2 Development and Eruption of the Teeth
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 chapter 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.
i1, Central incisor; i2, lateral incisor; c, canine; m1, first molar; m2, second molar.
* Universal numbering system for primary/deciduous dentition; see Chapter 1. See Tables 2-3 and 2-4 for detailed presentation of the data.
I1, Central incisor; I2, lateral incisor; C, canine; P1, first premolar; P2, second premolar; M1, first molar; M2, second molar; M3, third molar.
* See Tables 2-3 and 2-4 in Tooth Formation Standards for detailed presentation of the data.
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 interproximal 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.
(A from Ramfjord S, Ash MM: Periodontology and periodontics, Philadelphia, 1979, Saunders; B from Ash MM: Paradigmatic shifts in occlusion and temporomandibular disorders, J Oral Rehabil 28:113, 2001.)
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 chapter, 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).
Uncommon variations in the maxillary central incisors, which are shown in Chapter 6 (see Figure 6-12), reflect samples drawn from a population made up largely of EAa. 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. Hypocalcification 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 chapter.
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.
(A from Neville BW, Damm DD, Allen CM, et al: Oral and maxillofacial pathology, ed 3, St Louis, 2009, Saunders; B from Ash MM: Oral pathology, ed 6, Philadelphia, 1992, Lea & Febiger.)
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 chapter.
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 chapter. 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.
Figure 2-3 Development of the human dentition to the sixth year. The primary teeth are the darker ones in the illustration.
(From Schour L, Massler M: The development of the human dentition, J Am Dent Assoc 28:1153, 1941.)
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.8 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 neuromuscular system, and such factors as diet.
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 2-5 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.
(A from Ash MM: Oral pathology, ed 6, Philadelphia, 1992, Lea & Febiger; B from Avery JK, Chiego Jr DJ: Essentials of oral histology and embryology, ed 3, St. Louis, 2006, Mosby.)
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 chapter).
Crown and Root Development
Dental development can be considered to have two components: (1) the formation of crowns and roots and (2) the eruption of the teeth. Of these two, the former seems to be much more resistant to environmental influences; the latter can be affected by caries and tooth loss.11,12
After the crown of the tooth is formed, development of the root portion begins. At the cervical border of the enamel (the cervix of the crown), cementum starts to form as a root covering of the dentin. The cementum is similar in some ways to bone tissue and covers the root of the tooth in a thin layer. In the absence of a succeeding permanent tooth, the root of the primary tooth may only partially resorb. When root resorption does not follow the usual pattern, the permanent tooth cannot emerge or is otherwise kept out of its normal place. In addition, the failure of the root to resorb may bring about prolonged retention of the primary tooth. Although mandibular teeth do not begin to move occlusally until crown formation is complete, their eruption rate does not closely correlate with root elongation. After the crown and part of the root are formed, the tooth penetrates the alveolar gingiva and makes its entry (emergence) into the mouth.
Further formation of the root is considered to be an active factor in moving the crown toward its final position in the mouth. The process of eruption of the tooth is completed when most of the crown is in evidence and when it has made contact with its antagonist or antagonists in the opposing jaw. The root formation is not finished when the tooth emerges; however, the formation of root dentin and cementum continues after the tooth is in use. Ultimately, the root is completed with a complete covering of cementum. Additional formation of cementum may occur in response to tooth movement or further eruption of the teeth. Also, cementum may be added (repaired) and/or resorbed in response to periodontal trauma from occlusion. The covering of cementum of the permanent teeth is much thicker than that of the primary teeth.
The human dentitions are usually categorized as being primary, mixed (transitional), and permanent dentitions. The transition from the primary/deciduous dentition to the permanent dentition is of particular interest because of changes that may herald the onset of malocclusion and provide for its interception and correction. Thus of importance for the practitioner are the interactions between the morphogenesis of the teeth, development of the dentition, and growth of the craniofacial complex.
The first indication of tooth formation occurs as early as the sixth week of prenatal life when the jaws have assumed their initial shape; however, at this time the jaws are rather small compared with the large brain case and orbits. The lower face height is small compared with the neurocranium (Figure 2-6). The mandibular arch is larger than the maxillary arch, and the vertical dimensions of the jaws are but little developed. When the jaws close at this stage in the development of the dentition, they make contact with the tongue, which in turn makes contact with the cheeks. The shape of the prenatal head varies considerably, but the relative difference between the brain case, orbits, and lower face height remains the same. All stages of tooth formation fill both jaws during this stage of development.
DEVELOPMENT OF THE PRIMARY DENTITION
Considerable growth follows birth in the neurocranium and splanchnocranium. Usually at birth, no teeth are visible in the mouth; however, occasionally, infants are born with erupted mandibular incisors. Development of both primary and permanent teeth continues in this period, and jaw growth follows the need for additional space posteriorly for additional teeth. In addition the alveolar bone height increases to accommodate the increasing length of the teeth. However, growth of the anterior parts of the jaws is limited after about the first year of postnatal life.
SEQUENCE OF EMERGENCE OF PRIMARY TEETH
The predominant sequence of eruption of the primary teeth in the individual jaw is central incisor (A), lateral incisor (B), first molar (D), canine (C), and second molar (E), as seen in Table 2-1. Variations in that order may be the result of reversals of central and lateral incisors or first molar and lateral incisor, or eruption of two teeth at the same time.13 This subject is considered in more detail in the section on Tooth Formation Standards in this chapter and in Chapter 16, which addresses development of the primary occlusion.
Investigations of the chronology of emergence of primary teeth in different racial and ethnic groups show considerable variation,7 and little information is available on tooth formation in populations of nonwhite/non-European ancestry.14 World population differences in tooth standards suggest that patterned differences may exist that in fact are not large.14 Tooth size, morphology, and formation are highly inheritable characteristics.15 Few definitive correlations exist between primary tooth emergence and other physiological parameters such as skeletal maturation, size, and sex.16
EMERGENCE OF THE PRIMARY TEETH
At about 8 (6 to 10) months of age, the mandibular central incisors emerge through the alveolar gingiva, followed by the other anterior teeth, so that by about 13 to 16 months, all eight primary incisors have erupted (see Table 2-1). Then the first primary molars emerge by about 16 months of age and make contact with opposing teeth several months later, before the canines have fully erupted. Passage through the alveolar crest (Figure 2-7) occurs when approximately two thirds of the root is formed,17 followed by emergences through the alveolar gingiva into the oral cavity when about three fourths of the root is completed.18 The emergence data are consistent with those of Smith.14
Figure 2-7 Section of mandible in a 9-month-old infant cut through an unerupted primary canine and its permanent successor, which lies lingually and apically to it. The enamel of the primary canine crown is completed and lost because of decalcification. Root formation has begun. CEJ, Cementoenamel junction.
(Modified from Schour I, Noyes HJ: Oral histology and embryology, ed 8, Philadelphia, 1960, Lea & Febiger.)
The primary first molars emerge with the maxillary molar tending most often to erupt earlier than the mandibular first molar.19 Some evidence shows a difference by gender for the first primary molars, but no answer is available for why the first molar has a different pattern of sexual dimorphism.7
The primary maxillary canines erupt at about 19 (16 to 22) months (Figure 2-8), and the mandibular canines erupt at 20 (17 to 23) months. The primary second mandibular molar erupts at a mean age of 27 (23 to 31, boys) (24 to 30, girls) months, and the primary maxillary second molar follows at a mean age of 29 (25 to 33 ± 1SD) months. In Figures 2-8, A and B, the first molars are in occlusion.