Changes in the Maxillary Incisor Region Associated With the Transition of Maxillary Posterior Teeth*
As has been explained in the preceding chapters, a marked difference exists between the two jaws in the size and shape of the anterior and middle sections of the apical areas. The differences in tooth sizes, tooth positions before eruption, emergence sequences, and directions of eruption are also reflected in the changes in the maxillary incisor region associated with the transition of the maxillary posterior teeth. The combination of differences between the two jaws results in, among other things, the fact that in general the maxillary canine and incisors do not move distally in the dental arch and do not occupy some of the leeway space that becomes available, as is frequently seen in the mandible.
The relation between the size and shape of the anterior and middle sections of the apical area is discussed. The orientation of and the relations between the teeth involved before and after transition of the posterior teeth are described. Three patterns of changes in the incisor region associated with the transition of the posterior teeth are differentiated on the basis of the spatial conditions in the anterior region before the transition of the posterior teeth. In addition, the changes are related to the conditions in the canine-premolar region.
The combined anterior and middle sections of the apical area in the maxilla are considerably smaller than those for the mandible. This is understandable because both the anterior and middle sections of the apical areas are each smaller in the maxilla than in the mandible. The relatively larger maxillary permanent incisor and canine crowns lead temporarily to closer packing in the maxillary anterior section of the apical area before emergence than in the mandible. Later, the relatively small anterior section of the apical area in the maxilla will be sufficiently large to house the apices of the incisors. Further, the latter teeth are more labially inclined than mandibular incisors. As in the mandible, the permanent canine occupies a key position in the demarcation between the anterior and middle sections of the apical area. Its initial location lateral and adjacent to the piriform aperture is later reflected by the location of its apex. The distance by which the apex descends in the direction of the occlusal plane will depend on the amount of vertical growth of the face. As in the mandible, a large anterior section of the apical area is not always associated with a large middle section. The same holds true for a small section.
The variation in size of the anterior section of the apical area is restricted mainly in the transverse direction, and that of the middle section in the sagittal direction. The demarcation between the two (i.e., where the permanent canine crown is in close proximity to the lateral incisor root) is the region most susceptible to variations. The close proximity of the permanent canine and first premolar crowns before eruption complicates matters further. Both aspects play a special role in the changes in its position, inclination, and angulation that the lateral incisor undergoes when the canine erupts.
The size and anteroposterior position of the anterior segment of the dental arch as it is reflected by the perimeter of the incisal edges of the incisors and canines are affected less by the size and shape of the anterior and middle sections of the apical area in the maxilla than is the case in the mandible. As such the dental arch parameters play a more important role in the maxilla than in the mandible in relation to the changes in the incisor region associated with the transition of the posterior teeth.
The position of the maxillary incisors can vary markedly depending on the size of the anterior section of the apical area, local conditions in the dental arch, and influence of internal and external factors. The space available between their crowns can be considerable. However, contact can also exist and crowding can be present in different degrees. A comparable variation exists in the space available for the incisor roots; they can be far apart or close together. The permanent canine can overlap the root of the lateral incisor to a large extent or only slightly or not at all. In most instances the canine buccally overlaps the first premolar crown. Depending on the size of the middle section of the apical area this overlapping can be larger, smaller, or absent altogether. The second premolar crown does not contact the first premolar and is usually at some distance from the surface of the mesial root of the first permanent molar.
The changes experienced by the maxillary lateral permanent incisor in association with the eruption and emergence of the permanent canine have been discussed mainly in the previous chapter. The changes in position of the lateral incisor involve not only rotations and labiolingual movements, but also a mesial migration that will lead to establishing contact between the lateral and central incisor if that contact does not yet exist. The central diastema will become reduced and ultimately the central incisors will contact each other.
When diastemata are small there is only limited space available for the mesial migration of the lateral incisor. Particularly when diastemata are lacking, the lateral incisor may rotate and overlap the central incisor. When the displacement of the lateral incisor is limited the canine will tend to erupt more buccally. The abnormal buccal emergence of the maxillary permanent canine occurs usually when the space between the first premolar and lateral incisor is too small. The possibility of displacement of the two adjacent teeth is mostly limited and a canine that is excluded from the arch will result.
In general, the maxillary canine will emerge after the two premolars have replaced their predecessors. Some distal migration of the premolars may take place when not enough space exists for the erupting canine. Extra space will not usually be provided by the development of crowding in the premolar region; instead, the incisors will be moved closer together and crowding may occur or increase in this region. This difference between the premolar and the incisor region is probably related to three factors. First, the morphology of the incisor crowns, unlike the premolar crowns, facilitates the occurrence of overlapping and crowding. Second, the morphology of the incisor roots and their location within the jaw allow rotations to occur more readily. Third, occlusal interferences are less rigid for the maxillary incisors than for the premolars. It would appear that in critical conditions the incisor alignment will be the first to be sacrificed. Only in extreme conditions will the posterior region have to adapt. The latter is less likely to be the case in the mandible, probably because the limited transverse range of variation between the cortical plates does restrict the amount of possible variation.
Symmetry in the size and shape of the middle section of the apical area usually exists in the mandible and maxilla. However, the position of the permanent canine crown can vary. The maxillary c/>