THE TOOTH: Types, Function, Structure, and Nomenclature

The Tooth

Types, Function, Structure, and Nomenclature

Humans have two sets of teeth in their lifetime. The first set of teeth, the primary or deciduous dentition, starts forming in utero at about 14 weeks and is completed postnatally at about three years of age. At the age of six years, the first permanent teeth start emerging in the mouth and begin the transition or mixed dentition period, in which there is a mixture of deciduous and permanent teeth present in the oral cavity. By the age of 12 years, all the deciduous teeth will shed and ends the transition period.

Types of Teeth

The teeth are divided into four equal quadrants within the mouth. You can imagine a vertical line in the middle of the face and a horizontal line between your (upper and lower) jaws. These imaginary lines will leave you with four quadrants having up to eight permanent (adult) teeth in each of them.

Each quadrant has a similar set of eight teeth, the opposing pair in the same jaw and counter part in the opposite jaw. Each quadrant contains:

A Medial Incisor
A Lateral Incisor
A Canine
Two Premolars
Two to Three Molars

Incisors

A human mouth has eight incisors teeth; two of them in each quadrant. They are situated at the center. These teeth are characteristically thin, flat-bottom teeth that help us to make the initial bite of our food. We bite into food with our incisors, tugging and pulling into our mouths. They have a narrow edge and are adapted for cutting.

Cuspids / Canines

Canines are located on either side of the incisor teeth. They are sharp and pointed, used to tear into food and rip it apart. Canine teeth are the longest teeth, with a pointed end, and a single root. It also helps guide the mouth and other teeth into the best biting position.

Premolars / Bicuspids

The premolars are located after the canine and before the molar teeth. They are four in each quadrant. They are transitional teeth and are used to grind and break up the food and make it digestion ready.

Molars

Molars are the flat teeth at the rear of the mouth. Each molar typically has four or five cusps. They are used exclusively for crushing and grinding.

Structure of the Tooth

Crown and Root

The portion of the tooth that is visible in the oral cavity is known as the dental crown, and below it, we have the tooth root hidden beneath the gingiva and alveolar bone. The crown is covered with enamel, and the root portion is covered with cementum. The crown and root join at the cementoenamel junction (CEJ).

Enamel

The enamel is the outer layer of the tooth, and it is extremely hard avascular tissue with high mineral content. It is as hard as a crystal (7 on the Mohs scale of mineral hardness). Its color varies from white to yellow depending upon factors like age, thickness, oral hygiene, etc. It provides thermal insulation for a tooth and protects the internal vital tissues.

Cementum

The cementum is the tissue covering the surface of the tooth root. It connects the alveolar bone (the jaw bone) with the tooth by the periodontal ligament. These ligaments help to absorb and alleviate the force on the jaw.

Dentin

Underneath the enamel, the second layer is known as dentin. It is softer and slightly darker in colour. Dentine is a vital, innervated tissue, situated inside the enamel and cementum that forms the tooth from the dental crown to the tooth root. The part of the tooth where the dentine and enamel meet is called the dentino-enamel junction (DEJ).

The Pulp

The pulp chamber exists in the center of the tooth, through which the dental pulp, called the nerve, runs. This is the most inner layer of the tooth and contains the blood vessels and nerves, supplying nutrients to the dentin. The pulp chamber continues down from the crown into the roots and finishes at the end of the root called the apex where there is an opening (apical foramen) that allows the blood vessels and nerves to enter the dental pulp.

The enamel and the dentin must be intact for the tooth to stay alive and healthy, for once any bacteria enter the pulp chamber, the damage is irreversible. In this scenario the root canal treatment (click here to read more about root canal treatment) is the only option left to save the tooth.

The anterior teeth in both jaws and some premolars are single-rooted (with one apex) teeth. Whereas the maxillary second premolar and all molars have one to multiple roots.

Tooth Nomenclature

A tooth numbering system allows for easy identification, communication, and dental record maintenance in a precise manner. It makes it possible to identify and classify the condition associated with the concerned tooth both clinically and radiographically and communicate among dental professionals to assess and treat dental diseases.

Some commonly used systems are the Zsigmondy-Palmer system, Universal Numbering System, and the FDI system numbering system.

Dental Quadrants

Understanding dental quadrants are important for understanding tooth numbering systems. We have discussed this in the earlier section as well.

The whole dentition is divided into four quadrants or sections. Dental charts are prepared from the dentist's perspective, wherein the right side is shown on the left side of the chart and vice-versa.

FDI notation system

FDI stands for Féderation Dentaire Internationale. It is also known as the ISO system. It is the international standard for the numbering and naming of teeth, as defined by the World Health Organisation.

FDI Notation for Permanent teeth

Each tooth is denoted by two numbers.
The first number refers to the quadrant in which the tooth is located		The second number refers to the position of the tooth within the quadrant
Right upper/maxillary quadrant	1	Central Incisor	1
Left upper/maxillary quadrant	2	Lateral Incisor	2
Left lower/mandibular quadrant	3	Canine	3
Right lower/mandibular quadrant	4	1st Premolar	4
		2nd Premolar	5
		1st Molar	6
		2nd Molar	7
		3rd Molar	8

FDI Notation for Primary teeth

Each tooth is denoted by two numbers.
The first number refers to the quadrant in which the tooth is located		The second number refers to the position of the tooth within the quadrant
Right upper/maxillary quadrant	5	Central Incisor	1
Left upper/maxillary quadrant	6	Lateral Incisor	2
Left lower/mandibular quadrant	7	Canine	3
Right lower/mandibular quadrant	8	1st Molar	4
		2nd Molar	5

Universal Numbering System

The American Dental Association's Universal Numbering System is mainly used in the United States.
Permanent teeth are numbered 1 to 32; superior right to left, then inferior left to right.
Primary teeth are numbered A to T; superior right to the left are called A to J, then inferior left to right called as K to T.

Zsigmondy-Palmer System

Palmer notation (also called 'Zsigmondy notation') is the primary system used for the numbering of teeth in the United Kingdom.

Palmer Notation System for Permanent Teeth

Each tooth is denoted by one number and one symbol.
The first number refers to the position of the tooth within the quadrant		The symbol refers to the quadrant in which the tooth is located
Central Incisor	1	Right Upper/Maxillary Quadrant	┘
Lateral Incisor	2	Left Upper/Maxillary Quadrant	└
Canine	3	Left Lower/Mandibular Quadrant	┐
1st Premolar	4	Right Lower/Mandibular Quadrant	┌
2nd Premolar	5
1st Molar	6
2nd Molar	7
3rd Molar	8

Palmer Notation System for Primary Teeth

For primary teeth the arches are divided into quadrants, with the entire dentition being notated as follows:

References:

1. Wheeler's Dental Anatomy, Physiology and Occlusion 1/E South Asia Edition

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