Developmental data for primary and secondary teeth

SECTION II DEVELOPMENTAL DATA FOR PRIMARY AND SECONDARY TEETH

Dental students and dental hygiene students should become familiar with the eruption dates of primary and secondary teeth in order to adequately and correctly inform worried parents and patients about the normal times when teeth emerge or erupt. Tooth development and eruption dates for permanent teeth presented by the American Dental Association are in Table 6-1, and dates for primary teeth are in Table 6-2. The eruption time for a primary tooth can be considered normal if it is within 4 to 5 months (earlier or later) of the dates in these tables. Permanent tooth eruption can be within 12 to 18 months (early or late) of those dates and still be of no real concern. Early eruption of these teeth usually presents no problems other than a concern about instituting oral hygiene measures earlier.

TABLE 6-1 Permanent Tooth Formation and Emergence Times

Chart based on Logan WH, Kronfield R. Development of the human jaws and surrounding structures from birth to age fifteen. JADA 20:379–424, 1933/1935. Modified by McCall and Schour: Schour I, McCall JO. Chronology of the human dentition. In: Orban B, ed. Oral histology and embryology. St. Louis, MO: C.V. Mosby, 1944:240.

TABLE 6-2 Tooth Development and Eruption: Primary Teeth

From Lunt RC, Law DB. A review of the chronology of eruption of deciduous teeth. J Am Dent Assoc 1974;10:872–879. Copyright © 1974 American Dental Association. All rights reserved. Adapted 2010 with permission.

When a tooth has not emerged when expected, dental radiographs (x-ray films) are the best means for determining if the tooth is either unerupted or missing, particularly when it is considerably overdue (Fig. 6-6).

Two dental radiographs show maxillary and mandibular teeth.

FIGURE 6-6. Maxillary (top) and mandibular (bottom) dental radiographs of an 8-year-old child shows the first and second primary molars (I, J and K, L) and the adjacent first adult molars (14 and 19) erupted in the mouth. Notice the premolar crowns (12, 13 and 20, 21) are forming between the partially resorbed roots of the maxillary and mandibular primary molars. The smaller size, thinner enamel, and relatively larger pulp cavities are evident in the primary molars compared to the newly erupted, larger permanent molars just distal to them. The pulp chambers of these permanent molars will get even smaller as these teeth mature and the roots are completed. (Courtesy of Professor Donald Bowers, the Ohio State University.)

Description

A. IMPORTANT TIMES FOR TOOTH ERUPTION

Instead of memorizing the specific times of eruption of each tooth (which would be a daunting task), first divide the development of teeth into four time periods: (1) when there are no teeth, (2) when primary teeth are erupting and are the only visible teeth, (3) when the dentition is a mixed dentition (i.e., both primary and permanent teeth are visible), and (4) when there are only adult teeth. If you learn what happens during these important time periods, you will be well on your way to understanding the schedule for tooth eruption in both dentitions.

  1. NO TEETH (EDENTULOUS)
    • From birth to 6 months old (approximately): There are no teeth visible within the mouth.
  2. PRIMARY DENTITION ONLY
    • 6 months to 2½ years old (approximately): Over this time, the primary teeth are erupting into the child’s mouth.
    • 2½ to 6 years old (approximately): All 20 primary teeth are present; no permanent teeth are yet visible in the mouth.
  3. MIXED DENTITION

    From ages 6 through 12, primary teeth are being lost or shed (a process known as exfoliation) allowing room for the permanent teeth to erupt. During this time, the dentition is mixed with some primary teeth and some permanent teeth.

