Occlusion and posture

 SUMMARY

To understand the clinical relationship between dental occlusion and body posture, they need to be seen in the context of evolution.  In mammals, the posture of the mandible is dependent on the location of its fully braced (intercuspal) position. In hominids, the mechanics required for upright stance made mandibular posture inversely proportional antero-posteriorly with head posture. In modern humans, due to the softening of our diet in the last couple of centuries, average mandibular bracing and postural locations have shifted posteriorly and laterally (asymmetrically). The posterior mandibular shift may be an important cause of the accompanying average shift of head posture anteriorly. The lateral mandibular shift may be a common cause of a head tilt and other aspects of scoliosis. Dentists can contribute to treatment of forward and sidebent head posture by extending or shifting the mandibular bracing position, usually anteriorly or antero-medially, in conjunction with synergistic efforts to improve body posture. 

BACKGROUND

We know that posture can affect occlusion. Because of the precise bite registration techniques needed for prosthodontics, dentists have thoroughly documented how body posture can affect the mandibular closing trajectory.14-16 For example, it's been known since 1929 that extending the head retrudes the mandible, and flexing the head protrudes the mandible.3 More recent studies have shown that treatment of the cervical spine can cause a change in the dental occlusion.17

There is also a great deal of evidence that dental occlusion can affect posture. In animals, changing the occlusion changes the curve of the spine.18  In humans, changing the occlusion causes immediate changes in postural muscle activity19-27 and head posture.28-30

Dental occlusion has been correlated with postural variables in many studies.1-9 A recent 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.10 Occlusal parameters are correlated with craniofacial parameters, which are correlated with postural parameters.11-13, 79-80

However, researchers have been unable to find significant correlations between occlusal and postural parameters.31 The only occlusal parameter that can be correlated with head posture is Angle's class 2 (associated with extended head posture),32-34 but that association is not strong and has little specificity. The only dental feature that has been associated with a postural variable is incisor crowding (also associated with extended head posture),but that dental feature is an effect rather than a cause of the growth pattern.

Without a basis for understanding the connection between occlusal and postural features, the role of occlusion in posture is simply not recognized. Clinically we behave as if they are not connected.  The mandible has become the only postural component routinely left out of efforts to improve overall posture.

The reason for our failure to correlate occlusion and posture is that the parameters we use to measure dental occlusion have no relevance to almost any functional features, including body posture. We quantify and compare occlusions by measuring details of the way the teeth fit and slide together, but those details reveal very little information about relevant features such as the mandible's position relative to the cranial base and the cervical vertebrae or the cross sectional area of the pharyngeal airway.

To understand the role of occlusion in posture requires understanding the role of the mandibular bracing platform in determining mandibular posture, the role of mandibular posture in body posture, and how those roles have changed during the last couple of centuries as our dietary change to highly processed foods has changed facial growth patterns.

MANDIBULAR BRACING

In mammals, mandibular bracing plays a critical protective role, because a blow to a free floating mandible could drive its condyles posteriorly like hammers into the delicate and vital inner ear areas. Bracing the mandible immobilized it by clamping it up forcefully and immoveably against the underside of the skull through the medium of the occlusal table.

Mandibular bracing gained importance in hominids.  Canine and simian mandibles are protected from posteriorly directed impacts, even at rest, by overlapping canines. In hominids, the canines withdrew into the occlusal plane, leaving the vital inner ear areas protected only by a thin bony plate and neuromuscular reflexes that respond to danger by rapidly bracing the mandible. Functional swallowing requires a braced mandible to provide a stable base of operation for the circumhyoid and tongue muscles. Efficient postural maintenance requires bracing the mandible in order to fix it to the head and thereby enable the whole cranium to function as one structural unit so the anterior kinetic chain of skeletal muscles can pull down directly on its front end by pulling down on the mandible.

MANDIBULAR POSTURE

Mandibular bracing was so important in our evolution that our jaw muscles are programmed to hold our mandibles in a resting posture just beneath the bracing (intercuspal) platform in order to maintain fast easy access to bracing.  That resting posture may be located several millimeters beneath the bracing platform or almost pushed up against it, depending on the resting tensions of the elevator muscles. Still, the braced location of the mandible determines its postural location horizontally.

