THE ROLE OF BODY POSTURE
The following paper describes the unappreciated but vital relationship between body posture, TMJ disorders, and the bite table. A more detailed description, along with extensive footnotes, can be found in BITES AND BODY POSTURE under the FOR DOCTORS tab.
BACKGROUND - The posture of the mandible (the lower jawbone) affects the posture of the head, because the mandible is an integral member of the head posture mechanism; and head posture affects body posture, because the head and neck constitute about ten percent of the body's weight, so their position determines how the spinal column aligns beneath them to support the weight on top. Thus, the posture of the mandible affects the posture of the rest of the body. However, the mandible is the only bone routinely left out of postural evaluations, because dentists don't understand its orthopedic function, and orthopedists don't know how to deal with joint surfaces composed of jagged rocks.
To understand the connections between the bite and the postural system in TMJ disorders requires recognizing that mandibular posture is under two different and sometimes competing influences - one from the myofascial tonus of the postural system and one from reflexes designed to protect the teeth. When these influences are not aligned, mechanical strain is shared by the jaw and postural systems, and successful treatment requires addressing both. Treating the pain in one set of muscles usually just transfers it to a different set of muscles.
MYOFASCIAL TONUS - is a network of cooperative tensions (tensegrity) that maintains your habitual upright postural stance with a light steady skeletal muscle tonus of about one percent of maximum force. Collectively these tensions hold each bone in a neutral zone, balanced between opposing forces. If a bone is surgically moved away from its neutral zone, these tensions can move it back in, requiring metal fasteners to stabilize many orthopedic corrections. The mandible rests in a neutral zone within the myofascial curtain 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.
PROTECTIVE REFLEXES -The myofascial influence on mandibular posture in modern humans is often overridden by protective neuromuscular reflexes that were wired in to protect the teeth by always holding the mandible just beneath its most stable bite position so the mandible can be rapidly clamped up against the bite table. During evolution, the teeth were fragile, and the jaw muscles were strong; so the jaw muscles were programmed to protect the teeth, even if it must hold them in a strained postural position. As a result, the postural location of the mandible in a horizontal plane is controlled by the location of its most stable bite position, and any displacement of that most stable bite position causes a parallel displacement of mandibular posture, which is so thoroughly accommodated by the muscles that it feels normal.
BACKWARD MANDIBULAR POSTURE - resulting from a backwardly shifted bite,is almost always found in TMJ disorder patients, especially on the side of the most damaged TMJ or the side to which the chin is displaced by the bite. In most cases, the mandible has acquired a backwardly located position relative to the rest of the cranium due to a facial growth pattern in which inadequate jaw muscle development and/or a deep or steep overbite has prevented advancement of the mandible. The resulting backward mandibular posture pins the tongue back against the cervical spine.
FORWARD HEAD POSTURE - results from backward mandibular posture, because the mandible surrounds the airway on three sides; and a 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 upward and forward out of the airway space. However, the head cannot just extend, because the "righting reflexes" always keep it level with the horizon for the operation of its visual and balance systems. Thus the only way the head can extend is to simultaneously shift forward, as in the right side illustration below.
Of course, the causal chain goes both ways. Forward head posture can cause backward mandibular posture, because the mandible is tied to the clavicles and shoulder girdle by the pre-cervical muscles, and therefore it cannot shift as far forward as the head. However, longitudinal studies have indicated that the backward shift in mandibular posture usually comes before the forward shift of head posture.
EFFECT ON THE NECK - Forward head posture 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). The clockwise rotation of the vertical line from a 12:00 position to a 1:00 position shows the cervical spine tilting forward to follow the head, and the descent of the upper horizontal line shows the loss of vertical height that occurs when the head rotates forward.
NORMAL HEAD POSTURE FORWARD HEAD POSTURE EXTREME FORWARD HEAD POSTURE AND NORMAL JAW POSTURE. AND BACKWARD JAW POSTURE AND EXTREME BACKWARD JAW POSTURE
EFFECT ON THE SHOULDERS - The tilting of the lower horizontal line in the illustrations above shows the rotation of the inner and upper aspects of the shoulder blades following 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 rotate around a vertical axis, leaving their outer and lower portions sticking out like wings.
EFFECTS ON THE LUMBAR AND THORACIC SPINE - The head is a heavy weight perched on the top of the spine like a bowling ball on a broomstick. When it shifts forward, so the ball hangs off one side of the broomstick, it applies torque to 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
SIDEWAYS (LATERALLY SHIFTED) MANDIBULAR POSTURE - In some TMJ disorder patients, the mandible is also displaced laterally, almost always accompanied by backward mandibular posture on the side to which the mandible has shifted. The displacement usually 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. Subsequently, both 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 reflexively programmed to always bite in that position and to maintain a postural position just beneath it, wherever it is.
TILTED HEAD POSTURE - When the mandible shifts to one side, the head tips toward that side, because the shift is maintained by increased tonus of the temporalis muscle on the side of the shift, and the temporalis is a key postural muscle. The head tilt to the side of the shift makes the eye of the opposite side look higher in photographs, if you use a ruler to connect the pupils, and it also usually makes the face on the side of the shift look compressed.
IDENTIFYING DISPLACED MANDIBULAR POSTURE - requires wearing a front flat bite plate appliance to deprogram the jaw muscles by removing the flow of signals that programs them to always close the mandible into the most stable bite position. Once the jaw muscles are deprogrammed, they can hold the mandible in any position they choose. If they choose a position that is different from the one used in your old habitual bite, your old habitual bite is strained, and your jaw muscles would function better if your bite (and consequently your jaw posture) were located in the new position the jaw muscles have chosen. In people with strong jaw muscles, the deprogramming happens rapidly, especially if combined with myofascial release to stretch the fibers that were maintaining the old habitual bite. In people with jaw muscles that have been weakened by atrophy due to extensive orthodontics, chronic pain, or Botox; the muscles may need an exercise program along with bite stabilization before thay can consistently demonstrate a healthy deprogrammed jaw closing trajectory.
EXTENDING THE BITE TABLE - toward a more healthy bite and postural position 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 backward facing surfaces 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 TABLE - In some cases, the bite table cannot be easily extended as far as necessary without losing chewing ability, so the whole bite table can be shifted by adding artificial tooth structure behind the shift in addition to removing tooth structure from the path of the shift, requiring extensive dental work.
CONCLUSION - The goal is to get the mandible correctly positioned under the head and get the head centered over the long axis of the spine in a healthy balanced habitual postural stance. Dentists performing multiple crowns should recognize that stabilizing the existing natural bite also stabilizes the existing habitual body posture, and some patients may be best served by incorporating a period of time for improving their habitual 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 can help them advance the mandible, and ensuring proper alignment of the mandible during the treatment process can leave it properly aligned at the conclusion of treatment. Postural adjustments should be undertaken with the awareness that the interdigitation of upper and lower teeth can return the mandible to its pre-adjustment biting position in three dimensions within microns, which can cause other postural adjustments to relapse.