The Transition of the Posterior Teeth and the Emergence of the Second Permanent Molars—The Second Transitional Period
At approximately 10 years of age the first deciduous tooth in the posterior regions—usually a mandibular canine—sheds. This happens more than 1.5 years after the maxillary lateral incisor has emerged as the last of the permanent incisors and about one year after it has reached the level of the occlusal plane. In contrast to the situation for the incisors, a marked variation exists in the shedding sequence of the posterior teeth and the emergence of their successors. A deviating sequence of emergence normally does not occur in both anterior regions, as the localization of the permanent incisors prior to the beginning of eruption and the spatial conditions within the jaws impede this. A comparable impediment does not exist for the mandibular and maxillary posterior regions as the permanent teeth are further apart prior to emergence and no crowding exists within the jaws. An exception in this respect is the relation between the first premolar and permanent canine in the maxilla. During formation, these teeth are in close proximity to each other and overlap vertically. The already calcified distal corner of the permanent canine is directly adjacent to the forming mesial cervical region of the first premolar. Accordingly, the maxillary first premolar shows a concavity on the mesial surface at the cementoenamel junction (fossa canina). A comparable mesial concavity is absent, or present only in reduced form, at the maxillary second premolar whose crown morphology strongly resembles that of the maxillary first premolar. Conforming to the mutual relation between the maxillary permanent canine and first premolar described above, the premolar erupts first. As a rule, only when the spatial conditions are so ample that the situation is comparable to the one in the mandible can the maxillary permanent canine emerge prior to the first premolar. Under these conditions the maxillary first premolar does not act as an obstacle for the descent of the permanent canine.
The most frequently occurring sequence of emergence of the posterior permanent teeth with predecessors in the mandible is (1) canine, (2) first premolar, and (3) second premolar. In the maxilla it is (1) first premolar, (2) second premolar, and (3) canine.54 These sequences are applied in the design of Figure 6-1, which demonstrates the transition in the posterior regions and the emergence of the second permanent molars. The two teeth listed above as the first two to appear in each jaw may become visible in the mouth simultaneously or shortly after each other. In the mandible all types of variations may occur in the sequence of emergence of the permanent canine and the premolars. The same holds true for the maxilla with the restriction, indicated above, that as a rule the first premolar precedes the permanent canine in emergence. For frequencies in occurrence of the different emergence sequences46, 73, 87, 98 as well as for average emergence times44, 106 and other relevant numerical information, refer to Chapter 17.
Fig. 6-1 Survey of the transition of the posterior teeth and the eruption of the second permanent molars.
A The premolar crowns are located between the roots of their predecessors, which resorb in concert with the continuing eruption of the premolars. In the maxilla, the first premolar is nearest to the occlusal plane and the permanent canine is farthest away. The distal corner of the permanent canine is located in close proximity to the concavity at the mesial cementoenamel junction of the adjacent first premolar (fossa canina). A comparable concavity has not developed on the second premolars or has developed only slightly. In vertical direction, the second premolar is located between the first premolar and the permanent canine. Also in the lower jaw, the permanent canine is initially farther from the occlusal plane than the first premolar (see Fig. 4-1). However, both teeth are situated somewhat apart, unlike the situation in the upper jaw. Their mesiodistal angulation is about perpendicular to the occlusal plane, as is the case for the mandibular second premolar. The corresponding maxillary teeth are angulated slightly mesially.
In this drawing, the mandibular second deciduous molar has only a slightly larger mesiodistal crown dimension than its antagonist. Accordingly, the terminal plane shows a distinct mesial step. Hence, the first permanent molars intercuspate in maximal contact, and a solid sagittal occlusion exists.
The second permanent molars are distobuccally oriented in the maxilla and mesiolingually in the mandible, in conformity with the local spatial conditions in both jaws and the original location of the forming parts of these teeth.
In both jaws, the apices of the first permanent molars and the forming parts of the other permanent teeth are oriented in approximately one plane. Only the forming parts of the permanent canines, particularly in the maxilla, are located farther from the occlusal plane in accordance with the longer roots ultimately to be attained.
B Shortly after the mandibular deciduous canine is shed, its successor emerges as the first deciduous posterior tooth. In its eruption movement, the latter has passed the adjacent first premolar. Sufficient space is available in the dental arch for the emerging mandibular permanent canine to attain a good position. The extra space needed for the relatively large crown is partially provided by the diastema that was present distally to the deciduous canine. In the maxilla, the first premolar is further erupted than both adjacent permanent teeth. The first premolar has descended, lost contact with the permanent canine crown and more closely approached the crown of its predecessor.
C The first premolars in both jaws emerge shortly after the mandibular permanent canine did. They can attain a harmo/>
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