The accommodation of the posture of the mandible to the location of its bracing platform has been demonstrated in three planes. Vertically, an immediate increase in freeway space follows the first occlusal contact after placing a bite raising appliance, and an immediate return to the pre-treatment freeway space follows the first occlusal contact after removing the bite raising appliance.36  Similarly, shortening the face leads quickly to a new mandibular resting posture that maintains the pre-treatment freeway space. Laterally, children who develop unilateral cross-bite undergo a shifting of the mandible in both bracing and postural positions to the side of the cross-bite motivated by increased resting tension in the posterior temporalis on the side of the cross-bite; and their jaw muscle resting tensions normalize after correction of the cross-bite.37-40 Similarly, when monkeys have their occlusions altered by inclines that force their mandibles to one side, they undergo a shift of their mandibular resting posture to that side at the same time as the shift in bracing positions.41 Antero-posteriorly, monkeys who receive a protrusive occlusal interference exhibit an immediate increase in the tonus of the ipsilateral superior lateral pterygoid muscle that shifts the ipsilateral side of the mandible anteriorly to accomodate the interference.42-44 

However, mandibular posture is also influenced by the resting myofascial tensions produced by the habitual weight-bearing upright postural stance. In that posture, each skeletal member rests in a neutral zone at an equilibrium between opposing myofascial tensions. The mandible's posture should occupy a neutral zone within the curtain of myofascial tensions that extends between the front of the head and the clavicles – at least it would occupy that postural position if the teeth were not involved. 

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 reflex accomodation of the jaw muscles to the occlusion, a strain is shared between the masticatory and postural systems. In these cases, patients often report that their symptoms switch back and forth between jaw and neck or back muscles. If body posture is otherwise healthy, the symptoms can be relieved by changing the occlusion to make mandibular posture better fit body posture. If body posture is strained, it needs to be changed together with a change in mandibular posture.

However, because it is irreversible in some aspects, most dental authorities consider any occlusal treatment to be invasive and only to be used as a last resort. A change in occlusion may be irreversible because of the dentoalveolar adaptation that it triggers. Thus, if an occlusal change causes symptoms, those symptoms cannot always be eliminated by simply reversing the occlusal change. Actually, treating the adverse effects of a change in occlusion does not require precisely reproducing all the centric and bracing occlusal contacts that existed before the change but understanding the orthopedic aspects of that occlusion; and treating the occlusion to help improve body posture requires understanding the role of the mandible in head posture.

UPRIGHT POSTURE

FIGURE 1 - RESHAPING THE CRANIUM FOR UPRIGHT POSTURE

                            APE                                                        HOMINID                                                                                                       

This reshaping of the cranium made room for expansion of the brain on its back half, but compressed the face beneath its front half between the steady orientation of the orbital plane and the forwardly rotating cervical spine - necessitating changes in the mandible, tongue, soft palate, and epiglottis. The structural reinforcement for the mandible moved from its inner border to its outer front end where it formed a chin. Mandibular opening moved its center of rotation from an axis between the condyles, which would rotate the chin into the airway space, to an axis between the mandibular foramen, which maintains the stability of the airway borders and the neurovascular bundle entering the mandible. The tongue balled up. The epiglottis descended.

Unfortunately the hominid airway was still left vulnerable to obstruction in the pharynx. Between its upper end where it was protected by the bony nasal cavity and its lower end where it was protected by the cartilage of the larynx and trachea, the airway passed through a floppy tube surrounded by muscles and protected only by neuromuscular reflexes. Experimentally obstructing a pharyngeal airway causes all the muscles of the area to acquire unusual firing patterns, often in synchrony with respiration, in order to restore airway patency.46

The hominid head is surrounded by a curtain of muscles that keep it erect by pulling down all around its periphery much like stays pulling down on the mast of a sailboat or wire cables pulling down on a radio tower.  The pull down on each side prevents tipping to the opposite side, and selective relaxation of muscles provides the flexibility needed for movement.  

From side to side, this tower is symmetrical and stable. A series of parallel transversely extending structural components (two feet, a pelvic girdle, a shoulder girdle, a mandible, and a cranium) are connected by muscles at varied angles. A change in the orientation of any one of these structural components induces a change in the others.

