THE ROLE OF BODY POSTURE

The following paper descibes the unappreciated but vital relationship between body posture, TMJ disorders, and the teeth.  A more detailed description of the role of the bite in both TMJ disorders and body posture, along with extensive footnotes, can be found in BITES AND BODY POSTURE under the FOR DOCTORS tab.  

BACKGROUND

The mandible (the lower jawbone) is an integral member of the head posture mechanism, and head posture affects body posture; because the position of the heavy head on the top of the spinal column reflexively determines how that column aligns beneath it to support the weight on top.  In this manner, the posture of the mandible affects the posture of the rest of the body.  However, the mandible is the only postural component routinely left out of postural evaluations, because nobody seems to know what to do about it.  Dentists are more concerned with tooth mechanics than the function of the whole jaw system, and orthopedists don't know how to deal with joint surfaces composed of jagged rocks. 

To understand the role of the bite and the mandible in postural and TMJ disorders, it must be recognized that mandibular posture is under two different and sometimes competing influences - one from the tonus of the postural system and one from the neuromuscular reflexes designed to protect the teeth.  When there is a discrepancy between the mandibular posture that would be preferred by myofascial forces and the mandibular posture that is determined by the reflex protective mechanisms associated with the bite, mechanical strain is shared between the jaw and postural systems, and successful treatment requires addressing both systems.  Treating the symptomatic muscles often just transfers the pain to a different set of muscles. 

MYOFASCIAL TONUS is a network of light steady tensions,normally about one percent of each muscle's maximum voluntary contractile force, that maintains a habitual weight bearing postural stance in which each bone has resting place, a neutral zone, in balance between opposing tensions. Over time, the positions of the muscles and bones in the habitual stance becomes cemented in the fascia that connects them all.  The mandible rests in a neutral zone within the curtain of muscles draping from the front of the top of the cranium down onto the shoulder girdle and clavicles.  At least it would rest there if teeth were not involved.

PROTECTIVE REFLEXES  - The myofascial influence on mandibular posture in modern humans is often overridden by neuromuscular reflexes that were designed to protect the teeth.  During our evolution, the jaw muscles were strong, and the teeth were fragile; therefore the jaw muscles are programmed to protect the teeth by only closing the mandible where the teeth fit in a stable central bite position and to hold the mandible in a resting posture just beneath that stable central bite position so they can rapidly and securely brace the mandible at the first hint of danger by clamping it immoveably up against the underside of the cranium through the medium of the bite.  As a result of that protective reflex, the postural location of the mandible in a horiaontal plane is controlled by the location of its central bite position.

BACKWARD MANDIBULAR POSTURE - resulting from a backwardly shifted bite, is almost always found in TMJ disorder patients on at least one side, usually the side of the most damaged TMJ.  In most of these cases, the mandible has acquired a backwardly located position relative to the rest of the cranium, because its normal forward translation (advancement) has been blocked by a bite that locks it back posteriorly.  With its normal forward growth inhibited, the mandible gradually shifts backward relative to the rest of the cranium and the cervical spine.      

FORWARD HEAD POSTURE - results from backward mandibular posture, because the backwardly shifting mandible constricts the airway against the cervical spine (middle illustration below), which triggers the airway protective reflexes to extend (tip back) the head to rotate the mandible and hyoid bone forward out of the airway space. That reflex extension of the head causes forward head posture, because the head cannot just tip backward due to the "righting reflexes" that always keep it level for operation of the visual and balance systems.  Therefore, in response to backward mandibular posture, as in the middle illustration below,the head shifts forward while tipping back, as seen in the right side illustration below.

MANDIBULAR RETRUSION CAUSING FORWARD HEAD POSTURE

Of course, the causal chain goes both ways.  Forward head posture can cause the posture of the mandible to shift backward relative to the head, because the mandible is still tied to the clavicles and shoulder girdle by the muscles of the front of the neck and therefore cannot shift as far forward as the head.  However, the longitudinal studies that have looked at the shifting relationship between head posture and mandibular posture have indicated that the shift in mandibular posture usually comes first. 

The forward shifting of the head diminishes the normal cervical curve (lordosis) by bringing its top end forward over its base, as seen in the progression below, where the cervical lordosis flattens (middle illustration) and eventually reverses (right side illustration).  In the illustration, the 15 degree rotation of the vertical line shows the cervical spine bending forward and rotating clockwise from a 12:00 position to a 1:00 position relative to the shoulder girdle in order to support the forward head posture, and the descent of the upper horizontal line shows the loss of vertical height that occurs when the head rotates forward.     

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            NORMAL HEAD POSTURE                                    FORWARD HEAD POSTURE                                     EXTREME FORWARD HEAD POSTURE                              AND NORMAL JAW POSTURE.                              AND BACKWARD JAW POSTURE                          AND EXTREME BACKWARD JAW POSTURE

The tilting of the lower horizontal line in the illustrations above is due to the rotation of the inner and upper aspects of the shoulder blades as these areas follow the forward shifting of the base of the neck.  Since the outer and lower portions of the shoulder blades do not follow the forward movement of the base of the neck as closely as their upper and inner portions, the shoulder blades also tilt, leaving their outer and lower portions sticking out like wings.

