Formation of the Dentitions (Overview)

Humans have two sets of teeth in their lifetime. The first set of teeth to be seen in the mouth is the primary or deciduous dentition, which begins to form prenatally at about 14 weeks in utero and is completed postnatally at about 3 years of age. In the absence of congenital disorders, dental disease, or trauma, the first teeth in this dentition begin to appear in the oral cavity at the mean age of 6, and the last emerge at a mean age of 28 ± 4 months. The deciduous dentition remains intact (barring loss from dental caries or trauma) until the child is about 6 years of age. At about that time the first succedaneous or permanent teeth begin to emerge into the mouth. The emergence of these teeth begins the transition or mixed dentition period in which there is a mixture of deciduous and succedaneous teeth present. The transition period lasts from about 6 to 12 years of age and ends when all the deciduous teeth have been shed. At that time the permanent dentition period begins. Thus, the transition from the primary dentition to the permanent dentition begins with the emergence of the first permanent molars, shedding of the deciduous incisors, and emergence of the permanent incisors. The mixed dentition period is often a difficult time for the young child because of habits, missing teeth, teeth of different colors and hues, crowding of the teeth, and malposed teeth.

The permanent, or succedaneous, teeth replace the exfoliated deciduous teeth in a sequence of eruption that exhibits some variance, an important topic that will be considered in Chapter 16.

After the shedding of the deciduous canines and molars, emergence of the permanent canines and premolars, and emergence of the second permanent molars, the permanent dentition is completed (including the roots) at about 14 to 15 years of age, except for the third molars, which are completed at 18 to 25 years of age. In effect, the duration of the permanent dentition period is 12+ years. The completed permanent dentition consists of 32 teeth if none are congenitally missing, which may be the case. The development of the teeth, dentitions, and the craniofacial complex are considered in Chapter 2. The development of occlusion for both dentitions is discussed in Chapter 16.

Nomenclature

The first step in understanding dental anatomy is to learn the nomenclature, or the system of names, used to describe or classify the material included in the subject. When a significant term is used for the first time here, it is emphasized in bold. Additional terms will be discussed as needed in subsequent chapters.

The term mandibular refers to the lower jaw, or mandible. The term maxillary refers to the upper jaw, or maxilla. When more than one name is used in the literature to describe something, the two most commonly used names will be used initially. After that they may be combined or used separately as consistent with the literature of a particular specialty of dentistry, for example, primary or deciduous dentition, permanent or succedaneous dentition. A good case may be made for the use of both terms. By dictionary definition,¹ the term primary can mean “constituting or belonging to the first stage in any process.” The term deciduous can mean “not permanent, transitory.” The same unabridged dictionary refers the reader from the definition of deciduous tooth to milk tooth, which is defined as “one of the temporary teeth of a mammal that are replaced by permanent teeth. Also called baby tooth, deciduous tooth.” The term primary can indicate a first dentition and the term deciduous can indicate that the first dentition is not permanent, but not unimportant. The term succedaneous can be used to describe a successor dentition and does not suggest permanence, whereas the term permanent suggests a permanent dentition, which may not be the case due to dental caries, periodontal diseases, and trauma. All four of these descriptive terms appear in the professional literature.

Formulae for Mammalian Teeth

The denomination and number of all mammalian teeth are expressed by formulae that are used to differentiate the human dentitions from those of other species. The denomination of each tooth is often represented by the initial letter in its name (e.g., I for incisor, C for canine, P for premolar, M for molar). Each letter is followed by a horizontal line and the number of each type of tooth is placed above the line for the maxilla (upper jaw) and below the line for the mandible (lower jaw). The formulae include one side only, with the number of teeth in each jaw being the same for humans.

The dental formula for the primary/deciduous teeth in humans is as follows:

This formula should be read as: incisors, two maxillary and two mandibular; canines, one maxillary and one mandibular; molars, two maxillary and two mandibular—or 10 altogether on one side, right or left (Figure 1-2, A).

Figure 1-2 A, Casts of deciduous, or primary, dentition. B, Casts of permanent dentition.

