Multilevel Oral Appliance Treatment for Obstructive Sleep Apnea
SUMMARY - Obstructive sleep apnea (OSA) occurs when the pharyngeal airway gets plugged by the tongue base; sometimes with soft tissues such as the distal end of the soft palate filling in the space between the tongue base and the pharyngeal wall to complete the seal like a gasket in the nasopharynx; and sometimes with the epiglottis just beneath the tongue base closing the glottis like a trap door in the hypopharynx. Because the obstruction usually occurs at multiple levels of the pharynx, surgery is only predictably effective if it is multi-level. CPAP is usually effective, because it makes the pharynx too large to be obstructed at any level above the epiglottis; but it is difficult to tolerate and likely to interfere with glymphatic circulation. Mandibular advancement is more tolerable than CPAP; but it's not always effective, because the mandible is only loosely attached to the tongue base and soft tissues that produce the actual obstruction. Now there are new modalities that can be added to oral appliances to also control the soft tissues that actually produce the obstruction at each level of the pharynx. The soft palate can be elevated centrally to draw its distal end antero-superiorly out of the nasopharynx, the tongue body can be held to prevent it from dropping back into the oropharynx, and the tongue base can be pried by its lateral borders antero-inferiorly away from the pharyngeal wall in the hypopharynx. The new modalities are added one at a time to ineffective mandibular advancement appliances until follow-up sleep testing shows that the problem is eliminated. For edentulous patients, the new modalities can be carried on denture base plates; where the mandible is supported rather than advanced, and the tongue holding is especially effective, because the tongue gripping surfaces can extend all the way out to the cheeks, and tehy are cushioned by the light steady pressure of the tongue body.
RATIONALE - Current medical treatment for OSA is inadequate. Minor surgeries include cutting out areas of soft tissue such as prominent tonsils or the distal end of the soft palate (uvulopalatopharyngoplasty or UPPP), advancing the genioglossus muscle, suspending the tongue base along with the hyoid bone from a suture tethered to a screw in the lingual plate of the symphysis,1-2 implanting an electrical stimulator in the hypoglossal nerve to dilate the pharynx by activating the genioglossus and geniohyoid muscles to protrude the tongue base during each inhalation,3 and radiofrequency ablation or surgical reduction of the tongue base. These each work on some people; but it's difficult to predict which people. Predictable success surgically requires multi-level treatment, such as maxillo-mandibular advancement, which can advance the tongue base by 8-10 mm in both upper and lower jaws.4 However, even such an extensive structural correction only brings AHI to under 5 in less than half of the patients, and its effect can wane over time if not accompanied by a supportive change in the activity of the jaw muscles.
Most OSA patients are treated with CPAP; but it is difficult to tolerate for many people, who then do not use it regularly; and the continuous positive pressure which is central to its operation must eliminate or at least reduce the intermittent negative pressures that power the nightly drainage of cerebrospinal fluid from the cranium through glymphatic circulation.5-10 During normal sleep, each inbreath sucks between a tenth and a half of a drop of cerebrospinal fluid from your cranium. CPAP applies continual positive pressure in that same tubing system, making it very likely to interfere with the drainage. Also glymphatic circulation diminishes with age, which may be an important cause of neurodegenerative diseases, the biomarkers of which first show up in the cerebrospinal fluid, possibly due to poor drainage.
Current dental treatment is also inadequate. Mandibular advancement appliances are well tolerated; but they have limited effectiveness, because the mandible is only attached to the tongue base by muscles, which naturally lose tonus during sleep. Therefore, even extreme mandibular advancement cannot prevent the tongue base from falling back and obstructing the airway - it just has further to fall; and mandibular advancement has only a 50% success rate, as shown by remarkably similar results in hundreds of studies. Attempts to identify which patients are likely to respond have been generally unsuccessful, so the high failure rate makes mandibular advancement appliances a hard sell. More effective treatment requires more directly addressing the pathophysiology of OSA.
