The process of the formation of the teeth and their eruption to the functional position in the mouth involve a period of early development in a cellular state, followed by a period of formation of the hard tissue of the crown, and then by a period of root formation which generally coincide with the eruption of the tooth.
A consideration is made of the way on which these processes in the individual teeth are timed so as to permit the proper arrangement of the dentition both before and after eruption.
Although the small jaws of the child must accommodate the member of both dentition from the fetal life until about 7 year this is possible because of the following factors:
1. While the crowns of the deciduous teeth are forming in the jaws the permanent teeth are at the tooth germ stage.
2. By the time the crowns of permanent teeth are forming the deciduous teeth are erupted or erupting.
3. Since there is a period of about six years between the appearance of each of the permanent molars this allows sufficient time for jaw growth to accommodate the lengthening of the dental arch.
Schematic representation of mandibular growth, showing the fetal
Mandible superimposed upon the adult mandible.
During the period of eruption of deciduous teeth and their establishment as the functioning dentition the crowns of the successional permanent teeth, lying in their crypts, are forming. The 1st permanent molar start to calcify earlier, about the time of birth, before the beginning of the eruptive tooth movement of the deciduous teeth. The 2nd permanent molar begins to calcify towards the end of the 3rd year; and the 3rd molar at around 9 years.
Dental age 6 month
After the crowns of permanent teeth are formed there is an interval of quiescence, until with the commencement of eruption tooth formation again becomes active. For, as in the case of the deciduous teeth, root formation does not properly begin until the teeth start their movement towards the mouth cavity.
The roots of the permanent teeth are completed two to three years after the tooth first appears in the mouth.
While the crown of the permanent incisors and canines lie unerupted in the jaws they are situated lingual to the roots of their deciduous predecessors. The crowns of the canines, and particularly of the upper canine, are placed at a deeper level than those of the incisors. Moreover, the crowns of the lateral incisors are generally in a more lingual position than the other anterior teeth. This facilitates the packing of the crowns of the permanent teeth into the jaws beneath the much smaller deciduous incisors and canines.
The crowns of the permanent incisors (gray) lie lingual to the crowns of the primary incisors (yellow), particularly in the case of the maxillary laterals. Arrows point to the primate spaces
The developing premolar crowns are at first to be found between the roots of their respective deciduous predecessors, the deciduous molars, but by six years of age they lie at deeper level in the jaws relative to the deciduous molars, and beneath the apices of the molar.
Dental age is determined from three characteristics. The first is which teeth have erupted. The second and third, which are closely related, are the amount of resorption of the roots of primary teeth and the amount of development of the permanent teeth.
The permanent teeth tend to erupt in groups, and it is less important to know the most common eruption sequence than to know the expected timing of these eruption stages. The stages are used in the calculation of dental age, which is particularly important during the mixed dentition years.
The mesio-buccal cusp of the 1st permanent molar is calcified. The developing tooth germ of the permanent incisors, canine, and pre molars lie to the lingual side of the developing deciduous teeth in the same alveoli but they have not been begun to undergo calcification. In the upper jaw the developing teeth lie close beneath the orbital cavity, and the maxillary sinus lies on their inner side but has not yet grown out word over the teeth.
Dental age 1 year
The occlusal surface of the 1st permanent molar is complete. Calcification has commenced in the upper permanent central incisor, the lower incisors, and the canine, and just about to begin in the upper lateral incisor. The ramus of mandible increase greatly in height and width and attached to the body of mandible with a more acute angle.
Dental age 3 years
The crowns of permanent incisors are almost complete; that the canine about two-third completed; and calcification has just commenced in the premolars, which are embraced by the roots of the deciduous molars. The crown of the 1st permanent molars is virtually complete.
Dental age 6 years
The first stage of eruption of the permanent teeth, the transition from the primary to the permanent dentition at dental age 6, is illustrated in Figure below. The most common eruption sequence is the eruption of the mandibular central incisor, closely followed by the mandibular first permanent molar and the maxillary first permanent molar. These teeth normally erupt at so nearly the same time, however, that it is quite within normal variation for the first molars to slightly precede the mandibular central incisors or vice versa.