(A from Berkovitz BK, Holland GR, Moxham BJ: Oral anatomy, histology and embryology, ed 3, St. Louis, 2002, Mosby.)

A dental formula for the permanent human dentition is as follows:

Premolars have now been added to the formula, two maxillary and two mandibular, and a third molar has been added, one maxillary and one mandibular (Figure 1-2, B).

Systems for scoring key morphological traits of the permanent dentition that are used for anthropological studies are not described here. However, a few of the morphological traits that are used in anthropological studies² are considered in the following chapters, (e.g., shoveling, Carabelli’s trait, enamel extensions, and peg-shaped incisors). Some anthropologists use di₁, di₂, dc, dm₁, and dm₂ notations for the deciduous dentition and I₁, I₂, C, P₁, P₂, M₁, M₂, and M₃ for the permanent teeth. These notations are generally limited to anthropological tables because of keyboard incompatibility.

Tooth Numbering Systems

In clinical practice some “shorthand” system of tooth notation is necessary for recording data. There are several systems in use in the world, but only a few are considered here. In 1947 a committee of the American Dental Association (ADA) recommended the symbolic (Zsigmondy/Palmer) system as the numbering method of choice.³ However, because of difficulties with keyboard notation of the symbolic notation system, the ADA in 1968 officially recommended the “universal” numbering system. Because of some limitations and lack of widespread use internationally, recommendations for a change sometimes are made.⁴

The universal system of notation for the primary dentition uses uppercase letters for each of the primary teeth: For the maxillary teeth, beginning with the right second molar, letters A through J, and for the mandibular teeth, letters K through T, beginning with the left mandibular second molar. The universal system notation for the entire primary dentition is as follows:

The symbolic system for the permanent dentition was introduced by Adolph Zsigmondy of Vienna in 1861 and then modified for the primary dentition in 1874. Independently, Palmer also published the symbolic system in 1870. The symbolic system is most often referred to as the Palmer notation system in the United States and less frequently as the Zsigmondy/Palmer notation system. In this system the arches are divided into quadrants with the entire dentition being notated as follows:

Thus, for a single tooth such as the maxillary right central incisor the designation is . For the mandibular left central incisor, the notation is given as . This numbering system presents difficulty when an appropriate font is not available for keyboard recording of Zsigmondy/Palmer symbolic notations. For simplification this symbolic notation is often designated as Palmer’s dental notation rather than Zsigmondy/Palmer notation.

In the universal notation system for the permanent dentition, the maxillary teeth are numbered from 1 through 16, beginning with the right third molar. Beginning with the mandibular left third molar, the teeth are numbered 17 through 32. Thus, the right maxillary first molar is designated as 3, the maxillary left central incisor as 9, and the right mandibular first molar as 30. The following universal notation designates the entire permanent dentition.

The Zsigmondy/Palmer notation for the permanent dentition is a four-quadrant symbolic system in which, beginning with the central incisors, the teeth are numbered 1 through 8 (or more) in each arch. For example, the right maxillary first molar is designated as , and the left mandibular central incisor as . The Palmer notation for the entire permanent dentition is as follows:

Viktor Haderup of Denmark in 1891 devised a variant of the eight-tooth quadrant system in which plus (+) and minus (−) were used to differentiate between upper and lower quadrants and between right and left quadrants; in other words, +1 indicates the upper left central incisor and 1− indicates the lower right central incisor. Primary teeth were numbered as follows: upper right, 05+ to 01+; lower left, −01 to −05. This system is still taught in Denmark.⁵

The universal system is acceptable to computer language, whereas the Palmer notation is generally incompatible with computers and word processing systems. Each tooth in the universal system is designated with a unique number, which leads to less confusion than with the Palmer notation.

A two-digit system proposed by Fédération Dentaire Internationale (FDI) for both the primary and permanent dentitions has been adopted by the World Health Organization and accepted by other organizations such as the International Association for Dental Research. The FDI system of tooth notation is as follows.

