BITES AND BODY POSTURE
SUMMARY - Bites affect body posture, because the position into which the interdigitation of the teeth forces the mandible for bracing at least hundreds of times a day controls mandibular posture, and mandibular posture an integral component of head posture. Furthermore, the posture of the head affects how the spine aligns beneath it to support the weight on top. However, the mandible is the only postural component routinely left out of postural analysis, because dentists have never appreciated the orthopedic role of the bite, and orthopedists have never known how to manage joint surfaces composed of jagged rocks that are so delicately suspended their sockets that they have individual vascular pulses and so extensively wired with protective neuromuscular reflexes that they occupy an oversized portion of the brain. As a result, we allow developing malocclusions to produce progressively displaced jaw postures, which are bound to produce progressive displacement of spinal posture (scoliosis), even while straightening the teeth for esthetics. We need to understand how bites affect body posture so we can incorporate bite treatment into postural treatment.
To understand the relationship between bites and body posture requires seeing how they evolved together. Lloyd DrBrul used to comment, "To understand the teeth, you have to look at the feet." In mammals, the teeth operate collectively to form a structural platform on which the mandible exercises in chewing and braces for stability, and neuromuscular reflexes program the jaw muscles to hold the mandible in a postural position just beneath the center of that bite platform. In carnivores and simians, the canines project beyond the bite platform, where their contacts impact the mandible anteriorly and and protect against posterior posterior forces. In hominids, the protection of the vital areas behind the mandible by the simian canines was reduced to allow a broader range of mandibular movement, leaving those vital areas protected only by mandibular bracing, and the reshaping of the cranium necessary for upright posture made the postures of the mandible and the cranium inversely related in a sagittal plane. In modern humans, the softening of our diet has redirected our facial growth downward and backward. Canine contacts impact the mandible posteriorly instead of anteriorly. The posterior shift of average mandibular posture may be an important cause of the accompanying shift of average head posture anteriorly. In many people, the mandible has also shifted to one side, which tips the head to the same side as the shift of the mandible due to increased ipsilateral temporalis muscle tonus. Dentists can help other health care professionals restore postural health by extending or shifting the mandibular bracing position in conjunction with synergistic efforts to improve head and body posture.
THE MANDIBLE - is an integral component of the head posture mechanism, but researchers have been unable to establish any workable connection between dental parameters and posture parameters. It's been known since 1928 that extending the head retrudes the mandible, and flexing the head protrudes the mandible.1. In animals, experimentally changing the bite changes the curve of the spine.2 In humans, even minute changes in the bite can cause immediate changes in postural muscle activity and head posture.3 A literature review found mandibular position associated with the spine in 266 publications, head posture in 216 publications, pelvic tilt in 53 publications, and leg-length discrepancies in 35 publications.4 In growing rabbits, experimentally altering the plane of the bite table can bend the whole craniofacial skeleton.(Sergi H., and Farmand M.:Experiments with unilateral bite planes in rabbits. Angle Orthod vol 45 #2 p 108-114). However, the only bite parameters that have been correlated with postural parameters is general maxillo-mandibular relationship (class 1, 2, or 3 malocclusion) with cervical spine angle. Class 3 bites are associated with more flexed head postures, and class 2 bites (also incisor crowding) are associated with more extended head postures.
The reason for our failure to correlate bites and body posture is that the parameters we use to measure bites tell us nothing about their orthopedic features. Dentists compare bites by measuring static details of the way the teeth fit together and some details of how they slide together, but those details reveal very little information about where the teeth position the mandible relative to the cranial base and the cervical vertebrae or the cross-sectional area of the pharyngeal airway. To understand the role of the bite in body posture requires first understanding the importance of mandibular bracing.