PATHOPHYSIOLOGY - OSA has complex anatomic, physiologic, and sensorimotor aspects; but the event at its core is choking on the tongue base, - the gateway of airway passage between the oropharynx and the trachea and the only structure in the pharynx that is thick enough and strong enough to plug the airway. It can obstruct the oropharynx by directly contacting the pharyngeal wall, the nasopharynx with the help of soft tissues getting sucked into the narrow gap between it and the pharyngeal wall like a gasket, and the hypopharynx by pushing the epiglottis just beneath it closed like a trap door - with considerable overlap between these obstruction mechanics and the different levels of the pharynx. Imaging has found the location of the obstruction too variable to provide a basis for targeted treatment. During drug induced sleep endoscopy, the obstruction occurs most frequently in the nasopharynx, but it also occurs frequently in the oropharynx, behind the tongue, and at the epiglottis.11 It can even seesaw back and forth between these locations at different times.
Some researchers blame OSA on an inadequate neuromuscular arousal mechanism - commonly described as the muscles relaxing too much during sleep, and they target the neuromusculature for treatment, but muscles normally relax during sleep. As soon as a person with a healthy airway falls asleep, the tonus in all their pharyngeal dilator muscles plunges. In response to airway obstruction, these muscles reflexively increase their tonus as much as necessary to restore airway flow. In monkeys, experimental nasal airway obstruction causes the genioglossus to pulse in synchrony with inspiration.12 In humans, the increased carbon dioxide levels resulting from airway obstruction trigger a rapid increase in tonus of the pharyngeal dilator muscles (loop gain), until the arousal threshold is reached and breathing is restored. This arousal mechanism varies widely in its effectiveness. In some people, it successfully opens the airway with each breath (the model for Inspire). In other people, it allows hypopneas but prevents apneas, because rapid loop gain produces arousal that restores breathing before the airway is obstructed for long enough to produce hypoxia. Some people with high loop gain over-react to the increased carbon dioxide levels, which destabilizes the ventilatory system and prevents the return of normal breathing. Still other people with low arousal thresholds have their sleep repeatedly disturbed by minor desaturations or by events that are not associated with desaturation, such as snoring or wearing a CPAP mask. Precision medicine can be used to identify and treat some OSA by providing medications that alter arousal threshold and loop gain, but an inadequate neuromuscular response to the problem should not be considered the cause of the problem, and the goal of OSA treatment should be to provide an adequate resting airway passage that does not require a protective response.
Other researchers blame OSA on collapse of the pharyngeal walls; and they cite the model of a Starling resistor, in which the rate of flow through a tube within a pressurized chamber depends on the pressure in the chamber, and the flow through the tube stops if the pressure in the chamber is greater than the pressure in the tube. In airway flow through the pharynx, the pressure on the chamber is only the weight of the soft tissues surrounding the pharynx and the Bernoulli effect, which stops when the airway flow stops. After that, the Starling model fails, because the pressure from the weight of the soft tissues surrounding the pharynx is much less than the pressure produced inside the tube by a diaphragm desperately trying to inhale. Maintaining the obstruction requires a stopper. The tube does not get squeezed closed by an external force; it gets plugged from inside; and the only structure inside the tube with enough physical integrity to form a stopper is the tongue base. People don't choke on loose tissues like floppy pharyngeal walls, they choke on a piece of meat. The piece of meat in OSA is the 6 cm thick slab of muscle at the base of the tongue.
MANDIBULAR ADVANCEMENT - is one way to address the pathophysiology. The mandible serves as the structural framework for the lower face and the basal bone for the tongue, so advancing the mandible exerts an anterior bias on the tongue, stretches many of the pharyngeal soft tissues, and moves the muscle attachments. It has been shown to reduce AHI in clinical studies and increase Pcrit in experimental studies.
