e) Teeth

Structure

The teeth are firm calcified structures important for preparing food prior to swallowing. The teeth are divided into quadrants (right and left, mandibular or maxillary). Each quadrant has (from medial to lateral) two incisors, a single canine, two premolars (two cusps) and three molars (four – five cusps). The third mandibular molar teeth are colloquially known as wisdom teeth and may become impacted when they attempt to erupt.
A tooth may be divided into a crown (which projects above the gingiva) and a root (which projects into the alveolar process of the relevant bone). Teeth may possess up to three roots. The crown is covered initially by resilient enamel, whereas the root is covered by cementum. Beneath this layer is the dentine, and beneath this the pulp containing vessels and nerves.

Relations

The teeth extend from within the alveolar processes and erupt from the gingiva.

Macroscopic Appearance

The enamel is a white-transparent layer which extends above the gingiva.
The cementum is the covering of the root, and tends towards a yellowish hue.
The dentine is yellow and lies beneath the enamel and the cementum.
The pulp is loose tissue within the central part of the tooth containing vessels and nerves.
The incisors, canines and premolars typically have a single root, whereas the molars may have 2 or 3.

Microscopic Appearance

The enamel of teeth is acellular and is formed by highly concentrate apatites. Less than 1% of the mass is organic stroma which serves to provide some support for the apatite crystals.
Cementum, the external coating of the roots of the teeth, is about 50% inorganic hydroxyapatites. It is similar to bone except is avascular and lacks cells.
The dentine which underlies the enamel and cementum is characterised by small tubles which run perpendicular to the surface. This tubules are generated by odontoblasts which live adjacent to the dental pulp.
The pulp is loose connective tissue with significant vascularisation and innervation.

Physiological Variations

The teeth initially form beneath the epithelium covering the alveolar processes. From 6 months to 3 years of age, the five sets of deciduous teeth erupt. The deciduous teeth are lost between the ages of 6 and 12, again starting with the central / median incisor. The permanent teeth erupt from the age of 7-8 (central incisor) up to 13-25 (3rd molar). Teeth are frequently lost with advancing age, and may develop cavities (defects in the enamel) which permit infection to enter the dental pulp.

Neurovascular Supply

Arterial Supply

The mandibular teeth are supplied by the inferior alveolar artery, whereas the maxillary teeth are supplied by the superior alveolar arteries. The inferior alveolar and posterior superior alveolar arteries arise in the pterygopalatine fossa, whereas the middle and anterior superior alveolar arteries arise from the infraorbital nerve within the infraorbital canal.
Small dental vessels pierce the apex of each root to enter the pulp of the tooth.

Venous Drainage

Veins from the teeth form alveolar veins within the maxilla and mandible. They pass posteriorly to the pterygoid plexus or anteriorly to the facial or mental veins.

Lymphatics

Aside from the mandibular incisors which drain to submental nodes, the remaining teeth drain lymph to submandibular nodes.

Innervation

Dental branches of the posterior, middle and anterior superior alveolar nerves supply the maxillary teeth. The incisors and canine are usually supplied by the anterior, the premolars and part of the first molar by the middle and the remaining teeth by the posterior.
Dental branches of the inferior alveolar nerve supply the molars and premolars of the mandible. The incisive branch of the inferior alveolar supplies the incisors and canine teeth.

Routes of Cancer Spread

Odontogenic tumours are very, very rare.

Local Invasion

Odontogenic tumours typically invade the associated bone (mandible/maxilla), although they most commonly occur in the mandible. Invasion into the inferior alveolar nerve may lead to numbness of the lip and chin.

Lymphatic Spread

Most tumours of teeth are locally invasive only. If lymphatics are involved, then submandibular nodes would be the most common site for metastasis, except for the incisors which drain directly to submental nodes.

Neurological Spread

Tumours may track back along the alveolar nerves, to either the maxillary nerve or the mandibular nerve.

Haemotogenous Spread

Most odontogenic tumours spread to lung.


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