Orthodontics and TMJ Disorders
BACKGROUND
Orthodontics can control bites, and bites affect how faces grow; but orthodontists in the past have not been able to predictably improve how faces grow by altering the bite, and attempts to alter it by pushing on bones with screws and springs usually relapsed. Researchers identified some of the mechanisms involved in the growth processes that build the upper and lower jawbones, but they failed to recognize the important role of bite forces in stimulating those mechanisms and the adaptive systems needed to coordinate them, and they did not realize that jawbone growth continues during adulthood. More recently, all research on facial growth has been recently overtaken by the enormous demand for more effective esthetic teeth straightening mechanics.
Still today, in school, dentists are taught that orthodontics produces ideal bites, but they only learn about esthetic orthodontics, which employs braces (metal brackets and wires) or pre-formed plastic shells (clear aligners such as invisalign) to realign all the teeth into smoothly curved arches that interdigitate fully, because upper and lower archwires are matched for size. However, with esthetic orthodontics, there is no way to control where the fit of the teeth positions the mandible, which is the problem at the root of most TMJ disorders. As a result, in many of our children today, we align the teeth into two smoothly curved arches that fit together perfectly, while allowing the whole face to become progressively more crooked and disrupting postural balance due to a strained jawbone growth pattern.
Dental schools teach little about functional orthodontics, which has been popular in Europe for decades and is preferred by most TMJ specialists, because it is less concerned with the straightness of the teeth and more concerned with the positions and shapes of the jawbones and their ability to function as a healthy platform for the jaw to rest on and exercise against. Functional orthodontics employs removeable oral appliances which contain expansion screws, springs, and inclines designed to reposition the jawbones as well as the teeth. The appliances can be removed for cleaning, eating, and important social functions; but they must be worn most of the time during the period of active treatment.
Both of these orthodontic techniques, (esthetic and functional), have advantages and disadvantages in different situations. Therefore, if you are considering orthodontics, the following information should help you and your dentist understand your options. The first half of the following paper explains and contrasts these two different orthodontic styles. The second half of the paper describes the most common orthodontic problems we see today and how they are treated using each of the two orthodontic styles. The goal of the paper is to enable parents to look at their children and judge the type of orthodontics they will need.
ESTHETIC ORTHODONTICS
BRACES
In the 1950s, dentists believed that the shapes and positions of the jawbones are determined by genetics, and therefore the goal of orthodontics was simply to straighten the teeth within the existing bone structure. Dentists in the United States developed a clever system of metal brackets glued to the exact center of each tooth and connected by a series of pre-fabricated precisely curved arch wires to align all the teeth until they conform exactly to the curve of the arch wires. The technique provides excellent control of every aspect of each tooth's position, and it does not require patient cooperation, as long as there is sufficient underlying jawbone present and the child can be brought in for regular adjustments. If the jawbones are too small to accommodate all the teeth, permanent premolar teeth are removed, and the rest of the teeth are rearranged to fit within the small jawbones, or the jawbones are reshaped surgically.
More than half a century later, we know that the shape of the upper jawbone and the position of the lower jawbone (the mandible) are very much affected by the jaw muscles. Strong jaw muscle activity expands the upper jawbone and advances the tooth-containing front half of the mandible (the lower jawbone). When jaw muscle activity is weak, these growth patterns are not sufficiently stimulated. In addition, when a deep overbite or steep interdigitation of the teeth locks the upper and lower jawbones together, it inhibits both of their growth patterns. The upper jawbone cannot expand normally, because it is locked to a lower jawbone that can only translate; and the lower jawbone cannot translate normally, because it is locked to an upper jawbone that can only expand. The growth that is inhibited horizontally gets redirected vertically. As a result, narrow nasal cavities and backward rotation of the mandible have become common problems for orthodontics today.
Unfortunately, esthetic orthodontics can inhibit jaw muscle development, because pressure on teeth keeps them tender, which causes the jaw closing muscles to fire weakly, which can prevent them from gaining the strength they need to regulate facial growth. In addition, orthodontists usually place braces during the pubertal growth spurt, when jaw muscle development should be most rapid, and when inhibiting it has the most damaging effects.
CLEAR ALIGNERS
(such as invisalign) employ a series of removable computer generated plastic shells to align the upper and lower teeth in a manner very similar to braces but with more convenience and less control. The appliances cannot easily open or close spaces between teeth; but they can be removed for brushing, and they are usually worn for a shorter time period.
