JADA Continuing Education
Selecting sleep-disordered–breathing appliances
Biomechanical considerations
PETER T. GEORGE, D.D.S.
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ABSTRACT
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Background. Dentists who wish to provide sleep-disordered–breathing therapy have many different mandibular advancement devices, or MADs, from which to select. Documented research directly about the variations in MADs is sparse.
Types of Studies Reviewed. The author reviewed dental and medical literature dealing with biological and mechanical principles affecting the function of MADs.
Results. The author found that MADs vary in four major areas: freedom of mandibular movement, amount and rigidity of dental coverage, amount of mandibular advancement and amount of bite opening. Each of these areas appears to affect the appliance’s efficacy, safety or both. The main potential detrimental effect of MADs is occlusal shifting. The author presents biological and mechanical considerations in an attempt to determine the optimum parameters for each of the MAD variation areas. The MAD must be constructed in a manner and with material that secures the mandible in its optimum position. The optimum mandibular position needs to be captured and transferred to the articulator with an accurate construction bite.
Clinical Implications. MAD therapy may last a lifetime. Therefore, dentists must consider the efficacy and the safety of an MAD when selecting an appliance. Since occlusal shifting appears to be the main potential detrimental effect, dentists should consider all available means to monitor and minimize these changes.
Sleep-disordered breathing, or SDB, is a series of disorders ranging from primary snoring to severe obstructive sleep apnea-hypopnea syndrome, or OSAHS.1 Although primary snoring—without apneas, hypopneas or arousals—is benign to the snorer, OSAHS is a medical problem that should be referred to a sleep disorders clinic or a physician who treats sleep breathing problems. Daytime sleepiness is a key symptom of this syndrome; if it is not present, patients do not have OSAHS.1 Before treating a snoring problem, dentists should require the patient to complete an Epworth Sleepiness Scale, or ESS,2 which is a questionnaire used to determine the likelihood of dozing off in a variety of common daily situations. Scores on the ESS can range from 0 to 30. Patients scoring 10 or higher on the scale are considered hypersomnolent and should be referred to the appropriate medical center.3 In addition, dentists should inform physicians that they can provide oral appliances to patients to combat SDB. Responsibility for the patient and selection of the treatment modality for all patients with OSAHS, however, is the physician’s. Fortunately, the vast majority of snorers experience primary snoring.4
To evaluate sleep-disordered–breathing appliances, dentists need to question how and why they work.
It is well-established that oral appliances have a place in the treatment of SDB.5–7 Two appliance categories have been documented as being effective in this therapy: tongue-retaining devices, which retain the tongue anteriorly by means of an extraoral suction bulb, and mandibular advancement devices, or MADs, which retain the mandible anteriorly by means of dental anchorage. Because using an MAD can have dental consequences, the American Academy of Sleep Medicine, or AASM,8 indicated that this type of device should be fitted only by personnel with the knowledge and training possessed by dentists. I will be discussing MADs for the remainder of this article.
More than 55 different SDB appliances are available to dentists9; this number continues to increase. The purpose of each of these appliances is the same—to keep the pharyngeal airway open wide enough to prevent breathing efforts from causing snoring, apnea or arousal. Promoters of these various appliances all tout features that supposedly make their products more effective, more patient-friendly or kinder to the dental structures. To evaluate these appliances, dentists need to question how and why they work.
