03 - Equilibrium Theory and the Etiology of Malocclusion

Equilibrium Theory

Learning Objectives

Function influences form and vice versa, but the extent to which function contributes to the etiology of malocclusion remains controversial. Certainly it plays a role in many cases, and in at least some it is a major cause.

Function influences the form of the dental arches by changing the pattern of soft tissue pressures, affecting the equilibrium of forces against the teeth. The focus of this program is on potential environmental influences in the development of malocclusion. The environment of the teeth, in this context, is the soft tissues that surround them. You need to understand the role of potential etiologic factors that affect soft tissue pressures on the dentition and the pattern of jaw growth, with a special emphasis on what we know about the magnitude and duration of pressures against the dentition that can affect the position of the teeth.

Evidence for Equilibrium Effects on the Teeth

We already have reviewed the evidence that at least 50% of malocclusions are related to non-inherited influences, which means that the environment of the dentition is a factor in the etiology of the majority of malocclusions. Exactly what is meant by environmental influences?

In this context, we can say that the environment affects the developing dentition by changing the balance of natural-occurring pressures against the teeth.

These diagrams illustrates a simplified view of that balance of pressures or equilibrium, showing the incisors are in a position of balance between the front of the tongue and the lips (image 1), and the posterior teeth balanced between the side of the tongue and the cheeks (image 2). Equilibrium theory says that the teeth are located in a position of balance among the opposing forces that are brought to bear on them.

From the fact that teeth remain in stable positions most of the time although they are subjected to forces, and yet can be moved when additional forces (as from orthodontic appliances) are applied, we know that there is indeed an equilibrium. The question is “What defines the equilibrium?” in the context of both the magnitude and the duration of forces against the teeth.

Image 1: Tongue and lip pressures contributing to equilibrium.	Image 2: Tongue and cheek pressures contributing to equilibrium.

Evidence for Equilibrium Effects on the Teeth (cont.)

Looking at a child like this one, you can see how malocclusion might arise if, for instance, a patient thrusts his tongue forward constantly, increasing the pressure by the tongue against the incisors and pushing those teeth forward.

Could it be possible that in fact pressure against the teeth, created by normal or abnormal function, is a major contributor to malocclusion?

Pressure Against the Teeth During Chewing

Whatever your dental alignment and occlusion is at this moment, it’s quite stable—that is, nothing much is going to change in the near future unless you undergo orthodontic treatment. But the teeth are subject to heavy force during mastication. Biting down often puts tens of kilograms of force against the teeth. If a tooth is not quite in the right position, why doesn’t the force against it during chewing move it to the right place, or displace it even further? The force is more than enough to move a tooth. Why doesn’t it move?

The answer provides an important insight into the equilibrium conditions. Teeth do move during chewing, not because they are displaced within the periodontal ligament space, but because the alveolar bone bends. The fluid within the PDL acts like a shock absorber on an automobile. When you bite down, in the short term (seconds) the fluid is incompressible, the force is transmitted to the bone, and the bone bends. If the force is maintained more than a few seconds, the fluid begins to be squeezed out and the PDL begins to be compressed. That hurts, and you open up again to remove the force.

Why don’t you chew a tooth to a new place in the dental arch? Because the force isn’t maintained long enough to produce tooth movement.

Duration of Force / Pressure

The current concept is that the duration of any force against a tooth or teeth (which produces pressure within the periodontal ligament) must be at least 4 hours per day and perhaps a bit more than that, if it is to have any impact on the position of the teeth within the dental arches and thereby change the dental occlusion so that it could produce malocclusion.

There are two lines of evidence to support this concept. Some of the best evidence is derived from experiments that orthodontists run all the time, perhaps experiments in which you yourself participated during your own orthodontic treatment.

Orthodontists often use removable appliances—headgears, removable plates with springs (like the one shown here being used to bring central incisors together), retainers—and of course any appliance that can be removed will be. Some patients wear removable appliances faithfully, and others wear them very little or not at all.

How much does a patient have to wear a removable appliance in order for it to produce tooth movement? The answer is “Not all the time, but at least 4 to 8 hours per day.”

Image 1: Closure of a midline diastema can be accomplished with a removable appliance with fingersprings activated to move the incisors together.	Image 2: Occlusal view, with springs activated to bring the teeth together.
Image 3: How many hours per day did the patient have to wear the appliance to get this result? 12 hours per day would be enough for sure; the minimum would be 4-8 hours; and if the appliance was worn nearly full time, the tooth movement would be faster.

Duration of Force / Pressure (cont.)

