Knowledge of the development of the teeth and their emergence into the oral cavity is applicable to clinical practice, anthropology, demography, forensics, and paleontology. However, dental applications are considered primarily. This topic considers the development and eruption of the teeth, specific chronologies of both the primary and permanent human dentitions, dental age, tooth formation standards, and applications to dental practice (e.g., an understanding of both the chronology of dental development so that surgical intervention does not harm normal growth and the relationship between dental age and the effects of disease and environmental risks). The use of the terms primary and deciduous, or often, primary/deciduous, reflects the difference of opinion about the most appropriate term to describe the first dentition in humans. Readers of the literature are able to deal objectively with both terms.
It must be kept in mind that the dental practitioner sees in a "normal" healthy mouth not only the clinical crowns of the teeth surrounded by the gingival tissues, but also the number, shape, size, position, coloration, and angulations of the teeth; the outlines of the roots of the teeth; occlusal contacts; evidence of function and parafunction; and phonetics and esthetics. Most of the parts of the teeth that are hidden by the gingiva can be visualized radiographically. This can also be done by using a periodontal probe to locate the depth of normal or pathologically deepened gingival crevices or a dental explorer to sense the surfaces of the teeth within the gingival crevice apical to the free gingival margin as far as the epithelial attachment of the gingiva to the enamel. In addition, in pathologically deepened crevices, tooth surfaces can be sensed as far as the attachment of the periodontal ligament to the cementum. Perhaps the simplest example of clinical observation is the assignment of dental age or the assessment of dental development by looking into a child’s mouth to note the teeth that have emerged through the gingiva. In the absence of other data, however, the number of teeth present are simply counted.1
When observations from clinical and radiographic sources of information are coupled with sufficient knowledge of dental morphology and the chronologies of the human dentition, the clinician has the foundation for the diagnosis and management of most disorders involving the size, shape, number, arrangement, esthetics, and development of the teeth and also problems related to the sequence of tooth eruption and occlusal relationships. For example, in Figure 2-1, A, the gingival tissues are excellent; however, the form of the maxillary incisors and interdental spacing might be considered to be an esthetic problem by a patient. To accept the patient’s concern that a cosmetic problem is present and needs correction requires that the practitioner be able to transform the patient’s idea of esthetics into reality by orthodontics and cosmetic restorative dentistry. The situation in Figure 2-1, B demonstrates a periodontal problem (localized gingivitis of the gingival margin of the right central incisor), which is in part a result of the inadequate proximal contact relations of the incisors, leading to food impaction and accumulation of dental plaque and some calculus. For the most part, however, it is the result of inadequate home care hygiene. Most conservative correction relates to removal of the irritants and daily tooth brushing and dental flossing, especially of the interproximal areas of the central incisors. Even so, the risk factor of the inadequate proximal contact remains. If the form of a tooth is not consistent with its functions in the dental arches, then it is highly probable that nonfunctional positions of interproxi-mal contacts will lead to the problems indicated in Figure 2-1, B.
FIGURE 2-1 Clinical observations: clinical crowns. Note the difference in the shapes of the teeth in A and B, as well as the interdental spacing, and the presence and location of interproximal tooth contacts. Consider the contours of the roots (A), the occlusal contacts of the incisor, canine, and premolar teeth), and the gingiva of the maxillary right central incisor, and the esthetics presented in both A and B.
The form of every tooth is related to its position and angulation in the dental arch, its contact relations with the teeth in the opposing arch, its proximal contacts with adjacent teeth, and its relationship to the periodontium. An appreciation for the esthetics of tooth form and coloration is a requirement for the successful practitioner.
It is not enough to know just the "normal" morphology of the teeth; it is also necessary to accept the concept of morphological variability in a functional, esthetic, and statistical sense. Most of the data on tooth morphology are derived from studies of samples of population of European-American ancestry (EAa), and, for example, as indicated in the section on Tooth Formation Standards in this topic, a variety of sequences in eruption of the teeth exist depending on the population sampled. Because of the Immigration Reform Act of 1965, it is most likely that future tooth morphology standards will reflect the significant change in the ethnic makeup of the population of the United States (i.e., population samples of dentitions will reflect a greater variance).
It is possible to accept the incisors shown as being representative of this population, or perhaps "normal" for the EAa population at the time sampled. A shovel-shaped incisor trait is found in a Caucasoid population only infrequently (fewer than 5%); however, it is one of the characteristics found in patients with Down syndrome (trisomy 21) and normally in Chinese and Japanese individuals, Mongolians, and Eskimos. Statistically then, the shovel-shaped trait might be considered to be abnormal in the Caucasoid population but not so in the Mongoloid populations. The practitioner must be prepared to adjust to such morphological variations.
It is necessary to know the chronologies of the primary and permanent dentitions to answer questions about when disturbances in the form, color, arrangement, and structure of the teeth might have occurred. Dental anomalies are seen most often with third molars, maxillary lateral incisors, and mandibular second premolars. Abnormally shaped crowns such as peg laterals and mandibular second premolars with two lingual cusps present restorative and space problems, respectively.
