During a period of years, usually beginning at about age 6 and ending at about age 12 to 13, deciduous teeth are gradually replaced by 16 permanent (secondary) teeth in each jaw (Folder 16.2). Each side of both upper and lower jaws consists of the following:  A medial (central) incisor, which erupts at age 7 to 8  A lateral incisor, which erupts at age 8 to 9  A canine tooth, which erupts at age 10 to 12  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 either two roots (lower jaw) or three (upper jaw) and, on rare occasions, four roots. All teeth have the same basic structure, however.
 FOLDER 16.2 Clinical Correlation: Classification of Permanent (Secondary) and Deciduous (Primary) Dentition
Three systems are currently used to classify permanent and deciduous teeth (Fig. F16.2.1):  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 sys-tem is designated by angled lines: for upper right (UR), for upper left (UL), for lower right (LR), and for lower left (LL). For example, permanent canines are called number 3 in each quadrant, and the quadrant is desig-nated by its angled line.  International system, which uses two arabic numer-als to designate the individual tooth. In this system, the first numeral indicates the location of the tooth in a spe-cific 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 mid-line. For example, in this system, the permanent canines are named 13, 23, 33, and 43, and the deciduous ca-nines 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 numer-als, and the deciduous dentition is designated with up-percase letters. For permanent dentition, numbering begins in the UR quadrant, with the UR third molar des-ignated 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 num-bers of opposing teeth adds up to 33. For the decidu-ous 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 desig-nated 6, 11, 22, and 27, and the deciduous canines, C, H, M, and R. Also note that in Figure F16.2.1 the color outline demonstrates the relationship of the deciduous and per-manent dentitions. Examination of the table reveals that de-ciduous molars are replaced with permanent premolars after exfoliation and that the permanent molars have no de-ciduous precursors. continued next page
FOLDER 16.2 Clinical Correlation: Classification of Permanent (Secondary) and Deciduous (Primary) Dentition (Cont.)