MANDIBULAR BRACING – occurs when the powerful jaw closing muscles immobilize the mandible by clamping it up forcefully against the underside of the cranium. Mandibular bracing served a critical protective role during our evolution, because the vital inner ear structures in mammals are located just behind the mandibular condyles, and a blow to the prominent chin could drive the condyles into these delicate structures like little hammers. Bracing the mandible protected the inner ears from such blows. In carnivores and monkeys, the posteriorly facing surfaces of the lower canines fit against the anteriorly facing surfaces of the upper canines to brace the mandible against posteriorly directed forces. Also, the canines protruded beyond the bite plane formed by the rest of the teeth in order to extend the protection against posteriorly directed forces to the mandible's postural position. In hominids, the canines withdrew into the bite plane to increase adaptability by removing restrictions to the mandibular range of movement, which left the vital areas behind the mandibular condyles protected only by neuromuscular reflexes. These neuromuscular reflexes brace the mandible as soon as danger is sensed. For example, in a motor vehicle accident, you'll clench your teeth before your foot hits the brakes.
Mandibular bracing also serves a stabilizing role in postural dynamics. The braced mandible functions as a handle by which the postural muscles on the front of the body (the anterior kinetic chain) can pull down on the front of the cranium without applying forces to the face, where the sensory systems could not operate effectively if enclosed by bone thick enough for anchoring postural muscles. Therefore, to prevent postural muscle forces from reaching the face, they are absorbed by the long rigid mandible, which transfers them around to the sides of the head, where the zygomatic arches and temporal fossae provide large areas of bone with sufficient structural support for strong muscle attachments.
REFLEX NEUROMUSCULAR CONTROL OF MANDIBULAR POSTURE - Mandibular bracing was so important during evolution that our jaw muscles are programmed to always hold our mandibles in a resting posture just beneath its central bracing platform to ensure fast easy access to mandibular bracing. The distance by which the mandible rests below the central bracing platform (its vertical dimension) depends on the resting tonus of the jaw closing muscles. It may rest several millimeters below the central bracing platform when the jaw closing muscles are relaxed or pushed almost all the way up against it when central nervous system tension increases. Nonetheless, the horizontal dimension of the mandible in its resting posture is a function of the horizontal location of the mandible in its central bracing platform.
This accommodation of mandibular posture to the location of its central bracing platform has been demonstrated in many studies. An immediate increase in freeway space (the space between the teeth when the jaw is at rest in its postural position) follows the first bite contact after placing a bite raising appliance, an immediate return to the pre-treatment freeway space follows the first bite contact after removing the bite raising appliance, and shortening the face surgically or prosthodontically leads quickly to a new mandibular resting posture that maintains the pre-treatment freeway space.5 Children with a unilateral cross-bite that shifts the mandibular bracing position laterally show a parallel shift of mandibular posture, maintained by increased tonus in the ipsilateral posterior temporalis muscle, which normalizes after correction of the cross-bite.6-9 Monkeys fitted with crowns that force the mandible to brace in a laterally displaced position undergo a lateral shift of mandibular posture,10 and monkeys fitted with crowns that force the mandible to brace in an anterior position undergo an immediate increase in the tonus of the superior lateral pterygoid muscles to hold the mandible anteriorly.11-12
MYOFASCIAL CONTROL OF MANDIBULAR POSTURE - There is also another influence on mandibular posture - a network of cooperative background tensions that maintain the habitual upright postural stance with a light steady muscle tonus of about one percent of maximum voluntary contractile force. The resulting equilibrium (tensegrity) holds each bone in a resting place, called a neutral zone, balanced between opposing tensions. Neutral zone forces create fields of tension on the surfaces of the bone that either resorb bone or deposit new bone until the surfaces fit the neutral zone forces. If a bone is surgically moved away from its neutral zone, these opposing forces act to move it back in, requiring metal fasteners to stabilize many orthopedic corrections. The mandible rests in a postural neutral zone within the curtain of muscles draping down from the front of the cranium onto the clavicles and shoulder girdle. At least it would rest there if there were no teeth involved.