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 postcervical muscles attach directly onto large bony occipital prominences. 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. Thus, the front of the neck contains a series of generally small and nearly parallel bones (clavicles, hyoid, and mandible) connected by a large number of small muscles with varied directions of pull that maximize their leverage and 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 - HEAD POSTURE MECHANISM

THE ROLE OF THE MANDIBLE IN HEAD POSTURE

In this head posture mechanism, the mandible serves as an architectural beam that deflects postural forces from the face, where delicate sensory systems could not operate effectively if enclosed by bone thick enough for attaching large muscles. To prevent the traction from the anterior postural muscles from reaching the face, the long rigid mandibular corpus absorbs it and transfers it all the way 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.

The mechanics of this head posture mechanism makes the location of the mandibular corpus and the location of atlas inversely proportional antero-posteriorly. Shifting the head posteriorly shifts the corpus anteriorly (relative to the head) by compressing the tissues behind it.  Shifting the head anteriorly shifts the corpus posteriorly (relative to the head), because traction from the pre-cervical muscles and fascia that tether the mandible to the clavicles prevents it from shifting as far anteriorly as the head.47-48  Surgery to advance the corpus results in cranial flexion with retrusion of head posture, and surgery to retrude the corpus results in cranial extension with protrusion of head posture.49-50 

POSTERIOR DISPLACEMENT OF THE MANDIBULAR CORPUS

In modern humans, on average, the corpus has shifted posteriorly, because vigorous mastication no longer drives it anteriorly as far or fast as it used to. In our ancestors, the mandible elongated throughout life to continuously carry the roots of the mandibular teeth anteriorly into the maxillary teeth enclosing them in front and on both sides in order to continuously supply tooth structure at the occlusal table throughout life and thereby compensate for the occlusal wear that continuously removed tooth structure from the occlusal table throughout life. Typically, the elongation of the mandible produced enough anterior translation to steadily overcome the overjet and overbite, as illustrated in figure 3. Because the movement of the corpus was the primary motivator of this change in occlusion, the mandibular teeth moved slightly less far than their roots, causing the incisors to tip backwards on their bony bases.

 FIGURE 3  - TYPICAL CHANGE OF INCISOR RELATIONS WITH AGE IN PRE-INDUSTRIAL HUMANS

anterior wear.jpg

                                                                                   YOUTH                                  ADULT                                                        ELDER

Today the mandible still elongates throughout life, but some of its growth has been redirected posteriorly and inferiorly. Overbite and overjet usually persist, and they may even increase rapidly in some TMJ disorder patients. Mandibles have become shorter.50-66  Class 2 malocclusions, which were found in about 10% of pre-industrialized humans, are now found in at least 20% of modern humans; and many of those who now have class 1 occlusions actually have both a mandibular and a maxillary dentition that are located posteriorly relative to the cranial base, because most of the structural components of the craniofacial skeleton (including the occlusal table) follow the growth of the mandibular corpus, roughly in proportion to their distance from it, as can be seen by the relative lengths of the three arrows (representing mandibular corpus, mandibular dentition, and maxillary dentition) in figure 3.

ANTERIOR DISPLACEMENT OF HEAD POSTURE

The average change toward a more posterior mandibular posture may be an important cause of the accompanying average change toward a more anterior head posture. In population studies, posterior mandibular posture and anterior head posture are well correlated. 67-68 Furthermore, most longitudinal studies have found stronger associations between mandibular growth and subsequent body posture changes than between body posture changes and subsequent mandibular growth.69-70

Posterior mandibular posture probably causes anterior head posture by evoking adaptations to protect the airway. Because the mandible surrounds the pharynx on three sides and the cervical spine borders its fourth, posterior displacement of the mandible can diminish the cross-sectional area of the pharyngeal airway. In response, all the muscles of the area alter their resting postures to hold the bones in whatever positions are necessary to maintain an adequate airway.  Usually they tip the head back into extension in order to rotate the mandibular corpus anteriorly and superiorly and thereby increase its distance from the cervical spine. The increase in pharyngeal airway space produced by head extension has been demonstrated with imaging.71-72 The ability of pharyngeal airway blockage to cause head extension can be inferred from findings that most children with swollen tonsils have extended head posture which normalizes quickly after tonsillectomy.73-76

However, the head cannot just tip backward, because visual and vestibular reflexes keep it level with the horizon.  The regulatory effect of the visual orientation reflex can be seen by the effect of its absence in the increased variability of head posture in the blind,77 and its power to alter the resting postures of the craniofacial muscles can be seen in its ability to bend the whole cranium in growing bipedal mice and cause permanent head extension in people with palpebral ptosis.78  As a result of these reflexes, the head can only extend if it also translates anteriorly, as in figure 3.  

