Mandible – Anatomy, Bone, Fracture, Function

Also called the inferior maxillary bone, the mandible is the lowest, largest, and strongest bone in the face. It keeps the lower teeth in their place and constitutes the lower jaw. The anterior surface towards the midline features a minor ridge which is a sign of mandibular symphysis, marked by the bone formation via fusion of the left and right processes at the time of mandibular development. Just as other symphysis occurring in the body, this indicates a midline articulation wherein the bones connect via fibrocartilage. This articulation however fuses the bones together during early childhood.

Mandible: Structure, function, and components

The mandible is made up of:

• A horizontal curved section which is the base or the body.
• The alveolar process, which forms the upper mandible body bears the lower tooth.
• Rami, which are 2 perpendicular portions that connect with the body’s ends at nearly right angles. The angle occurring at this junction is known as gonial angle.
• Coronoid process is an anterior and superior projection from the rami. It offers connection to the temporalis muscle.
• Condyle is posterior and superior rami projection that permits temporo-mandibular joint or attachment with the temporal bone. 2 temporal bones are articulated by the mandible at these joints.
• Nerves: Inferior alveolar nerve is a branch of Trigeminal (V) nerve belonging to the mandibular division provides sensation to the lower teeth. It passes into the mandibular foramen and continues forward into the mandibular canal. It divides into the mental and incisive nerves, or terminal branches, at the mental foramen. The mental nerve comes out of the mental foramen and offers sensation to the lower lip, while the incisive nerve continues onward in the mandible and delivers to the front teeth.
  • In rare cases, people may feature a bifid inferior alveolar nerve. Such patients may feature a more inferiorly positioned 2^nd mandibular foramen. On a radiograph, it is seen by notation of a doubled mandibular canal.
• Foramina: Mental foramen are present as paired, laterally to the chin or the mental protuberance on the mandible body, and normally at the lower end of the apices of the 1^st and 2^nd premolars on the mandible. Growth of the mandible in children is marked by changes in mental foramen opening from front to postero-superior. It also permits the entry of blood vessels and mental nerve into the mandibular canal.
  • The mandibular foramen occurs as paired, medially to the inner section of the mandible and above the mandibular angle occurring in the center of the ramus.
• The development of the mandible: Mandible ossification refers to growth of new bone substance in the fibrous membrane that encloses the Meckel’s cartilages’ outer surfaces.
  • These cartilages occur in the left and right and form, the mandibular arch’s cartilaginous bar. The cranial or proximal ends are joined with ear capsules, while their distal extremities are connected together by mesodermal tissue at the symphysis. They immediately run onwards under the condyles and later curve downwards to lie in a ridge next to the inferior boundary of the bone. They curve upwards to the symphysis anterior of the canine tooth.
  • The incus and malleus, which are two middle ear bones develop from the proximal tip of each cartilage. The next portion gets replaced by fibrous tissue nearly till the lingula; the tissue persists and develops into the spheno-mandibular ligament.
  • The cartilage vanishes between the canine tooth and the lingula, while some part of it present anterior and under the incisors ossify and incorporate with this section of the mandible.
  • Ossification occurs in the membrane enclosing the outer aspect of Meckel’s cartilage’s ventral tip. Each half of the bone develops from one center next to the mental foramen and which occurs around the 6^th week of fetal life.
  • The section of Meckel’s cartilage lying behind and under the incisors is invaded and enclosed by the membrane bone. The cartilage’s accessory nuclei form a little later and feature, tinier nuclei anterior to both walls of the alveolar and along the anterior of the inferior bone border; nucleus shaped like a wedge in the condyloid process that continues downward across the ramus; a tiny strip along the front boundary of the coronoid process.
  • The above mentioned accessory nuclei do not consist of any distinctive ossific areas, but get invaded by nearby membrane bone and are eventually absorbed. The inner alveolar boundary, normally said to be arising from the splenial center, develops in the mandible via an ingrowth of the primary mass of bone.
  • At the time of birth, the bone is made up of 2 parts, joined by a fibrous symphysis, wherein ossification occurs in the 1^st

Fracture and other problems of the mandible

• A major chunk of facial injuries consist of mandibular fracture. The condition often tends to occur along with a ‘twin fracture’ on the opposite or contralateral side. There is no standard method of treating mandibular fractures and doctors differ on the varied applicable techniques of correcting the fractured facial anatomy. In most cases, doctors may opt for attachment of metal plates on the fractures area so to as aid the process of healing.
  • Percentage wise, mandibular fractures normally occur due to motor vehicles accidents and assault (40 percent of the times), falls (10 percent), and sports and other injuries (5 percent); and the location of mandibular fracture are condyle (30 percent), body and angle (25 percent), symphesis (15 percent), ramus (3 percent) and coronoid process (2 percent).
• Dislocation of the mandible usually occurs downwards or inferiorly, or towards the front or anteriorly. In rare cases, mandibular dislocation may occur backwards or posteriorly.
• The alveolar process of the mandible can get resorbed when fully edentulous in the arch of the mandible, and sometimes when partially edentulous. Such resorption may occasionally happen extensively marked by occurrence of mental foramen on the mandible’s superior border in place of opening on the front surface, thereby altering its relative position. However, the lower sections of the mandible body remain unaffected, circular, and thick. With loss of tooth and age, the alveolar border gets absorbed causing the mandibular canal to come closer to the superior boundary. Occasionally, excessive absorption of the alveolar process can cause the complete disappearance of the mandibular canal, thus leaving the lower alveolar nerve without any protection by the bones; it is however still enclosed in soft tissue.