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Synonyms: lower jaw, lower jaw bone
German: Mandibula

1 Definition

The mandible is the largest and strongest bone of the facial skull. It consists of an U-shaped curved body, the Corpus mandibulae, and two branches ascending on the right and left of the jaw angles (Anguli mandibulae), the Rami mandibulae.

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2 Corpus mandibulae

The Corpus mandibulae is curved in a hoof-shaped form. Its upper delimitation is the alveolar process (Pars alveolaris mandibulae), which contains the 16 bony tooth sockets and can hugely variate in its form according to the respective dentition. When elderly people lose their teeth, the alveolar process therefore almost disappears, so that this is no variety, but a physiological alteration.

The lower edge of the Corpus mandibulae transitions into the Angulus mandibulae in dorsal direction on both sides. The Corpus has 2 surfaces:

2.1 Facies externa

The convex side of the mandible is called Facies externa (external surfaces). In the midline, a small bony ridge marks a place where the right and left part of the lower jaw bone grow together during the embryonic phase. Like the part of the pubic bone, it is called symphysis. The bony ridge is divided in its caudal course, and there, it transitions into the triangular elevation, the Protuberantia mentalis (chin ledge). It has an elevation on the left and right each (Tuberculum mentale), which later transitions into a fine ridge situated diagonally across the bone (Linea obliqua).

The Linea obliqua ascends until the front edge of the respective Ramus mandibulae in cranial and dorsal direction. It also represents the origin of the triangular muscle and the depressor labii inferioris muscle, the platysma muscle inserts caudally. Cranial to the Linea obliqua, on the alveolar process directly below the molars, you can find the insertion of the buccinator muscle.

The Fossa incisiva, which serves as the origin of the mentalis muscle and of a part of the orbicularis oris muscle, passes below the incisors. Caudal to the second premolar, the mental nerve, the artery and the mental vein leave the skull through the mental foramen.

2.2 Facies interna

The concave side of the mandible is the Facies interna (inner surface). Near the lower part of the symphysis, there are small protrusions, the mental spines, which represent the origin of the genioglussus muscle and geniohyoideus muscle. However, the spines can also fuse to a bony ridge or a tuberosity. Below the spines, there is the small oval digastric fossa of the mandible, which represents the origin of the digastric muscle. In dorsal and cranial direction, you can see the mylohyoid line of the mandible ascending on both sides, imitating the course of the Linea obliqua on the external side. It is the origin of the mylohyoid muscle, in the posterior part of the constrictor pharyngis superior muscle. Above and below the line, you can see two indendations (Fossae), where the sublingual gland or the submandibular gland, respectively, are settled to the bone.

3 Ramus mandibulae

The mandible branch (Ramus mandibulae) originating from the Anguli mandibulae on both sides, has a square basic shape with 2 surfaces, 4 edges and 2 protrusions (processus). On its inner side, it harbors the entry point of the inferior alveolar nerve and the inferior alveolar artery in the alveolar canal, the mandibular foramen.

3.1 Processus coronoideus

The coronoid process is a thin, triangular, bone protrusion flattened on both sides. Its anterior edge is convex and transitions into the anterior margin (Margo anterior) of the Ramus mandibulae. The posterior edge is concave and forms the anterior edge of the mandibular notch. The lateral surface is smooth and serves as insertion for the masseter muscle and the temporal muscle. The medial surface also serves as muscle insertio for the temporal muscle. It has a small bony crest (temporal crest of mandible) that draws to the last molar in anteroinferior direction. Between this and the anterior edge of the Ramus mandibulae, there is a small triangular surface, the Trigonum retromolare (retromolar triangle), which serves as insertion or origin, respectively, to the temporal muscle and some fibers of the buccinator muscle.

3.2 Processus condylaris

The condylar process is more massive than the Processus coronoideus, and it consists of two parts: The mandibular neck (Collum mandibulae) and the mandibular condyle (Caput mandibulae).

The Caput mandibulae forms the mandibular joint (Articulatio temporomandibularis) together with the articular meniscus and the mandibular fossa of the temporal bone. It has a surface convex in longitudinal and transverse direction. Its longitudinal axis points to the medial and slightly to the dorsal direction. On the lateral spur of the condyle, you can find a small tuberculum, which serves ans insertion for the temporomandibular ligament.

The Collum mandibulae is relatively delicate. It is reinforced by bony ridges passing on the lateral and frontal sides. The front is concave and forms the dorsal delimitation of the mandibular notch. On the medial side, there is the insertion of the pterygoideus lateralis muscle.

4 Biomechanics

You can differentiate between various movements of the mandible:

  • Protrusion: Protrusion of the mandible from its resting position
  • Retrusion: Retrusion of the mandible from its resting position
  • Protraction: Protrusion of the mandible from its retrusion position into the resting position
  • Retraction: Retrusion of the mandible from its protrusion position into the resting position
  • Laterotrusion: Lateral movement away from the midline
  • Mediotrusion: Lateral movement towards the midline

Laterotrusion and mediotrusion each refer to one half of the mandible. They always occur together with each other.

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