MANDIBULAR POSTURAL STRAIN - If the mandibular posture determined by the resting myofascial tensions of the habitual upright stance is not coincident with the mandibular posture determined by the neuromuscular reflexes that program it to fit just below the bite table, a strain is shared between the jaw and postural systems, and the symptoms often switch back and forth between jaw and neck or back muscles. Treatment that relieves symptoms in one set of muscles usually just transfers them to the other set of muscles. Permanently relieving the source of the strain requires correcting the jaw and postural systems together, which requires understanding how they are connected, which requires understanding how they became connected in the first place. UPRIGHT BODY POSTURE - The mandible became incorporated into the posture mechanism when hominids went upright. The spines of quadrupeds are supported at front and back like a suspension bridge - with the skull hanging from the well supported shoulders and the mandible hanging from the skull. The forward ends of the food and air channels, hanging in sequence from the front of that bridge, are easily kept separated and scarcely affected by movements of the head. The neck muscles assist in mouth opening and closing by extending and flexing the head.
Uprighting a quadruped required revamping the skeleton. The spine developed alternating curves. The foot acquired a concave arch. The rib cage flattened. The pelvis acquired a concave front surface to support the abdomen resting on it and a thick bony crest for enlarged buttock muscles to help balance the weight over the legs. The cranium had to be reshaped. The weight of the face shifted backward by hollowing out the facial bones and moving them posteriorly until they ran into the airway.15 The connection between the cranium and the spine moved to under the center of mass of the cranium.
FIGURE 1 - RESHAPING THE CRANIUM
APE HOMINID
This reshaping of the cranium made room for expansion of the brain on its back half but also compressed the face under its front half between the steady orientation of the orbital plane and the forwardly rotating cervical spine - necessitating changes in orofacial and cervical anatomy. The tongue balled up. The structural reinforcement for the mandible moved from its inner border, where it limited the size of the airway, to its outer front end where it formed a chin. The center of rotation of the mandible moved from an axis between the condyles, which could rotate the shortened mandible into the airway space during opening, to an axis between the mandibular foramen, which preserved the airway and protected the neurovascular bundle where it enters the mandible.
To stabilize the hominid cranium on the top of the spine, it was surrounded by a cooperative system of jaw and cervical muscles that keep it erect by pulling down all around its periphery like stays pulling down on the mast of a sailboat or wire cables pulling down on a radio tower. The jaw and postural muscles developed coordinated firing patterns.13-14 The pull down on each side prevents tipping to the opposite side, and selective relaxation provides the flexibility needed for movement. From side to side, this tower is symmetrical and stable, supported by a series of parallel transversely extending structural components - cranium, shoulders, hips, and two feet side by side. However, from front to back, stability is far more difficult for a body so tall and flat. Symmetry is lost right from the top. In back, thick straps of post-cervical muscles attached to large bony occipital prominences pull straight down on the back of the head. However, in front, thick muscles and large bones could interfere with the freedom of movement necessary for yawning, swallowing, coughing, vomiting, spitting, talking, and turning the head. Therefore, the pull down in back is counterbalanced by multiple small muscles with varied directions of pull connecting a series of generally small parallel bones (clavicles, hyoid, and mandible) to ensure that each bone is capable of independent but coordinated action. The balance between this sophisticated pre-cervical kinetic chain and the simple thick post-cervical muscle mass is illustrated below.
FIGURE 2 - THE HOMINID HEAD POSTURE MECHANISM
THE ROLE OF THE MANDIBLE– The airway passing through the middle of this mechanism and bending around the mandible makes the postural locations of the head and the mandible inversely proportional in a sagittal direction. Moving either one anteriorly or posteriorly moves the other in the opposite direction. For example, surgery to advance the mandible causes the head to shift posteriorly, and surgery to move the mandible posteriorly causes the head to shift anteriorly.16-18. Shifting the head posteriorly shifts the mandible anteriorly (relative to the head) by compressing the tissues behind the mandibular rami. Shifting the head anteriorly shifts the mandible posteriorly (relative to the head), because the mandible is tethered to the clavicles and shoulder girdle and thus cannot shift as far anteriorly as the head.
