Internal surface topography

The digastric fossa (fossa digastrica) is a flat depression on the inferior border of the mandibular body at the symphysis. It is the attachment point for the digastric muscle.

The mental spines (spinae mentales) lie very close to the symphysis, directly above the digastric fossa.These four bony projections are divided into the lower spines (spinae musculi geniohy-oidei) for the geniohyoid muscle and the upper spines (spina musculi genioglossi) for the genio-glossus muscle.

The mylohyoid line (linea mylohyoidea) is a ridge of bone that, starting very close to the mental spines, extends diagonally in a posterosupe-rior direction. It is the attachment for the geniohyoid muscle and forms the border of the floor of the mouth. When the floor of the mouth rises during swallowing, a mandibular denture whose border is too deep can cause pressure points; the denture border should therefore be shortened in keeping with the mylohyoid line. Once again, denture reduction is a necessity.

The sublingual fovea (fovea sublingualis) is the flat impression of the sublingual gland in the region of the premolar roots above the mylohyoid line. The submandibular fovea (fovea sublingualis) is the slight impression of the submandibular gland in the region of the molars below the mylohyoid line.

The alveolar part (pars alveolaris) of the mandibular body bears the mandibular set of teeth and has the same topographic features as the alveolar process of the maxilla: alveoli, interalveo-lar septa, interradicular septa, and the aforementioned alveolar juga. The alveolar part is inclined lingually in line with the inclination of the posterior teeth.

The retromolar trigone (trigonum retromolare) demarcates the alveolar part from the mandibular ramus. The trigone is a small, porous bony support posterior to the last molar on the alveolar ridge. After tooth loss, this portion of bone is not resorbed, much like the tuberosity of the maxilla. This area of bone can also be used to support a denture base. The topographic border of the ret-romolar trigone is marked by the temporal ridge, which starts on the internal surface of the mandibular ramus and is divided into two limbs or crura: the lateral crus (crus laterale) and the medial crus (crus mediale), which runs beside the oblique line.The elevation of mucous membrane over the retromolar trigone is often given the same name, which can be confusing when this variable mucosal elevation is used as a reference point for the occlusal plane. The retromolar trigone lies about 2 to 3 mm below the occlusal plane.

The internal surface topography of the mandible is shown in Fig 6-25.