RETENTION
After esthetic orthodontics, retainers are needed to prevent relapse. One cause of this relapse is the "neutral zone" which holds the teeth in a balance between forces from the lips, cheeks, and tongue, as well as by the bite pushing them in opposite directions, as shown below. If a tooth moves outside of its neutral zone, these forces will move it back in. Esthetic orthodontics was not designed to take neutral zone forces into account.
The other cause of post-treatment relapse, especially crowding of the lower front teeth, is continuing adult jawbone growth. Esthetic orthodontics usually leaves the upper and lower teeth steeply interdigitated, because it was developed before we realized that the jawbone growth slows down about 90 percent after the second decade; but then continues slowly throughout adulthood. Because the upper and lower jawbones grow in different types of bone, at different rates, and in slightly different directions; continued adult jawbone growth requires some slippage in the bite to prevent it from causing progressive mechanical strains between the jawbones and teeth.
Esthetic orthodontics with braces or clear aligners can sometimes help treat TMJ disorders that are caused by jaw muscle tightness due to a strained bite by deprogramming the jaw muscles, like a FRONT FLAT PLATE APPLIANCE does. Then, if the jaw muscles can hold the lower jawbone in a better location for the duration of the treatment, the teeth will end up fitting there. Physical therapy and jawbone exercises can help. However, braces and clear aligners cannot be considered treatments for TMJ disorders, because TMJ disorders are caused by the malpositioning of the jawbones rather than the straightness of the teeth. Therefore, esthetic orthodontics is for people who want very straight teeth and already have well developed jaw muscles, good jawbone structures, and a healthy symmetrical facial growth pattern.
FUNCTIONAL ORTHODONTICS
While American orthodontists were developing more efficient methods for straightening the teeth, European orthodontists were developing removeable oral appliances to expand the upper jawbone and advance the lower jawbone. The functional orthodontic appliances they developed have certain advantages.
EARLY TREATMENT – While esthetic orthodontics usually waits until age 12, when the second molars have arrived, functional orthodontics can begin early, when it has a greater effect on craniofacial growth. Even before the adult teeth have begun to erupt, young children can chew for short periods of time on specially molded rubber appliances, such as the Myobrace or the Myomunchie. In cooperative children, these specially shaped silicone rubber chewing pads can be very effective in a surprisingly short period of time. Anterior growth guidance appliances, (AGGAs) employ a functional orthodontic appliance that can be glued in the mouths of young children. Removeable versions of AGGA appliances can be used on young children if small retentive dots of composite resin are first bonded onto their short teeth in order to give the appliance some grip on them. These appliances recently received FDA clearance, and they are now being promoted for treating airway problems such as obstructive sleep apnea in adults, but the devices have been problemmatic, and the FDA clearance may have been based on misinformation.
Recently, some functional orthodontic treatments have been repackaged for marketing to adults. ALF (advanced lightwire functional) appliances, which are made of wires soldered to bands glued onto the teeth, are thin and easy to wear; but they must be worn for a long time. DNA, RNA, and HOMEOBLOCK APPLIANCES are traditional screw type palate expansion appliances with finger springs that are sometimes marketed as treatments for obstructive sleep apnea; however, they cannot be expected to improve obstructive sleep apnea in most people, because the expansion occurs more than an inch from the obstruction. They are also sometimes marketed as, "epigenetic orthodontics"; which is a misleading term; because there is no evidence or likelihood that straightening the teeth or widening the palate of an adult will lead to straighter teeth or wider palates in their offspring.
ORTHOTROPICS is a branch of functional orthodontics that focuses on tongue strength as a key to wide palates and healthy mandibular advancement. The tongue plays an important role in natural palatal expansion, because its resting posture guides the erupting upper teeth into the "neutral zone" around it. When people are born without tongues, the palate becomes very small and collapsed. However, that finding does not mean the tongue is responsible for palatal expansion or that stronger tongues are the key to wider palates. In fact, bottle fed infants get much less tongue exercise early in life than breast fed infants, and they can be expected to have weaker tongues; but they do not have significantly narrower palates. The tongue muscles are part of a network of craniofacial and neck muscles which all work together, and they all must be strengthened together. Orthotropic dentists generally use conventional palate expanders (bioblocs) along with tongue exercises and stretches.
