The human dentition is composed of two sets of teeth – primary and permanent. Teeth are organized into two opposing arches – maxillary (upper) and mandibular (lower). These can be divided down the midline (mid-sagittal plane) into left and right halves. Teeth are positioned in alveolar sockets and connected to the bone by a suspensory periodontal ligament.
In this article, we shall explore the structure of teeth, the identification of teeth, and the differences between primary and permanent dentition.
The primary dentition is composed of twenty teeth, with ten in each arch. There are five teeth in each quadrant, composed of two incisors (central and lateral), a canine, and two molars. These teeth are referred to as letters A, B, C, D and E. The primary teeth begin to erupt at six months of age.
The permanent dentition is composed of 32 teeth with sixteen in each arch. There are eight teeth in each quadrant, composed of two incisors (central and lateral), a canine, two premolars, and three molars. These teeth are referred to as numbers - from 1 (central incisor) to 8 (3rd molar or ‘wisdom’ tooth). The permanent teeth begin to erupt, and replace the primary teeth, at six years of age. The permanent teeth complete eruption by approximately age thirteen years – with the exception of the 3rd molar ‘wisdom’ teeth, which usually erupt by the age of 21 years.
A tooth can be divided into two main parts – the crown and the root. The part of a tooth which is visible in the mouth is referred to as the clinical crown, while the part which is not visible is, by definition, the clinical root.
Anatomically, the crown and root can be distinguished based on their structure and the type of hard tooth tissue covering the external surface. Generally, the anatomic crown is covered by enamel, whilst the anatomic root is covered by cementum.
A layer of dentine lies beneath the enamel and cementum, throughout the crown and root. Dentine is a vital, innervated tissue that accounts for the majority of the hard tooth structure. The part of the tooth where the dentine and enamel meet is called the dentino-enamel junction (DEJ). The boundary where the anatomic crown meets the anatomic root (where the enamel meets the cementum) is called the cemento-enamel junction (CEJ). The pulp cavity is the space within a tooth root that is filled with the vital dental pulp, a pink mass of innervated, vascular tissue. The end of the root is called the apex. The apical foramen is the space at the apex through which blood vessels and nerves enter the dental pulp, and through which pulp infection may enter the alveolus and surrounding soft tissues.
There are four main types of teeth – incisors, canines, premolars, and molars. The premolars are only present in the permanent dentition.
Note - There is an accepted order that is used when naming teeth – dentition, arch, quadrant, tooth type. E.g. permanent mandibular right lateral incisor.
There are eight incisors in both the primary and permanent dentition; four maxillary and four mandibular.
The human teeth are categorised by number, from the single central incisor (tooth number 1) to the third molar or 'wisdom' tooth (tooth number 8). These permanent teeth typically begin to erupt at the age of six, and by age thirteen, the permanent teeth have completely erupted, with the exception of the third molar 'wisdom' teeth, which usually erupt by age twenty-one.
A tooth can be divided into two main parts - the crown and the root. The crown can be further divided into four distinct layers - the enamel, cementum, dentin, and the pulp cavity. The enamel is the hardest and most external layer of the tooth, covering the crown of the tooth, and is composed mainly of calcium and phosphate. It is designed to provide thermal insulation for the tooth, and to protect the internal vital tissues from destruction. The enamel is highly susceptible to dental caries, tooth wear and acid dissolution.
The cementum covers the root and is a bonelike material also composed of calcium and phosphate. Beneath the enamel and cementum lies a layer of dentin, which makes up the bulk of the tooth structure. It contains dentinal tubules, which lead to the pulp cavity in the centre of the tooth. This cavity contains connective tissue, nerves, and blood vessels that nourish the tooth. The junction between the enamel and cementum is known as the cemento-enamel junction (CEJ), and the junction between the dentin and enamel is known as the dentino-enamel junction (DEJ). The tip of the tooth is called the apex, and the tip of the root contains the apical foramen through which the blood vessels and nerves enter the tooth.
The part of a tooth which is visible in the mouth is referred to as the clinical crown, while the part which is not visible is, by definition, the clinical root. Generally, the anatomic crown is covered by enamel, whilst the anatomic root is covered by cementum.
There are four main types of teeth - incisors, canines, premolars, and molars. The premolars are only present in the permanent dentition.
The identification and care of teeth is an important part of oral health care, and knowing the different types of teeth helps to ensure adequate care for each tooth. It is important to note that the teeth are at risk of damage from traumatic injuries, cavities, gum disease and other issues, and regular dental visits are key to maintaining healthy teeth and gums.
Canines have deep-rooted stability that is able to tolerate stronger pressures than that which incisors are capable of. Teenagers who are affected and experience dental overcrowding, which is when the total width of the teeth is bigger than the width of the arch they are meant to settle into, may present with unerupted canines. These can often be identified using radiography in the palate or high in the buccal sulcus. In order to align these teeth correctly, oral surgery may be necessary.
The permanent dentition includes 8 premolars that generally contain two cusps, though this is not consistently the case. They fit in between the canines and molars, and share some of the attributes of those teeth. There are no premolars in the primary dentition; they're commonly extracted by dentists to promote dental crowding, specifically before undergoing orthodontic treatment.
The primary permanent dentition carries 8 molars, 4 maxillary and 4 mandibular. For the permanent dentition, there are 6 molars in each category, with the number of cusps ranging from 3 to 5. They're situated in the back of the mouth and are responsible for crushing and chewing food before swallowing. Molar teeth are more vulnerable to dental caries or decay, due to fine grooves in the occlusal surface and an increased area of contact between adjacent molars - making them more difficult to clean than other surfaces such as the labial, buccal, lingual and palatal.
