Teeth and supporting tissues

Teeth are a major component of the oral cavity and are essential for the beginning of the digestive process. Teeth are embedded in and attached to the alveolar processes of the maxilla and mandible. Children have 10 deciduous (primary, milk) teeth in each jaw, on each side, consisting of

• A medial (central) incisor, the first tooth to erupt (usually in the mandible) at approximately 6 months of age (in some infants, the first teeth may not erupt until 12 to 13 months of age)

• A lateral incisor, which erupts at approximately 8 months

• A canine tooth, which erupts at approximately 15 months

• Two molar teeth, the first of which erupts at 10 to 19 months and the second of which erupts at 20 to 31 months

Over a period of years, usually beginning at about age 6 and finishing at about age 12 to 13, deciduous teeth are gradually replaced by 16 permanent (secondary) teeth in each jaw. Each side of both upper and lower jaws consists of

• A medial (central) incisor, which erupts at age 7 to 8

• A lateral incisor, which erupts at age 8 to 9

Three systems are currently used to classify permanent and deciduous teeth (Fig. 15.6):

• Palmer system, which was the most commonly used notation worldwide. In this system, uppercase letters are used for the deciduous teeth, and arabic numerals are used for the permanent teeth. Each quadrant in this system is designated by angled lines: -1 for upper right (UR), L for upper left (UL), -| for lower right (LR), and r for lower left (LL). For example, permanent canines are called number 3 in each quadrant, and the quadrant is designated by its angled line.

• International system, which uses two arabic numerals to designate the individual tooth. In this system, the first numeral indicates the location of the tooth in a specific quadrant. The permanent quadrants are designated UR = 1, UL = 2, LL =3, and LR = 4; the deciduous quadrants are designated UR = 5, UL = 6, LL = 7, and LR = 8. The second numeral designates the individual tooth, which is numbered beginning from the dental midline. For example, in this system, the permanent canines are named 13, 23, 33, and 43, and the deciduous canines would be 53, 63, 73, and 83.

• American (Universal) system, which is the most commonly used notation in North America. In this system, the permanent dentition is designated by arabic numerals, and the deciduous dentition is designated with uppercase letters. For permanent dentition, numbering begins in the UR quadrant, with the UR third molar designated number 1. Numbering continues across the maxillary arch to the UL third molar, designated tooth number 16. Tooth number 17 is the third molar located in the LL quadrant inferior and opposite to tooth number 16. Then, the numbering progresses across the mandibular arch and terminates with tooth number 32, the LR third molar. In this system, the sum of the numbers of opposing teeth adds up to 33. For the deciduous dentition, the same pattern is followed, but the letters A to T are used to designate the individual teeth. Thus, in this system, the permanent canines are designated 6, 11, 22, and 27, and the deciduous canines, C, H, M, and R.

Also note that in Figure 15.6 the color outline demonstrates the relationship of the deciduous and permanent dentitions. Examination of the table reveals that deciduous molars are replaced with permanent premolars after exfoliation and that the permanent molars have no deciduous precursors.

• Two premolar teeth, which erupt between ages 10 and 12

• Three molar teeth, which erupt at different times; the first molar usually erupts at age 6, the second molar in the early teens, and the third molar (wisdom teeth) during the late teens or early twenties

Incisors, canines, and premolars have one root each, except for the first premolar of the maxilla, which has two roots. Molars have three and, on rare occasions, four roots. All teeth have the same basic structure, however.

Teeth consist of several layers of specialized tissues

Teeth are made up of three specialized tissues:

Enamel is an acellular mineralized tissue that covers the crown of the tooth. Once formed it cannot be replaced. Enamel is a unique tissue because unlike bone, which is formed from connective tissue, it is a mineralized material derived from epithelium. Enamel is more highly mineralized and harder than any other mineralized tissue in the body. The enamel that is exposed and visible above the gum line is called the clinical crown; the anatomic crown describes all of the tooth that is covered by enamel, some of which is below the gum line. Enamel varies in thickness over the crown and may be as thick as 2.5 mm on the cusps (biting and grinding surfaces) of some teeth. The enamel layer ends at the neck or cervix of the tooth at the cemen-toenamel junction (Fig. 15.7); the root of the tooth is then covered by cementum, a bone-like material. Dentin lies deep to the enamel and cementum.

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