Usually, the mandibular molar will precede the maxillary molar. The beginning eruption of this group of teeth characterizes dental age 6.
Dental age 6 years
Dental age 7 years
In the second stage of eruption at dental age 7, the maxillary central incisors and the mandibular lateral incisors erupt. The maxillary central incisor is usually a year behind the mandibular central incisor, but erupts simultaneously with the mandibular lateral incisor. At dental age 7, root formation of the maxillary lateral incisor is well advanced, but it is still about 1 year from eruption, while the canines and premolars are still in the stage of crown completion or just at the beginning of root formation.
Dental age 8 years
Dental age 8 is characterized by the eruption of the maxillary lateral incisors. After these teeth come into the arch, there is a delay of 2 to 3 years before any further permanent teeth appear.
Dental age 8 years
Dental age 9, 10 years
Since no teeth are erupting at that time, dental ages’ 9 and 10 must be distinguished by the extent of resorption of the primary canines and premolars and the extent of root development of their permanent successors. At dental age 9, the primary canines, first molars, and second molars are present.
Approximately one third of the root of the mandibular canine and the mandibular first premolar is completed. Root development is just beginning, if it has started at all, on the mandibular second premolar. In the maxillary arch, root development has begun on the first premolar but is just beginning, if it is present at all, on both the canine and the second premolar.
Dental age 10 is characterized by a greater amount of both root resorption of the primary canines and molars, and root development of their permanent successors. At dental age 10, approximately one half of the roots of the mandibular canine and mandibular first premolar have been completed; nearly half the root of the upper first premolar is complete; and there is significant root development of the mandibular second premolar, maxillary canine, and maxillary second premolar.
Teeth usually emerge when three fourths of their roots are completed.24 Thus a signal that a tooth should be appearing in the mouth is root development approaching this level. It takes 2 to 3 years for roots to be completed after a tooth has erupted into occlusion. Another indicator of dental age 10, therefore, would be
completion of the roots of the mandibular incisor teeth and near completion of the roots of the maxillary laterals. By dental age 11, the roots of all incisors and first permanent molars should be well completed.
dental age 9 years
Dental age 11 years
Dental age 11 is characterized by the eruption of another group of teeth: the mandibular canine, mandibular first premolar, and maxillary first premolar, which all erupt more or less simultaneously. In the mandibular arch, the mandibular canine most often appears just ahead of the first premolar, but the similarity in the time of eruption, not the most frequent sequence, is the important point. In the maxillary arch, on the other hand, the first premolar usually erupts well ahead of the canine. At dental age 11, the only remaining primary teeth are the maxillary canine and second molar, and the mandibular second molar.
Dental age 11 years
Dental age 12 years
At dental age 12, the remaining succedaneous permanent teeth erupt. Succedaneous refers to permanent teeth that replace primary predecessors; thus a canine is a succedaneous tooth, whereas a first molar is not. In addition, at age 12 the second permanent molars in both arches are nearing eruption. The succedaneous teeth complete their eruption before the emergence of the second molars in most but by no means all normal children. Although mineralization often begins later, it is usually possible to note the early beginnings of the third molars by age 12.
dental age 12 years
Dental age 13, 14, 15 years
Dental ages 13, 14, and 15 are characterized by the extent of completion of the roots of permanent teeth. By dental age 15, if a t h i rd molar is going to form, it should be apparent on the radiographs, and the roots of all other permanent teeth should be complete.
Dental age 15 years
Like all other developmental ages, dental age correlates with chronologic age but the correlation for dental age is one of the weakest. In other words, the teeth erupt with a considerable degree of variability from the chronologic age standards.
It remains true, however, that the teeth erupt in the stages described above. A child who has precocious dental development might have the mandibular central incisors and first molars erupt at age 5 and could reach dental age 12 by chronologic age 10. A child with slow dental development might not reach dental age 12 until chronologic age 14.
A change in the sequence of eruption is a much more reliable sign of a disturbance in normal development than a generalized delay or acceleration. The more a tooth deviates from its expected position in the sequence, the greater the likelihood of some sort of problem. For example, a delay in eruption of maxillary canines to age 14 is within normal variation if the second premolars are also delayed, but if the second premolars have erupted at age 12 and the canines have not, something is probably wrong.