A plot of tooth movement, as a function of the number of hours per day that a removable appliance is worn, looks something like this.

Wearing a removable appliance nearly all the time results in tooth movement that is very similar to full-time force from a fixed appliance. If the patient wears the appliance half the time, there’s less response but the appliance still works.

Note that there’s a threshold, which occurs in humans somewhere between 4 and 8 hours. The concept that force must be applied for several hours per day in order to produce a change in tooth position has well been established.

Duration of Force / Pressure (cont.)

The second line of evidence for a duration threshold has come from animal experiments, largely carried out by Davidovitch and co-workers.

In the experimental preparation they used, it is possible to monitor cellular activity and to follow the cascade of chemical changes that lead to remodeling of the alveolar bone around a tooth and tooth movement.

In experiments of this type, it can be observed that after about 4 hours, the “second messengers” that trigger the cellular changes necessary for tooth movement begin to appear.

If it takes 4 hours or so to induce the formation of cyclic AMP and other second messengers, it seems reasonable to presume that this establishes the duration threshold, and of course this result coincides rather nicely with what has been observed in human patients wearing removable appliances.

Habits?

Now let’s consider habits or other behavior patterns as influences on the position of teeth. Can you affect the position of the teeth with a habit like thumb sucking? It’s apparent that you can—but it’s a common observation that some children who suck their thumb have much more displacement of their teeth than others who seem to suck in the same way. How could you explain that?

That’s right, it’s probably how long (hours/day) Susie sucks her thumb, not how hard she sucks when doing it. It would make a lot of difference whether or not the thumb was in the mouth all night.

The same thinking would apply to other habits or behaviors. They would have an effect on the dentition only if their duration exceeded the 4-6 hour minimum. Could the pressure on your teeth generated while playing your musical instrument (trumpet, saxophone, flute, violin, etc.) displace them? Yes—but you’d have to practice and perform a lot more than the average person to get that effect. It’s interesting that some professional musicians do show an effect on the shape of their dental arches from their long hours playing the instrument. Even for them the effects often are subtle.

Duration vs Force / Pressure in Orthodontic Tooth Movement

It looks as if the duration of force against the teeth is a critical variable in whether it affects tooth position, and therefore could be an etiologic agent for malocclusion. How much force does it take to move a tooth, if it’s maintained long enough?

The answer is “not very much”. Even a force of just a few grams can lead to tooth movement if it’s maintained long enough. A plot of tooth movement versus pressure in the periodontal ligament looks like this figure. If there is a threshhold, it’s very low. As pressure increases, the rate of tooth movement also increases, up to a point. Above that, tooth movement occurs at about the same rate as pressure increases, up to and beyond the point of pain.

Duration vs Force / Pressure

That leads to an important concept: for orthodontic tooth movement or for environmental influences on the developing dentition, the duration of pressure is more important than its magnitude. Almost any pressure, maintained for enough hours per day, can affect the dentition, and above a certain level, it doesn’t matter how high the pressure is.

Tongue vs Lip / Cheek Pressures: Tongue Thrust?

Measurement of Tongue and Lip Pressures

We have seen that force against the teeth during chewing can be ignored in equilibrium calculations, because it’s too heavy to be tolerated for more than a few seconds. After that it exceeds the shock-absorbing capacity of the periodontal ligament. We also have seen that habits, if continued for a long enough time, could displace teeth. What about pressures against the teeth from tongue and lip / cheek function?

We can measure tongue and lip pressures using miniature pressure-measuring devices (called transducers) of the type shown in this image. The active elements in the device, used here to measure lip pressues against the lower teeth, are the strain gauges that you see in the incisor and canine regions. Thin lead wires extend out the corner of the mouth to the recording equipment. Devices of this type can be placed immediately in front of and behind the lower incisors so that tongue and lip pressures can be measured simultaneously.

The fact that the measuring devices extend labially or lingually from the teeth is a potential problem, but experiments suggest that if the measuring device can be kept within 2 mm of the tooth surface, major errors are avoided.

Tongue vs Lip / Cheek Pressure: Equilibrium?

It is interesting that when the data from direct pressure measurements are examined, it is very difficult to demonstrate a balance of tongue and lip pressures.

In fact, almost never is there any indication that the tongue-lip/cheek pressures really are balanced as our original diagram suggested. For instance, if the pressures are measured during swallow, the tongue pushes forward to create a pressure of about 50 gm/cm2 against the upper and lower incisors, while the lip pushes back with a pressure of only 20 gm/cm2.

