What is the mandible?
The lower jaw bone consists of a body (corpus mandibulae), the rear ends of which merge into an ascending branch (ramus mandibulae) on both sides at the angle of the jaw (angulus mandibulae). The angle formed by the body and branch (angulus mandibulae) varies between 90 and 140 degrees depending on the strength of the masticatory apparatus – in newborns it reaches 150 degrees. It decreases with strong development of the chewing muscles.
The basis of the mandible is the basal arch, which includes the base, the middle part of the branch and the articular process. The basal arch becomes narrower towards the top, where the alveolar arch rests, which carries the tooth compartments of the lower row of teeth. It is somewhat smaller and narrower than the basal arch and is set back from the chin.
If teeth are missing, the alveolar arch changes its shape. In the case of complete tooth loss, it can even disappear completely, because a bone that is not functionally used perishes (atrophy of inactivity). As a result, the body of the lower jaw appears narrower and lower, the mouth looks “sunken” – unless the shape is restored with dentures.
The outer surface of the mandibular body
The mental foramen, an exit point for nerves and vessels that lead from the mandibular canal to the skin, is located between the base and the alveolar margin at the level of the first to second molar.
A small elevation on the outer surface of the mandibular body, the linea obliqua, runs diagonally upwards to the ramus (ascending branch of the mandible). Two muscles attach to it: One pulls the corners of the mouth downwards, the other pulls the lower lip downwards and to the side.
Slightly below this is the insertion of a muscle that extends from the neck to the second rib and is part of the mimic musculature. Above this, on the alveolar process and directly under the molars, is the muscle that pulls the corners of the mouth to the side and presses the lips and cheek against the teeth. It helps with sucking by stiffening the cheeks and forces food between the teeth when chewing.
The inner surface of the mandibular body
Close to the bony ridge where the two bones of the lower jaw grow together, there are two small, strong bony protrusions that serve as reinforcement and as an attachment point for two muscles – the muscle that stretches out the tongue and a muscle in the floor of the mouth. This bony reinforcement means that the lower jaw always breaks to the side of the chin area in the event of an impact.
The lower jaw carries the compartments for the tooth roots in the alveolar arch. As in the upper jaw, the individual compartments are separated by bony septa; in teeth with several roots, the individual root compartments are further subdivided by bone. The bone of the alveolar processes has a structure of fine bone beams, whereby the pressure generated during chewing is transferred from the teeth to the jaws.
The mandibular branches
There are two protrusions on the mandibular branches: the articular process and the ossified attachment of the temporal muscle.
The condylar process has a joint head and a neck. The muscle that pulls the lower jaw forward and to the side attaches to the neck in a pit. The joint head forms the temporomandibular joint in a fossa of the temporal bone, together with a joint disk (meniscus articularis) located in between.
The ossified insertion of the temporal muscle (processus coronoideus) is the second projection on each branch of the mandible. The temporal muscle pulls up the pinna and tensions the skull plate. The muscle that allows the mouth to close and the lower jaw to move forward also attaches to the coronoid process. This process is pointed in adults and curves backwards with age.
What is the function of the lower jaw?
The lower jaw is the only movable bone in the skull. Its movements against the upper jaw help to chew and crush bites of food. It also helps with sound formation.
Movements of the lower jaw
The mandible can perform various movements: In addition to opening and closing the mouth, the mandible can be pushed forward (protrusion) and pulled back (retrusion), sideways away from the midline and back towards the midline.
Where is the lower jaw located?
The lower jaw forms the lower part of the facial skull. Its two lateral branches are movably connected to the temporal bone in the temporomandibular joint.
What problems can the mandible cause?
A mandibular fracture can be accompanied by fractures of the tooth roots.
Progenia is the term used by doctors to describe a jaw misalignment in which the lower incisors bite over the upper incisors. Those affected have a protruding chin.
With lockjaw, the mouth can no longer be opened and with lockjaw, it can no longer be closed. Possible causes are inflammatory processes (as in mumps), a dislocation or fracture of the temporomandibular joint, scars or tumors.