HERBST MANDIBULAR ADVANCEMENT APPLIANCES - have the longest history of effective treatment for OSA and the best control of mandibular position; but the basic hardware was designed more than a century ago for bite jumping in children. When used for adults with OSA, the bolted connectors protrude into the lips and restrict lateral mandibular movements, which leads to frequent breakage; the downward vector of force on the mandible can open the lips and lead to habitual mouth breathing; and adjustments require a special tool and have limited range. To solve those problems, Dr. Summer has developed and tested (now in FDA review) improved telescopic hardware. The high-push Herbst has a lower profile (doesn't push into the lower lip), freely moveable connectors that cannot bind or restrict lateral movements, an upward vector of force on the mandible that has less tendency to push the mouth open, and an unlimited range of adjustment just by sliding the rod out of the tube, rotating the tubing assembly on the externally threaded offset connector arm, and re-inserting the rod into the tube to lock in the adjustment.
PALATE EXPANSION - can be included in the high push Herbst appliance by using detachable telescopic components. The upper appliance member, containing the expansion screw, is used by itself all day. Then every night, the lower appliance and telescopic components are added. Some dexterity is required, but most people acquire the needed skills rapidly.
TITRATING - mandibular advancement to a position of maximal tolerable benefit improves effectiveness. Some people can tolerate extreme mandibular advancement. In others, it can damage teeth or their supporting structures, and its beneficial effects on the shape of the airway diminish slowly over time due to osseous remodeling. To maintain support for the mandible as it advances, the bite platform should extend anteriorly well beyond the labial surfaces of the upper anterior teeth, as shown in the photos below; because loss of anterior support when the mandibular anterior teeth advance past the anterior bite platform greatly increases the forces applied to the connectors and can thereby lead to breakage of any Herbst appliance.
TESTING - After titrating a mandibular advancement appliance, its effectiveness should be tested. Even partial reductions of OSA can feel like successful treatment to the patient, but partial successes usually fade over time, because the remaining incidents keep stretching out the pharyngeal soft tissues. Multi-night home sleep testing eliminates the first night effect and night-to-night variability.14
Unfortunately, follow-up testing shows that mandibular advancement has limited success. Advancing the mandible creates space anterior to the tongue, but it cannot advance the tongue into the space created, because the mandible is only attached to the tongue by muscles, which naturally lose their tonus during sleep. As a result, even extreme mandibular advancement cannot prevent the tongue base from falling back into the airway - it just has a little further to fall; and mandibular advancement only cures about half of the patients treated, as shown by remarkably similar results in hundreds of studies. Attempts to identify anatomical features that indicate which patients are likely to respond to mandibular advancement have been generally unsuccessful, so the high failure rate makes mandibular advancement appliances as a solo treatment a hard sell. More effective treatment requires also clearing the soft tissues from the different levels of the pharynx, as described below.
CLEARING THE NASOPHARYNX - In the nasopharynx, the loose pharyngeal soft tissues can contribute to the obstruction by getting sucked into the narrow space between the tongue base and the pharyngeal wall to form a seal that plugs the airway like a gasket. In children, the loose soft tissues often come from the lateral pharyngeal walls, such as swollen tonsillar tissues. In adults, they usually come from the distal end of the soft palate.
The distal end of the soft palate is untouchable, because it is full of gag reflexes; but it can be drawn away from the obstruction by elevating the center of the soft palate, where there are no gag reflexes. The soft palate is a flexible flap of thin muscles suspended from a keel-like midline aponeurosis, and its muscles lose their tonus during sleep, leaving its distal end swinging back and forth due to airway flow.13 In this state, the soft palate can be "tented" by elevating the center of the aponeurosis about 1/4" using a silicone rubber bulb on the end of a thin flat arm. The distal end of the soft palate probably only moves a fraction of that distance, but any opening of the gasket can restore airway flow. The light pressure is easily tolerated, and the arm is flexible enough to comfortably accommodate the functional movements of the soft palate during swallowing, when breathing stops anyway. Most patients don’t even know it’s there.