If pressure by the side of the tongue and the cheek against the molar teeth is examined, both tongue pressure and lip pressure are significantly greater, but the ratio is about the same, i.e. tongue pressure during swallowing is 2-3 times greater than the corresponding lip or cheek pressure.

Does that mean that there’s no equilibrium? No, because if any object (like a tooth) is subjected to pressures (which it is, as you chew, swallow, whatever) but does not move, there is an equilibrium.

So we have not been careful enough in considering what determines the equilibrium. It appears that simple measurements of pressures during one type of oral function (like swallowing) are not enough—there must be other components of the equilibrium.

Tongue vs Lip / Cheek Pressure: Equilibrium? (cont.)

Tongue and lip pressures against the teeth also occur during speech and when a person is sitting quietly at rest. Pressure during speech lasts only a fraction of a second for most sounds. For instance, every time you say / th /, your tongue presses against your upper incisors with a force of about 40 grams, but for only 0.1 second.

In contrast, pressure created by the drape of the lips and cheeks against the teeth, or pressure created by the tongue resting against the teeth, is quite small, in the range of 5-10 grams. But these resting pressures last for hours.

Equilibrium: Resting Pressures Only?

It is easy to see that the dentition is designed to withstand forces of short duration, such as the heavy forces encountered during chewing, and of course that would enable the teeth to withstand other short-duration forces like those from swallowing and speaking.

What would happen if we looked for balance just in the resting pressures against the teeth? That’s difficult because the magnitude of resting pressures is small and the patient must be still and relaxed in order to measure them, but it can be done.

Equilibrium: Resting Pressures Only? (cont.)

Interestingly, the resting pressures do not balance either. At some places, as for example the lower incisors, resting tongue pressure tends to be greater than resting lip pressure, often by a ratio of 1.5:1.

Resting tongue pressure also is greater against lower molars than is the balancing cheek pressure, by about the same ratio.

But in other locations, as for instance on the lingual of upper molars or canines, there is little or no resting tongue pressure, at least while the patient is awake, and resting cheek pressure is higher than tongue pressure.

Even if only the long duration pressures are considered, therefore, it is very difficult to demonstrate a balance between tongue and lip / cheek pressures.

But we know there is an equilibrium. The only conclusion would be that there is more to the balance of pressures than just tongue and lip / cheek pressure. What could the other elements be?

Equilibrium Components

Let’s examine again the possible contributors to equilibrium that we have considered up to this point.

What could we add to this chart?

Equilibrium Components (cont.)

Force that can move teeth also is generated within the periodontal ligament by the mechanism that produces eruption. Erupting teeth move, and the force that moves them is developed within the periodontal ligament. Furthermore, eruption can occur at any age, which means that force continues to be produced within the PDL after eruption apparently has stopped. For example, if a lower molar is extracted and the upper molar does not have an antagonist, the upper molar will begin to erupt again, even in patients in their 40’s and 50’s.

Equilibrium in the Eruption of Teeth

In Level 1, you already learned that as teeth erupt, light pressures of long duration opposing eruption (those from the soft tissues at rest) affect the amount of eruption, while heavy pressures (those from chewing or clenching the teeth together) do not.

The vertical equilibrium, in short, is like the equilibrium in other planes of space: the duration of pressure opposing the eruption of a tooth is more important than its magnitude.

Periodontal Ligament Contributions to Equilibrium

The present view is that force generated within the periodontal ligament, by the same mechanism that produces tooth eruption, also plays a role in stabilizing the teeth in other planes of space, and that this is the missing element in the equilibrium equation.

Dentists often observe that teeth in adults begin to drift to new locations when periodontal breakdown occurs. Perhaps that happens because stabilization by the periodontal ligament is lost. There has been no direct experimental demonstration of that, however, and so it must be labeled at this time as a plausible theory.

How much force can the eruption mechanism produce? That’s really difficult to measure, but it’s clear that it’s quite small, probably less than 5 grams. So how much of an imbalance in tongue vs lip / cheek pressures could PDL stabilization overcome? That’s right, you could think that if resting lip pressure was enough greater than 5 grams or so, the teeth would move toward the tongue until the opposing resting lingual pressure was close enough to balance for the PDL to be effective.

This concept of active stabilization by the periodontal ligament does explain why sometimes it appears that there is a threshhold for orthodontic force (5 grams or so) below which the tooth doesn’t respond, and why sometimes it appears there is a lower or no threshold. Do you understand that?

That’s right, if the PDL’s stabilizing capacity was not being used, you’d have to add some light force to overcome it before tooth movement began. But if all the stabilizing capacity was being used, adding even a little more force in the direction that was being opposed by the PDL would result in tooth movement.

