Teeth

The teeth (Latin: dentes) are small structures that consist of solid, bone-like tissue on the outside and soft connective tissue on the inside. They are a part of the digestive system and are located in the dental alveoli of the maxilla and mandible. Their primary function is to aid in the mechanical processing of food during the digestion process.

 

Types of teeth

Humans have two types of dentition throughout their lifetime. The first set is known as primary dentition or milk teeth, also referred to as deciduous teeth. And they are slowly replaced by the secondary dentition or permanent teeth during childhood.

Primary teeth are smaller in size, have smaller roots, and larger pulp chambers compared to permanent teeth. Children have 20 milk teeth that begin to appear at the age of six to eight months. The process of replacing milk teeth usually starts around the age of 6 to 7 years and continues until the age of 12 to 14 years. The dentures of the permanent teeth lie between the roots of the milk teeth until they are ready to replace them. The last permanent teeth to appear are the last molar teeth, also known as the wisdom teeth. They typically emerge between the ages of 17 and 24.

Permanent teeth are divided into two groups. The first group includes the incisor, canine, and premolar teeth, which replace the milk teeth. The second group is made up of the molar teeth, which do not have counterparts in the primary dentition. The permanent teeth increase the number of teeth from 20 to 32 as each side of the upper and lower jaw contains three molar teeth. Molar teeth of a child corresponds to the premolars of an adults.

 

Teeth eruption times

The first teeth to appear in the primary dentition are the medial incisors around 6 - 8 months, followed by the lateral incisors. The first molar tooth of the primary dentition can be visible around 12 to 15 months, the canine teeth emerge around 16 to 20 months, and the next premolar tooth can be visible only at 20 to 24 months of age.

The first molar tooth of the secondary dentition appears around the age of 6 - 7 years. Medial incisors replace milk teeth around eight years, lateral incisors - 9 years, first premolar tooth around ten years, canine around 11 to 13 years, while the second molar tooth appears only around 13 to 16 years. Wisdom teeth or the last molar teeth can emerge anytime between 17 to 30 years or even later, but usually, it happens till 24 years.

 

Teeth anatomy

Each tooth consists of three parts - the crown, the neck, and the root.

The crown is the visible part of the teeth that is elevated above the gingiva and have a white or yellowish-white color depending on several factors such as the age of a person or unhealthy habits. The roots of the teeth are located in the dental alveoli of the alveolar arches of the mandible and maxilla.

The neck of a tooth connects the crown and the root and is located in the middle. The neck connects with the marginal gingiva, and its height varies between 1 and 1.5 mm (0.039 to 0.059 inches).

The root of the tooth is fixated by a gomphosis, a continuous bone connection or synarthrosis type called fibrous joint type, that binds the teeth to the dental alveoli. The movement in gomphosis is minimal, although it can increase due to changes caused by aging, various disorders, and pressure changes.

Each tooth is also fixated in its place by the periodontium, which is a specialized fibrous tissue between the root of the tooth and the bone. Collagen fibers stretch between the periodontium and tooth's root that also help in fixation.

 

Teeth surfaces

Teeth have several surfaces.

The surface facing the lips and oral cavity opening is the labial surface. It is found only on incisors and canine teeth. The surface facing the cheeks is the buccal surface, and it is found on the premolars and molar teeth. The labial and buccal surface together form the facial surface.

The surface facing the tongue on lower teeth is the lingual surface, while the surface facing the palate on upper teeth is the palatal surface. The mesial surface is the side of the teeth closest to the midline of the jaws, while the distal surface is the side farthest from the midline.

The occlusal or biting surface is found only premolars and molar teeth, and has several cusps and grooves. The incisal surface is the sharp and pointed top part of the incisors and canine teeth.

Each surface has a unique design that serves a specific function. For example, canines and incisor teeth have a single sharp top surface for cutting food, while premolars and molar teeth have larger and flatter occlusal surfaces with several cusps, grooves, fissures, and pits designed for food grinding and chewing.

 

Teeth microanatomy

The primary structure of a tooth is composed of dentin, which form the bulk of the tooth.The crown of the tooth is covered by enamel on exterior, while the outside of the root is covered by cement. The transition between cement and enamel is the neck of the tooth, where cement thinly layers on the enamel. This junction is known as the cementoenamel junction.

Cement and dentin make up what is known as bone tissue, while enamel is special mineralized secretion of epithelial cell. Although teeth appear similar to bone, they have different histology.

The enamel of a newly erupted tooth is covered by a delicate membrane called Nasmyth’s membrane, also known as the primary enamel cuticle. This membrane is removed by the mastication process, but typically remains intact on the sides of the crown close to the gingiva. It serves a protective role and is acid-resistant.