The "tenting" of the soft palate can be seen in the X-rays below, where the black arrows mark the top of the soft palate and the thin white line in the right side X-ray is the metal arm of the soft palate elevator. A tongue holding device is also used in the right side X-ray.
BEFORE INSERTING THE SOFT PALATE ELEVATOR AFTER INSERTING THE SOFT PALATE ELEVATOR
Tenting the soft palate also tightens the nearby pharyngeal wall, as was demonstrated below in a patient who had previously undergone UPPP, after horizontal lines were stamped on the pharyngeal wall with gentian violet. Inserting the device caused the straight lines on the left side photo below to become curved on the right side photo. The resultant tightening of the pharyngeal wall makes it less easily sucked into obstructive contact with the tongue base and less prone to vibrate during snoring.
The soft palate elevator is usually employed together with mandibular advancement in the first phase of treatment, because these two modalities work better together than either one alone. Simply removing the gasket without also making changes in the functional environment that led to gasket formation invites relapse due to formation of a new gasket, and simply changing the functional environment by advancing the mandible may not produce enough change in the anatomy to restore airway flow without also removing the gasket.
CLEARING THE OROPHARYNX
Generally, the oropharynx gets plugged by the tongue base directly contacting the pharyngeal wall, and clearing the oropharynx requires holding the tongue in manner that prevents its base from directly contact the pharyngeal wall.
PREVIOUS TONGUE HOLDING DEVICES - Until now, there have been a couple of different kinds of tongue holding appliances, but they require extreme tongue advancement. The Tongue Stabilizing Device, Tongue Retaining Device, and Good Morning Snore Solution all employ a silicone rubber squeeze bulb that uses suction to grasp the tongue tip; however the bulb is too large to fit in the mouth so it must be held all the way out in front of the lips, producing extreme tongue advancement that is difficult to tolerate; and the tongue tip is far from the obstruction, so they are only about as effective as mandibular advancement. Also, pulling the tongue forward can exacerbate the OSA in some people by pulling a thicker portion of the tongue base up into the area of obstruction. The Full Breath Solution and Zyppah Appliances push down on the middle of the tongue base, which can move it down and away from an obstructing contact superiorly, but into an obstructing contact inferiorly; and they have limited posterior reach, because they contact the tongue base in its midline, where the gag reflexes are located.
THE NEW TONGUE HOLDING DEVICE – holds the entire tongue body and the lateral borders of the tongue base between upper and lower tongue gripping surfaces, which each contain thousands of tiny bristles that are all slanted forward at a 45 degree angle, to prevent the tongue from shifting posteriorly into the pharynx. The bristles on the upper tongue gripping surface are sized to fit between the filiform papillae that cover the dorsal surface of the tongue, and the tiny bristles on the lower tongue gripping surface grip the sensitive tissues on the underside of the tongue like a non-skid surface. Upper and lower carpets of this “tongue Velcro” are molded to fit each patient's anatomy.
STEEL MODEL FOR MOLD FOR LOWER TONGUE GRIPPING SURFACE LOWER BASE APPLIANCE
Their grip is so effective that the only compressive force they need to hold the tongue between them without discomfort all night is that of a thin wire compression spring that biases one tongue gripping surface, (the upper in dentate patients and the lower in edentulous patients) toward the other. With so little pressure on them, the tongue gripping surfaces effectively float on the surfaces of the tongue. Their flexible anterior tethering to the base appliance allows tongue and mandible movement in every direction except posteriorly. The thin wire spring is long enough to create a cushioned grip which persists until the mouth is open wide, so it cannot be interrupted by the submaximal mandible and tongue movements that occur during normal sleep, including bruxism. Opening wide releases the tongue, as shown in the bottom illustration below.
