POSTURE AND THE BITE

 SUMMARY

The lower jawbone is an integral component of the head posture mechanism, and head posture determines how the spine aligns beneath it to support the weight on top; therefore improving your body posture requires a synergistic change in your jaw posture.  Ideally the posture of the lower jawbone should fit in with the constant background myofascial forces of the rest of the postural system.  However, lower jawbone posture is determined by the way the teeth fit together in the full natural bite. Wherever this bite position is located, the lower jawbone acquires a resting posture just beneath it.  Commonly, because the lower jawbone is forced backward into the full natural bite, the postural position of the lower jawbone is also forced backward.  Backward lower jawbone posture causes a forward head posture because of the inverse relationship between jaw position and head position in upright posture. Most TMJ disorder patients have both backward jaw posture and forward head posture. These two conditions arise together, and they should be treated together.

TWO JAW POSTURES

In the body's habitual weight bearing posture, each bone has natural resting place, called a neutral zone, at an equilibrium between opposing background tensions within the curtain of muscles and fascia that hold all the skeletal components together in one big continuous myofascial sheath.  Within this sheath, the lower jawbone rests in a postural neutral zone between the face and the clavicles. At least it would rest there if there were no teeth involved.

However, because of neuromuscular reflexes that were designed to protect the teeth, the lower jawbone's resting posture is also determined by the bite - in particular by the exact position that the lower jawbone acquires each time the back teeth of both sides are closed firmly together in a full natural bite.  This stabilizing of the lower jawbone by bracing it in the full natural bite was so important in evolution that the jaw muscles are programmed to hold the lower jawbone in a postural position just below its full natural bite position in order to maintain fast easy access to bracing.   In this manner, the bite determines lower jawbone resting posture. Therefore, if that full natural bite position is displaced, lower jawbone resting posture also becomes displaced in the same general direction.

When there is a discrepancy between the lower jawbone posture determined the myofascial neutral zone and the lower jawbone posture determined by the bite, mechanical strain is shared between the jaw and postural systems.  Frequently muscle soreness and pain fluctuate back and forth between the jaw and postural systems.  If overall body posture is otherwise good, the symptoms can be relieved by making jaw posture better fit body posture by altering the bite in a way that moves the bracing position of the lower jawbone toward a location just above its neutral zone.

If there is no discrepancy between the lower jawbone posture determined the myofascial forces of the neutral zone and the lower jawbone posture determined by the bite, your jaw and body posture may be at an equilibrium that provides stability, but they may both be strained.  In that case, improving them requires a change in all the postural components, including the lower jawbone. Because lower jawbone posture is determined by lower jawbone position in the full natural bite, changing lower jawbone posture requires changing your natural bite. 

THE FRONT FLAT BITE PLATE APPLIANCE 

The best way to determine if there is a discrepancy between the lower jawbone posture that would be determined by the resting forces of the weight bearing body stance and the lower jawbone posture that is determined by the full natural bite requires wearing a front flat plate appliance during sleep.  The flat plate removes the flow of signals from the teeth that constantly program the jaw muscles to always bring the lower jawbone closed wherever the teeth fit.  After this deprogramming of the jaw muscles, we can see the jaw posture that is preferred by the muscles, without any influence from the teeth.

BACKWARD JAW POSTURE

In the vast majority of TMJ disorder patients, the bite and the jaw posture that is automatically located just beneath the bite are located rearward from the postural neutral zone. Thus the jaw muscles have to pull the lower jawbone backward to fit all the teeth together in a full natural bite, and they hold the lower jawbone in a backwardly strained postural position.

Backward lower jawbone posture causes the face to grow backward as well.  Experimental studies have shown that a backward bite causes an increase in the postural tension of the posterior temporalis muscles and also that light continuous forces like background postural tensions control bone growth.   A light continuous backward pull on the lower jawbone changes the direction of growth of the jawbone from forward to down-and-backward. The rest of the face follows the lower jawbone. The midface (including the upper jawbone) grows downward and backward instead of outward. As a result, it does not fully unfold, leaving the palate and the nasal cavity too narrow.

 FORWARD HEAD POSTURE

Backward lower jawbone posture then causes forward head posture by triggering reflex airway protective mechanisms. The face grows around its airway.  The lower jawbone surrounds the airway in front and on both sides, and the cervical spine borders it from behind.  Backward lower jawbone posture constricts the airway against the cervical spine (middle illustration below). The muscles automatically respond by tipping the head backward (extending it) in order to rotate the lower jawbone forward out of the airway space.  However, the head cannot just tip, because it has to stay level for the visual and balance systems, so it tips backward while shifting forward, resulting in forward head posture, as seen in the illustration on the right below.

