Factors Influencing the Development of the Dentition
The development of the dentition is closely associated with the morphology and growth of the face and the way in which the functions of the orofacial region are exercised.
Purposely, the morphology and growth of the face have been omitted as much as was reasonably possible here.* However, without further explanation it will be clear that the size of the two jaws, their anteroposterior relation, the vertical dimensions of the mandible and the maxilla, as well as the changes occurring during growth in these components play an essential role in the development of the dentition.
The functions of the orofacial region can be classified as those exclusively related to the dentition and face (chewing and partial shaping of the facial configuration) and as those that have a broader character, as respiration, speech and deglutition. Chewing, shaping of the face, and speech will not be discussed here. A few remarks will be made regarding respiration and deglutition.
A restricted passage of the nose usually involves continuous mouth breathing or alternate periods of nose and mouth breathing. However, mouth breathing also can be based on an abnormal habit, while nasal passage is adequate. To attain a good functional relation between the tongue and the lips and cheeks, a closed mouth is a first prerequisite. In non-closed mouth situations, not only the lip seal is lacking, but the tongue and lips cannot fulfill the role that they have to play in maintaining a good relation between the individual teeth, particularly in the incisor region. In the review of open bites (Chapter 13), the effects of abnormal tongue position were discussed. Some comments are made here regarding the role that the tongue and lips have to play in maintaining the position of the mandibular and maxillary incisors.
The position of the incisors is not only determined by the mutual contacts between the pertinent mandibular and maxillary teeth. The tongue and lips have a special function in this respect, as explained in Figures 16.1 and 16.2, together with the relevancy of the dental contacts. The tongue and the buccal musculature not only play a role in the position of the teeth in the incisor region, they are also of importance in the determination and maintenance of the position and buccolingual inclination of the posterior teeth. In comparison with the anterior region, the influence of the tongue and buccal musculature is relatively small for the posterior teeth, where the intercuspation of the premolars and molars and particularly the transverse interdigitation are more essential in this respect.
Fig. 16-1 Variations in the vertical and sagittal support of the maxillary permanent incisors in normal and abnormal situations.
A In a Class I condition and normal sagittal and vertical relation in the anterior region, the maxillary incisors are supported vertically by the mandibular incisors on the lingual side and by the lower lip on the labial side. Further, the lower lip also lies against part of the labial surface of the maxillary incisors. The middle of the lipline (stomion) is situated about 1–3 mm superiorly to the incisal edge of the maxillary central incisors. The upper lip exerts a function in preventing the labial displacement of the maxillary incisors. The lower lip also plays a role in this respect, albeit a less important one.