MOUTH CLOSED
PARTLY OPEN
MOUTH WIDE OPEN
The tongue gripping surfaces hold the patient's tongue in a target treatment position, with its tip between the incisors, as seen below. Because the tongue cannot escape during sleep, patients awake with the tongue in the same position. More advanced tongue positions can be used, but they are rarely necessary. The tongue holding device for dentate patients includes bite stops, small flat stable plateaus of acrylic located over the disto-buccal cusps of the terminal molars, where they absorb all bite forces before those forces can hurt the tongue. In the photo below right, the upper bite stops are made of yellow acrylic, and the lower bite stops are made of green acrylic.
The X-rays below show a patient's tongue with a radiopaque paint before and after inserting a tongue holding device. In the top X-ray, the radiopaque paint forms a puddle that looks like a white V shape in the foramen cecum. In the bottom X-ray, the radiopaque puddle in the foramen cecum has been replaced by a larger white V shape where the tongue has curled around the end of the upper tongue gripping surface alongside the distal root of the second molar.
TONGUE IN RESTING POSTURE TONGUE HELD ANTERIORLY
EDENTULOUS - people have greater prevalence and severity of OSA, because they lack a platform against which the mandible can be braced to prevent it from dropping back into the pharyngeal airway during sleep. Also, edentulism has been shown to produce various functional and sensory deficiencies in the jaw system, increased collagen in the extracellular matrix of the superior pharyngeal constrictor muscle, and reduced tonicity in the pharyngeal musculature.15-19 Wearing dentures during sleep appears to reduce OSA in some people.20-22 Tongue gripping surfaces carried on denture base plates are even more effective at holding the tongue than tongue gripping surfaces carried on mandibular advancement appliances, because they can extend much further laterally, and the constant light pressure from cushioning by the tongue keeps the denture base plates well seated. Also, the base plates need little retention, because they are not used for chewing. However, advancement of the tongue or mandible in its resting posture used during sleep could apply directional pressures to the edentulous ridges from passive stretch of the muscles, therefore the goal of treatment is just to hold the tongue up against the upper denture base plate to prevent it from dropping back into the pharynx; and the cushioned grip is produced by biasing the lower tongue gripping surface upward rather than biasing the upper tongue gripping surface downward, as in dentate patients.
In the photo below, the lower tongue gripping surface is attached to a full lower denture. A polyester mesh tether attaches the anterior end of the lower tongue gripping surface to the denture, and the posterior end of the lower tongue gripping surface is biased upward by a pair of orthodontic elastics that each connect a cleat on the lingual flange of the lower tongue gripping surface with a ball clasp on the buccal side of the lower denture. The bias is maintained until the cleats reach the same plane as the bite table, as seen in the right-side photo below. Any further opening releases the tongue.
BOTTOM ENDS OF ELASTICS ON CLEATS TOP ENDS OF ELASTICS ON BALL CLASPS ELASTICS SUSPEND LOWER TONGUE GRIPPING SURFACE
CLEARING THE HYPOPHARYNX - If further treatment is needed for dentate or edentulous patients, the upper tongue gripping surface can be amended with tongue base titraters to extend protection down into the lower oropharynx and the hypopharynx. The tongue base titraters are small adjustment mechanisms that can gradually shift the tail segments of the upper tongue gripping surface further down and back or down and forward. The body of the upper tongue gripping surface, which has been stabilized by the grip of the entire tongue body and the bite stops bracing the mandible, provides a good anchoring structure from which to extend the tail segments in progressive 1.3 mm increments, as the patient gets used to them.
BEFORE SHIFT AFTER SHIFT
The effect of shifting the tongue base titraters down and back is shown below in X-rays of a patient's tongue painted with a radiopaque paste before (top) and after (bottom) shifting the tail segments of the upper tongue gripping surface and thereby also the tongue base further down-and-back, which has shifted the white V-shaped area marking the end of the upper tongue gripping surface and the end of the wire framework of the tongue base titrater in the same direction and has further increased the sagittal width of the airway space in the lower oropharynx, seen between the arrows. The thin white line is made by the arm of a soft palate elevator.