Forward head posture diminishes the normal cervical curve (lordosis) of the neck by bringing its top end forward over its base, as seen in the progression below.  First the cervical lordosis flattens, as seen in the middle illustration.  Eventually the cervical lordosis may reverse, as seen on the right side illustration.  The tilting of the vertical line shows the cervical spine bending forward to support the forward head posture, the descent of the upper horizontal line shows the loss of vertical height that occurs when the position of the head rotates from a 12:00 position to a 1:00 position, and the tilting of the lower horizontal line shows 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 the upper and inner portions, the shoulder blades rotate around a largely vertical axis, often leaving their outer and lower portions sticking out like wings (winged scapulae). 

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NORMAL HEAD POSTURE         FORWARD HEAD POSTURE                EXTREME FORWARD HEAD POSTURE
 
 THE EFFECTS ON THE BACK

Forward head posture can produce strains throughout the 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 off center, torque is produced along the length of the broomstick.  However, forward head posture also triggers an adaptive change that maintains physical balance by shifting the lower portions of the spine forward beneath the forward positioned head. Typically, flexion of the hip joints thrusts the abdomen forward under the head, and the thoracic curve (kyphosis) increases as the chest sinks backward. These changes in the spine can be seen accompanying backward jaw posture from left to right in the illustrations below.  Usually these changes occur together.  The causal relationship can go both ways. 

 
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 NORMAL POSTURE                                   FORWARD HEAD POSTURE                    EXTREME FORWARD HEAD POSTURE
 

Contributing to the treatment of forward head posture by adjusting the bite requires shaving down portions of teeth that collide first because they are too high when the lower jawbone comes forward, building up portions of teeth that are too low when the lower jawbone comes forward, or a combination of the two.  The choice usually depends on face height and dental situation.  

When a deep overbite is present, it is often the contact between the top front edges of the lower front teeth and the middle of the back sides of the upper front teeth that most directly locks the lower jawbone backward.  This contact can usually be felt by placing a finger on the front edges of the upper front teeth while trying to tap only the back teeth together.  Solid contact on the back teeth should not be felt strongly on the front teeth. If it is, short term relief can be obtained by shaving a little off the top front edges of the lower front teeth, which does not harm those teeth.  Long term relief can be obtained by wearing a front flat bite plate that redirects nocturnal clenching forces onto the front teeth to gradually drive them back into the bone and thereby reduce the overbite.

SIDEWAYS (LATERALLY SHIFTED) JAW POSTURE

In some TMJ disorder patients, the lower jawbone is also shifted sideways. This shift can be estimated by comparing the locations of the midlines between upper and lower front teeth in a mirror.  It is only an estimate, because the dental midlines are not reliable guides due to the many factors that can shift them, including orthodontics.

Typically, in these cases, placing a grid over a picture of the face shows that the chin is shifted toward one side and the eye of the opposite side appears higher. The opposite eye looks higher not because the eye socket has moved but because the head has tilted toward the side of the lower jawbone (and chin) shift due to increased postural tensions in the temporalis muscle on that side, the muscle responsible for jawbone posture. 

The sidebending of the head disrupts the normal parallel relationship between the laterally extended postural members - head, jawbone, shoulders, hips, and two feet spread slightly apart.  Typically, as the lower jawbone shifts and the head tilts to one side, the head tilt pulls upward on the shoulder girdle of the opposite side.   The hips often follow, depending on the adaptation of the spine to the shifting of the weight on top.  

The asymmetric lower jawbone position forces the face to keep growing asymmetrically during adulthood.  Our jaws were designed to keep growing in adulthood to compensate for tooth wear that kept shortening the teeth during adulthood in our ancestors. Without such compensatory growth, the chins of our ancestors would have steadily approached their noses, and they would become unable to chew effectively, a skill on which their lives depended. Thus the growth pattern of the face does not stop after the second decade when other growth processes stop.  It just slows down to about ten percent of its previous rate.  An asymmetric or retrusive facial growth pattern tends to gradually worsen the problem.   Adjusting the bite toward the midline does not reverse all the asymmetry, but it at least normalizes subsequent facial growth. 