In most TMJ disorders, the bite has shifted the mandible posteriorly, which has shifted the head anteriorly by triggering reflex neuromuscular responses that extend the head to protect the airway. The face grows around the airway, because the muscles acquire whatever resting postures are necessary to maintain airway passage. Resting postures, in turn, produce the type of light steady forces that shape bones. As a result, pugging one nostril of a rat make its whole craniofacial structure grow asymmetrically, and plugging the nostrils of a monkey makes it grow a long narrow face, because the monkey lowers its mandibular posture to create an oral airway passage. In humans, the mandible surrounds the pharynx on three sides, and the cervical spine borders its fourth; therefore shifting the mandible back toward the cervical spine diminishes the cross-sectional area of the pharyngeal airway, which reflexively causes the muscles of the area to tip the head back (extension) in order to rotate the mandibular corpus upward and forward away from the cervical spine in order to restore the lost pharyngeal airway space. The increase in pharyngeal airway space produced by such head extension has been demonstrated with imaging.45-46 The ability of pharyngeal airway blockage to trigger head extension can be inferred from findings that most children with swollen tonsils have extended head posture which normalizes quickly after tonsillectomy.47-49
However, the head cannot just tip back, because visual and vestibular reflexes (the so-called righting reflexes), keep it level with the horizon. The regulatory effect of the visual orientation reflex can be seen in the increased variability of head posture in the blind,50 and its power to alter muscle resting postures can be seen in its ability to bend the whole cranium in bipedal mice and also to produce extreme head extension in people with palpebral ptosis (eyelid droop).51 As a result of these righting reflexes, the head can only extend by simultaneously translating forward, as illustrated below.
THE RECENT POSTERIOR SHIFT OF THE MANDIBLE - During the last couple of centuries, our sudden dietary change to liquids and refined foods has shifted our bites and mandibular postures posteriorly.19-32 The average mandible now rotates down and back rather than up and forward, as it did in our ancestors. Mandibles have become shorter and more posteriorly positioned. Class 2 bites, in which the lower dentition is located posteriorly relative to the upper dentition, have become twice as frequent as they used to be; and many of those with class 1 bites actually have both an upper and lower dental arch that are located posteriorly relative to the cranial base, because most of the structural components of the craniofacial skeleton (including the bite table) follow the growth of the mandibular corpus, roughly in proportion to their distance from it.
One cause of this posterior shift of the mandible is the failure to stimulate jawbone growth by chewing forces. Chewing is the major stimulus to growth in all animals studied. The role of jaw muscles in upward and forward mandibular rotation can be seen by the extreme downward and backward mandibular rotation that occurs in people with myotonic dystrophy.
The other cause of this posterior shift of the mandible is a partial maxillo-mandibular synostosis due to retained overbite and steep cuspal interdigitation. In our ancestors, overjet and overbite slowly but steadily disappeared with age. In modern people, they generally persist throughout life, furthering the posterior shift by locking the tooth containing portion of the mandible (the corpus) back behind the upper front teeth.
Much of the facial growth that has been inhibited horizontally has been redirected vertically. When the mandible's advance is blocked so it cannot grow anteriorly, it grows down and back. In some people, strong jaw closing muscles are able to prevent the downward component but keeps the mandible locked posteriorly back behind a steep and usually deep overbite.
The change toward decreased horizontal growth and the increased vertical growth in modern humans is easy to see in skeletal remains. The study below left compares a population before and after they discovered agriculture about 10,000 years ago. The study below middle compares the average facial structures of traditional Australian Aborigines and modern Swedes. The same changes can be seen all over the planet when tribal people abandon their traditional diets and acquire a diet of soft processed foods.
The dependence of this change on jaw muscle strength has been shown by numerous experiments. Very similar changes have been produced in several species of animals just by raising them on soft diets.33-40 The same types of changes can also be seen in natural experiments when people lose jaw muscle strength due to myotonic dystrophy, as seen below right.
THE SECULAR CHANGE IN CRANIOFACIAL PROPORTIONS
SOLID LINE NUBIANS BEFORE AGRICULTURE SOLID LINE AUTRALIAN ABORIGINES SOLID LINE NORMALS
DOTTED LINE AFTER AGRICULTURE DOTTED LINE MODERN SWEDES DOTTED LINE MUSCLE DISEASE
ANTERIOR SHIFT OF HEAD POSTURE - The average change toward a more posterior mandibular posture is likely an important cause of the accompanying average change toward a more anterior head posture. In population studies, these two variables are well correlated, and most longitudinal studies have found that the changes in mandibular growth usually come first.41-44 The ability of posterior mandibular posture to cause anterior head posture can be seen in children who experience TMJ ankylosis, which prevents the mandible from advancing and also causes extreme forward head posture, simply as a result of their extreme posterior mandibular posture.