PHARYNX WITH TONGUE HOLDING DEVICE WITH TONGUE BASE TITRATER MOVED DOWN AND BACK
The tongue base titraters can depress the tongue base in a range of directions that can be varied by up to 25 degrees, as illustrated below.
BEFORE ADJUSTMENT ADJUSTED DOWN-AND-BACK ADJUSTED DOWN-AND-FORWARD
This range of adjustment enables the tongue base titraters to first shift the tail segments down-and-back, to get them as far as possible behind the tongue base (below left), and then to rotate them forward (below right) to pry the tongue base anteriorly off the lower pharyngeal wall and away from the epiglottis. The green acrylic build-ups over the third molars are the bite stops.
TAIL SEGMENTS DOWN AND BACK TAIL SEGMENTS DOWN AND FORWARD
The photos below show one tail segment rotated down-and-back and the other rotated down-and-forward.
CLINICAL CONSIDERATIONS
VERTICAL DIMENSION - the height of an oral appliance, or the interarch space when the appliance is worn, affects facial growth and airway dimensions in a manner that varies widely depending on facial form. Increasing vertical dimension increases airway space in some people and decreases it in others. Most oral appliances should be no taller than necessary to ensure structural integrity. For patients with very strong jaw muscles and significant anterior overbite, their mandibular advancement appliance can include a front flat bite plate to reduce the forces of nocturnal bruxism and redirect the remaining forces to gradually reduce the overbite. For patients with weak jaw muscles and excessive vertical dimension (usually accompanied by difficulty maintaining a lip seal at rest), any reduction of the vertical dimension that can be achieved by selectively grinding down teeth will make appliance wear more comfortable. Equilibration scares patients but rarely damages teeth.
MOUTH BREATHING - prevents the nose from moistening, filtering, and warming the air before it hits the throat - making mouth breathers prone to upper respiratory problems. It also prevents the release of nitric oxide from the paranasal sinuses into the lungs, where it helps widen blood vessels and improve oxygen intake. In some ways, the nose acts like a little lung. When volunteers temporarily wear nose clips to force mouth breathing, their arterial oxygen levels drop.
Obligate mouth breathers have nasal cavities that are too small to allow adequate resting nasal airway flow, and they cannot sit comfortably with their lips sealed for more than a couple of minutes. At rest, their lips remain slightly parted to maintain an oral airway passage. Many people whose nasal cavities are barely large enough to allow adequate resting nasal airway flow become obligate mouth breathers whenever allergy or rhinitis even slightly reduces the internal volume of their nasal cavity.
One way to treat obligate mouth breathing is to widen the palate non-surgically using a removeable expansion screw appliance. In adults, the maxillary bones don’t separate at the midline suture, but they respond to light steady expansion forces by unfolding, accompanied by lowering and adaptive remodeling of the midline suture.23 The expansion is maintained post-treatment by nightly wear of a retainer with a bite table that is broadly loaded during nocturnal bruxism and by ensuring a stable natural bite to function as a daytime retainer. Changing from mouth breathing to nose breathing can improve many sleep parameters; but it rarely relieves OSA, because the expansion occurs so far from the obstruction.24
Habitual mouth breathers use an oral airway despite having an adequate nasal airway. Usually the habit arises from an open-mouth mandibular resting posture due to a framework of bones and teeth (and sometimes the plastic of an oral appliance) at the front of the face that longer than the drape of relaxed soft tissues hanging down from the forehead and temple areas. When the muscles in the drape are fully relaxed, the lips are separated, creating an oral airway passage during sleep that can easily become the habitual route for breathing.
Habitual mouth breathers can become obligate mouth breathers due to the effects of mouth breathing on facial growth. Mouth breathing lowers mandible and tongue posture to create an oral airway passage, and postural forces provide the light steady forces that shape bones. As a result, lowered mandible and tongue posture makes the face grow vertically long, which also makes it grow narrow by pushing in on the cheeks. This effect can be seen dramatically in the ability of Frankel appliances to expand the palate using only plastic bumpers that hold the cheeks out away from the teeth.