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EFFICACY OF MANDIBULAR ADVANCEMENT DEVICES
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The answer to the question "How do MADs keep the airway open?" appears obvious. The back of the tongue is part of the airway’s front wall, and the front of the tongue is attached to the mandibular symphysis. As the mandible moves forward, so does the tongue, and thus the airway enlarges. While this explanation is accurate, it is an oversimplification. There is a lack of concurrence among authors as to whether a sagittal expansion of the airway always can be detected on lateral view radiographs during successful MAD therapy.10 Also, it has been found that when the mandible is advanced surgically, the advancement must be at least 15 millimeters before a therapeutic effect can be expected.11,12
An MAD’s efficacy is based on more than a simple cause-and-effect relationship between advancement of the mandible and tongue. Magnetic resonance images indicate that the airways in patients who have SDB are constricted more laterally than anteroposteriorly than in patients who do not have SDB.13 Further, the narrowest part of the pharyngeal airway usually is not behind the tongue but behind the soft palate; it is in the latter area that obstruction is most likely to start.14,15
In September 1983, before the previously described research was conducted, an orthodontist modified a functional orthodontic appliance in an attempt to prevent the breath stoppages in a patient with OSAHS. The patient wore the appliance while sleeping, and the mandible was advanced 7 mm. Polysomnography, or PSG, showed a reduction in apneas per hour of sleep from 79 to 5.3,16 and improvement in daytime symptoms was dramatic.17 The patient, as of this writing, continues to wear an MAD nearly every night with no apparent decline in efficacy and no significant side effects.
Although MADs have been successfully used in treating SDB for nearly two decades, peer-reviewed evidence of their success is scarce. Objectively testing the efficacy of an MAD is relatively simple, but it often is not done because of the expense of PSG. The cost of this overnight sleep test, which should be done before and while wearing an MAD, is around $1,200; grants for this research have been infrequent. Insurance often pays for PSG if it is used for diagnosis, but seldom if it is used to test treatment results.
Most commercially available MADs’ efficacy is not documented. Even less evidence is available to help dentists evaluate how the features and modifications of these appliances contribute to their success or detract from their safety. Research to establish the optimum parameters of any of the MAD features would require a minimum of two polysonograms for each subject. This hard evidence, therefore, may not be available any time soon. Until then, guidelines will have to be based on empirical observations and mechanical and physiological principles; dentists will have to continue to exercise their professional judgment that distinguishes them from technicians.
In the following discussion, I do not present hard and fast rules. Instead, I present clinical considerations that may change with the introduction of new evidence. The four main areas of variability among MADs are freedom of mandibular movement, amount and rigidity of dental coverage, amount of mandibular advancement and amount of bite opening.
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FREEDOM OF MANDIBULAR MOVEMENT
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Some clinicians state that an MAD that allows the mandible to move several millimeters laterally or vertically is less likely to produce temporomandibular disorder, or TMD, and is tolerated more readily by bruxers. They believe the appliance should be constructed as a two-unit assembly holding the mandible and maxilla separate but joined by a moveable connection rather than as a monoblock holding mandible and maxilla as a single, rigid unit.9 There is no published scientific evidence that an MAD that immobilizes the mandible is more likely to cause TMD or that it is less tolerable to bruxers in the short or long term than one that allows the mandible some freedom of movement. A recent article concluded that two years of treatment with an MAD that rigidly immobilized the mandible produced no adverse effects on the subjects’ craniomandibular status and function.18
Relieving temporomandibular pain.
I have fitted more than 700 MADs, each rigidly immobilizing the patient’s mandible in a protruded position. TMD complaints have been uncommon. They can, however, occur at any time during treatment, initially or after years of nightly use. TMD problems resulting from MAD use are myofascial, not intracapsular. Therefore, I manage those that occurred in my patients with communication and analgesics, using the principles advocated by Dr. Charles Greene.19 I reassure the patients that wearing the MAD is not harming the joint and inform them that dental authorities advocate such appliances in TMD.20,21 I also tell them that the muscles around the joint are resisting the protruded position and becoming very sore, but the muscles will learn to relax in that position and the pain should disappear.
I also reassure patients about any anxiety they may have about temporomandibular joint problems. Most patients respond that if no permanent damage is being done to the joint they can tolerate the pain. I tell them, however, not to tolerate it, as muscle pain can become self-perpetuating.22 If pain is experienced during the night or on awakening in the morning, I instruct them to remove the appliance and take aspirin, ibuprofen or acetaminophen every four hours until the pain disappears. Most often the pain disappears within the first half-hour and does not return. If analgesics are required until the next bedtime, however, I tell them not to wear the appliance that night. I instruct them to repeat this routine until they can wear their appliances the entire night without pain. Although I have lost a few patients to follow-up, all those who reported following these instructions are able to resume regular nightly wear of their appliances. If, however, after a week of this routine the pain does not begin to subside, it is not likely that the patient can adjust to the appliance, and discontinuation of therapy should be considered.