 

FIGURE 3 - MANDIBULAR RETRUSION CAUSING FORWARD HEAD POSTURE

The ability of posterior mandibular posture to cause anterior head posture can also be seen in natural experiments. When a child's mandible suddenly stops growing anteriorly with the rest of the face due to an injury such as TMJ ankylosis, the resulting severe class 2 malocclusion is accompanied by extreme anterior head posture simply as a result of the extreme posterior mandibular posture.  

Forward head posture progressively diminishes and may even reverse the cervical lordosis by shifting its top end (atlas) forward over its base, as seen from left to right in figure 4. The vertical line rotating from a 12:00 position to a 1:00 position shows the long axis of the cervical spine rotating forward to support the anteriorly shifting head above the posteriorly shifting mandible. The descent of the upper horizontal line shows the loss of vertical height that accompanies forward head posture. The lower horizontal line shows the rotation of the medial and superior aspects of the shoulder girdle as they follow the anterior shift of the base of the neck. Because the lateral and inferior aspects of the scapulae do not follow the base of the neck as far anteriorly, the scapulae also tend to rotate around a largely vertical axis, leaving their lateral and inferior aspects sticking out like wings (winged scapula).

FIGURE 4 -PROGRESSION OF FORWARD HEAD POSTURE

fhp with arrows.jpg

NORMAL POSTURE                                  FORWARD HEAD POSTURE                                    EXTREME FORWARD HEAD POSTURE

 

EFFECTS ON THE BACK

Cantilevering the head anteriorly from the top of the spine initially produces strains along the length of the spine, but it also triggers adaptive realignment of the spine beneath the shifting weight on top in order to maintain physical balance. Typically the hips flex to thrust the abdomen out beneath the head. Between the anteriorly displaced head and abdomen, the chest shifts posteriorly (sinks), increasing the thoracic kyphosis, as seen accompanying mandibular retrusion from left to right in figure 5. 

FIGURE 5 - SPINAL RESPONSE TO FORWARD HEAD POSTURE

                                         NORMAL POSTURE                     ANTERIOR HEAD POSTURE                                   

posture deets low res.jpeg

 

LATERAL DISPLACEMENT OF THE MANDIBLE

 

 

The mandible and head posture also get displaced in a frontal plane. Lateral displacement of the mandible, which can be approximated by comparing the midlines between maxillary and mandibular central incisors, has been correlated with scoliosis and other postural asymmetries.81-83 Facial asymmetry has been found to be correlated with shoulder imbalance and adolescent idiopathic scoliosis.84

Much like the posterior displacement of the mandible may precede and therefore cause anterior head posture in a sagittal plane, the lateral displacement of the mandible may precede and therefore cause the scoliosis that occurs in a frontal plane. Studies of healthy subjects have shown that asymmetric occlusal interferences produce asymmetric firing of neck muscles, 85-86 lateral displacement of the cervical spine,87-88 and increased body sway.89

One common cause of lateral mandibular displacement has been the narrowing of the maxilla that has accompanied the posterior shift of the mandible in modern human facial growth. The width of the maxilla is determined largely by functional forces. For example, pre-industrial populations who chewed very forcefully often developed maxillae that were so wide that the mandible could only brace unilaterally on either side. In contrast, monkeys raised on soft diets develop narrow palates much like modern children. Yet the width of the mandibular dental arch is determined mainly by genetics. As a result, many modern children have a mandibular dentition that is wider than the maxillary dentition. In response to this misfitting of facial parts, the mandibular corpus may be prevented from growing anteriorly, the mandibular teeth may tip lingually, and/or the mandible may shift laterally to achieve intercuspation, often accompanied by a unilateral cross-bite.