Forward head posture progressively diminishes and then reverses the cervical lordosis by shifting its top end forward over its base, as seen from left to right below. The vertical line rotating from a 12:00 position to a 1:00 position shows the long axis of the cervical spine rotating clockwise to support the forward shifting cranium. The descent of the upper horizontal line shows the loss of vertical height that accompanies the rotation of the cervical spine. The rotation of the lower horizontal line shows the shoulder girdles following the base of the neck anteriorly. Because the upper and inner aspects of the scapulae follow the advancing base of the neck more closely than their outer and lower aspects, the scapulae also often rotate inward around a vertical axis, leaving their lower and outer aspects sticking out like wings (winged scapula).
FIGURE 5
-PROGRESSION OF FORWARD HEAD POSTURE
NORMAL POSTURE FORWARD HEAD POSTURE EXTREME FORWARD HEAD POSTURE
AND BACKWARD JAW POSTURE AND EXTREME BACKWARD JAW POSTURE
EFFECTS ON THE SPINE - Forward head posture reflexively triggers adaptive realignment of the spine to restore physical balance. Typically the hips rotate down in front to thrust the abdomen anteriorly out beneath the anteriorly shifted head while the chest between them sinks backward, increasing the thoracic kyphosis, as seen from left to right in figure 6.
FIGURE 6 - SPINAL RESPONSE TO FORWARD HEAD POSTURE
NORMAL POSTURE FORWARD HEAD POSTURE
LATERAL DISPLACEMENT OF THE MANDIBLE – In addition to modern bites displacing mandibular postures posteriorly, they also frequently displace mandibular postures laterally. Because the lateral displacement is maintained by increased tonus in the ipsilateral temporalis muscle, as shown convincingly in children with unilateral cross-bite, and because the temporalis is the postural muscle for the mandible; lateral mandibular displacement tips head posture toward the side of the mandibular displacement, which makes the eye of the opposite side appear higher in photographs. It also compresses the face on the side of the displacement, which frequently makes the eye of that side appear smaller than the eye of the opposite side in photographs.
The lateral displacement of the mandible then disturbs symmetry throughout the spine, just as asymmetry in body posture can produce lateral displacement of the mandible. Crooked body posture and mandibular posture are reflections of each other. Tipped head posture has been correlated with scoliosis, asymmetric firing of neck muscles, shoulder imbalance, lateral displacement of the cervical spine, and increased body sway.52-60
One common cause of lateral mandibular displacement is a misfit between a narrow maxilla and a wide mandible. While the width of the mandible is determined almost entirely by genetics, the width of the maxilla is determined largely by functional forces. Pre-industrial populations who chewed very forcefully sometimes developed maxillae which were so wide that the mandible could only bite on the back teeth of one side at a time, while animals raised on soft diets develop narrow maxillae like modern children. When the lower dental arch cannot fit inside the narrowed upper dental arch to achieve normal interdigitation of the teeth, the mandible shifts to achieve meshing of the teeth for chewing. In unilateral cross bite, it shifts laterally far enough to reverse the interdigitation of the teeth on that side.
A significant lateral displacement of the mandible can even be caused by one malpositioned tooth. For example, a permanent canine enters the mouth of an adolescent as a very large structure at the end of a long eruption pathway which cannot be very precisely controlled and therefore needs to be refined by functional forces as it enters the mouth. If it erupts off to one side and the functional forces produced by the tongue, lips, cheeks, and jaw muscles are too weak to realign it; the displaced canine can shift the habitual bracing position used by the mandible instead of being shifted by it, which causes the rest of the erupting permanent teeth also acquire interdigitation in the same displaced mandibular position, until it becomes normalized by the neuromusculature. As a result, many people have a signficantly displaced mandibular posture and do not know it.
