Anatomy

Detailed consideration of all craniofacial bones is impossible in a text of this sort but by focussing on the maxilla and mandible alone, this chapter offers a view of the processes affecting facial bones as a whole and how specimens derived from them might be handled.

The maxilla is the largest bone of the upper facial skeleton and houses the maxillary sinus. It articulates with a large number of other bones, relating to a number of clinically important anatomical areas, including the nasal cavity, the pterygomaxillary space, the infratemporal fossa and the orbit. It is composed of a body and four processes, namely alveolar, frontal, nasal and zygomatic processes. The body of the maxilla is pyramidal in shape with four surfaces, namely anterior, nasal, orbital and posterior (or infratemporal) surfaces. The anterior surface extends from the alveolar process of the upper anterior teeth below to the infraorbital margin, while the orbital surface forms the floor of the orbit. The nasal surface articulates with the ethmoid, lacrimal and palatine bones and the inferior turbinate to complete the lateral nasal and medial orbital walls. The infratemporal surface is convex and projects posteriorly and laterally.

The upper alveolar process projects from the inferior aspect of the maxilla and contains the sockets of the maxillary teeth. The roots of teeth posterior to the first premolar may be intimately related to the maxillary sinus; teeth and sinus may each become involved in diseases originating in the other. The slightly thickened posterior end of the alveolar process is called the maxillary tuberosity. The frontal process projects superiorly between the nasal and lacrimal bones, articulating with the frontal bone and contributing to the medial wall of the nasal cavity. The palatine process projects medially from the inferior aspect of the maxilla and forms most of the palatal vault and the nasal floor. The zygomatic process is a pyramidal projection from the lateral aspect of the maxilla where anterior, infratemporal and orbital surfaces converge, articulating with the zygomatic bone.

The mandible is composed of an arched body, which runs posteriorly on each side to attach to the flat ramus. The body of the mandible has an external surface, an internal surface, an upper border and a lower border. The lower border is rounded and well defined, outlining the profile of the lower jaw. The external surface bears the mental foramen between the premolar roots. The internal surface of the body is indented by the sublingual and submandibular glands. The superior border, more usually referred to as the lower alveolar ridge, contains the sockets of the mandibular teeth. The posterior aspect of the body joins the ramus behind the last molar tooth. The anterior surface of the ramus extending from the abrupt change in angulation of the bone is called the ascending ramus while the area around the junction of the body and ramus is called the angle. The coronoid process extends upwards and slightly forwards from the anterosuperior aspect of the ramus and bears the attachment of temporalis. The condylar process extends upwards and posteriorly from the posterior aspect of the ramus and bears the knuckle-shaped articulating condylar head on the narrow condylar neck. The sigmoid notch lies between these two processes. Near the centre of the medial surface of the ramus lies the mandibular foramen, where the inferior alveolar branch of the mandibular nerve and its accompanying vessels enter the mandible. The lateral and medial surfaces of the mandible bear several shallow fossae and roughened elevations corresponding to the attachments of muscles both of facial expression and of mastication.

Each fully developed tooth is composed predominantly of dentine. The crown, the portion visible within the mouth, is covered by a layer of hard translucent enamel while a thin layer of bone-like substance called cementum covers the conical root, which may be single or multiple. The crown and root join at a slight narrowing called the cervical margin. The dentine encloses a central cavity called the pulp canal; the portion towards the crown is dilated to form the pulp chamber, while the pulp canal narrows at the end of the root, the apex, into an apical foramen. The pulp chamber and canal contain the neurovascular supply to the tooth, passing through the apical foramen. The tooth is suspended in the alveolar bone by the periodontal membrane, composed of thick bundles of collagenous tissue running between the cementum and the bone. At the cervical margin, the periodontal ligament merges with the gingival mucosa; a narrow sleeve of epithelium continuous with the gingiva called the epithelial attachment surrounds the cervical margin.

The crown of each tooth has five surfaces. The biting surface is called the occlusal surface; on incisor teeth, this is termed the incisal edge. The surface closest to the tooth in front is called the mesial surface; that closest to the tooth behind is called the distal surface. The surface lying closest to the cheek is called the buccal surface (or the labial surface on anterior teeth); the surface lying closest to the tongue is called the lingual surface (or the palatal surface on upper teeth). Incisor teeth have a relatively broad crown with flattened edge for cutting food, while canine teeth have a single point or cusp. Premolar teeth have two cusps on the occlusal surface, one buccal and one lingual, while molars have four or five cusps.

There are 20 deciduous (or milk) teeth in the primary dentition - two incisors, a canine and two molars in each quadrant of the jaw. The teeth of the primary dentition begin their development in the first trimester of pregnancy as epithelial ingrowths from the lining of the oral cavity. The epithelial component of the tooth bud forms the enamel while the mesenchymal element gives rise to the remaining parts of the tooth. The crown of the tooth forms first but the root only forms after the crown is complete; root development is closely linked with eruption into the mouth. The deciduous incisors begin to appear in the mouth at around six months of age, usually the central before the lateral, followed by the first deciduous molar at around 12 months. The deciduous canine appears around 18 months and finally the second deciduous molar at around 24 months of age. The precise timing of eruption into the mouth is variable although the sequence is relatively unchanging and lower teeth tend to appear before the uppers.

There are 32 teeth in the permanent or secondary dentition - two incisors, a canine, two premolars and three molars in each quadrant of the jaw. There is an ordered pattern of replacement of the deciduous dentition by the permanent dentition. Beginning around six years of age, the first permanent molar erupts distal to the second deciduous molar, soon followed by shedding and replacement of the incisors. The deciduous molars are replaced by the premolars, while the eruption of the permanent canines straddles the eruption of the second permanent molar with the upper canine appearing latest, usually around 13 years of age. The process ends with the eruption of the third permanent molar or wisdom tooth around 18 years.

0 0

Post a comment