    • 6 years old (approximately): Permanent teeth start to appear, beginning with the first molars (also called 6-year molars) just distal to the primary second molars. These are followed closely by the loss of the primary mandibular central incisors, which are soon replaced by the permanent mandibular central incisors. During mixed dentition from 6 years old through 12 years old, there are most often 24 teeth visible (20 primary teeth or their permanent (succedaneous) replacement, plus the 4 secondary first molars).
    • 6 to 9 years old: All eight permanent incisors are replacing primary incisors that are exfoliated (shed).
    • 9 to 12 years old: All four permanent canines and eight premolars are replacing primary canines and molars.
  4. ADULT DENTITION ONLY
    • After 12 years: All primary teeth have been exfoliated and replaced, and second molars (also called 12-year molars) have emerged distal to the permanent first molars. After the eruption of the 12-year molars, there are normally only permanent teeth in the mouth, 28 of them.
    • 17 to 21 years old: Third molars (if present) emerge, resulting in all 32 teeth visible in the mouth.

B. CROWN AND ROOT DEVELOPMENT

With these basic time periods in mind, one must keep in mind that much more is taking place during the development of human teeth than just the eruption and exfoliation of primary teeth, or the eruption of permanent teeth. Prior to eruption, primary tooth crowns are forming and calcifying within the jawbones. As crown calcification is completed, each tooth root begins to form and the tooth moves through bone toward the surface and eventually through the oral mucosa into the oral cavity (eruption or emergence). After eruption, the root continues to form until root formation is completed. At the same time that primary teeth are forming and erupting, permanent teeth are already beginning to form within the jawbones. These permanent teeth develop and calcify, and their roots form as they move occlusally to replace the primary teeth, or erupt distal to the primary teeth.

Now let us look at this entire process in more detail, discussing it step by step. All of the following information is derived from Tables 6-1 and 6-2.

1. Crown Calcification of Primary Teeth

The crowns of all 20 primary teeth begin to calcify between 4 and 6 months in utero (seen developing in Fig. 6-7). Crown completion of all primary teeth occurs within the first year after birth, taking an average of 10 months from the beginning of tooth calcification.B

Photo A and B show developing human primary molars.

FIGURE 6-7. Developing human primary molars illustrating that the mesiobuccal cusps of both the maxillary and mandibular molars are the first to form and mineralize. A. Occlusal view of an in utero 19-week maxillary right first molar that already has a well-developed mesiobuccal cusp, which is covered with a mineralized enamel cap (original magnification 36×). B. Buccal view of an in utero 20-week mandibular right first molar with its mesiobuccal cusp dominating the mesial portion of the tooth. The mesiolingual cusp is the second to differentiate and shows incipient mineralization (original magnification 36×).

Description

2. Roots Form and Primary Teeth Erupt

Root formation for primary (and permanent) teeth begins once the enamel on the crown is formed, and at this time, the tooth starts its occlusal movement through bone toward the oral cavity. After the primary tooth crowns erupt into the oral cavity from age 6 months to 24 to 30 months (about 2 to 2½ years),C they continue to erupt until eventually they reach the level of an ideal occlusal plane. These teeth also continue to erupt slightly to compensate for wear (attrition) on the incisal or occlusal surface and/or when there are no opposing teeth.

3. Order of Emergence of Primary Teeth (from 6 Months to about 2 Years Old)

According to data currently distributed by the American Dental Association (summarized in Table 6-3), the first primary teeth likely to erupt are the mandibular central incisors, at about 6 to 10 months of age (Fig. 6-8), followed by the maxillary central incisors, then maxillary lateral incisors, and finally the mandibular lateral incisors (Fig. 6-9). (Note that in previous editions of this book, a second study was also cited where the mandibular central incisors are likely to erupt first, but they are followed by the mandibular lateral incisors, then the maxillary central incisors, and finally the maxillary lateral incisors.) Next to erupt through about 19 months are the primary first molars (Fig. 6-10), then canines, and finally second molars. Thus, the last primary teeth to emerge, thereby completing the primary dentition, are the maxillary second molars, at about 2 to 2½ years (24 to 33 months) of age.

TABLE 6-3 Chart Representing Order of Primary Tooth Eruption Based on Data in TABLE 6-2
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Sep 12, 2021 | Posted by in General Dentistry | Comments Off on Developmental data for primary and secondary teeth
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