Habits? Tongue Thrust?

Tongue vs Lip / Cheek Pressure: Measurement

In our discussion of known causes of malocclusion, we have already discussed thumbsucking as an etiologic agent. Equilibrium considerations make it easier to understand why some children create a major effect on their teeth with a sucking habit while others do not, even though they seem to be doing the same thing.

It’s not how hard you suck, it’s how long—the same effect of duration being more important than magnitude.

Many children clench and grind their teeth. Whether this is a habit—an extraneous and learned activity—or a developmental stage is not understood as well as we’d like. A pertinent question in a discussion of equilibrium, however, would be: “Can you decrease the amount of tooth eruption with a clenching / grinding habit?”

The answer would be:

“Clenching, maybe, if you succeeded in having enough hours with pressure against the teeth”. More than four hours of clenching would be needed, and it would be hard to reach that level.

“Grinding, no. You’d never get hours of pressure during grinding the teeth.” Enamel would be worn away, but eruption wouldn’t be affected.

Tongue Thrust Swallow

One oral activity that has been labeled as a cause of malocclusion for a long time is “tongue thrust swallow”.

Tongue thrust swallow receives confusing labels in the dental literature, but usually it is defined as extension of the tip of the tongue between the incisor teeth during swallowing. When the lips are pulled apart as a child swallows, this can be seen—and almost always, in such a child, there is a space between the incisors, with excessive overjet and/or open bite. It’s easy to jump to the conclusion that the tongue thrust is the cause of this malocclusion.

In young children (up to age 2-4), it’s normal to place the tongue tip against the lower lip during swallow. All children do it at those ages, and the percentage who are labeled as tongue thrusters declines with advancing age. About half have what could be called a tongue thrust swallow at the time they begin school.

The first misconception about tongue thrust swallow is that it’s a habit—an extraneous learned activity. It’s not. Instead, it’s a normal developmental stage, one that may be retained after it should have been replaced by the next stage. The concept that an activity is a habit leads immediately to the thought, “We need to break that habit”. That’s not the way to start thinking about tongue thrust swallow.

Prevalence of Malocclusion in Tongue Thrusters

Look again in the graph on this screen at the data for the prevalence of tongue thrust swallow in a reasonably typical American population, and note how the number of children with a tongue thrust compares to the number with anterior open bite. Tongue thrust swallow is about 10 times as prevalent as the malocclusion it is supposed to cause! If it’s an etiologic agent, it’s not a very potent one.

We would conclude that the epidemiologic data support what you would expect from consideration of equilibrium theory. It’s not the swallow—the duration isn’t right—and it doesn’t look like there’s much of a tongue posture effect either.

Note the prevalence of open bite and thumbsucking in this group of children. Open bite also is less prevalent than thumbsucking, which we know can cause it. How would you explain that? Thumbsucking wouldn’t make a difference either, unless you had a thumb in your mouth long enough each day, and it appears that a lot of children don’t.

Finally, note the difference in open bite between black and white children. That is a reflection of inherited skeletal proportions. Open bite is more prevalent in those of African descent; deep bite (not shown on this chart because nobody relates that to how you swallow) is more prevalent in those of European descent.

Tongue Thrust Habit?

Could you displace your incisors with a tongue thrust swallow? The duration of a single swallow is about 1 second. You swallow a few hundred times per day. Even if the shock absorber effect in the periodontal ligament didn’t totally accommodate this activity by the tongue, you’d be nowhere close to 4-6 hour duration needed for tooth movement.

So tongue thrust is not a habit, and it shouldn’t affect tooth position anyway. Does that mean that a child with a tongue thrust swallow could have no equilibrium effects on the teeth? Not necessarily. Although the swallow pattern is irrelevant, if a child like that had a different resting posture of the tongue, there could be a long enough duration of resting pressure to make a difference.

It does mean that trying to break a “tongue thrust habit” or teach a child to swallow differently would not be an effective way to remove a potential cause of malocclusion. In fact, it’s almost impossible to teach a child to swallow differently—and it wouldn’t make any difference if you did, unless resting posture also changed.

Effects from Speech?

Could you displace your incisors with tongue pressure during speech? Some children with a speech problem do show a lot of tongue during speech, and sometimes the speech problem is blamed for the development of a malocclusion. With a duration of tongue contacts during speech of only 0.1 second, nobody could talk that much.