Habitual mouth breathing can usually be eliminated by almost any way of holding the mouth closed and restoring the lip seal; including chin straps, medical tape (mouth taping), thick foam cervical collars, or interarch orthodontic elastics attached to orthodontic ball clasps, cleats, or buttons on upper and lower appliances that fit on the teeth tightly enough to resist being pulled off by the weight of the mandible.
NOCTURNAL BRUXISM - occurs in everyone as a side effect of normal sleep, when the brain sends motor signals to the jaw muscles, usually during transitions to lighter sleep stages. It is intensified by stress, but it is not affected by bite conditions.25-26 Nocturnal bruxism is also not correlated with TMJ disorders.27-29 In some people, nocturnal bruxism may be an adaptive response that prevents OSA by bracing the mandible against the cranium to prevent it from slipping back into the pharynx. In other people, nocturnal bruxism can be caused by OSA, because the obstructions trigger sympathetic activity to stimulate bodily movement to restore breathing, and bruxism is one of those movements.
TMJ DISORDERS - usually resolve by middle age when OSA usually begins, so they are rarely significant barriers to oral appliances for OSA treatment. It's rare to find an inflamed TMJ beyond middle age. Many older people being treated for OSA with a mandibular advancement appliance experience tight jaw muscles due to bite destabilization, because instability in any joint reflexively increases tonus in the muscles crossing that joint, and the periodontium is wired like a joint between the jawbones. There are many ways toreduce the tightness, including restabilizing the bite, as discussed further below.
BITE CHANGES - The posterior open bites that commonly result from mandibular advancement are not due to shortening of the superior lateral pterygoid muscles, TMJ inflammation, or any other pathology. They have two causes.
One cause is the forces generated anteriorly on the lower dentition and posteriorly on the upper dentition by pushing the lower dentition forward off the upper dentition to advance the mandible. These forces can tip the teeth and/or move them bodily.
The other cause is continuing adult jawbone growth. After the second decade of life, when elongation of the long bones stops, jawbone growth slows down 90 percent and then continues at about a 10 percent rate. The most rapid feature of continual adult jawbone growth is continual advancement of the mandibular corpus, with gradual reduction of overbite and overjet throughout life, which can be seen in all museum collections, although it cannot be proven without longitudinal studies. The slower feature of continual adult jawbone growth is maxillary expansion. These jawbone growth processes were designed to continue slowly during adulthood to reduce resistance to airway flow as muscles simultaneously lose strength at about 5% per decade. Also mandibular advancerment helped compensate for continuous tooth wear by continuously bringing the lower dentition antero-superiorly into the upper dentition. The bone growth that produces mandibular advancement is stimulated by bite forces, so that our ancestors who chewed extensively and experienced more tooth wear also underwent more mandibular advancement to compensate for that wear.30-33 Today, in people who use mandibular advancement appliances on a long-term basis during sleep, the increased bite forces from passive stretch of the jaw closing muscles and the distraction osteogenesis created by holding the mandible down and forward all night can stimulate the bone growth that produces natural adult mandibular advancement until it propels the mandibular corpus past the habitual bite.
The loss of a stable habitual central bite position removes the normal reference point for clinical dental work; therefore, many dentists see it as a serious adverse side effect, and they try to prevent it by having their mandibular advancement patients chew on hard gum or small wedges for an hour every morning (morning occlusal guides) to create an "occlusal return" or "reprogramming" process that forces the mandible back towards its previous central bite position. While use of these morning occlusal guides can trigger regressive remodeling that enables the condyles to seat more posteriorly than they otherwise would; simply adjusting the bite anteriorly to create a more anterior central bite position is much easier than trying to reverse growth that has already occurred or prevent further growth in a functional environment that stimulates it.