Preventing antitongue-biting reflex.
The objective in keeping the mandible motionless is to prevent the antitongue-biting reflex. This reflex detracts from MAD’s efficacy. Movement of the mastication muscles activates trigeminal proprioceptive afferents that initiate a reflex suppressing the tongue protruding muscles.23 This allows the tongue to fall back away from the teeth, but it also makes it more vulnerable to the suction of the airway. This effect can be verified by noting that MAD efficacy can be enhanced or reduced by increasing or decreasing the retention of the clasps.
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AMOUNT AND RIGIDITY OF DENTAL COVERAGE
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When a mandible is held in a protruded position, muscles and elastic connective tissue are stretched, exerting a force to return the mandible to centric relation. The MAD uses the teeth as anchorage against this force, pushing the mandibular arch forward and the maxillary arch backward (Figure 1
). Such forces over time may change the angulation of the teeth, especially the incisors.24,25 The more teeth held, the less force against each tooth. All the teeth in both arches should be recruited into the anchorage. This is particularly important in patients with missing teeth or reduced alveolar bone support.
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Figure 1. Stretched muscles and elastic connective tissues exert a force to return a displaced mandible to centric relation. The mandibular advancement device resists this force by pushing forward on the mandibular teeth and backward on the maxillary teeth (arrows).
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While most MADs are constructed with hard acrylic, the use of thermoplastic material is becoming more common. The dentist immerses the thermoplastic-constructed appliance in hot water to soften it before inserting it in the patient’s mouth. As the material cools to mouth temperature it stiffens. It does not become as hard as acrylic; instead, it has a slight yield, enabling it to slip beyond small undercuts, thus enhancing retention and obviating the need for clasps. With nightly wear, however, this slight yield also can permit incremental changes in tooth angulation. Any malocclusion resulting from wearing a thermoplastic MAD is more likely to be the result of the inclination of the teeth rather than of changes in the temporomandibular joint.26 Such resultant malocclusion usually is reversible if treatment is discontinued.25
MADs constructed from hard acrylic offer excellent dental support, and the implacability of their fit indicates if any tooth movement has occurred. A recent cephalometric study was conducted to determine changes in mandibular posture after two years of treatment using a hard-acrylic, full-occlusal-coverage, monoblock MAD.27 The study found occasional slight shifts in occlusion, but none were attributed to dental shifting. Instead, the shifts were attributed to changes in condylefossa relationship.
There is no direct correlation between the amount of mandibular advancement and the efficacy of a mandibular advancement device.
Advocates of using the softer material to construct MADs contend that it is gentler on the teeth and causes less pain.9,28 There is no published scientific evidence to support that contention. Dental pain caused by hard acrylic is an indication of a defect in construction and usually can be eliminated by a small amount of selective grinding at the initial seating. Occasionally, patients leave the initial appointment saying all is well but return claiming that a tooth is sore. Sometimes they will point to a clasp and say that it is too tight. Tightness of clasps very seldom causes discomfort; most often a little acrylic grinding around the clasp will relieve the problem.