Again, this doesn’t mean that an underlying adaptation in tongue posture would make no difference. It does mean that tongue posture, leading to resting pressure, would be the important thing in etiology. What you do with your tongue as you move it rapidly around makes no difference to the dentition. Where you position it at rest and while asleep could be a different matter.

The bottom line, however, is that lessons on how to swallow or speech therapy would be ineffective as a way to prevent or correct malocclusion. Some speech therapists do offer to help out the dentist and correct or prevent malocclusion, by correcting poor tongue position related to speech or swallowing. That doesn’t work clinically, and from what you now know about equilibrium, you wouldn’t expect it to.

Effects on Posture: Mouth Breathing?

Posture as a Determinant of Resting Pressure

So far in our examination of equilibrium we have established that only pressures of long duration should be important in affecting the equilibrium situation, and have shown that this theoretical prediction seems to be borne out rather well by clinical experience.

But resting pressures could be important, and they seem to be. What determines resting pressures? The answer to that would be, more than anything else, the posture of the head, jaw and tongue. Like everybody else, you have a characteristic posture of your head—and an equally characteristic, though harder to observe, posture of your mandible and tongue.

Moving the head, or the jaw, or the tongue to a different location and keeping it there, would change the pattern of resting pressures of both the tongue and the lips.

Influences on Posture: Respiration

We could assume, therefore, that influences on posture might well change a normal pattern of development to an abnormal one leading to malocclusion.

And we know that a major influence on posture of the head, jaws, and tongue is respiration. Respiration has a very high physiologic priority. If you can’t breathe, very quickly it does not matter what else you could have done if you had survived.

The influence of respiration on posture goes back literally to the first moments of life after birth. When you are delivered into this world, you have only a few minutes (not more than 4 or 5) to take the first breath of life. To do that, you have to open up an airway, which requires:

• lifting the head, so that it isn’t held down against the chest as it was throughout intra-uterine life
• bringing the mandible down and forward
• bringing the tongue forward

The First Breath of Life

In a series of experiments in the early 1960s that will never be repeated because of concern now about the effects of even small amounts of radiation, Bosma made x-ray movies of the first breath of life of a series of infants. That required taking the infant as it emerged from the birth canal and going quickly to the cine-radiographic machine that was in the delivery room.

His x-ray movies clearly show the sequence of events that are necessary to open up the airway so that the newborn can breathe. In this view of a single image from the movie and a tracing from it of the airway, you can see the head elevation that is part of opening the airway so the new-born child can breathe.

Influence of Respiratory Mode on Posture

Once you have established an airway, of course you must maintain it for the rest of your life. It’s easy to demonstrate to yourself that posture of the head, jaw and tongue are adjusted to meet respiratory needs.

Try this experiment right now: pinch your nostrils shut and hold it so you can’t breathe through your nose. Keep holding it. After 30 seconds or so, you will have an irresistible urge to tilt your head up and move your mandible away from the maxilla, which also drops your tongue down. You’re opening up an oral airway so you can breathe through your mouth as long as your nose is blocked. The figure with this screen shows the results of an experiment with dental students, whose head posture was measured while their nose was blocked. Note that when the obstruction was removed, head posture immediately went back to what it was before the experiment.

Interestingly, newborn babies can’t breathe through their mouth. It takes a few weeks to develop that ability. So a totally blocked nose for a newborn is a medical emergency.

The bottom line, however, is that from the first minutes of life, respiratory needs are a major determinant of head, jaw and tongue posture.

Does Respiratory Mode Affect Facial Growth?

Could the way you breathe affect the way your jaws grow and the way your dental arches develop, and thereby be an etiologic agent for malocclusion?

The answer is, “Yes, it could be”.

When we speak of “mode of respiration” we are referring to the extent to which an individual breathes through the nose as opposed to the mouth. Nasal respiration is a mode of respiration, and so is mouth breathing.

The current concept is that the mode of respiration can be an etiologic factor in malocclusion, because changing the respiration mode affects posture, and thereby changes resting pressure. Let’s examine the two parts of that concept individually, starting with whether the mode of respiration makes a difference. Then we’ll consider how it makes a difference.

Adenoid Facies

For over 100 years, there have been descriptions of the “adenoid facies” (image 1) in the dental literature. The concept was that if you have large adenoids, you can’t breathe well through your nose, therefore have to breathe through your mouth, and this will give you a long narrow face.

The strongest evidence to support the idea that nasal obstruction by enlarged adenoids can produce something like the adenoid facies (and an associated malocclusion) comes from observations in Sweden by Linder-Aronson. He studied children who required surgery for medical reasons to remove enlarged adenoids that presumably were at least partially blocking the airway and causing an increased proportion of mouthbreathing. Often tonsils were removed as well.