Another common cause of lateral mandibular displacement has been the weakening of our jaw muscles. In humans, the jaw muscles regulate facial growth, and our average jaw muscle strength is only about half of what it used to be. Experiments have shown that jaw muscle weakening causes increased variability and asymmetry in all the craniofacial components.90-94

Lateral mandibular displacement is typically accompanied by a tilting of the plane of occlusion superiorly on the side of the shift95 due to the increased tonus of the ipsilateral temporalis muscle. The temporalis muscles are responsible for posturing the mandible in a frontal plane. In unilateral cross-bite, the ipsilateral temporalis undergoes increased resting tension.28-29 Because this muscle has a superior as well as a lateral vector, an increase in its resting tension can drive the ipsilateral side of the occlusal plane superiorly. The reciprocal force generated by the increased resting tension in the ipsilateral temporalis muscle can also pull the ipsilateral temporalis fossa inferiorly, which is probably the cause of a head tilt toward the same side, which typically makes the eye of the opposite side appear higher in photographs of patients with lateral displacement of the mandible. Light continuous forces, like those that result from postural tensions, are very effective at shaping bones.

STRAINS ON JOINTS

Changing body posture can create mechanical strains in the intervertebral joints as the compressive forces from weight bearing become unevenly distributed among the spinal segments.  Intervertebral disks may be displaced.  Degenerative changes in the intervertebral joints can limit their ranges of motion, depriving the specialized articular surfaces of the rubbing movements they need for weeping (hydrostatic) lubrication and circulation. Manipulations that restore ranges of motion and exercises that maximize movement while minimizing loading are helpful.

STRAINS ON MUSCLES

Inflammation in any joint triggers reflex protective bracing in the muscles which cross that joint. Muscles automatically go into a state of protective guarding to stabilize an injured joint - increasing their contractile activity to hold it more tightly during rest and decreasing power during exercise.  However, if the protective guarding becomes chronic, it can diminish resting circulation.96-98 Local loss of resting microcirculation in portions of muscles may produce trigger points – small localized intramuscular areas that are exquisitely sensitive to manual pressure. Clinically they can cause pain at distant locations (referred pain), and they often persist long after their original cause has been eliminated.  At the same time, the decreased contractile activity during exercise diminishes the beneficial pumping effect of healthy function.

Muscles all over the body may be affected, because the postural muscles function as members of long myofascial chains running up and down the length of the body.  A change in the resting length of one muscle affects all the muscles in the chain as well as the balance between chains on opposite sides of the body.  

THE ROLE OF STRESS

Increased central nervous system stress can exacerbate the effects of muscle tightness in any part of the body by increasing resting muscle tonus all over the body.  If a muscle is already operating at borderline resting circulatory levels because of chronic tightness due to peripheral conditions, even a slight increase in tonus from central nervous system stress can lower that muscle's resting circulation far enough to cause pain.

The masticatory system is uniquely susceptible to the effects of stress because of the extreme imbalance between jaw opening and closing muscles.  In other parts of the body, the bones are held in place between muscles of similar mass pulling in opposite directions.  When overall resting tonus increases, the bones do not move, - they are just held more tightly in their neutral zones between opposing myofascial tensions.  However, in the masticatory system, where the jaw closing muscles dwarf the jaw opening muscles, increasing central nervous system stress holds the mandible further closed.  The teeth may even rest in contact, and they may go into a clench whenever a further increase in stress causes increased overall resting muscle tensions. Occlusal contact can then increase elevator muscle activity by evoking masticatory reflexes and thereby sustain the pathophysiology.

