2.1 Nomenclature, numbering system, and dental tissues
For studying dental anatomy, a common language is required. Humans have two dentitions in their lifetime that support the anatomical structures and orofacial functions such as mastication, speech, and give shape and beauty to the face. Teeth are highly calcified structures with individualized tissues, supported in the upper and lower jaw by bony sockets also called alveolus. Teeth are comprised of four types: incisors, canines (cuspids), premolars (bi-cuspids), and molars. The first set of teeth is the primary or deciduous dentition, which begins to form prenatally at about 14 weeks and is completed postnatal at about 3 years of age. In total, 20 teeth, 10 maxillary and 10 mandibular teeth, and 5 teeth on each side of the jaw are consisting the deciduous dentition. The teeth in deciduous dentition are grouped on maxillary and mandibular jaw as follows: four incisors, two canines, and four molars (Table 1). World Dental Federation proposed a numbering system for deciduous and permanent dentitions, and the system was adopted by the World Health Organization and accepted by other organizations such as the International Association for Dental Research [1].
Morphological characteristics of deciduous teeth are different from permanent ones. Deciduous teeth have smaller crowns and roots, more prominent cervical ridges, and narrower “neck” or cervical diameter. The roots are narrower and flare, and the buccolingual diameters are smaller than the permanent’s diameters. Crowns of deciduous anterior teeth have a mesiodistal diameter higher in comparison with the cervical-incisal diameter. The roots of deciduous anterior are long and narrower, and the roots of deciduous molars are longer, slender, and flare to allow more room for the development of permanent crowns. The cervical ridges of the deciduous anterior teeth are more prominent, the crowns and roots of deciduous molar are more slander mesiodistally, the buccal and lingual surfaces of deciduous molars are flatter, and the shade of deciduous is whiter [2].
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The teeth supported by the upper jaw are called maxillary teeth and the ones supported by the lower jaw are called mandibular teeth. The deciduous dentition lasts until about 6 years of age when the first succedaneous or permanent teeth are emerging into the oral cavity. In this stage, the transitional dentition or mixed dentition is formed, which is present in the oral cavity until 12 or 13 years of age and ends when all the deciduous teeth are lost and all the permanent ones have emerged in the oral cavity. Mixed dentition, present between 6 and 12 years of age, can be a difficult period of time because of missing teeth, different shades and hues of the recently emerged teeth, malposition, and crowning, which may need orthodontic treatment.
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In total, 32 teeth, 16 maxillary and 16 mandibular teeth, and 8 teeth on each side of the jaw are consisting the permanent dentition. Teeth in permanent dentition are grouped on each jaw as follows: four incisors, two canines, four premolars, and six molars. The evolution of permanent dentition ends at about 14-15 years of age when the roots have completed their development. The age of emergence of permanent teeth is described in Table 2. The exception is related to third molars, which are emerging at 18-25 years of age [1, 3].
Teeth have two primary components, one crown—exposed in the oral cavity—and one or more roots supported by the alveolar socket. The tooth has four primary tissues—enamel, dentine, cement, and pulp. The main component of a tooth is a bone-like tissue called dentine.
Enamel covers the visible part of the crown in the oral cavity and is a highly calcified tissue (95.5% inorganic, 0.5% organic, and 4% water). The upper incisors reach the highest density of enamel, which is increasing progressively during development. The thickness of the enamel layer varies from about 2.5 mm in the cusps area, 2.0 mm in the incisal edge, and 0.5 mm at the cervical enamel. The color is semitranslucent and depends on the enamel thickness. In the thick opaque area, the color appears bluish-white and yellow-white in the area where the enamel layer is thinner. Enamel is the hardest structure of the body, having a value of 5-8 on the Moch scale (diamond-10 Moch). The high hardness influences the tensile strength and compressibility, which indicate that enamel is extremely brittle [4, 5].
