Changes in the Mandibular Incisor Region Associated With the Transition of Mandibular Posterior Teeth *
Replacement of the deciduous canines and molars by their successors influences the position of the teeth distally and mesially of them. The first and second permanent molars will migrate more mesially. The extent of this migration depends on the alterations that take place during the transition. The changes in position of the incisors are more complex and thus justify separate discussion.
The relation between the size and shape of the anterior and the middle section of the apical areas is discussed. The orientation of and relations between the five most mesial permanent teeth before and after transition of the posterior teeth are described. Three patterns of changes in the mandibular incisor region associated with transition of the posterior teeth are presented. These patterns are distinguished on the basis of the spatial conditions in the anterior region before the transition of the posterior teeth. In addition, the changes involved are analysed with reference to the conditions that can be distinguished in the canine premolar region.
The morphogenesis of the mandible and its subsequent growth, together with the position of the forming teeth, determine the shape and size of the apical area. The correlation of the apical area with the size of the deciduous and permanent teeth is limited. It seems that the ultimate size and shape of the apical area is determined by control mechanisms other than the size of the teeth.
The site where the permanent canines form determines the demarcation of the anterior and middle sections of the apical area. In further development this role is taken by the mesial side of the crown of the permanent canine before emergence and by that of its root and apex subsequently. In situations with anterior and middle sections of the apical area of standard size, a relative mesial position of the canine crown before emergence will consequently lead to a small anterior section of the apical area and a large middle section. A relatively distal position of the canine will result in the reversed situation. Generally, a large anterior section of the apical area is associated with a large middle section. However, a small anterior section of the apical area can occur in combination with a large middle section and the reverse.
The position of the mandibular incisors at the end of the first transitional period can vary considerably, as has been indicated in Chapter 2. The local conditions in the anterior segment and the manner of transition of the posterior teeth determine how the position of the incisors will alter in that respect. The position of the not-yet-emerged permanent canine and its relation to the adjacent lateral incisor are crucial. However, the relation between the canine and the first premolar is important too. This also applies to the position of the second premolar and the first and second permanent molars. Whether the extra space that becomes available with the transition of the posterior teeth is used for an improvement of the position of the incisors also depends on the sequence of emergence of the canine, both premolars, and the second permanent molar.
In the mandible the incisors are most susceptible to influences within and outside the dental arch. Rotations present at emergence usually correct subsequently, whenever space is available, as a result of pressures exerted by the tongue and lips. However, mandibular incisors seem to be the most vulnerable teeth regarding increases in malposition, crowding, and overlapping. This holds true for the development of the dentition and also applies to postadolescent changes in the position of teeth. With increasing age mandibular incisors that were originally perfectly aligned may begin to overlap. Existing crowded conditions can worsen. The combination of (1) the large susceptibility to influences originating within and outside the dental arch, (2) the relatively fragile contact points, and (3) the room within the jaws that allows the teeth to approximate to each other and to rotate leads to these undesirable changes in the position of the mandibular incisors.
Crowded conditions in the anterior region can improve by utilising the space that becomes available with the transition of the posterior teeth. The contacts between the incisors and the contact that will develop between the lateral incisor and permanent canine are essential for changes associated with the transition of the posterior teeth. Favourable contacts can lead to considerable improvement; unfavourable ones lead to a worsening of the situation. A mesially angulated and erupting permanent canine can emerge mesially in the dental arch, labial to the lateral incisor. The lateral incisor cannot move labially, and its lingual position is maintained during the continuing eruption of the canine. Particularly in instances of small leeway space, the canine will be limited in its possible distal movement, leading to a continuation of the existing situation. The contact between the canine and the premolar is less important because the cross section of the the premolar is round, not flat. This applies even more to the contact between the two premolars, because the contact is formed by two rounded structures. The round section of the premolar crowns is, however, responsible for the fact that premolars emerged in a rotated position will not be corrected by pressures exerted by the tongue and buccal musculature. Occasionally, occlusal contacts can improve initially rotated premolars to some extent.
Symmetry exists in the size and shape of both middle sections of the apical area. However, mandibular permanent canine crowns are not always formed in corresponding locations on the left and right sides. Their relation to the premolars and already emerged incisors will vary accordingly. In addition, positions of the incisors frequently are not symmetrical. Changes in the left and right side of the incisor region, usually evoked by the transition of the posterior teeth, are not independent of each other. Both sides interact in that respect.
The changes are divided into three patterns, based on differences in the space available in the incisor region. Special attention will be paid to the contact between the permanent canine and the lateral incisor. In addition, the influence of the early and late emergence of the second permanent molar will be dealt with.
Changes in the mandibular incisor region associated with the transition of the posterior teeth will be treated on the basis of the conditions present in the canine premolar region.
Pattern A is characterized by excess space available in the anterior segment of the dental arch (Figs. 4-1 and 4-2). More space is available for the four permanent incisors than is needed. Diastemata are present. The mandibular canine emerges without contacting the lateral incisor, the position of which is not affected. The middle section of the apical area and the leeway space are large. The premolars can emerge without spatial limitations. Diastemata between the permanent teeth anterior of the first permanent molar will be present initially; they gradually become reduced and in most instan/>