The mandible develops from a paired primordium, laterally from Meckel's cartilage. Desmal ossification takes place, with the exception of the condyle which develops by chondral ossification.
In the newborn, both halves of the mandible are connected to each other in the region of the chin by the mental symphysis, fibrous cartilage that will have ossified completely by the end of the first year of life, though it will remain a visible site of fusion of the two halves for a while in the child's skull.
In the area of the mental symphysis, small wormian, or sutural, bones may be present, remnants of Meckel's cartilage that have a part in the development of the mental tuberculae.
The mandible is a freely movable horseshoe-shaped bone plate that is connected to the temporal bone by the temporo-mandibular joint (TMJ). It consists of the body of the mandible and the ascending paired mandibular ramus #pic# #pic#.
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Shape and structure
The horseshoe-shaped body of the mandible, a paired bone at first, fuses into one skeletal unit.
The mandible consists of various parts:
- Body of the mandible #pic#
- Alveolar process #pic#
- Angle of the mandible #pic#
- Ascending ramus #pic#
- Articular (condylar) process #pic#
- Muscular (coronoid) process #pic#
The mandibular angle connects the ramus and body.
- Oblique line
- Mylohyoid line
- Buccinator crest
- Mental protuberance
Body of the mandible
The body of the mandible consists of the basal arch, the base of the mandible #pic# #pic#, and the tooth-bearing parabolic alveolar arch, or alveolar process (processus alveolaris mandibulae / pars alveolaris mandibulae) #pic# #pic#.
Base of the mandible
The protuberance of the chin protuberantia mentalis #pic# #pic# #pic# is located in the midline of the external side of the base.
The mental tubercle tuberculum mentale on both sides at the lower edge of the protuberance is more pronounced in men than in women. The chin probably developed later on during evolution as a functional adaptation to the increased activity of the protrusive muscles.
The mental foramen foramen mentale #pic# lies below the second premolar and from which exits the mental nerve nervus mentalis (pressure point of the mandibular branch of the trigeminal nerve) and the accompanying mental artery arteria mentalis as a lateral branch (not a terminal branch!) of the inferior alveolar artery. Since the pathways enter the skin obliquely backwards, the mental foramen has developed a sigmoid-shaped, sharp anterior edge. The posterior edge, on the other hand, levels off continuously towards the surface. In rare cases, two or three mental foramina are present. Such anatomical variations are of practical interest in local anaesthesia and dental implantology.
On the inner side, there are 4 small bony spines in the area of the chin that are referred to as spina mentalis #pic# #pic#, or separately as spinae genii superiores (for the origin of the musculus genioglossus) and spinae genii inferiores (for the origin of the musculus geniohyoideus). Lateral to, and below these spines, there is a paired, shallow depression, the fossa digastrica, from which the anterior belly of the >b>musculus digastricus originates.
The line of origin of the musculus mylohyoideus, referred to as linea or crista, mylohyoidea #pic#, ascends backwards in an oblique direction. Above this line, there is a shallow depression in which lies the sublingual gland referred to as fovea sublingualis, and below the posterior part of the line, another shallow depression against which lies the submandibular gland, fovea submandibularis.
Behind the first molar, an oblique line, linea obliqua, starting externally, ascends backwards and passes into the anterior sharp edge of the mandibular ramus, margo anterior rami mandibulae #pic#.
The alveolar process #pic# #pic# #pic# #pic# contains tooth sockets referred to as alveoli dentales and protrudes on the vestibular side (juga alveolaria) clearly visible especially in the anterior region. The alveolar arch follows the curvature of the mandibular base though not completely as posteriorly it curves more lingually. This inward curvature results in the laterally positioned linea obliqua so that the anterior edge of the ascending mandibular ramus is also more lateral and does not lie directly behind the alveolar process.
Angle of the mandible
The shape and strength of the rounded angle of the mandible #pic# depend mainly on the functional status of the dentition and the masticatory muscles. The size of the angle changes until growth is complete.
The mandibular ramus #pic# #pic# has its origin in the rounded angle of the mandible. The outer surface of the mandibular ramus is rough, this part of the ramus being referred to as the tuberositas masseterica #pic#, and is the attachment area of the musculus masseter. Significant activity of the masticatory muscles results in wave-like surface irregularities of the mandibular angle on the lateral surface, with a similar appearance on the medial surface #pic# #pic# due to the attachment of the medial pterygoid muscle musculus pterygoideus medialis, sometimes referred to as the pterygoid tuberosity. The entrance to the mandibular canal, the foramen mandibulae #pic# #pic#, is located centrally on the medial surface of the adult mandibular ramus, approximately level with the occlusal plane. The opening leads the inferior alveolar nerve and the inferior alveolar artery into the mandibular canal #pic# #pic#, the inferior alveolar vein exits here.