For the primary teeth:

Numeral 5 indicates the maxillary right side, and 6 indicates the maxillary left side. The second number of the two-digit number is the tooth number for each side. The number 8 indicates the mandibular right side, and the number 7 indicates the mandibular left side. The second number of the two-digit system is the tooth number. Thus, for example the number 51 refers to the maxillary right central incisor.

For the permanent teeth:

Thus, as in the two-digit FDI system for the primary dentition, the first digit indicates the quadrant: 1 to 4 for the permanent dentition and 5 to 8 for the primary dentition. The second digit indicates the tooth within a quadrant: 1 to 8 for the permanent teeth and 1 to 5 for the primary teeth. For example, the permanent upper right central incisor is 11 (pronounced “one one,” not “eleven”).

THE CROWN AND ROOT

Each tooth has a crown and root portion. The crown is covered with enamel, and the root portion is covered with cementum. The crown and root join at the cementoenamel junction (CEJ). This junction, also called the cervical line (Figure 1-3), is plainly visible on a specimen tooth. The main bulk of the tooth is composed of dentin, which is clear in a cross section of the tooth. This cross section displays a pulp chamber and a pulp canal, which normally contain the pulp tissue. The pulp chamber is in the crown portion mainly, and the pulp canal is in the root (Figure 1-4).The spaces are continuous with each other and are spoken of collectively as the pulp cavity.

Figure 1-3 Maxillary central incisor (facial aspect). A, Apex of root; R, root; CL, cervical line; C, crown; IE, incisal edge.

Figure 1-4 Schematic drawings of longitudinal sections of an anterior and a posterior tooth. A, Anterior tooth. A, Apex; AF, apical foramen; SC, supplementary canal; B, bone; C, cementum; PM, periodontal ligament; PC, pulp canal; G, gingiva; GC, gingival crevice; GM, gingival margin; PCH, pulp chamber; D, dentin; E, enamel; CR, crown. B, Posterior tooth. A, Apices; PC, pulp canal; PCH, pulp chamber; PH, pulp horn; F, fissure; CU, cusp; CEJ, cementoenamel junction; BI, bifurcation of roots.

The four tooth tissues are enamel, cementum, dentin, and pulp. The first three are known as hard tissues, the last as soft tissue. The pulp tissue furnishes the blood and nerve supply to the tooth. The tissues of the teeth must be considered in relation to the other tissues of the orofacial structures (Figures 1-5 and 1-6) if the physiology of the teeth is to be understood.

Figure 1-5 Sagittal sections through the maxillary and mandibular central incisors.

Figure 1-6 Section through the second maxillary molar and adjacent tissues.

The crown of an incisor tooth may have an incisal ridge or edge, as in the central and lateral incisors; a single cusp, as in the canines; or two or more cusps, as on premolars and molars. Incisal ridges and cusps form the cutting surfaces on tooth crowns.

The root portion of the tooth may be single, with one apex or terminal end, as usually found in anterior teeth and some of the premolars; or multiple, with a bifurcation or trifurcation dividing the root portion into two or more extensions or roots with their apices or terminal ends, as found on all molars and in some premolars.

The root portion of the tooth is firmly fixed in the bony process of the jaw, so that each tooth is held in its position relative to the others in the dental arch. That portion of the jaw serving as support for the tooth is called the alveolar process. The bone of the tooth socket is called the alveolus (plural, alveoli) (Figure 1-7).

Figure 1-7 Left maxillary bone showing the alveolar process with three molars in place and the alveoli of the central incisor, lateral incisor, canine, and first and second premolars. Note the opening at the bottom of the canine alveolus, an opening that accommodates the nutrient blood and nerve supply to the tooth in life. Although they do not show up in the photograph, the other alveoli present the same arrangement.

The crown portion is never covered by bone tissue after it is fully erupted, but it is partly covered at the cervical third in young adults by soft tissue of the mouth known as the gingiva or gingival tissue, or “gums.” In some persons, all of the enamel and frequently some cervical cementum may not be covered by the gingiva.