Patients who have a disturbance such as the ones shown in Figure 2-2 not only want to know what to do about it, they want to know when or how the problem might have happened. How the problem came about is the most difficult part of the question. Enamel hypoplasia is a general term referring to all quantitative defects of enamel thickness. They range from single or multiple pits to small furrows and wide troughs to entirely missing enamel. Hypocalcifica-tion and opacities are qualitative defects. The location of defects on tooth crowns provides basic evidence for estimating the time of the development of the defect with an unknown error and potential bias.2-5 One method of estimating is provided in the section on Tooth Formation Standards in this topic.
In a cleft palate and lip, various associated malformation of the crowns of the teeth of both dentitions occurs. The coronal malformations are not limited to the region of the cleft but involve posterior teeth as well.6 A number of congenital malformations involving the teeth are evident, with some the result of endogenous factors and others the result of exogenous agents. When a malformation has some particular characteristics (e.g., screwdriver-shaped central incisors) and is consistent with a particular phase of dental development, it may be possible to determine the cause of the disturbance. This aspect is considered further in the section on Dental Age in this topic.
FIGURE 2-2 A, Hypoplasia of the enamel. B, Defect in tooth structure caused by trauma to the primary predecessor during development of the permanent central incisor.
Chronology of Primary Dentition
The chronology of the primary teeth presented in Table 2-1 is based on data derived from Tables 2-3 and 2-4 in the section on Tooth Formation Standards in this topic. The universal numbering system is used in Table 2-1. The pictorial charts (Figures 2-3 and 2-4) are not intended to be used as ideal standards of normal development. Their use is directed toward showing patients the general aspects of development rather than providing precise guidance for clinical procedures.
Development and Eruption/ Emergence of the Teeth
Historically, the term eruption was used to denote the tooth’s emergence through the gingiva, but then it became more completely defined to mean continuous tooth movement from the dental bud to occlusal contact.7 Not all tables of dental chronologies reflect the latter definition of eruption, however; the terms eruption and emergence are used here at this time in such a way as to avoid any confusion between the historical use of eruption and its more recent expanded meaning.
Emergence of the primary dentition takes place between the sixth and thirtieth months of postnatal life. It takes from 2 to 3 years for the primary dentition to be completed, beginning with the initial calcification of the primary central incisor to the completion of the roots of the primary second molar (see Figure 2-3).
The emergence of the primary dentition through the alveolar mucous membrane is an important time for the development of oral motor behavior and the acquisition of masticator skills.
Table 2-1 Chronology of Primary Teeth*
iI, Central incisor; i2, Lateral incisor; c, canine; ml, first molar; m2, second molar. ^Universal numbering system for primary/deciduous dentition;
At this time of development, the presence of "teething" problems suggests how the primary dentition can affect the development of future neurobehavioral mechanisms, including jaw movements and mastication. Learning of mastication may be highly dependent on the stage and development of the dentition (e.g., type and number of teeth present and occlusal relations), the maturation of the neu-romuscular system, and such factors as diet.
Figure 2-3 Development of the human dentition to the sixth year. The primary teeth are the darker ones in the illustration.
Enamel organs (Figure 2-5) do not all develop at the same rate; some teeth are completed before others are formed, which results in different times of eruption for different groups of teeth. Some of the primary/deciduous teeth are undergoing resorption while the roots of others are still forming. Not all the primary teeth are lost at the same time; some (e.g., central incisors) are lost 6 years before the primary canines. Groups of teeth develop at specific rates so that the sequence of eruption and emergence of the primary/ deciduous teeth is well defined with few deviations. Even so, for the individual child, considerable variation in the times of emergence of the primary dentition may occur. The primary dentition is completely formed by about age 3 and functions for a relatively short period of time before it is lost completely at about age 11. Permanent dentition is completed by about age 25 if the third molars are included (see Figures 2-3 and 2-4).9
Figure 2-4 Development of the human dentition from the seventh year to maturity. Note the displacement of the primary teeth.
Calcification of the primary teeth begins in utero from 13 to 16 weeks postfertilization. By 18 to 20 weeks, all the primary teeth have begun to calcify. Primary tooth crown formation takes only some 2 to 3 years from initial calcification to root completion. However, mineralization of the permanent dentition is entirely postnatal, and the formation of each tooth takes about 8 to 12 years. The variability in tooth development is similar to that for eruption, sexual maturity, and other similar growth indicators.10
Crown formation of the primary teeth continues after birth for about 3 months for the central incisor, about 4 months for the lateral incisor, about 7 months for the primary first molar, about 8.5 months for the canine, and about 10.5 months for the second primary molar. During these periods before and after birth, disorders in shape, pigmentation, mineralization, and structure sometimes occur (fluorosis is considered later in this topic).
Figure 2-5 Enamel organ. A, 1, Beginning of first primary molar; 2, bell stage of second primary molar; 3, dental lamina of first permanent molar. B, Partially developed primary incisor and, lingually, the developing permanent incisor.