The position of the mandibular foramen changes during growth and aging of the mandible and this should be remembered when giving a mandibular regional block. In the newborn, it is located below the occlusal plane while in the elderly with a reduced alveolar arch, it will be far above the edentulous occlusal plane.
The bow-shaped mandibular canal is covered by a thin layer of compact bone and passes from the spongy bone of the mandible with ascending lateral rami up to the roots of the teeth, forwards approximately to the level of the first premolar where it curves obliquely back and up and exits in the mental foramen #pic#.
Its narrowed final part continues as canaliculus incisivus below the canine and incisor teeth with lateral rami to the roots and does not have a blind end but is connected to the opposite canaliculus.
The main mandibular artery passing through the mandibular canal is not responsible for all the blood supply but is supported by accessory blood vessels entering the mandible via nutritious foramina. There are fewer foramina in the mandible than in the maxilla but their diameters are larger and accommodate larger vessels. They are characteristically located on the inner surface of the coronoid process in the regions of the fovea retromolaris (as foramen retromolare) and of the spina mentalis (as foramina linguales). The mandibular foramen is covered by a bony plate varying in size and referred to as lingula mandibulae which serves as an area to which is attached the ligamentum sphenomandibulare. Opposite the lingula, on the outer surface of the ramus, a small bony prominence may occasionally be found, the antilingula.
The sulcus mylohyoideus originates at the lower edge of the mandibular foramen and passes forwards underneath the mylohoid line, containing the nervus mylohyoideus and an artery with the same name.
The upper proximal end of the mandibular ramus is separated by the sigmoid notch, referred to as incisura mandibulae, into the coronoid and condylar processes.
The coronoid process #pic# #pic# #pic# serves as an area for attachment of the tendon of the temporalis muscle and has a basically triangular shape with an apex that is frequently bent backwards. The coronoid process will be thin and pointed if the temporalis muscle is not very well developed, thicker and curved with a more developed muscle.
Its anterior edge is in continuity with that of the mandibular ramus. A slip of muscle, attached to an area of the coronoid referred to as crista temporalis mandibulae #pic# #pic# #pic# #pic#, extends along the inner surface of the coronoid, gently curving as it does so, and serves as an area for attachment of the deep tendon of the temporal muscle.
Behind the third molar, this strip is separated into two branches (crus mediale and crus laterale), thus enclosing a triangular area referred to as trigonum retromolare. The lateral branch of the temporal crest is more pronounced; it continues to the alveolar process and is also referred to as crista buccinatoria because the buccinator muscle originates from it. In the area of bifurcation of the temporal crest, there is the bony attachment area of the pterygomandibular, or buccopharyngeal, raphe (= ligament) that is the origin of the buccinator muscle. Laterally from the temporal crest, there is a shallow depression referred to as fovea retromolaris that is limited externally by the margo anterior rami mandibulae.
The condylar process carries the barrel-shaped head of the mandibular condyle, caput mandibulae #pic# #pic# #pic# #pic# connected to the mandibular ramus by a narrow neck referred to as collum mandibulae. The lower head of the musculus pterygoideus lateralis is attached to the fovea pterygoidea, a slight depression located in the anterior inner region of the condylar neck.
Age-related changes of the mandible
The mandible and also the associated maxillary alveolar process #pic# #pic# may be almost completely resorbed in elderly edentulous patients.
The funnel-shaped lip of the mandible in old age causes the prominent chin and leads to a reduced height of the lower face. In old age, the mandible is shaped like a clasp and flattened so that it looks similar to the mandible of a newborn.
Severe atrophy results in the mental foramen becoming relatively more superior and closer to the upper edge of this clasp and no longer visible from the vestibule. Atrophy of the alveolar process significantly reduces the distance from the mandibular canal to the upper edge of the mandible so that in rare cases the inferior alveolar nerve is located directly underneath the mucosa.
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- Gerlach KL, Pape HD  Studies on preventive antibiotics in the surgical treatment of mandibular fractures Dtsch Z Mund Kiefer Gesichtschir 12:497-500