THE ROLE OF ATTITUDE

While stress increases all resting skeletal muscle tensions, attitude selectively alters resting skeletal muscle tensions. The motor end plates are anatomically and physiologically extensions of the brain. By reflecting the emotional state of the brain, they determine which muscles are held more tightly and which more loosely.  Feeling sad causes a sunken chest, a forward and downward shifted (hanging) head, and mandibular retrusion. The extremely retrusive mandibular posture in grief probably causes a lump in the throat by triggering an airway protective response that extends the head, opens the glottis, and protrudes the hyoid bone. There is evidence that pain itself can cause anterior head posture. Conversely, feeling proud or determined causes thrusting of the chest, standing tall, and mandibular protrusion. There is evidence that pain itself can produce anterior head posture.99

PASSIVE PHYSICAL SUPPORT

Passive physical support influences posture. Most furniture is designed for style and comfort rather than orthopedic support.  In order to accommodate the anterior head posture that has become endemic in modern society; most head rests on planes, trains, and buses force the head anteriorly.  Backsleeping on a soft mattress such as a waterbed, an air mattress, or memory foam allows the body to sink down at its center of mass and thereby place the spine in flexion as the hips and abdomen drop down below the head and feet.  In contrast, a mattress made of simple foam rubber (without memory) constantly pushes the spine toward a straighter axial alignment.  When backsleeping, the back of the head should be in contact with the mattress, and the pillow should provide support under the neck rather than under the head. Neck support can be provided by U shaped pillows (such as travel pillows), a rolled up towel, or a pillow with a hole in the middle. Side sleeping is posturally neutral as long as the pillow is about as thick as the distance between the ear and the shoulder.

ACTIVE PHYSICAL SUPPORT

Improving posture requires strengthening or shortening some postural muscles and weakening or stretching others.  The muscles that need strengthening can be selectively exercised by repeatedly pulling the body up into an ideal posture and then allowing it to slump back into the old posture before pulling it up again.  An ideal posture can usually be approximated by holding the head as tall as possible, because anterior head posture and the accompanying changes in spinal curvatures generally cause loss of vertical height, as can be seen in figures 3 and 4, and restoring some of that height can help restore some of the lost spinal support. Some people may need strengthening of the jaw and circum-hyoid muscles in order to improve the effectiveness of mandibular bracing by giving the anterior chain of postural muscles better purchase on the front of the head.

OCCLUSAL STABILITY

Strengthening the jaw muscles may require occlusal stabilization. Before the elevator muscles can fire strongly, the brain needs to receive proprioceptive signals indicating stable occlusal contact. As a result, occlusal stability and elevator muscle strength are well correlated with each other and inversely correlated with TMJ disorder symptoms.100 Stabilizing an occlusion can lead to more symmetrical and healthy development of the jaw muscles, which in turn can have a beneficial impact on facial growth, because the jaw muscles regulate facial growth. However, even a very stable occlusion can still be displaced.

DEPROGRAMMING THE JAW MUSCLES

To find out if an occlusion is displaced in a horizontal plane requires deprogramming the jaw muscles to stop the flow of afferent periodontal feedback that has been continually programming them to aim all mandibular closing trajectories at the old habitual bracing (intercuspal) position. The muscles can be deprogrammed by a Lucia jig, a cotton roll, or even anesthetizing the teeth; however the most reliable method is by wearing a flat anterior bite plate appliance during sleep. The orientation of the anterior bite plate should be roughly parallel to the orientation of the lateral pterygoid muscles to enable them to easily locate the mandible in their preferred position on the bite plate.

After the jaw muscles have been deprogrammed, the mandible opens and closes on a trajectory determined only by resting myofascial forces without the influence of neuromuscular reflexes designed to protect the teeth. The distance between where this posturally determined mandibular closing trajectory and the old occlusally directed mandibular closing trajectory intersect the occlusal plane is a measure of postural bite strain. The old occlusally directed mandibular closing trajectory can also be compared with a mandibular closing trajectory produced by a more ideal body posture to project further improvements.

EXTENDING THE MANDIBULAR BRACING PLATFORM

The easiest way to bring mandibular posture into a better fit with the existing or desired body posture is by extending the mandibular bracing platform. The process can be performed gradually in concert with other modalities to improve body posture. It usually leads to a shift of the mandibular postural location in the direction of the extension, as long as the jaw muscles are healthy and the rest of the postural system supports the shift. Extending the bracing area anteriorly in the presence of deep overbite usually requires reducing the occlusal contacts between the labial-incisal edges of the mandibular anterior teeth and the palatal surfaces of the maxillary anterior teeth. In the presence of a steep curve of Spee, it may also require removing contacts between the anterior facing inclines of the distal mandibular molars and the posterior facing inclines of the distal maxillary molars. Extending the bracing area medially from a laterally displaced location usually requires removing tooth structure from the side opposite the displacement as well as any balancing side interferences that occur on the side of the displacement.