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AMOUNT OF MANDIBULAR ADVANCEMENT
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As I indicated previously, there is no direct correlation between the amount of mandibular advancement and the efficacy of an MAD. Three-fourths of the distance between centric occlusion and full protrusion was the amount the mandible was advanced initially on the assumption that this was the most protruded, unstrained position.16,29 This position has worked well in the treatment of a large majority of patients with SDB. In some cases, however, it did not work at all.30,31 I assumed that the failures were the result of an improper mandibular position, and if the ideal position for each patient could be determined, each appliance could yield excellent results. An MAD with adjustable mandibular advancement, theoretically, should be able to accommodate the ideal position in each patient. Several adjustable MADs have become available that can advance and retain the mandible 14 to 19 mm, positions exceeding 100 percent of the protrusive range registered at the initial visit.32–34 Unfortunately, these devices’ results have been approximately the same as those obtained with a monoblock MAD at 70 percent protrusion.7
Some patients with SDB will not respond favorably to any amount of mandibular advancement.31,32 The amount of airway expansion resulting from each millimeter of mandibular advancement varies among patients; in some cases advancements of more than 75 percent cause decreases in anteroposterior airway size.35 The variation may be the result of structural or soft-tissue differences.35,36
In awake subjects, mandibular advancement usually expands the airway laterally and anteroposteriorly.37,38 But in anesthetized OSAHS patients whose muscles were paralyzed, Isono and colleagues36 found that if suction was applied to the airway it was more likely that it would collapse anteroposteriorly if the mandible was in centric occlusion and laterally if the mandible was protruded. In some patients with very small airways, 70 percent advancement may be insufficient to maintain patency, but further protrusion may elongate the airway anteroposteriorly and collapse it laterally.
The force pulling the protruded mandible back to centric relation increases the pressure against the teeth with every millimeter of advancement (Figure 1
). Graber and Neumann39 indicated that if a 5-mm forward positioning generates 500 grams of force, a 10-mm forward positioning will generate about double that amount. Mandibular advancement of 70 percent causes minimal or no tooth movement when the teeth are held in hard acrylic,27 but greater advancement by an appliance constructed of softer material appears to cause teeth to shift.26
Orthodontists use MADs to correct Class II malocclusions in children through growth stimulation in the temporomandibular joint. Increased growth in the condyle and remodeling in the glenoid fossa appear to be a reaction to the stretching of retrodiskal tissue and are dependent on the amount of mandibular advancement.40 In young adults, however, nearly full-time mandibular advancement results in only slight growth changes.41 It is not likely that six to eight hours of nightly mandibular advancement will result in much if any growth in the temporomandibular joints of typically middle-aged patients with SDB.
Bondemark27 found a slight, but statistically significant, forward and downward repositioning of the mandible after two years of treatment with an MAD at 70 percent mandibular advancement in a pilot study of middle-aged and elderly patients. Finding no difference in the fit of the hard-acrylic, full-arch occlusal coverage MAD, he ruled out dental shifting. He also found no increase in dual bites after treatment. He attributed the change in mandibular position to changes in the temporomandibular joint.
A possible contributor to the observed jaw shift is myostatic contracture. This condition occurs when a muscle is held in a shortened position over a sustained period42; the muscle loses sarcomeres in series, preventing it from extending to its former full length.43 When the MAD is worn, the inferior lateral pterygoid muscle is shortened. If the mandible is advanced more than 100 percent of its normal range, this muscle is compressed. A shortening of this muscle will position the mandible forward and will not allow the condyle to be manually manipulated to its previous position in the fossa.
Occasional full extension and relaxation of a muscle prevents this shortening.42 The inferior lateral pterygoid muscle is stretched to its fullest and is most relaxed when biting firmly on the posterior teeth.42 Most patients who wear an MAD all night report that the bite does not feel right on awakening, but that it normalizes after breakfast. This is the result of the condyle’s being held forward by the partially contracted inferior lateral pterygoid muscle and the accumulation of retrodiskal blood.44 Firm chewing during breakfast solves both those problems. Some patients normally skip breakfast, and if the bite does not feel right, will not firmly occlude their teeth, prolonging the period of muscle shortening. Patients who wear MADs and who do not regularly eat breakfast should be advised to isometrically clench their teeth three times for six seconds each shortly after awakening.
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AMOUNT OF BITE OPENING
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An MAD with full occlusal coverage obviously increases the vertical dimension. As the mouth opens, the anterior attachment of the tongue swings not only down but also backwards (Figure 2). This backward movement carries the tongue toward the airway. For this reason, the authors of a 1997 study concluded that MADs should keep jaw opening to an "absolute minimum."35 In another study, the authors also concluded that jaw opening should be minimized, because they found that the pharyngeal airway was more likely to obstruct when the mandible opened 15 mm at the incisors than when it remained in centric occlusion.45 There are no published polysomnographic studies that establish the optimum vertical dimension for the MAD; the most commonly selected bite opening is about 2 mm between the incisors.
