The influence of the tongue on dentofacial growth

In 1907, Edward Angle¹ said this about the tongue: “… there are as many variations as there are cases met with, resulting and corresponding with variations in the malocclusion”. At the time, these were recognized as wise words, and in 1918 A.P.Rogers² created a series of exercises intended to train the tongue and lips. Unfortunately, few clinicians, either then or since, have been very successful in influencing the action or position of the tongue and, possibly as a result, soft tissue training or Myofunctional Therapy, as it is currently referred to, is hardly used in current Orthodontic practice. It is not easy to understand why, as orthodontists both then and now accept that the teeth and alveolus are in a position of balance between the soft tissues. I think the probable reason is that training the soft tissues is not considered to be part of an orthodontist’s job and, in addition, it is tedious and unreliable. Might that attitude change?

In fact, there is much evidence to suggest that unusual function of the soft tissues in general, and the tongue in particular, are related to malocclusion. Rare cases of aglossia show major collapse of the dental arches and Harvold³ showed that the surgical removal of part of a monkey's tongue caused a related reduction of arch size. Malocclusion is frequently associated with unusual form or function of the soft tissues which may range from a simple sucking habit to a 'full fan' tongue thrust with only the posterior molars in contact. It seems that most of the speciality are of the opinion that the soft tissues respond to the shape of the hard tissues rather than the reverse, although there is little evidence to support this.

That a relationship does indeed exist between them can easily be verified by putting a blob of acid-etched resin on the buccal surface of an unopposed molar tooth and watching it move lingually. This alone must suggest that the oral soft tissues have the power to change alveolar form. Cases of muscular-dystrophy demonstrate bizarre mandibular shapes and, if the IX or XI nerves are damaged, major changes will be seen in the dental skeleton.

It is difficult to measure tongue function and almost impossible to measure its posture. Sadly, in today’s scientific world if something cannot be measured, it is at risk of being ignored. Why, we should ask, are orthodontists so sure that the tongue, which is 95% muscle, is not an important factor in malocclusion?

The issue was raised in the UK by Rix⁴ in 1946 who noticed that many patients with malocclusion had tongue-between-tooth swallows and several orthodontists including Ballard⁵ and Tully⁶ emphasized the influence of the tongue and lips. However despite this revival of interest, the topic of oral function and posture subsequently faded in the UK and elsewhere. Currently, I see few references to the tongue in The Angle Orthodontist, and often its influence does not appear to have been considered. Indeed the word ‘tongue’ rarely appears, even when discussing issues such as the inclination of the incisor teeth and open bites.

Research provides us with information about experimental situations and, on this basis, we form our theories and hypotheses. However, this information is always specific to the particular conditions of the research and cannot necessarily be extended to a wider population. To act on the basis of research, we have to use logic. For instance, there is good evidence to suggest that the faster you drive over a cross road, the less likely you are to hit another vehicle; however, you would be wise to apply some logic to the evidence before you act on it.

Denied significant research about tongue posture, can we apply logic to the miscellaneous information that does exist concerning the tongue? Several papers suggest that malocclusion is reduced when the mouth is closed^7–9 and I know of no evidence to the contrary. Equally, there is a lot of evidence to suggest that there is a higher ratio of malocclusion when the tongue is away from the palate, especially if it is postured between-the-teeth or low.^10–15 Often this is of the open bite or Class III type and, again, I know of no evidence to the contrary. I have never seen a case of anterior or lateral open bite where the tongue does not flow into the space. Clearly this is either coincidental or one facilitates or precipitates the other. It is commonly assumed that the tongue moves to fill a space created by adverse growth or defective eruption; however, I know of no evidence to confirm that either is causative.

On the evidence available, I think it is unsafe to ignore the influence of the tongue and there are many, including myself, who go to great efforts to train the tongue to contact the palate both at rest and during swallowing. Myofunctional therapy has yet to prove itself as a viable technique and many orthodontists will need more evidence before they will accept it. Others will see it for what it is: an intermittently successful way of improving soft tissue posture. That it often fails is probably due to lack of technique rather than failure of principle and, as techniques improve, its need is likely to become more widely recognized.

Over time, clinical experience works on the minds of enthusiastic young clinicians, many of whom grow to realize that the soft tissues are in ultimate control. For instance, most orthodontists think of Charles Tweed as a ‘four on the floor’ man, dedicated to mechanics. This is why he was severely criticized in his time by many members of the American Association of Orthodontists. He was an active student in Angle’s school, constantly creating new ideas, often contradicting Angle himself. Not many people know that, in his later years, he rejected much of his own previous work. He decided that early treatment was essential and only accepted mixed dentition cases in his practice. Not long before he died, he pronounced, “Knowledge will gradually replace harsh mechanics, and in the not-too-distant future the vast majority of orthodontic treatment will be carried out during the mixed dentition period of growth and development prior to the difficult age of adolescence.” Has that time come yet?