The adult/permanent dentition consisting of 32 teeth is completed from 18 to 25 years of age if the third molar is included. Apparently there are four or more centers of formation (developmental lobes) for each tooth. The formation of each center proceeds until a coalescence of all of them takes place. During this period of odontogenesis, injury to the developing tooth can lead to anomalous morphological features (e.g., peg-shaped lateral incisor). Although no lines of demarcation are found in the dentin to show this development, signs are found on the surfaces of the crowns and roots; these are called developmental grooves. Fractures of the teeth occur most commonly along these grooves.
The follicles of the developing incisors and canines are in a position lingual to the deciduous roots.
The developing premolars, which eventually take the place of deciduous molars, are within the bifurcation of primary molar roots. The permanent incisors, canines, and premolars are called succedaneous teeth because they take the place of their primary predecessors.
The central incisor is the second permanent tooth to emerge into the oral cavity. Eruption time is quite close to that of the first molar (i.e., tooth emergence occurs between 6 and 7 years). As with the first molar, at age 6 years, 50% of individuals have reached the stage considered the age of attainment of the stage or, more specifically, the age of emergence for the central incisor. The mandibular permanent teeth tend to erupt before maxillary teeth. The mandibular central incisor usually erupts before the maxillary central incisor and may erupt simultaneously with or even before the mandibular first molar. The mandibular lateral incisor may erupt along with the central incisor.
Before the permanent central incisor can come into position, the primary central incisor must be exfoliated. This occurs through the resorption of the deciduous roots. The permanent tooth in its follicle attempts to move into the position held by its predecessor. Its influence on the primary root evidently causes resorption of the root, which continues until the primary crown has lost its anchorage, becomes loose, and is finally exfoliated. In the meantime, the permanent tooth has moved occlusally so that when the primary tooth is lost, the permanent one is at the point of eruption and in proper position to succeed its predecessor.
Mandibular lateral incisors erupt very soon after the central incisors, often simultaneously. The maxillary central incisors" erupt next in chronological order, and maxillary lateral incisors make their appearance about 1 year later. The first premolars follow the maxillary laterals in sequence when the child is about 10 years old; the mandibular canines (cuspids) often appear at the same time. The second premolars follow during the next year, and then the maxillary canines follow. Usually, the second molars come in when the individual is about 12 years of age; they are posterior to the first molars and are commonly called 12-year molars.
The maxillary canines occasionally erupt along with the second molars, but in most instances of normal eruption, the canines precede them somewhat.
The third molars do not come in until age 17 or later. Considerable posterior jaw growth is required after age 12 to allow room for these teeth. Third molars are subject to many anomalies and variations of form. Insufficient jaw development for their accommodation complicates matters in the majority of cases. Individuals who have properly developed third molars in good alignment are very much in the minority. Third-molar anomalies and variations with the complications brought about by misalignment and subnormal jaw development comprise a subject too vast to be covered here. Figure 2-15 shows an anatomical specimen with a full complement of 32 teeth.
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