Chapter 7

Variations in Functioning

7.1 Introduction

Some of the functions operating in the facial region may vary in the manner in which they are provided. In particular, this applies to the air passages in breathing and in the way in which swallowing is performed. There can be differences, too, in the “at-rest” relationship of the tongue and lips with the teeth. Habits, such as thumb or finger sucking, can also be considered as variants of function.

7.2 Respiration

When respiration starts after birth, there is an airway adequate for the passage of air to the lungs. The air passage is held open through activity of muscles in the tongue and the walls of the pharynx and through posturing forward of the mandible. Normally, babies are able to breathe through their noses, although mouth breathing can develop in later years as a reaction to some kind of obstruction of the nasal passages and/or nasopharynx. Such an obstruction can be due to allergy, hypertrophy and inflammation of tonsils or adenoids, deviation of the nasal septum, enlarged conchae, and hypertrophy of the nasal mucous membrane. Enlargement of the adenoid tissue of the nasopharynx in children is frequently responsible for nasal obstruction. The adenoid tissue is clearly present after 6–12 months of age. It can then increase considerably, and by 2–3 years of age, nearly half the nasopharynx can be occupied by adenoid tissue. Normally, the adenoid tissue increases further. Before puberty is reached, it starts to reduce gradually. Usually, facial growth—specifically, increase in the distance between cranial base and palate—is sufficient to preserve an adequate air passage. If a real discrepancy develops, whether by abnormal increase in adenoid tissue, by reduction in rate of growth in posterior face height, or by a combination of both, then the passageway can become inadequate. A child in such circumstances would take to mouth breathing so as to reduce the difficulty of adequate respiration. A number of secondary effects would result from this. The lips would be held apart and the mandible would be held forward and downward. The soft palate would be lifted. The tongue would be held lower and more ventrally in the mandible and would no longer maintain contact with the palatal vault. These changes always take place and are independent of the cause of restriction of the airway. In cases where the impediment is of a permanent character, the changes in position of the above structures will be continuous. Where the restriction is of a temporary nature, such as with colds and allergies, the postural changes will generally be impermanent. But, it also can happen that such a situation does not revert to normal after the obstruction has gone, resulting in habitual mouth breathing.

A mouth-breathing habit of long duration, with the consequent changes just mentioned, might lead to the following modifications in the growth pattern of the face and in the natural posture of the head: the posterior margin of the palate moves relatively more downward; the lower facial height, particularly anteriorly, increases; the mandibular lower border runs at a steeper angle; the head is held tilted back as another means of increasing the airway.

Removal of the cause of the obstruction to breathing results in a reversal of those alterations.138 149 261 270 320 In Figure 7-1A, adenoids are illustrated increasing in extent; the differences in facial morphology between children with and without long-lasting obstruction by adenoids are described in Figure 7-1B.

7.3 Swallowing

The swallowing mechanism operates differently in newborns than in adults. When an infant swallows, the jaws do not come together; the tongue is kept between them. After the first deciduous molars come into contact, the child begins the passing to a “mature swallow,” a process that takes some months to achieve. The “mature swallow” generally involves overall tooth contact, with the tongue in touch with the anterior part of the palate, above and behind the incisors, and with only minimal contraction of the lips.194 195

There exists a great diversity in the way in which children swallow. Even one child exhibits variations in the way in which he swallows. Children between 3 and 10 years of age display many intermediate patterns of swallowing between the “infantile” and “mature” swallow. Adults are more likely to swallow according to a set pattern.164 229

After many years during which normal and abnormal swallowing patterns have been talked about, it is now felt more acceptable to consider the whole gamut of variations in swallowing to be normal. This conclusion assumes that humans swallow in the way best suited to them. As long as food does not enter the larynx, there is no reason to speak of abnormal swallowing, even if the tongue protrudes in the process. The principal cause for an infantile swallowing pattern persisting is probably related to the instinctive need to maintain patency of the airway when not swallowing. The bringing forward of the tongue and placing it between the dental arches is associated with that instinct. The resulting open bite is closed by the lips and tongue while swallowing, in a typical movement (Figs. 7-2C and D). This generally prevents the child from learning the proper swallowing action.

In certain conditions, impediments in the patency of the nasal passages can be relieved, without converting to mouth breathing, by adaptations of structures which, to a large extent, might correspond with the changes described above as being associated with mouth breathing. The tongue is placed between the dental arches in a comparable manner; however, the lips can remain closed. Thus, open bites are not always paired with mouth breathing.

Adult swallowing patterns are not uniform, either. When swallowing saliva or soft food, not everyone swallows with teeth in occlusion. However, the swallowing of a solid bolus, without placing the teeth together, is generally considered abnormal. Stabilizing the mandible by occluding appears to be unnecessary in swallowing fluids, but to be essential with solid food.57 183 237

7.4 Tongue and lip forces

In normal occlusions, the forces applied to the dental arches by the tongue are greater than those generated by the lips.145 In swallowing, greater forces are exerted than when no specific activity is undertaken. In the anterior part of the dental arches, Proffit gives values as norms for swallowing forces of 50 g/cm² for the force generated by the tongue, and 20 g/cm² for the force from the lips. At rest, the corresponding forces are 15 and 10 g/cm², which values by and large also apply to the posterior region.230 231

In increasing degrees, more weight has been given to the position of the tongue and lips at rest than in activity when swallowing, insofar as it is concerned with influencing the form of the dental arches and the morphology of the face. The forces exerted in swallowing are indeed about two to three times as great as at rest. The total length of time each day that these forces are active is, as a rule, hardly more than 10 minutes.71 Forces that operate for such short periods don’t produce tooth movement. This is unlike those which are more or less continuously evident, despite being only small in value.312 Analogous to the situation in swallowing, teeth are not displaced in response to the forces generated in speaking.

The most important forces determining the labiolingual position of the incisors are those developed by the tongue and lips at rest. The contact with adjoining teeth and antagonists makes its contribution, also. Moreover, other forces, such as those of eruption and those within the periodontal membrane, including the supra-alveolar fibers, are recognized as being of importance.231 As indicated previously, the interdigitation of the posterior teeth fulfills a specific function in adjusting the relative positions of the opposing teeth until they assume full occlusion. This is accomplished by the working of the “cone-funnel” mechanism after the first attainment of contact between antagonists.

7.5 Open bite

In the creation and maintenance of an open bite, it is unlikely that forces involved in speech and swallowing are of primary concern. The forces which the tongue (and lips) at rest between and against the teeth may exert along their long axes probably have the most influence (Figs. 7-2 and 7-3).