Abnormalities in the Dental Arches
A number of abnormalities that may occur in the dental arches are treated in this chapter. These abnormalities are not discussed in terms of their developmental characteristics. This treatise is restricted to the description of various deviations in the dental arches, supported by systematic drawings.
In view of the intricate processes typical for the development of the dentition, the occurrence of abnormalities in the dental arches is not astonishing. On the contrary, it is amazing that the developmental processes so often come to a good end and that relatively few deviations evolve.
The most frequently occurring abnormality in the dental arches is a discrepancy between the needed and the available dental arch perimeter. Such a discrepancy means that the space available for the teeth within a dental arch is either too small or too large to contain all teeth harmoniously and in contact with each other. These incongruities are indicated by the term “Arch Length Discrepancy” (ALD). This term leaves undecided whether the incongruity is caused by an excess of one component or a shortage of the other. The same applies to the terms “crowding” and “spacing.”*
Crowding rarely occurs in the complete deciduous dentition, but appears frequently in later phases of the development of the dentition. In particular, crowding in the mandibular incisor canine region is rather common in the adult dentition.
Spacing is normally present in the deciduous dentition but rather infrequently in the permanent one. In the latter, it occurs primarily in the maxillary incisor canine region. Specifically, space in the median plane in the permanent dentition (a central diastema) is often perceived as disturbing.
Concerning the individual teeth, abnormalities in the development of the dentition can be categorized as deviations in number, size, shape, and position.
Either too many or too few teeth can be formed. Deviations in the number of teeth in the deciduous dentition are rare; they are regularly encountered in the permanent dentition. Absence of teeth based on nonformation (agenesis) occurs quite often. The sequence of diminishing frequency in agenesis is3: third molars (16%), the mandibular second premolars (4.4%), the maxillary lateral incisors (1.7%), and the maxillary second premolars (1.6%). Formation of other permanent teeth can fail (see Fig. 9-1) and more than one tooth can be missing in one individual (multiple agenesis). For statistical information on agenesis and other topics presented here, refer to Chapter 17.
Fig. 9-1 Examples of the absence of teeth because of nonformation (agenesis).
A The third molar (16%) is most frequently not formed; next the mandibular second premolar (4.4%).
B The maxillary lateral incisor is the tooth most frequently not formed in the maxilla (1.7%).
C The maxillary second premolar is occasionally missing (1.6%).
D An example of multiple agenesis.
Supernumerary teeth can be encountered everywhere in the dental arches and particularly in the mandibular and maxillary incisor regions. Most frequently, a supernumerary tooth occurs between the two maxillary central incisors adjacent to the median maxillary suture (mesiodens). Such a tooth often deviates in size and shape and can be oriented in all possible directions. Mesiodentes can be present two-fold or plural (Fig. 9-2).