The teeth, aka, dentition are accessory digestive organs housed within the margins of the upper and lower jaw bones.
There are 32 permanent teeth in the adult, which replace the 20 deciduous teeth, (aka, milk teeth or baby teeth) that are lost during childhood.
Deciduous teeth
Lower teeth appear 1-2 months earlier than upper teeth.
- Upper and lower central incisors appear between 6-9 months.
- Upper and lower lateral incisors appear between 7-11 months.
- Upper and lower canines appear between 16-20 months.
- Upper and lower 1st molars appear between 12-16 months.
- Upper and lower 2nd molars appear between 20-30 months.
Adult teeth
- Upper and lower central incisors appear between 6-8 years.
- Upper and lower lateral incisors appear between 7-9 years.
- Upper and lower canines appear between 9-12 years.
- Upper and lower 1st molars appear between 6-12 years.
- Upper and lower 2nd molars appear between 10-12 years.
- Upper and lower 3rd molars ("wisdom teeth) appear between 17-25 years.
The shapes of our teeth reflect their functions in biting and chewing (mastication).
8 incisors, aka the front teeth, which are chisel-shaped and slice and bite through foods.
4 canines, which have pointed tips that puncture and tear foods,
8 premolars, which are transitional teeth with sharp cusps and flattened surfaces.
The incisors and canines have single roots, which is the part of the tooth that lies deep in the gums; depending on their position, the premolars have 1 or 2 roots (the upper first premolar typically has 2 roots, the rest typically have 1 root).
12 molars, which are broad and flat with cusps that grind foods; they have two or three roots (the upper often have 3 roots, the lower often have 2 roots).
Dental Formula
The dental formula is how we indicate the numbers and positions of the teeth.
It is written as a ratio, with upper teeth over lower teeth for one half of the mouth multiplied by two.
So, in an adult with a full set of teeth, we would indicate for both the upper and lower teeth: 2 incisors, 1 canine, 2 premolars, and 3 molars (often shortened to 2123). When we multiply this ratio by 2 to include both sides of the mouth, we get 32 teeth.
The adult dental arches comprise the teeth in anatomical position.
The dentition can be divided into quadrants along the sagittal plane:
Upper (aka, maxillary) quadrants – right and left upper.
Lower (aka, mandibular) quadrants – right and left lower.
In the upper dental arch, from anterior to posterior, we indicate the following:
The central incisors, which are colloquially referred to as the "two front teeth."
The lateral incisors, which are smaller than the central incisors.
The canines, which are relatively sharp and pointy, like fangs.
The posterior teeth are the premolars and molars:
The 1st and 2nd premolars are flatter and broader than the anterior teeth.
The 1st, 2nd, and 3rd molars are large and flat; the third molars are commonly referred to as the "wisdom teeth" because they typically erupt during late adolescence or early adulthood.
This same pattern is replicated in the bottom dental arch:
The central incisors are the medially located, anterior teeth.
Then, the lateral incisors, canines, 1st and 2nd premolars, and 1st, 2nd, and 3rd molars.
The teeth are numbered from 1-32.
The first tooth is the 3rd molar in the right upper quadrant; teeth 8 and 9 are the right and left upper central incisors, and the 16th tooth is the 3rd molar in the upper left quadrant.
The 17th tooth is the 3rd molar in the lower left quadrant, teeth 24 and 25 are the lower right and left central incisors, and the 32nd tooth is the 3rd molar in the right lower quadrant.
Notice that the 1st and 32nd teeth occlude (they fit together when chewing) and the 16th and 17th teeth occlude.
Tooth surfaces
The surfaces of the teeth are named according to adjacent structures:
The vestibular surface faces the lips and cheeks; the space between the teeth and overlying soft tissues is the vestibular cavity.
The vestibular surface can also be called the labial or buccal surface, which depends on whether the tooth faces the lips or cheeks.
The lingual surface faces the tongue ("lingual" refers to the tongue).
The occlusal surface of the premolars and molars faces the matching tooth of the opposite jaw; it is where the teeth from the superior and inferior jaws meet.
The occlusal surfaces of the premolars and molars have prominent ridges, called cusps, that mash the foods.
Though we've simplified in our diagram, premolars typically have two cusps (and are therefore referred to as the bicuspids), and molars typically have 3-5.
The incisors and canines have incisal edges that are used for cutting and tearing food).
The mesial surface faces the midline of the mouth and the distal surface faces away from the midline. The mesial and distal surfaces of the teeth are collectively called proximal surfaces.
Next, we'll draw the anatomy of a bisected molar.
The alveolar socket (aka, tooth socket) is the space within a jawbone (the
maxilla comprises the upper jaw, the mandible, the lower jaw). Neurovascular structures pass through it; each tooth has its own blood supply and innervation.
The periodontium comprises the connective tissues that attach the tooth to the bone.
The gingivae (aka, gums), which are the outermost layer of the periodontium; the gums overlie the jawbone and attach to the neck of the teeth.
The crown is the exposed portion of the tooth above the gums.
The neck is the constricted region of the tooth.
The root is the portion of the tooth that articulates with the bone of the jaw.
The cementum comprises the outer protective cover of the root; periodontal ligaments connect the cementum to the alveolar socket (forming a
gomphosis joint).
Features of the tooth:
The enamel is the thick protective layer of the crown; the enamel is formed by calcium salts and is the hardest substance in the body.
Deep to the enamel, dentin surrounds the pulp cavity, which is a space within the tooth. Dentin comprises most of the tooth and is similar to bone tissue, but much harder; it protects the pulp cavity contents.
Outline the pulp cavity and label the root canal.
Neurovascular branches enter and exit the teeth via the apical foramen and travel through the root canal to the pulp cavity.
Review
tooth histology
Finally, let's learn about how the accumulation of plaque can lead to tooth loss.
Dental plaque comprises a thin layer of sugars, bacteria, and other materials that adhere to the teeth; flossing and brushing the teeth helps to prevent plaque build-up.
Dental caries (aka, cavities) form when the dental enamel and dentin are broken down by demineralizing acids, which are produced when bacteria metabolize dental plaque.
Plaque accumulations calcify to form calculus (aka tartar). These hard mineral deposits cannot be removed by normal brushing or flossing and must be removed by professional dental cleanings.
Over time, calculus and the associated inflammation breaks the seal between the teeth and the gingivae, which facilitates infection called gingivitis. Patients with gingivitis have red, swollen, bleeding gums.
Without treatment, gingivitis can lead to periodontal disease (aka, periodontitis).
In periodontal disease, inflamed pockets of infection form between the teeth and gums and trigger an immune response that leads to the erosion of the bone surrounding the tooth. Without treatment, periodontal disease causes tooth loss.