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Figure 2. The mandible can displace by translating forward and downward under the articular eminence and by rotating downward and backward around the condyles. The arrow through the symphysis indicates the displacement of the anterior tongue attachment, and the arrow distal to the third molar indicates the displacement of the lateral pharyngeal wall mandibular attachments. The checkered squares indicate the ratio of downward to backward displacement.
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These two studies notwithstanding, there is considerable evidence that the downward stretch of the pharynx is at least as effective as forward expansion in maintaining airway patency. One study showed good apnea reduction with an MAD that advances the mandible 6 to 9 mm while opening it vertically 17 mm.46 There are several possible mechanisms by which mandibular depression can enhance MAD efficacy.
MADs expand the airway not only behind the tongue but also behind the soft palate.36,37 The mechanism for this velopharyngeal expansion is the pull on the palatoglossus muscle.37 In an upright person, this muscle runs down and forward from the palate to the tongue (Figure 3). If the MAD simply pulled the tongue forward, a smaller part of its force vector would be in line with that of the palatoglossus muscle. If the appliance lowered and advanced it, the tongue would pull more in line with these muscle fibers and its effect on the palate would be proportionately greater (Figure 3).37 This pull advances the palate and increases its tension, making it less likely to collapse or vibrate as in apnea or snoring.37
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Figure 3. Effect of bite opening on palatoglossal muscle. As the protruded mandible opens, the pull of the tongue (lower solid line) more closely approaches that of the palatoglossal muscle (broken line).
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The superior pharyngeal constrictor muscle attaches directly and indirectly to the mandible.47 Opening the mouth, therefore, exerts a downward force on the lateral walls of the pharyngeal airway, stretching them longitudinally (Figure 2). Animal studies indicate that this stretching improves airway patency by reducing folds, compliance and extrinsic compression.48,49 This is verified in humans by observing the decrease in airway resistance as the "tracheal tug" is activated with each downward contraction of the diaphragm.50
Another advantage of increasing the jaw opening beyond 2 mm is that it helps part the lips allowing a passage for oral breathing. This is important because patients with SDB need assistance in breathing most when the nasal airway is inadequate. One cannot mouth-breathe with lips sealed, but lips apart does not make mouth breathing obligatory.51 The breathing route is determined by the veloglossal relationship.52 While sleeping, the usual breathing route is through the nose; the soft palate contacts the tongue, preventing air passage through the mouth. Only when insufficient air comes in through the nose does the soft palate pull away from the tongue.52
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CONSTRUCTION BITE
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The position in which a nonadjustable MAD holds the mandible is determined by the construction bite. A mandible in centric occlusion when viewed from the side can reposition in three directions, forward, downward and backward; it can translate forward and downward under the articular eminence and can rotate downward and backward around the condyle (Figure 2). Before selecting the parameters of the construction bite, the dentist must carefully weigh the advantages and disadvantages of each millimeter of mandibular forward translation and each degree of downward rotation.
Forward mandibular translation provides beneficial longitudinal and sagittal stretching of the pharyngeal airway; however, each millimeter of movement forward increases the forces applied against the teeth and in the temporomandibular joint. These forces may alter the occlusion by changing the angulation of the teeth, remodeling the condyle and fossa, and shortening the inferior lateral pterygoid muscle. Mandibular advancement of about 70 percent of the protrusive path has been shown to be effective in reducing SDB symptoms with minimal occlusal changes.27,29
Downward mandibular rotation provides beneficial longitudinal stretching, but also causes detrimental backward compression of the pharyngeal airway. As the mandible rotates, the attachment of the tongue on posterior surface of the mandibular symphysis describes an arc (Figure 2). At the start of mouth opening, the arc is more vertical and the movement is mostly downward, but as it opens wider the arc becomes more horizontal and the movement is mostly backward. The position of the mandible on this arc varies with the facial structure. A mandible in a person with a short lower face would start opening in the more vertical segment of the arc, whereas one in a person with a long lower face would start opening in the more horizontal segment. Five mm appears to be a good compromise, since that amount of opening would give a short-faced person ample downward stretch; more than 5 mm would cause excessive backward compression in a long-faced person.