What Linder-Aronson demonstrated was that on the average, children who required adenoidectomy had longer faces than normal children, because the mandible was rotated downward and backward. In the computer-generated composite superimposition of the normal and T&A children (image 2), you can see the different facial proportions. The average difference was not large, but it was statistically significant, and it fits with the adenoid facies concept.

Image 1: Adenoid facies: long, narrow face—usually accompanied by anterior open bite.	Image 2: Computer-generated composites of children with and without adenoidectomy-note the downward-backward of the rotation in the adenoidectomy group

Growth After Adenoidectomy

In this graph, also from Linder-Aronson, you can see that after the adenoids were removed, the mandibular plane angle, which decreases during normal growth, decreased more in the children who had this surgery. There was a tendency for these children to return toward the facial proportions of the normal group, although the recovery was not complete.

A weakness in the study is that nasal obstruction was presumed, not measured. One other group of researchers who repeated a similar study had the same findings. Another group were unable to replicate the result. But it does seem reasonable that some degree of nasal obstruction and increased mouth breathing was present in the children who needed T&A.

Nasal vs. Oral Respiration

One of the things that makes it difficult to evaluate the impact of respiratory mode on dentofacial development is that all humans breathe partially through the mouth.

You may think that you are a 100% nasal breather. If you do, run up four flights of steps, and recheck your mode of breathing. Even serious mental concentration, something we hope is occurring while you study this material, can increase oxygen consumption to the point that nasal breathing cannot totally supply it, and then a switch to partial mouth breathing occurs. At maximum effort in a person with no nasal obstruction, the air flow is about 50% through the nose, 50% through the mouth.

Probably what counts in the effect on growth of oral vs. nasal breathing is how many hours per day a high percentage of oral breathing is maintained. In the laboratory, it is possible to measure the percentage of oral breathing while physical activity is minimal—as it would be at rest and during sleep, when long-duration soft tissue pressures would be most likely to affect both growth and tooth positions.

This graph shows the percentage of a group of adolescent subjects with normal facial proportions and a long-face group, as a function of their nasal-oral ratio during a period of time in the laboratory when total air flow and nasal air flow were measured. The difference would be the amount of oral air flow. Note three things: (1) one-third of the long face group had low or very low nasal-oral ratios, i.e., were largely mouth breathers; (2) one third of the long face group had high ratios, i.e., were largely nasal breathers; and (3) the majority of the group with normal facial proportions, but by no means all, were largely nasal breathers. It looks as if impaired nasal respiration may be a risk factor for growth in the long face pattern, but does not seem to be its major determinant.

Severity of Nasal Obstruction

There is no doubt that total nasal obstruction, which is very rare in humans, can lead to severe malocclusion. The striking downward-backward rotation of the mandible shown in this cephalometric superimposition shows what happened in a patient with cleft palate and leakage of air into the nose during speech. A pharyngeal flap was placed surgically to reduce the nasal leakage, and it inadvertently completely blocked off the nose.

We have every reason to believe that if partial nasal obstruction is severe enough, there also can be an impact on growth. What we do not know is how severe the obstruction must be to significantly alter growth, and how much effect could be expected from varying degrees of partial obstruction.

Detection of Mouth Breathing

Often it is presumed that individuals who have their lips separated at rest are breathing through their mouth. If you look like a mouth breather, are you one? Not necessarily, because the rear of the oral cavity can be closed off with the tongue, and 100% nasal breathing is quite possible when the lips are separated. You can’t tell who’s breathing through their mouth just by looking at them.

The best way to characterize the mode of respiration is in terms of the nasal-oral ratio. This requires measuring the amount of air that passes through the nose as a percentage of the total air flow. Both of these things can be measured, using a body plethysmograph that fits around the chest for total air flow, and a flow meter over the nose for nasal air flow.

In a study at UNC, the percentage of nasal respiration in long-face vs. normal adolescents was measured using the nasal mask / body plethysmograph technique. In this graph of the results, note that one quarter of the long face group had less than 40% nasal respiration, while none of the normals had nasal respiration percentages that low. But you also can see that most of the long face group were predominantly nasal breathers. The data suggest that impaired nasal respiration may contribute to the development of long face / open bite problems, but this is not the sole or even the major cause.

Influence of Respiratory Mode

The second part of the concept that respiratory mode can be a factor in the etiology of malocclusion is that it acts largely by changing head, jaw and tongue posture, and this leads to changes in resting pressures against the dentition. Does that really happen?