The problem is that dental training has created such a box of thinking around centric relation that most dentists have never adjusted a bite anteriorly, and they will use medications, physical therapy, or trigger point injections to treat the jaw muscle tightness that was caused by the bite destabilization rather than restabilizing the bite. The most frequent mechanical barrier to natural mandibular advancement is incisor overbite, and a couple of millimeters can usually be removed from the labial-incisal edges of the mandibular incisors to allow some mandibular advancement without requiring anesthesia or causing any damage. In fact, those teeth benefit periodontally from the decreased crown/root ratio. Also, most of the patients who are wearing an oral appliance to treat OSA are not concerned with the small esthetic change that would result from permanently advancing the mandible a few mm; and they don't need a perfectly intercuspated bite or dozens of simultaneous centric contacts. They just need a stable bite platform, with at least a few contacts on each side, which they can use for bracing and chewing. It’s time to get over our fear of adjusting the bite.
More important, adjusting the bite anteriorly can significantly benefit OSA patients by shifting their mandibular resting postures anteriorly. The mandible always acquires a resting posture just beneath its central bite (intercuspal) position due to neuromuscular reflexes designed to maintain fast easy access to bracing, which was a critical protective feature in our evolution. The adaptation of the mandibular postural position to its bracing position has been demonstrated in numerous experiments.34-40 Adjusting the bite anteriorly improves resting airway flow during the day, and it can reduce dependency on the appliance in our patients by preventing the mandible from dropping all the way back to its pre-treatment position if the mandibular advancement appliance is not worn for a short time, such as during a nap.
APPLIANCE THICKNESS - The unnecessary bulk, especially in the anterior regions, of nearly all current oral appliances can work against our goals by distalizing tongue posture, which distalizes mandibular posture, which prevents natural mandibular advancement. A study of growing monkeys showed that a block of acrylic cemented in their palates lowered their tongue and mandibular posture to avoid the block, which redirected their facial growth vertically, causing them to grow long narrow faces. In humans, the center of rotation of the mandible is within the shortened ramus instead of far above it, therefore lowering tongue and mandibular posture also rotates the mandibular corpus down and back into the pharyngeal airway space. To prevent such adverse effects on facial growth, our upper oral appliances include a space hollowed out to fit the tongue tip just beneath the anterior bite table at the front of the palate behind the maxillary incisors.
TIGHTNESS - The tightness of commercially made acrylic dental appliances also works against our goals by squeezing the upper teeth inward (palatally) due to the shrinkage of acrylic, especially the front teeth in the middle of the squeeze. Healthy teeth at rest are each suspended in the middle of metabolically hyperactive sockets; which contain so many blood vessels that they give each tooth an individual arterial pulse and so many sensory nerves that they occupy an oversized portion of the brain. When the teeth remain in the middle of their sockets all night, these tissues remain passive. Displacement of a tooth from the middle of its socket by the pressure of an appliance triggers a collection of adaptive responses designed to make the socket once again fit the tooth position. One of those responses is increased jaw muscle tonus, because noxious afferent input from any joint triggers increased tonus in the muscles which cross that joint. As a result, any pressure on the teeth increases tonus in the jaw closing muscles; which can add to the increased jaw muscle tonus caused by a TMJ disorder or an unstable bite.
Even if a dentist could fabricate a resinous oral appliance which perfectly fit all the teeth in their rest positions, a tight fit would impair the functional circulation to the teeth and their supporting tissues by limiting their functional ranges of motion, which extend about 1/4 mm vertically and bucco-lingually and about a tenth of that distance mesio-distally in the presence of adjacent teeth. Restricting this range of motion creates discomfort and prevents optimal circulation, like immobilization of other joints does. What assures compliance in appliance wear is not tightness that makes it difficult to remove, but comfort that makes the patient not want to remove it. An appliance should apply no pressure to teeth. It only needs enough mechanical retention to resist the forces of gravity.
FOOTNOTES
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