Development and Eruption of the Teeth (Dental Anatomy, Physiology and Occlusion) Part 5

Duration of Root and Crown Formation

The onset and duration of crown and root formation of the primary dentition are illustrated in Table 2-9, which answers questions about the relationship between onset and completion of tooth formation from start to finish.

Summary of Chronologies

Compared with older, descriptive chronologies based on dissection and those based on radiological plus statistical methods to produce developmental data, newer methods tend to avoid attributing discrepancies to population differences because of methodological or sampling effects. The data in Tables 2-7 and 2-8 have been recommended for deciduous tooth development.

Cumulative distribution functions and probit analysis are recommended for generating statistical solutions for schedules of age of attainment of growth stages.14,35

Clinicians can use chronologies to avoid treatment that can damage developing teeth (attainment schedules), to assess an unknown age of a patient (e.g., age prediction in forensics, demographics), and to assess growth (maturity).14

Sequence of Eruption

The sequence of eruption of the primary teeth does show some variation. Such timing is a result in large part of heredity and only somewhat of environmental factors. Jaw reversals in eruption of canines and first molars have been found to be important in increasing the variety of sequences.13,60 When differences according to jaws are considered, Lunt and Law19 conclude that the lateral incisor, first molar, and canine tend to erupt earlier in the maxilla than in the mandible. Sato and Ogiwara60 found the following characteristic order in about one third of their sample of children:

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However, this arrangement of mean ages of eruption to yield a mean order of eruption was found to occur only in a small percentage of the subjects in the study by Lysell et al.13 The sequence and age of eruption of primary teeth are illustrated in Table 2-10.

Table 2-10 Sequence and Age at Eruption of Primary Teeth

Teeth

Age

Mean Number of Teeth Present

Distribution in Number of Teeth

A-

6 mo

1-3, 33%

A

9 mo

3

1-6, 80%

B

12 mo

6

4-8, 50%

D

18 mo

12

9-16, 85%

B, D

C

24 mo

16

15-18, 60%

C

-, E

30 mo

19

20, 70%

A, Central incisor; B, lateral incisor; D, first molar; C, canine; E, second molar; underscore indicates maxillary teeth; overscore indicates mandibular teeth.

Estimating Time of Enamel Hypoplasia

To estimate the time of enamel hypoplasia, measure in millimeters the distance from the CEJ to the midpoint of the enamel defect. As a comparison, note in Table 6-1 that the cervicoincisal length of the crown of the permanent maxillary central incisor is 10.5 mm. In Table 2-3 and Table 6-1, the first evidence of calcification is 3 to 4 months. Assuming that rate of development is constant and that a maxillary central incisor develops over a period of 4 to 5 years, the age of development of the defect is related inversely to the distance from the CEJ to the enamel defect and can be computed as follows according to a first-degree polynomial for estimating the chronological age at which the enamel defect occurred5,61:

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Here, ADF = age at which the formation of the enamel hypoplasia occurred; and ACF = age at which crown is completed. For example, assume that the cervical-incisal length of the crown of the permanent maxillary central incisor is 10.5 mm, the crown is completed at 4 to 5 years, and the midpoint of the defect is 6.6 mm from the CEJ. Plugging in the data for both 4 years and 5 years and obtaining an average, the age of formation of the defect is estimated to be at about 2 years, keeping in mind that for estimating time here, dividing the development of enamel into intervals of one or even a few months is an accuracy that is not justified, and 6-month or yearly periods are more realistic. Computed estimated ages of hypoplasia vary somewhat, depending on the chronological tables used.

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