THE EFFECTS ON THE LUMBAR AND THORACIC SPINE

The head is a heavy weight perched on the top of the spinal column like a bowling ball on a broomstick. When it shifts forward relative to the long axis of the spine, (when the ball hangs off one side of the broomstick), it applies torque to the whole length of the spine, which triggers neuromuscular reflexes that reflexively act to restore physical balance by shifting structural support forward beneath the forwardly shifted head.  Usually the hip joints rotate forward (clockwise in the illustration below) to thrust the abdomen forward under the head, while the thoracic curve (kyphosis) increases as the chest sinks backward. These changes in the spine can be seen in the illustrations below.   

                                         NORMAL POSTURE                     FORWARD HEAD POSTURE                                   

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SIDEWAYS (LATERALLY SHIFTED) MANDIBULAR POSTURE

In some TMJ disorder patients, the mandible is also laterally displaced, because it has grown toward one side, usually accompanied by backward mandibular posture on just that side.  The crooked jaw posture commonly begins in adolescence when a large canine or other permanent tooth erupts into a position that forces the mandible to shift sideways in order to close fully.  Subsequently, the closing pathway and bracing position of the mandible quickly adopt the new laterally displaced location, which guides the rest of the erupting permanent teeth into positions that also fit only when the mandible is in the same laterally displaced position.  The sideways shift into that displaced location will feel normal, because the jaw muscles become programmed to always use that position to protect the teeth and to maintain a postural position in line with it.  

TILTED HEAD POSTURE  - Because the lateral mandibular shift is maintained by increased tonus of the temporalis muscle on the side of the shift, the head tips toward the side of the shift, making the opposite eye look higher in photographs.  The lateral mandibular shift also becomes embedded in the subsequent facial growth pattern.  Typically it lengthens that side of the face opposite the shift while compressing the face on the side of the shift, which makes the eye on the side of the shift look smaller than the other eye

IDENTIFYING DISPLACED MANDIBULAR POSTURE - requires wearing a front flat bite plate appliance nightly for a period of time during sleep in order to deprogram the jaw closing muscles that control mandibular position by removing the flow of signals that constantly programs them to always close the mandible wherever the teeth fit.  While wearing the front flat bite plate appliance, the jaw muscles can hold the mandible in any position they choose.  If they choose a position and posture that is different from your normal bite, your bite is probably strained, and your jaw muscles would function better if your bite (and consequently your jaw posture) were located in the new position they have chosen. 

CORRECTING DISPLACED MANDIBULAR POSTURE - usually requires making small adjustments to the bite, in conjunction with muscle retraining to  reposition head posture back over the long axis of the spine, strengthen the muscles needed to hold the new mandible and head postures, and stretch the fascial restrictions that maintained the old mandible and head postures. People with strong muscles will need more stretching, and people with weak jaw muscles will need more jaw muscle strengthening, which requires a stable bite.  

EXTENDING THE BITE TABLE -  can be accomplished by shaving down portions of teeth that are too high (and therefore collide when the mandible shifts toward the improved position), building up portions of teeth that are too low (and therefore leave a gap when the mandible shifts toward the improved position), or orthodontically moving teeth. When a deep or steep overbite is responsible for backward mandibular posture, the contact between the top front edges of the lower front teeth and the backs of the upper front teeth is usually the feature that locks the mandible back, therefore short-term relief can be obtained by shaving down those contacts; and long-term relief can be obtained by wearing a front flat bite plate that redirects nocturnal clenching forces onto the front teeth to slowly reduce the overbite.  In deeply curved (front to back) dental arches, the rear ends of the second molars may also get in the way and need to be shaved down.

MOVING THE BITE TABLEIn some cases, the bite table cannot be easily extended as far as necessary without losing chewing ability, and thus the whole bite may need to be altered by rebuilding the teeth using crowns or onlays or by removing tooth structure from the path of the mandibular shift and adding artificial tooth structure behind the path of the mandibular shift.  

CONCLUSION - Mandibular posture and head posture function together and should be treated together to get both the mandible and the head centered over the long axis of the spine in a habitual postural stance.  Dentists performing multiple crowns should recognize that stabilizing the existing natural bite also stabilizes the existing body posture, and some patients may be best served by incorporating a period of time for improving body posture before locking it in by finalizing the bite. Orthodontists treating class 2 malocclusions should recognize that postural treatment to shift the head back to a position more nearly over the top of the spinal column could be helpful in keeping the mandible properly aligned during the treatment process, which will leave it properly aligned after treatment.  Postural adjustments should be undertaken with the awareness that steep interdigitation of upper and lower teeth can return the mandible exactly to its pre-adjustment biting position, which can cause other postural adjustments to relapse.