Experiments with monkeys have shown that placing an obstruction in the roof of the mouth, so that tongue and jaw posture must be altered, has an effect on both how the jaw grows and on the form of the dental arches. Also in monkeys, it has been demonstrated that blocking the nose so that the monkey has to breathe through the mouth leads to a change in the jaw relationship and affects tooth positions.

In humans, such experiments are not possible, but if posture changes, resting pressures against the teeth would also change. We are left with the same question: how great a change in posture would be needed to generate a significant change in resting pressures? On the average, the postural change in Linder-Aronson’s adenoidectomy patients was quite small. All the data suggest that in the more severely affected individuals, changes would be expected, while there would be little or no effect in the less severely affected ones. How do you determine who’s severely affected enough? We don’t know.

Should a dentist suggest tonsillectomy and adenoidectomy for the snotty-nosed kid with a long face and anterior open bite? When the best data say that nasal obstruction is not the major cause of the facial growth pattern, that wouldn’t be a good idea.

Summary

Summary: Equilibrium Concepts

When environmental influences on the developing dentition are considered, the key concept is that the duration of pressure against the teeth is much more important than its magnitude. Examining possible contributors to pressure from this perspective shows that resting pressure and a contribution from metabolic activity in the periodontal ligament are the key items.

Summary: Tongue Thrust?

On clinical examination it can seem obvious that a tongue thrust swallow habit creates protrusion of incisors and/or anterior open bite. The problems with that:

• tongue thrust swallow isn’t a habit, it’s a normal developmental stage that may be maintained into the primary school years or later
• tongue pressure during swallow isn’t maintained long enough to affect the position of the teeth
• therapy (usually called myofunctional therapy) to break this “habit” or teach a child to swallow “correctly” doesn’t work

The resting posture of the tongue may be different in children with a tongue thrust swallow. If this tongue posture is necessary in order for the child to breathe, trying to change it isn’t going to work either.

Summary: Mouth Breathing?

Total nasal obstruction that is maintained all the time (which is very rare in humans) can lead to marked changes in the pattern of growth and severe malocclusion.

The long face, open bite (adenoid facies) type of malocclusion is:

• more prevalent in adolescents with a low percentage of nasal respiration
• found most often in adolescents with normal percentages of nasal respiration
• best considered as possibly due to impaired nasal respiration but only in a minority of the long face children

Recommending tonsillectomy and adenoidectomy primarily to improve the pattern of facial growth rarely is a good idea.

Referral to Self-Test

Before you take the self-test, be sure to do the assigned reading for this module: pages 133-145 in the 5th ed. of Contemporary Orthodontics; pages 145-158 in the 4th ed. Then use the self-test to be sure you have understood this important topic, and be prepared to discuss it with your seminar leader.

Self-Test

Question 1

The best evidence is that environmental factors explain at least what percentage of malocclusion?

1. <10%
2. 25%
3. 50% ✓
4. 75%

5. 90%

Correct

That’s right, twin studies indicate that no more than 50% of malocclusion can be attributed to heredity, so at least 50% of malocclusion must be attributed to environmental causes, and it may be greater.

Incorrect

No, that’s not the best answer. Twin studies indicate that no more than 50% of malocclusion can be attributed to heredity, so at least 50% of malocclusion must be attributed to environmental causes, and it may be greater.

Question 2

(A) Under most circumstances, the teeth are in a position of equilibrium because (B) tongue and lip pressures against the teeth are in balance.

1. A true, B true, A and B related
2. A true, B true, A and B not related
3. A true, B false ✓
4. A false, B true
5. A and B false

Correct

That’s right, the first statement is true even though the second one is false. The teeth are in a position of equilibrium, but the equilibrium isn’t defined just by tongue and lip forces.

Incorrect

No, that’s wrong. The first statement is true even though the second one is false. The teeth are in a position of equilibrium, but the equilibrium isn’t defined just by tongue and lip forces.

Question 3

During swallow, how does tongue pressure against the incisors usually relate to lip pressure?

1. 2-3 times greater ✓
2. about 60% greater
3. about the same
4. about 50% less
5. 2-3 times less

Correct

That’s correct. Tongue pressure against the incisors during swallowing usually is 2-3 times greater than lip pressure, so they don’t balance.

Incorrect

No, that’s wrong. Tongue pressure against the incisors during swallowing usually is 2-3 times greater than lip pressure, so they don’t balance.

Question 4

(A) The dental equilibrium is explained by a balance of resting tongue and lip pressures against the teeth because (B) the dental apparatus is designed to resist forces of short duration.