Any extension of the mandibular bracing platform must be used to be preserved. Simply providing a pathway along which the mandible can shift does not make it shift. Sometimes the neuromusculature is able to maintain a habitual bracing position and/or a functional range of motion in spite of the occlusion. If the jaw muscles do not begin bracing the mandible in the newly extended portion of the occlusal table, they may not distribute enough axial forces onto the involved teeth to maintain their positions. For example, if anterior guidance is shallowed by equilibration of the anterior teeth and the anterior extension is not used, the anterior teeth may extrude until their incisal edges reach the same location they had before the equilibration.

MOVING THE MANDIBULAR BRACING LOCATION

In some cases, the mandibular bracing location may need to be moved rather than merely extended. In addition to eliminating occlusal barriers in the path of the desired mandibular shift, artificial tooth material is added behind the shift. In most dentitions, shifting the mandibular bracing location anteriorly requires adding material posteriorly, and shifting the mandibular bracing location medially requires adding material laterally.

Because adding to natural teeth can be expensive, the mandibular bracing location is only moved when extending it cannot produce the desired results or creates other problems. One problematic result of extending the mandibular bracing location beyond the mandible's functional range can be loss of some of the dentition's cutting and tearing capacity. One indication for moving the mandibular bracing location rather than just extending it is a tendency to keep returning to the same old bracing location in spite of the extension.

The mandibular bracing location is usually moved together with the surrounding inclines of tooth structure that guide the mandible into and out of it. The occlusal anatomy defines an exercise template for the jaw muscles, and its contours affect their health. Any tooth structure that impedes direct access to mandibular bracing may reflexively trigger increased resting tension in the jaw muscles.

However, there are problems with changing occlusions. One problem is that dentists are trained to duplicate occlusions, not to change them.  Most dental authorities view any occlusal change as risky because it is irreversible and therefore to be avoided whenever possible. Another problem is that the limitations of prosthodontic techniques usually force us to produce the full occlusal 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 position right away, their tendency to brace the mandible in its old intercuspal position can fracture the added barriers of artificial tooth material or stress the periodontal support for the teeth bearing them.

 

CONCLUSION

In hominids, the mandible became an integral part of the postural system, the jaw and postural muscles developed coordinated firing patterns,101-102 and the bracing location of the mandible became an important determinant of mandibular posture. Shifting that bracing location can be a tool in the treatment of displaced head posture. Today dentists performing full mouth reconstruction, dentures, or orthodontics must recognize that stabilizing the existing occlusion also stabilizes the existing body posture, and some patients may be best served by incorporating a period of time for improving body posture before stabilizing it by finalizing the occlusion. Conventional edgewise orthodontics produces an intercuspal bracing position in whatever location the mandible used most frequently for bracing during the process; therefore exercises or physical therapy could be incorporated before and during the treatment period in order to maintain muscle strength and prevent inadvertent lateral displacement of the mandibular midline. Postural treatments should be undertaken with the awareness that, unless the dental occlusion is addressed, the treatment may relapse shortly after the patient resumes bracing the mandible in its old intercuspal position, which then causes a return of the previous mandibular posture and subsequently also the previous head and body posture. 

FOOTNOTES 

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3. Schwartz AM. Positions of the head and malrelations of the jaws. Int J Orthod Oral Surg Radiol. 1928;14:56-68.

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  64. Solow B, Sandham A. Craniocervical posture: a factor in the development and function of the dentofacial structures. Eur J Orthod. 2002;24:447-456.

  65. Springate SD. A re-investigation of the relationship between head posture and craniofacial growth. Eur J Orthod. 2012; 34(4):397-409.

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  70. Choi J-K. A study of the effects of maximal voluntary clenching on the tooth contact points and masticatory muscle activities in patients with temporomandibular disorders. J Craniomandib Disord Facial Oral Pain 1992;6:41-46.

  71. Zafar H. Integrated jaw and neck function in man: studies of mandibular and head-neck movements during jaw opening-closing tasks. Swed Dent J Suppl. 2000;143:1-41.

