Normal mandibular anatomy

On periapical radiographs of the central incisors the mental fossa appears as a radiolucent depression extending laterally from the midline and between the alveolar ridge and the mental ridge. Due to the thinness of the bone in the area, the mental fossa appears slightly radiolucent compared to adjacent bone and may be mistaken for periapical disease.

The mental foramen is seen on some periapical radiographs and has a varying appearance; sometimes round or oblong, sometimes slitlike. Typically it is positioned halfway between the lower border of the mandible and the alveolar crest, in the region of the apex of the second premolar. It may appear over the apex of a tooth, mimicking periapical pathoses. A second radiograph from another angle will likely cause the appearance of the foramen to shift in relation to the apex and confirm its identity.

The mandibular canal appears inconsistently and is seen as a dark linear shadow with thin radiopaque borders. The canal extends radiographically from the mandibular foramen to the mental foramen.

Nutrient canals appear in a small number of patients as radiolucent lines extending vertically from the inferior dental canal to the interdental space between the mandibular incisors. Occasionally the canals may appear as small round radiolucencies perpendicular to the cortex and can be mistaken for pathology.

The mylohyoid ridge appears as a radiopaque line running from the area of the third molars to the premolar region, occasionally superimposing the molar roots. The margin of the ridge is varies and is often not well defined.

The submandibular gland fossa is located below the mylohyoid ridge in the molar area and appears as a radiolucent area with a sparse trabecular pattern.

The external oblique ridge is the continuation of the anterior border of the mandibular ramus which disappears in the area of the first molar. On periapical radiographs it appears superior to the mylohyoid ridge, running nearly parallel to it. Radiographically it appears as a radiopaque line with varying width, density, and length.

The coronoid process is often seen in the molar region and appears as a triangular opacity superimposed on the area of the third molar. Trabecular pattern may or may not be visible.