In these cases of sideways displacement of lower jawbone and head, adjusting the bite to reduce or eliminate the displacement almost always involves repositioning the lower jawbone toward the midline by either building up teeth on the side of the displacement, shaving down teeth on the side opposite the displacement, or some combination of the two.  The choice depends on factors like your face height and the condition of your teeth.

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 spinal segments. Localized areas of high pressure can squeeze a portion of an intervertebral disk backward out of the intervertebral joint space and into the spinal nerves, causing pain, numbness, and loss of muscle strength.  

Degenerative changes in the intervertebral joints can limit their ranges of motion, depriving the specialized articular surfaces of the rubbing movements they need for local circulation.  Because the weight bearing surfaces of joints function under compression, they cannot be directly supplied by blood vessels.   Instead, they receive their nourishment from a hydrostatic process that circulates fluids like repeatedly squeezing out a sponge in one spot at a time.  Waste products are driven out of the area under compression, and then new fluids flow back in to that area as soon as the point of compression moves to a different area. When movement of the bones at the joint is smooth and variable enough to spread this compression and release process widely around the surface of the joint, it keeps the entire surface of the cartilage healthy. Because of this functional circulation process, passively moving a damaged joint has been shown to dramatically reduce its healing time, and immobilizing a healthy joint causes it to undergo arthritic changes.  Restoring normal ranges of motion to injured joints is one of the mechanisms by which chiropractic adjustments provide relief.

STRAINS ON MUSCLES

Muscles far from the source may be affected, because the postural muscles, including the jaw 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.  Backward jaw posture and forward head posture can upset the balance between the chains of postural muscles on the front and back of the body and thereby lead to the loss of a single central upright resting posture.  Subsequently the muscles and bones no longer fit together well enough in a single weight-bearing stance to allow them to rest simultaneously.  They may avoid discomfort by frequently shifting stances such as rocking back and forth to alter the muscle groups receiving the strain.   

MUSCLE CONTRACTURE

When muscles have been held tight for long periods of time, they shorten anatomically and acquire a decreased resting length in a process known as contracture.  When your jaw muscles are in contracture, your mouth no longer hangs so far open at rest. For example, when you fall asleep in a chair, it may be only open slightly or the teeth may be held touching, and they may go into a tight clench whenever overall resting muscle tensions are increased even slightly by stress (as explained below).  Muscles in contracture have diminished resting circulation.  For short term relief, they can be massaged to flush waste products from their capillary beds.  For long term relief, they need stretching (along with their fascia) to restore their resting lengths.  

An easy way to stretch the jaw muscles is by icing them while the jaw is mechanically propped open.   The jaw can be propped open using a block such as a wine cork or a piece of wood placed between the front teeth.  It does not need to be open as wide as possible.  At the same time, the jaw muscles are iced by holding ice packs or bags of frozen vegetables against the sides of the face. The ice with stretch is maintained for 5 to 10 minutes. Then the block is removed and the ice is replaced by a hot wet towel. The heat will feel good after the ice, and the ice will feel good after the heat. The two modalities (ice with stretch, and heat with relaxing) can be alternated several times while watching a movie.   

TRIGGER POINTS

Muscles in contracture often develop trigger points - commonly described as pea shaped nodes or knots that are exquisitely sensitive to manual pressure.  They can cause pain at locations surprisingly far away in typical "pain referral" patterns, and they can persist long after their original cause has been eliminated.   For short term relief, trigger points can be treated by forceful compression of the site using a finger, knuckle, or elbow. The greater the force used, the more sustained the relief.  There are many dentists and doctors who inject trigger points with novocaine or saline. The immediate pain is intense, but the relief can last a long time.

THE ROLE OF STRESS

Increased central nervous system stress can worsen a muscle related condition in any part of the body by increasing resting muscle tensions all over the body.  When a muscle has already been operating at borderline resting circulatory levels due to tightness from automatic bracing to protect an injured joint, even a slight increase in overall stress can bring its resting circulation below a threshold level that results in pain. For that reason, a period of increased central nervous system stress often precedes the onset of postural muscle symptoms, and relief can often be obtained from stress lowering measures like relaxation, meditation, humor, spiritual inspiration, or feeling loved.

