The Emergence of the First Permanent Molars and the Transition of the Incisors—The First Transitional Period
The first emerging tooth of the permanent dentition is usually the mandibular first molar, which appears in the mouth at about six years of age.* The preceding continuous increase in size of the jaw regions posteriorly to the deciduous molars provides the space needed for the addition of the first permanent molars to the dental arches. In both jaws the first permanent molars erupt more or less in a perpendicular orientation to the occlusal plane. This perpendicular orientation exists in the mesiodistal as well as in the buccolingual direction and is in accord with the position of the first permanent molars within the jaws prior to emergence. Frequently the mesial surface of the permanent first molar and the distal surface of the adjacent deciduous second molar are not in close proximity. A diastema between the two teeth present after full eruption of the former can be preserved for a few years.
The anteroposterior relation between two opposing permanent first molars after emergence depends on their positions previously occupied within the jaws, the sagittal relation between the mandible and maxilla, and the ratios of mesiodistal crown dimensions of the mandibular and maxillary deciduous molars. Regarding the latter, a distinct variation exists among individuals, particularly concerning the deciduous second molar. If the mesiodistal crown dimensions of the mandibular and maxillary deciduous second molars are about the same, the sagittal relation between corresponding mesial and distal surfaces will be similar. A mesial shoulder will be present anteriorly at the mesial surfaces and posteriorly at the distal surfaces of the teeth. The terminal plane will have a mesial step (Fig. 4-1A). If the mandibular second deciduous molar is considerably larger in mesiodistal crown dimension than the maxillary one—and this is often the case—then the distal surfaces of the two opposing second molars will lie more or less in one plane. Under these occlusal conditions, a flush terminal plane will be present (Fig. 4-1C). The consequences of the varying forms of terminal planes for the initial contact of the first permanent molars and the resulting occlusion are indicated in Figure 4-1. However, when deciduous molars are lost prematurely or the mesiodistal dimensions of their crowns are reduced by decay, other rules apply (Chapter 15).
Fig. 4-1 Influence of the mesiodistal crown dimensions of opposing second deciduous molars on the terminal plane of the deciduous dentition and on the position of the first permanent molars before and after emergence.
A Situation at about 4 years of age with opposing second deciduous molars of approximately the same mesiodistal crown dimensions. Under normal conditions, the terminal plane will have a mesial step. The sagittal relation between the adjacent, not yet erupted first permanent molars corresponds. The mandibular one is slightly more ventrally positioned than the maxillary one.
B Upon emergence at about 6 years of age, the first permanent molars may attain maximal intercuspation immediately. The sagittal relation between the mandibular and maxillary first permanent molars, conforms to the situation normally present in the dentition of a young adult, when all permanent teeth have attained occlusion.
C Developmental stage corresponding to the one shown in A. The mesiodistal crown dimension of the mandibular second deciduous molar is a few millimeters larger than that of the maxillary one. The opposing second deciduous molars occlude normally and demonstrate the associated mesial shoulder at their mesial surfaces. However, because of the excess in mesiodistal crown width of the mandibular second deciduous molar, the terminal plane is flush. The sagittal relation of the not yet erupted first permanent molars conforms; they are oriented in an end-to-end position.
D After emergence, the first permanent molars do not attain maximal contact. However, a more or less correct transverse relation is established because the large mesiopalatal cusp of the maxillary first permanent molar becomes guided between the buccal and lingual cusps of the mandibular one by means of the cone-funnel mechanism (see Fig. 2-2). The maxillary first permanent molar becomes slightly more buccally placed than its antagonist. In mesiodistal direction, the two teeth are in an end-to-end relation.
Not before the deciduous molars are replaced by the premolars can good sagittal relation and maximal contact and intercuspation of the first permanent molars be achieved. The mandibular first permanent molar is then able to migrate mesially over a larger distance than the maxillary one as the crowns of the opposing premolars have by and large the same mesiodistal crown dimensions. The difference of the deciduous molars in this respect provides the extra space needed for an improvement of the occlusion of the first permanent molars.
The transition of the mandibular and maxillary incisors is explained in Figure 4-2. In this illustration, the teeth have been drawn in one plane. Such a representation has a disadvantage in that it is difficult to gain a good impression of the spatial relations. The inclination of the erupting teeth and the relation between the forming permanent teeth and their predecessors are difficult to grasp. The connection between the tooth dimensions and the size of the associated region in both jaws is not detectable, and differences in size and shape of the lower and upper jaw do not appear. For the indispensable three-dimensional insight in the transition of the mandibular and maxillary incisors, reference is made to the Atlas of van der Linden and Duterloo.107
Fig. 4-2 Survey of the normal transition of the incisors by means of drawings representing the incisors and canines of both dentitions in one plane.
A Situation at approximately 5 years of age, prior to beginning of eruption of the permanent incisors. The differences in height of the permanent teeth have changed little since formation started. The incisal edges of the maxillary lateral permanent incisors are situated nearer the occlusal plane than those of the central ones. The distoincisal corners of the maxillary central permanent incisors are in contact with the mesial surfaces of the roots of the adjacent deciduous laterals. In the lower jaw, the incisal edges of the centrals are slightly nearer to the occlusal plane than those of the laterals. Here, the crowns of the permanent centrals are situated lingually to the roots of their predecessors and partly overlap the lingually located crowns of the permanent laterals. In both jaws, the amount of overlap of the central and lateral incisor crowns depends mainly on the relation between their mesiodistal crown dimensions on the one side and the space available within the jaws on the other side.5 The localization of the crowns of the forming canines determines the lateral demarcations of the space available for the permanent incisors.109
B The mandibular central deciduous incisors shed first. Some time later, their successors emerge. In their eruption movement, they pass the laterals. The contact between the distoincisal corners of the maxillary central permanent incisors and the mesial surfaces of the roots of the adjacent lateral deciduous incisors results in a distal displacement of the latter. The diastemata mesial to the maxillary deciduous canines reduce in width, and more space becomes available in the dental arch for the central permanent incisor crowns still to emerge. This increase in available space is needed as the crowns of the central permanent incisors have substantially larger mesiodistal crown dimensions than those of their predecessors.
The mandibular synchondrosis originally present at birth has disappeared; no remnants are left. The synostosis has been established and the previously morphologically distinguishable left and right mandibular sides form a solid bony unity. The left and right mandibular central permanent incisors can be positioned in close proximity before emergence, as is often the case.
Such a close proximity never exists in the ma/>