The enamel is structured in enamel rods (prisms), rod sheaths, and cementing interred substance. Generally, enamel rods are aligned perpendicular to dentine-enamel junction and usually in cervical area the rods become twisted. The rods are aligned in planes best suited to withstand the occlusal forces as long as the forces are perpendicular to the tooth surface. Rod sheath is an area identified in histologic sections of a tooth, found where enamel rods, the functional unit of enamel, meet interrod enamel. Both types of enamel meet at sharp angles and form the appearance of a space called the rod sheath. The rod sheath consists of more protein and the rod sheath is characterized as being hypomineralized in comparison to the rest of the highly mineralized enamel. The rod sheath is Inorganic matrix tying the enamel rods together. The interred substance cement the rods together. Enamel forms junctions with dentine and cement. The dentine-enamel junction (DEJ) has a pitted aspect and in the rounded pits fit the enamel rod, which generates a strong bond between the two tissues [3, 4].
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Underlying the enamel tissue is the dentine root, a bone-like tissue that consists of a spongy bone-like tissue, which consists of the root, and is covered by cementum which is present only on the tooth surface. The crown delimited by the enamel and the root delimited by the cement join in a junction called cementoenamel junction (CEJ), which is also called the cervical line. The cementum is specially adapted to anchor and support the tooth in the bony socket and covers the dentine root. Both enamel and cement are covering the dentine, and three variations in their link may exist: the cervical enamel is covered by cementum (65%), contact line between enamel and cementum (25%), or enamel and cement do not touch, and in this situation, dentin is exposed (10%) [3, 6].
Dentin is the largest tissue of a tooth, gives the basic shape of the crown and root, and forms the walls of the pulp cavity in the crown and root area. The color is light yellow in deciduous teeth and yellow in permanent teeth, less hard than enamel, and harder than the cementum. In contrast to the enamel, dentine is highly elastic, can support the non-resilient enamel, and is highly permeable. In its chemical composition, dentine has 70% inorganic matter, 17% organic matter (collagen, proteins, and citric acid), and 13% water. Dentine is structured into dentinal tubules that are ending beneath the enamel and contain the Tome’s fibers also called processes of odontoblasts cells present in the dental pulp. For this reason, dentine is a living and sensitive hard tissue of the tooth. Dentine tubes have an “S” curved shape and start perpendicular to the pulp chamber surface but are straight under the cusps and in the root area. The dentine encloses the dental pulp, the soft tissue formed by blood vessels, nerves, and specialized tissue, all responsible for the tooth’s vitality [3, 6].
Dental pulp is contained within the pulp chamber of the crown, close to the cervical line and in the cervical third part of the crown, and is followed by the pulp canal along the root. The pulp chamber and root canal form together the pulp cavity. Pulp has the main role in forming the dentine, has a sensory function through which the tooth is felt, and also has a nutrient function by supplying nutrition to the dentine through the blood vessels and odontoblastic processes with an important role in maintaining the tooth’s vitality. The last function is the defensive one, through which the pulp is producing secondary dentine to maintain the vitality of the tooth in case of damaging factors such as cavities, occlusal overloading, and aging. In young teeth, the pulp chamber is bigger and follows the shape of the crown through the extension of pulp chamber into the cusps. These extensions are called pulp horns. By aging, the pulp chamber is reducing its size because of secondary dentine deposition. The root canal and apical foramen are wide in case of young teeth and become narrow by aging. Root canals can be straight or curved, single or not, with accessory lateral canals usually in the apical foramen or at the floor of the pulp chamber. Apical foramen or root apex is a small foramen through which the blood vessels and nerves enter the pulp chamber and may be located on the lateral side of the apex. A tooth can have more than one foramina. Through apical foramen, there is a communication with periodontal space. Endodontic and periodontal pathology are interconnected and influenced one by another through apical foramen or accessory foramens and root canals [7].
The three hard tissues of the tooth, enamel, dentine, and cementum, and the soft tissue represented by dental pulp have to be considered in relation with the oral-facial structures, oral cavity, and all surrounded soft tissues and anatomical structures.
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