THE ROLE OF ATTITUDE

While stress increases all resting muscle tensions, attitude selectively alters resting muscle tensions.  The motor nerves are anatomically and physiologically extensions of the brain. By controlling their resting tensions, the state of the brain determines which muscles are held tightly and which are held loosely.  Feeling sad makes the lower jawbone drop back, the chest sink, the head hang, and the shoulders slump. The extremely retrusive lower jawbone posture in grief causes a lump in the throat by triggering an airway protective response to compensate for airway space lost due to extreme lower jawbone retrusion. Conversely, feeling proud or determined causes forward thrusting of the chest, standing tall, and jutting the lower jawbone, which can also be seen in facial expressions of anger or determination.

PASSIVE PHYSICAL SUPPORT

The passive physical support provided by your furniture influences your posture.  In order to accommodate the "average" person, most seats on planes, trains, and buses have head rests that force the head forward.  Sleeping on your back on a soft mattress such as a waterbed, an air mattress, or memory foam also produces forward head posture, because it allows your body to sink down at its center of mass (usually at the hips or abdomen). A mattress made of inexpensive foam rubber (without memory) or a futon reinforces good body posture by constantly pushing your spine towards a straight axial alignment that supports your legs and head in the same plane as your abdomen, hips, and shoulders.

Pillows can profoundly influence head posture.  When back sleeping, the back of your head should be in contact with your mattress. A pillow that holds the back of your head above your mattress reinforces forward head posture. Instead, your pillow should provide support under your neck rather than under your head. The 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. A roll under your neck can be combined with pillows on each side of your head so that, when you roll over to either side, your head rolls up onto a pillow that is roughly as thick as the distance between your ear and your shoulder.

ACTIVE PHYSICAL SUPPORT

The same improved skeletal alignment that you maintain with passive physical support must also be acquired habitually by your skeletal muscles in a normal weight bearing stance.  A simple way to improve your standing posture is by holding yourself as tall as possible.  Both the flattening of the cervical spine and the increased thoracic and lumbar curvatures that accompany forward head posture cause loss of vertical height, and restoring some of that height can restore some of the lost spinal support.  

EXERCISE

You can't just hold your skeleton in an ideal alignment, or your muscles will rapidly become exhausted. Before you can hold an improved posture, you will need to strengthen the muscles that can pull you out of your old posture and into a better one.  One way to selectively strengthen those muscles is by making an exercise of repeatedly allowing yourself to slouch and then pulling yourself back up out of the slouch and into ideal posture. 

NECK exercises are particularly important for improving head posture.  You can work your head back against resistance provided by a head rest in a car or your hands held behind your head without taking time out of your life. The neck muscles can't be strengthened without also strengthening the muscles of the shoulders and back, which form the base of operation for the neck muscles.  Dynamic exercises (involving movement) are generally healthier for joints than isometric exercises, because movement provides weeping lubrication and circulation at the joint surfaces.

Exercises used to rehabilitate muscles for health are different from exercises used to strengthen muscles for maximal muscle power output. Rehabilitation exercises should be performed in rhythmically alternating contractions of about a second each to mimic the muscle firings during functional activities like chewing, walking, climbing, hitting, etc. While one muscle group (agonists) is contracting, its antagonists relax and receive a quick shot of fresh blood.  In this manner, healthy exercise rhythmically flushes tissues and pumps circulation.

STRETCHING

Just as some muscles need to be strengthened or tightened, other muscles need to be stretched and loosened. The balance depends on the individual. Generally muscular people need more stretching, and frail people need more strengthening.  To have longlasting effects, stretching also has to include the tough sheets of fascial webbing that bind all the muscles together into long functional units (myofascial release). 

COMBINING stretching and strengthening can be accomplished by exercises such as Yoga, Tai Chi, swimming, or pulling yourself upward using overhead bars. Even if you don't pull yourself off the ground, applying some traction to your spine by pulling upward with your arms serves to distract the compressed vertebral segments and thereby allowing new blood to flow in.  Pulling upward rhythmically alternating with full release creates an exercise which strengthens the arms and shoulders while pumping circulation to the vertebral segments by alternating distraction and compression.

CONCLUSION

Lower jawbone posture and head posture always function together and they should be treated together. Dentists performing multiple crowns, dentures, or orthodontics, should recognize that stabilizing the existing natural bite also stabilizes the existing body posture, and the patient may be best served by incorporating a period of time for improving the posture before finalizing the bite.  Postural adjustments should be undertaken with the awareness that, unless the bite is addressed, they will tend to relapse shortly after the patient resumes biting on his or her natural teeth, which causes a return of the previous lower jawbone posture and subsequently also the same head and body posture.