Tooth

Dens

Also known as: teeth, dentition

Teeth are calcified organs held in the alveolar sockets of the jaws that cut, tear and grind food. Humans have two sets: 20 deciduous (primary) teeth and 32 permanent teeth.

Show on body map Skull — Lateral View Tooth: Molar in Longitudinal Section

Overview

Deciduous teeth20 (2-1-0-2 per quadrant)

Permanent teeth32 (2-1-2-3 per quadrant)

First deciduous tooth≈6 months (mandibular central incisor)

First permanent molar≈6 years

Third molars≈17–21 years (often impacted)

Each tooth has three regions. The crown projects above the gum and is covered by enamel. The neck (cervix) is at the cemento-enamel junction. The root or roots sit in the socket and are covered by cementum. Most of the tooth is dentin, which surrounds the pulp cavity. The pulp cavity widens into a pulp chamber in the crown and narrows into root canals that open at the apical foramen, where nerves and vessels enter.

The tooth is slung in its socket by the periodontal ligament. This tooth-to-socket joint is a special fibrous joint called a gomphosis. The periodontal ligament, cementum, alveolar bone and gingiva together make up the periodontium.

There are four tooth types. Incisors have a chisel edge for cutting. Canines have a single cusp for tearing and the longest root. Premolars have two cusps for crushing. Molars have 3–5 cusps and 2–3 roots for grinding.

The deciduous dentition has 20 teeth, with formula 2-1-0-2 per quadrant: two incisors, one canine, no premolars and two molars. They erupt from about 6 months (mandibular central incisors) to about 2–2½ years (second molars). The permanent dentition has 32 teeth, with formula 2-1-2-3 per quadrant. The first permanent molar erupts at about 6 years behind the deciduous teeth without replacing any of them. Incisors follow at 6–8 years, canines and premolars at 9–12, second molars at 11–13, and third molars (wisdom teeth) from 17 to 21 years or later; third molars are often impacted. Between about 6 and 12 years the mouth contains a mixed dentition.

Location: Two dental arches, in the alveolar processes of the maxillae (upper arch) and the mandible (lower arch).

Parts & subdivisions

Enamel
Hardest tissue in the body; covers the crown.
Dentin
Bulk of the tooth; sensitive, tubular, living tissue.
Pulp
Vascular, innervated connective tissue in the pulp chamber and root canals.
Cementum
Bone-like layer on the root that anchors the periodontal ligament.
Periodontal ligament
Fibrous suspensory ligament between cementum and alveolar bone.
Gingiva
Masticatory mucosa that seals the tooth neck.
Crown, neck, root
Crown: enamel-covered part above the gum. Neck: the cemento-enamel junction. Root: the part in the alveolus, ending in the apex with its apical foramen.

Anatomy

Innervation: Maxillary teeth: anterior, middle and posterior superior alveolar nerves (V2). Mandibular teeth: inferior alveolar nerve (V3), which enters the mandibular foramen; it is the target of the "inferior alveolar block", which also numbs the lower lip and chin via the mental nerve. Pulpal afferents carry essentially pain only.
Arterial supply: Superior (anterior, middle and posterior) and inferior alveolar arteries, all from the maxillary artery.
Lymphatic drainage: Mainly to the submandibular nodes. The mandibular incisors drain to the submental nodes.
Development: Odontogenesis begins in week 6 with the dental lamina and passes through bud, cap and bell stages. Oral ectoderm forms the enamel organ (ameloblasts → enamel). Neural-crest ectomesenchyme forms the dental papilla (odontoblasts → dentin, and the pulp) and the dental follicle (cementum, periodontal ligament, alveolar bone).

Function

Mastication: incisors cut, canines tear, premolars and molars crush and grind. This increases the surface area available to digestive enzymes.
Articulation of speech (dental and labiodental consonants).
Support of the lips and cheeks, and maintenance of the vertical dimension of the face.

Clinical significance

Dental caries

Plaque bacteria, mainly Streptococcus mutans, ferment dietary sugars to acid. Enamel demineralises once the plaque pH falls below about 5.5. The lesion advances through enamel into dentin and can reach the pulp (pulpitis). Fluoride, diet control and fissure sealants prevent it.

Dental abscess and spread of infection

Pulpal necrosis leads to a periapical abscess. Lower molar roots lie below the mylohyoid line, so infection spreads to the submandibular space (risk of Ludwig angina). Upper molar and premolar infection can reach the maxillary sinus.

Periodontitis

Plaque-induced inflammation destroys the periodontal ligament and alveolar bone. It is the main cause of tooth loss in adults and is associated with diabetes and cardiovascular disease.

Mnemonics

Dental formula

Permanent 2-1-2-3, deciduous 2-1-0-2 per quadrant, multiplied by 4 for the whole mouth (32 and 20).

Test yourself

The parotid (Stensen) duct opens into the oral vestibule opposite which tooth?

Maxillary second molar
Mandibular second molar
Maxillary first premolar
Mandibular central incisor

Answer: Maxillary second molar

The duct crosses the masseter, pierces the buccinator and opens on the parotid papilla opposite the upper second molar. The submandibular duct opens at the sublingual caruncle beside the lingual frenulum.

What is the dental formula (per quadrant) of the permanent dentition?

2-1-0-2
2-1-2-2
2-1-2-3
1-1-2-3

Answer: 2-1-2-3

The permanent formula is 2 incisors, 1 canine, 2 premolars and 3 molars per quadrant, giving 8 × 4 = 32 teeth. The deciduous formula is 2-1-0-2 (20 teeth); there are no deciduous premolars.

Approximately how far from the incisor teeth is the gastro-oesophageal junction (diaphragmatic constriction)?

15 cm
22.5 cm
27.5 cm
40 cm

Answer: 40 cm

The oesophageal constrictions lie about 15 cm (cricopharyngeus, C6), about 22.5 cm (aortic arch), about 27.5 cm (left main bronchus) and about 40 cm (oesophageal hiatus, T10) from the incisors. These distances guide endoscopy and explain where foreign bodies lodge.