Mandibular Posterior Landmarks
Mental foramen – The mental foramen, the primary landmark of this area, is a circular radiolucent structure located below the roots of the mandibular premolar teeth. This structure is the opening for passage of the mental nerve and vessels and can be observed on mandibular premolar and the lateral aspect of canine periapical images. This bilateral radiolucency can be misinterpreted as a periapical lesion. However, it is easily differentiated upon closer examination of the tooth and its supporting structures.
External oblique ridge – The external oblique ridge or line is the bony anterior border of the ramus located on the outer aspect of the mandible. This ridge has a downward diagonal course and is seen on most mandibular molar periapical images and molar bitewings. It is more prominent and appears more frequently than the internal oblique ridge which will be discussed next. This bilateral radiopaque landmark gives the mandible and the dentition a smile appearance.
Internal oblique ridge - The internal oblique ridge is the bony ridge found bilaterally on the lingual aspect of the posterior mandible. This radiopaque ridge is variable in its appearance ranging from highly defined to barely visible. When recorded on molar periapical images, it runs parallel to but below the external oblique ridge. The internal oblique ridge is sometimes referred to as the mylohyoid line.
Mandibular canal – The mandibular canal is the pathway in bone where the inferior alveolar nerve and blood vessels course through the mandible. The canal extends from the mandibular foramen within the ramus anteriorly to the mental foramen. This tubular bilateral radiolucency often demonstrates fine radiopaque boundaries. The mandibular canal is recorded on mandibular premolar and molar periapical images. It is also referred to as the inferior alveolar nerve canal. The mandibular foramen is not recorded on mandibular periapical images.
Submandibular fossa – The submandibular fossa is a depression in bone on the lingual aspect of the posterior mandible. The fossa is located bilaterally below the internal oblique ridge or mylohyoid line. This concavity is where the submandibular salivary gland rests. The submandibular fossa presents as a diffuse bilateral radiolucency typically with few trabeculae. It can appear unusually radiolucent, enticing the novice clinician into thinking a bony lesion is present. The submandibular fossa is recorded on premolar and molar periapical images. This structure is also referred to as the submandibular gland fossa or mandibular fossa.
Lower border – The lower border of the mandible appears as a radiopaque band of dense cortical bone demarcating the inferior aspect of the mandible. This structure can be observed on any mandibular periapical view, especially when the x-ray beam angulation is excessive.