Once the desired parameters for the construction bite are selected, they can be captured and transferred to an articulator with the use of a George Gauge (Great Lakes Orthodontics Ltd.), which is used to register the jaw position in the construction of MADs (Figure 4).53 This instrument’s millimeter scale can be used to measure the range of the protrusive path. The operator can then preset the moveable incisal notches so that when the patient bites into them the mandible automatically is brought to the desired point on the protrusive path. The vertical dimension is determined by the height of the incisal notches, which may be 2 mm or 5 mm. Impression material placed on the bitefork registers this bite position (Figure 5).
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Figure 4. When teeth are in the incisal notches of the George Gauge (Great Lakes Orthodontics Ltd.), anteroposterior mandibular movement slides the millimeter indicator over a scale indicating length of the protrusive path. A millimeter indicator then can be secured to an appropriate position on the protrusive path, placing incisal notches in position to capture the desired bite.
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Figure 5. Patient closing into preset incisal notches, registering the desired anteroposterior and vertical mandibular position in impression material on a bitefork.
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ANTICIPATED RESULTS
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Results in the treatment of SDB with an MAD are not uniform. When using a properly constructed MAD, practitioners should anticipate 90 to 95 percent success in treating primary snoring and about 60 percent success in treating moderate to severe OSAHS, depending on how success is defined. Success in snoring treatment may be considered its elimination or reduction to a level tolerable by a bed partner. Success in OSAHS treatment may be considered a 50 percent reduction in the number of sleep-related obstructive breathing events per hour, as well as a severity no greater than mild. The AASM recommends that OSAHS be rated mild when the apneas and hypopneas per hour of sleep are five to 15, moderate when 15 to 30 and severe when more than 30.1
Most authors believe that MADs should be used primarily in treating snoring and in mild-to-moderate, but not severe, OSAHS.6,54,55 Because they are closer to mild cases, moderate cases are more likely than severe cases to be successfully treated, but the percentage of improvement cannot be predicted by the severity of the SDB. Figure 6 shows the results of 32 moderate to very severe cases of OSAHS in patients I treated with a monoblock MAD that rigidly held the mandible at about 70 percent protrusion and at a bite opening of 5 mm between the incisors. Two of the most severe cases in this group had excellent results. The respiratory disturbance index, or RDI—the number of obstructive breathing events per hour—in one patient was reduced from 94 to eight and in the other patient from 79 to five. The patient with the mildest severity rating in this group was treated unsuccessfully based on the previous definition of success; the RDI was reduced from 18 to 10, only 44 percent.
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Figure 6. Results obtained with monoblock mandibular advancement device that held mandible rigid at about 70 percent protrusion and 5 millimeter bite opening. Note that treatment response cannot be predicted from initial severity.
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CONCLUSIONS
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Although MADs have earned an important place in SDB therapy, the mechanisms by which they achieve their results have not been completely determined. Currently, a wide variety of MADs are available to dentists; however, there is a scarcity of documented research on MADs. Since the efficacy of the various commercially available MADs varies, dentists must subject the claims of their promoters to scientific questioning and reasoning. Also, since MAD therapy may last a lifetime, dentists should be concerned not only with MADs’ efficacy but with their safety, as the major untoward side effect appears to be a change in the occlusion. Dentists should consider all available means to monitor and minimize these changes.
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Dr. George is an assistant professor, John A. Burns School of Medicine, University of Hawaii. Address reprint requests to Dr. George at HEA Building, Suite 204, 1649 Kalakaua Ave., Honolulu, Hawaii 96826-2494, e-mail "ptgeorge{at}pacinfo.net".
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FOOTNOTES
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Dr. George holds the patent on the George Gauge, a device mentioned in this article. He also hold the patent on the Nocturnal Airway Patency Appliance, a mandibular advancement device used in the treatment of sleep-disordered breathing.
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ABSTRACT
EFFICACY OF MANDIBULAR...