  72. Eriksson PO, Haggman-Henrikson B, Nordh E, Zafar H. Coordinated mandibular and head-neck movements during rhythmic jaw activities in man. J Dent Res. 2000;79:1378-1384.

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  76. Solow B, Tallgren A. Dentoalveolar morphology in relation to craniocervical posture. Angle Orthodont.1977;47:157-164.

  77. These anterior and posterior kinetic chains have firing patterns that are coordinated with the jaw muscles to stabilize the head during chewing and swallowing.35

  78. Ferrario VF, Sforza C, Serrao G. The influence of crossbite on the coordinated electromyographic activity of human masticatory muscles during masticaion. J Oral Rehabil 1999;26:575-581.

  79. Karlsson S, Ch SA, Carlsson GE. Changes in mandibular masticatory movements after insertion of nonworking side interference. J Craniomandib Disord 1992;6:177-183.
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  83. Kibana Y, Ishijima T, Hirai T. Occlusal support and head posture. J Oral Rehabil. 2002;29(1):58-63.

  84. Katsaros C, Masticatory muscle function and transverse dentofacial growth. Swed D J. Suppl 2001;151:1-47.

  85. Menegaz, Rachel. "Craniofacial developmental instability and masticatory behavior [abstract]." 2009 Health Sciences Research Day (MU) (2010).

  86. Kiliaridis S. The importance of masticatory muscle function in dentofacial growth. Seminars in Orthod. 2006;12(2):110-119.

  87. Solow B, Sandham A. Craniocervical posture: a factor in the development and function of the dentofacial structures. Eur J Orthod. 2002;24:447-456.

  88. Springate SD. A re-investigation of the relationship between head posture and craniofacial growth. Eur J Orthod. 2012; 34(4):397-409.

  89. Rasmussen O C, Bonde-Petersen F, Christiensen L V, Moller E. Blood flow in human mandibular elevators at rest and during controlled biting. Arch Oral Biol. 1977;22(8-9):539-43.

  90. Moller E, Rasmussen OC, Bonde-Petersen F. Mechanism of ischemic pain in human muscles of mastication: intramuscular pressure, EMG, force and blood flow of the temporal and masseter muscles during biting. Advances in Pain Research and Therapy vol 3. John Bonica et al. (eds) Raven Press, N.Y. 1979.

  91. Bonde-Petersen F, Christiensen LV. Blood flow in human temporal muscle during tooth grinding and clenching as measured by Xenon clearance. Scand J Dent Res. 1973;81:272-275.

  92. Greenbaum T, Dvir Z, Reiter S, Winocur E. Cervical flexion-rotation test and physiological range of motion – A comparative study of patients with myogenic temporomandibular disorder versus healthy subjects. Musculoskeletal Science and Practice 2017;27:7-13.

  93. Choi J-K. A study of the effects of maximal voluntary clenching on the tooth contact points and masticatory muscle activities in patients with temporomandibular disorders. J Craniomandib Disord Facial Oral Pain 1992;6:41-46.

  94. Zafar H. Integrated jaw and neck function in man: studies of mandibular and head-neck movements during jaw opening-closing tasks. Swed Dent J Suppl. 2000;143:1-41.

  95. Eriksson PO, Haggman-Henrikson B, Nordh E, Zafar H. Coordinated mandibular and head-neck movements during rhythmic jaw activities in man. J Dent Res. 2000;79:1378-1384.

  96. Visscher CM, Huddleston SJ, Lobbezoo F, Naeije M. Kinematics of the human mandible for different head postures. J Oral Rehabil 2000;27:299-305.

  97. Huggare J. Postural disorders and dentofacial morphology. Acta Odontol Scand. 1998;56(6):383-386.

  98. Solow B, Tallgren A. Head posture and craniofacial morphology. Am J Phys Anthropol. 1976; 44: 417-436.

  99. Solow B, Tallgren A. Dentoalveolar morphology in relation to craniocervical posture. Angle Orthodont.1977;47:157-164.

  100. These anterior and posterior kinetic chains have firing patterns that are coordinated with the jaw muscles to stabilize the head during chewing and swallowing.35