1. A true, B true, A and B related
2. A true, B true, A and B not related
3. A true, B false
4. A false, B true ✓
5. A and B false

Correct

That’s correct, the first statement is false but the second one is true. Although only resting pressures have a long enough duration to affect the equilibrium, they don’t balance, and so don’t explain the equilibrium.

Incorrect

No, that’s incorrect. The first statement is false but the second one is true. Although only resting pressures have a long enough duration to affect the equilibrium, they don’t balance, and so don’t explain the equilibrium.

Question 5

What is the threshold for force duration to produce tooth movement?

1. <1 hour
2. 4-8 hours ✓
3. about 12 hours
4. 18-20 hours

Correct

That’s correct. The duration threshold is somewhere between 4 and 8 hours, probably at about 6 hours.

Incorrect

No, that’s incorrect. The duration threshold is somewhere between 4 and 8 hours, probably at about 6 hours.

Question 6

(A) Tongue thrust swallow has little or no effect on the position of the teeth because (B) the duration of the pressure during swallowing is too short to have an effect.

1. A true, B true, A and B related ✓
2. A true, B true, A and B not related
3. A true, B false
4. A false, B true
5. A and B false

Correct

That’s correct. The statements are true and related. Tongue thrust swallow doesn’t push the teeth around because the pressure during each swallow lasts for only one second, so hundreds of swallows per day produce only a few minutes of pressure.

Incorrect

That’s wrong. the statements are true and related. Tongue thrust swallow doesn’t push the teeth around because the pressure during each swallow lasts for only one second, so hundreds of swallows per day produce only a few minutes of pressure.

Question 7

(A) The mode of respiration can be an etiologic factor in malocclusion because (B) respiratory mode affects posture and thereby changes resting pressure.

1. A true, B true, A and B related ✓
2. A true, B true, A and B not related
3. A true, B false
4. A false, B true
5. A and B false

Correct

That’s right, the statements are true and related. Respiratory mode can be significant for malocclusion, and almost certainly its’ because posture and thereby resting pressures are affected.

Incorrect

That’s wrong. The statements are true and related. Respiratory mode can be significant for malocclusion, and almost certainly it’s because posture and thereby resting pressures are affected.

Question 8

To what extent do you have to be a mouthbreather instead of a nasal breather to produce significant effects on your dental occlusion?

1. 10-20%
2. 20-50%
3. 50-75%

4. 75%

5. nobody knows ✓

Correct

That’s right, although we know there’s a relationship between the percentage of nasal vs oral respiration and effects on malocclusion, at this point nobody knows what the threshold percentage is.

Incorrect

That’s either wrong or a lucky guess. Although we know there’s a relationship between the percentage of nasal vs oral respiration and effects on malocclusion, at this point nobody knows what the threshold percentage is.

Question 9

What happens to head posture when the nose is abruptly closed off?

1. head tilts up
2. mandible drops down
3. tongue assumes lower posture
4. 1 only
5. 1 and 2
6. 1 and 3
7. 2 and 3
8. 1, 2, and 3 ✓

Correct

That’s right, When the nose is abruptly closed off, head posture changes as the head tilts up and the jaw goes down, and tongue posture changes too.

Incorrect

No, that’s wrong. When the nose is abruptly closed off, head posture changes as the head tilts up and the jaw goes down, and tongue posture changes too.

Question 10

(A) Long face adults have about the same biting force as normal children because (B) the long face grop fail to gain biting strength during adolescence.

1. A true, B true, A and B related ✓
2. A true, B true, A and B not related
3. A true, B false
4. A false, B true
5. A and B false

Correct

That’s right, the statements are true and related. Why the long face group fail to gain strength in the masticatory muscles at puberty remains a mystery, but the long face condition can be recognized before muscle strength becomes abnormal.

Incorrect

No, that’s wrong, the statements are true and related. Why the long face group fail to gain strength in the masticatory muscles at puberty remains a mystery, but the long face condition can be recognized before muscle strength becomes abnormal.

Question 11

(A) Measuring biting force is an important diagnostic procedure for children with growth problems because (B) bite force is the major determinant of how much face height the patient will develop.

1. A true, B true, A and B related
2. A true, B true, A and B not related
3. A true, B false
4. A false, B true
5. A and B false ✓

Correct

That’s right, both statements are false. Measuring bite force isn’t an important diagnostic procedure, because its role in determining fact height is modest at best.

Incorrect

No, that’s wrong, both statements are false. Measuring bite force isn’t an important diagnostic procedure, because its role in determining fact height is modest at best.