DEPROGRAMMING THE JAW MUSCLES - To find out if the mandible's central bracing position is displaced from its postural neutral zone requires temporarily deprogramming the jaw muscles to stop them from reflexively aiming all mandibular closing trajectories at the old habitual central bracing platform. This is best accomplished by wearing a flat front bite plate appliance during sleep. The orientation of its bite plate parallel to the superior lateral pterygoid muscles supports their role in determining the horizontal location of the mandible by pulling on its condyles like steering a bike by its handlebars. After deprogramming, the mandible opens and closes on a trajectory determined only by resting myofascial postural forces. The distance and direction between where this deprogrammed mandibular closing trajectory and where the habitual mandibular closing trajectory land on a flat bite table provides a measure of the distance and direction of bite strain.
REHABILITATING THE JAW MUSCLES - Unhealthy jaw muscles may need strengthening or stretching before they can clearly and consistently demonstrate a healthy jaw closing trajectory. Jaw muscles that have become shortened due to chronic tightening (contracture) may need stretching, and jaw muscles that have become hypotonic or atrophic due to a longstanding reduction of functional forces from protecting an inflamed TMJ or tender teeth may need strengthening. Strengthening the jaw muscles requires a very stable bite table, because that forms its exercise template. However, even a very stable bite can still hold the mandible in a very displaced location and impair muscle health by displacing the template they exercise against, like riding a bike with the seat much too low or lifting weights with your feet planted off to the side.
EXTENDING THE CENTRAL BRACING PLATFORM - toward the postural neutral zone can be accomplished by reducing (shaving down) portions of teeth that are too high (called equilibration) or building-up portions of teeth that are too low when the mandible is moved toward the neutral zone. Extending the central bracing platform anteriorly in the presence of deep or steep overbite usually requires reducing the contacts between the labial-incisal edges of the lower front teeth and the palatal surfaces of the upper front teeth or building up all the other teeth. Extending the central bracing platform medially from a laterally displaced position usually requires reducing teeth on the side opposite the displacement, as well as any balancing side contacts on the side of the displacement, or building up the teeth on the side of the displacement.
Extension of the central bracing platform must be used to be preserved. Simply providing a pathway along which the mandible can shift does not make it shift. If the jaw muscles do not begin bracing the mandible in the newly extended portion of the bite platform, they may not distribute enough axial forces onto the involved teeth to maintain their positions. For example, if the central bracing platform is extended anteriorly by equilibration of the front teeth and the extension is not used functionally, such as by incising or bracing there, the front teeth may extrude until their incisal edges reach the same location they had before the equilibration, which is controlled by lower lip posture. Synergistic measures to improve head posture can also increase the use of the extended area of the bite platform and thereby help the change "take".
MOVING THE CENTRAL BRACING PLATFORM – may be preferred when simply extending it far enough to fit healthy resting myofascial postural forces threatens to damage teeth or make them too flat to cut and tear food. Moving the central bracing platform requires adding tooth structure behind the mandibular shift in addition to removing tooth structure from the path of the shift. The central bracing position is usually moved together with the surrounding inclines of tooth structure that define a functional range of motion which has been established by the jaw muscles.
However, moving the central bracing platform by means of dental work such as crowns forces us to produce the full bite change at a single appointment, requiring a relatively sudden adaptation by the rest of the postural system, while most postural system corrections occur more gradually. If the jaw muscles do not fully adopt the new bracing platform right away, their tendency to brace the mandible in the position of the old central bracing platform can fracture the added artificial tooth material or stress the periodontal support for the teeth bearing them.
DENTAL WORK – Should recognize the orthopedic role of the bite in body posture. Dentists performing full mouth reconstruction must recognize that stabilizing the existing bite also stabilizes the existing body posture. Conventional orthodontics produces a central bracing platform in whatever location the mandible used for bracing most frequently during the process, therefore jaw muscle strength should be maintained during the process to prevent inadvertent displacement of the central bracing platform. Also, if the patient had a deep or steep overbite before orthodontic treatment, adding a front flat bite plate to the retainer (Hawley), is important to prevent the overbite from recurring post-treatment.
FOOTNOTES
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