The Mandible

Anatomy

At birth, the mandible consists of two lateral halves united in the midline at the symphysis by a bar of cartilage ( Fig. 22.1 , e-Fig. 22.2 , and Fig. 22.3 ). Bony fusion of the symphysis usually occurs before the second year, but segments of the fissures may persist beyond puberty. The body of the mandible is large at birth compared with the relatively short vertical rami with poorly differentiated coronoid and condylar processes. The rami form an angle of about 160 degrees with the body at birth.

The mandible is the only freely movable bone of the face; it articulates with the temporal bone in the temporomandibular fossa anterior to the external auditory canal (see Fig. 22.3 ). The range of motion is free in all directions, and the condyle moves downward and forward in the articular fossa upon opening of the jaw.

The temporomandibular joint (TMJ) (see Fig. 22.3 ) is a complex joint in which a biconcave fibrous disk divides the articular space into upper and lower compartments. Gliding movements occur in the upper compartment, whereas the lower compartment functions as a true hinge joint. The articulating bony surfaces are not covered by hyaline cartilage as in other synovial joints but by fibrocartilage separated from the underlying bone of the condyle by growth cartilage.

Dentition

By approximately the sixth week of embryologic development, the dental lamina develops along the length of the upper and lower jaws. It is the predecessor to dental bud formation and ultimately tooth development.

Teeth are situated in bony prominences along the mandible and maxilla known as alveolar processes . The alveolar processes and mandibular body atrophy when teeth are absent ( e-Fig. 22.4 ). There are two sets of dentition, primary/deciduous teeth and permanent teeth. There are 20 total primary teeth and 32 total permanent teeth in the mouth in a normal situation ( Fig. 22.5 ). The developmental absence of some or all teeth, hypodontia and anodontia respectively, is demonstrated in a rare inherited disorder called anhidrotic/hypohidrotic ectodermal dysplasia ( Fig. 22.6 ). Hyperdontia or supernumerary teeth may occur sporadically or in association with conditions such as Gardner syndrome and cleidocranial dysplasia. Other dental abnormalities in cleidocranial dysplasia include markedly delayed shedding of primary teeth and delayed or failed eruption of permanent teeth.

Natal and neonatal teeth are uncommon but do occur in 1 : 2000–3000 live births. In the majority of cases, this is an isolated, nonsyndromic occurrence with early premature eruption of a normal primary tooth/teeth. Complications with suckling may occur for baby and mother.

Embryology

The branchial apparatus, also known as pharyngeal apparatus , comprises a series of paired embryologic structures—arches, pouches, clefts, and membranes—associated with the pharynx from which many adult craniofacial structures are derived. Branchial apparatus neural crest cells contribute to skeletal development in the face.

Between weeks 4 and 5 of development, the stomodeum (rudimentary mouth) forms in the center of the face surrounded by the first and second branchial pairs. The first branchial arch, the so-called mandibular arch, and its corresponding neural crest cells give rise to the mandible.

First branchial arch neural crest cell insufficiency results in mandibular insufficiency (i.e., micrognathia). Micrognathia describes an undersized hypoplastic mandible ( Fig. 22.7 ). Retrognathia describes a retropositioned mandible. Often micrognathia and retrognathia go hand-in-hand; a small jaw is also a retropositioned jaw. Prognathia describes an oversized mandible. Partial duplication of the mandible is very rare.

Transformation of the first branchial apparatus explains a developmental relationship between the jaw and the ear. The principal derivatives of the first arch include not only the bony mandible but also middle ear ossicles malleus and incus. The endodermal-lined first branchial pouch derivatives include the pharyngotympanic (Eustachian) tube and middle ear cavity. The ectodermal-lined first branchial cleft principal derivative is the external auditory canal. The intervening first branchial membrane develops into tympanic membrane. Genetic conditions and environmental factors can adversely affect the complex transformation of the branchial apparatus. Gone awry, the interrelated transformations of the first and second branchial pairs can result in isolated mandibular anomalies (rare) to major syndromic categories of craniofacial malformation termed the otomandibular dysplasias , which include Goldenhar (hemifacial microsomia), Treacher Collins (mandibulofacial dysostosis), branchio-oto-renal syndrome, Nager preaxial/radial acrofacial dysostosis, and Pierre Robin sequence (PRS) (see also Chapter 20 ).

Craniofacial and more specifically mandibular development is increasingly assessed in utero using prenatal magnetic resonance imaging (MRI) as an adjunct to screening ultrasound (US). MRI-based reference data for fetal mandibular growth with validated metrics to quantitatively evaluate fetal mandibular size and position (jaw index and inferior facial angle) have been derived improving the antenatal diagnosis of micrognathia and retrognathia. Improved antenatal diagnosis of micrognathic conditions such as PRS, characterized by cleft palate, small jaw, and occlusive retropositioned tongue, could preclude perinatal airway emergencies and improve survival.