relationship between impacted third molar and the mandibular canal.