Enamel is the hardest substance in the human body, forming a durable protective layer that protects the tooth from various harmful factors. The thickness of the enamel varies, with the top part being the hardest. The hardness gradually decreases closer to the dentin. A tooth cannot heal by itself, except for the enamel portion.

Every tooth has a pulp cavity or chamber located in the middle of the crown and neck, containing blood vessels and nerves. This cavity becomes the pulp or root canal in the root of the tooth. Every root canal ends with a small opening called the root end opening or apical foramen. It is through this opening that nerves and blood vessels pass through the root canals to the pulp chamber.

 

Teeth classification

Adults have 32 permanent teeth, divided into four groups based on their functions, number of roots, and crown shape. These groups are incisors, canines, premolars, and molars. The 16 upper teeth are known as maxillary teeth, while the lower 16 teeth are mandibular teeth. Although teeth from the same group are similar between the upper and lower jaws, they are not identical.

The oral cavity is divided into four quadrants, with each quadrant having eight teeth. These teeth are made up of a central or medial incisor, a lateral incisor, a canine tooth, two premolars, and three molars. Each alveolar arch is divided in two parts in the midline, contributing to the overall organization of the teeth in the oral cavity.

 

Incisor teeth

In total, each person has eight incisors. Four of them are located in the middle of the lower jaw, while four are found in the middle of the upper jaw. Overall, every jaw has two central (medial) incisors and two lateral incisors.

Incisors are used to cut, pluck, and bite food during chewing. They also help to hold food. Incisors have a single, long conical root and a chisel-shaped crown.

 

Canine teeth

Canines are also known as eyeteeth or cuspids. Each person has four canine teeth. Every jaw has two canines, and each of them is located between the lateral incisor and the first premolar.

This type of teeth has a single, long and deep root that is flattened and grooved on the sides. It is longer than other teeth roots. The crown has a sharpened conical shape with spear-like cutting edges. This teeth type is used for gripping, cutting, and tearing food.

 

Premolars

Premolar teeth or premolars are also known as bicuspids. Each person has 4 maxillary and 4 mandibular premolars, and they lie between the canines and first molars. This teeth type provides food crushing and grinding during chewing.

Premolars have one usually bifurcated root and a cuboid crown with two elevations or cusps.

 

Molar teeth

Molars are the last teeth in each jaw and they are used for grinding and crushing food. Every human has a set of 12 molars, six in each jaw and three in each quadrant.

The molars have a cuboid-shaped crown and a square-shaped occlusal surface with four or five cusps. Usually, the upper molars have three roots and five cusps, while the lower molars have two roots and 4 cusps.

The last molar teeth or the third molar teeth on every side of the upper and lower jaw are called wisdom teeth. These are the last teeth to erupt. For some individuals, it does not happen during their lifetime.

 

Teeth functions

The primary function of teeth is to aid in the chewing of food. Teeth cut, bite, grip, and grind food into smaller pieces to form a food bolus, making easier to swallow.

However, teeth play a significant role in articulation process and speech formation. They shape the face and support surrounding tissue, giving the oral cavity a unique and personal appearance.

 

Neurovascular supply of the teeth

Arterial blood supply

The arterial blood supply to the teeth is provided by the maxillary artery, which is a terminal branch of the external carotid artery.

  • The anterior and middle parts of the upper teeth are supplied by the anterior superior alveolar arteries originating from the infraorbital artery.
  • The posterior part of the upper teeth are provided with the arterial blood by the posterior superior alveolar arteries.
  • In contrast, the lower teeth are supplied by the inferior alveolar arteries.

 

Venous drainage

Venous drainage is provided by veins with similar names as arteries mentioned above.

The upper teeth are drained by the anterior and posterior superior alveolar veins, while the lower teeth are drained by the inferior alveolar veins. All veins drain into the pterygoid plexus or the facial vein.

 

Lymphatic drainage

Lymphatic drainage of upper and lower teeth occurs via submandibular and deep lymph nodes, except the mandibular incisors, as they are usually drained to the submental nodes.

 

Innervation

The teeth are innervated by the maxillary and mandibular branches of the trigeminal nerve.

The upper teeth are innervated by branches arising from the maxillary nerve. To be more precise - the superior alveolar nerves from the infraorbital nerve.

  • The upper incisor and canine teeth are innervated by the anterior superior alveolar nerves.
  • The upper premolar teeth get innervation from the middle superior alveolar nerves, while upper molar teeth - the posterior superior alveolar nerves.

The lower teeth are supplied by the branches from mandibular nerves - the inferior alveolar nerves and the incisive nerve.