The ideal functional orthodontic treatment is strong chewing, because it provides the type of exercise that ensures healthy jaw muscles, which regulate facial growth naturally. Babies need to nurse forcefully; and they need to transition from breast milk to food with consistency (rather than baby food) as they can handle it (baby led weaning), so their jaw muscles keep developing as the teeth are erupting. Once all the baby teeth are in, tough foods like dried fruit, whole nuts, and jerky act like a gym for the jaw muscles, because they prevent the opposing teeth from making physical contact, which stops the firing of the jaw muscles. In healthy jaw muscle exercise, the mandible pivots and slides on the resistant bolus without allowing the opposing teeth to penetrate it.
COMPARATIVE ORTHODONTICS
The two different styles of orthodontics are completely different tools, and they have very different applications. Braces can align all the teeth into perfectly curved upper and lower arches; but they also usually retract the jawbones, so they should be used for people who are mostly concerned with appearance and already have a good horizontal jawbone growth pattern. Functional orthodontics should be used when the pattern of facial growth needs to be improved for long term health. Ideally functional orthodontics is followed by braces or clear aligners like framing carpentry is followed by finish carpentry. The following text describes the way both of these different types of orthodontics deal with each of the common orthodontic problems that predispose a child to have TMJ disorders later in life - the narrow upper jawbone, the downwardly and backwardly rotating lower jawbone, excessive vertical face length, and the deep or steep overbite.
COMMON ORTHODONTIC PROBLEMS
1. THE NARROW UPPER JAWBONE
The width of the upper jawbone is determined largely by the activity of the jaw muscles, and many modern children have narrow upper jawbones due to weak jaw muscle activity. Frequently the upper dental arch is more V-shaped than U-shaped. The upper teeth come in crooked due to lack of space. The mandible cannot advance normally, because the lower dental arch is locked behind an undersized upper dental arch. The lower teeth are tipped inward to fit inside the narrowed line of upper teeth. The tongue cannot fit up in the front of the palate where it belongs, so it acquires a downward and backward resting posture, often resting on top of the lower teeth, which tips them further inward. The lowered tongue posture also lowers mandibular posture, which is at the root of airway problems and TMJ disorders.
MOUTH BREATHING is the most damaging consequence of a narrow upper jawbone. The narrowing that results in the V-shaped upper dental arch typically occurs in the premolar area just under the anterior nasal airway, where the roof of the mouth (the palate) is the floor of the nose, and the narrowing in the front of the palate produces a narrow nasal cavity just above it. When these structures are too narrow to allow adequate nasal airway passage, people become obligate mouth breathers. Their lips are always parted to provide airflow. However, that air has not been filtered, warmed, and moistened by the nose before it hits the back of the throat; leading to frequent upper respiratory infections. Many people with palates of borderline width become obligate mouth breathers whenever a cold or allergies cause even minor swelling of the nasal passages.
CROSSBITE occurs when the upper jawbone is so much narrower than the lower jawbone that some of the lower teeth fit outside of the upper teeth instead of inside them. In bilateral crossbite, both sides have this reversed interdigitation. In unilateral crossbite, the lower jawbone shifts to one side to create a reversed interdigitation only on that side, while the other side looks normal.
TREATMENT of narrow upper jawbones using braces traditionally requires extracting some permanent teeth (usually first premolars), so the remaining teeth can be rearranged to fit within the existing narrow upper jawbone structure. This practice is no longer nearly so common as it once was, because people now recognize the importance of nasal breathing as well as the esthetic value of a wide smile. Today, braces in children are often combined with a palate expander that is glued to the teeth and turned by the parents on a regular schedule. Treatment of narrow upper jawbones using functional orthodontics usually employs a removeable palate expansion appliance, as described in PALATE EXPANSION under the tab TREATMENTS.
2. THE DOWNWARDLY AND BACKWARDLY ROTATING LOWER JAWBONE
A second orthodontic problem that has become common in modern children and is also correlated with TMJ disorders is the downwardly and backwardly rotating lower jawbone. Mandibles need to continue advancing slowly during adulthood to reduce airway resistance to compensate for the loss of muscle strength that also occurs slowly during adulthood. When the advancement of the mandible is blocked by a deep overbite or steep interdigitation, this mandibular growth gets redirected down and back, as seen below left. The front of the mandible rotates down and back into the space needed for the pharyngeal airway.
DOWNWARD AND BACKWARDLY ROTATING MANDIBLE FORWARD ROTATING MANDIBLE WITH DEEP OVERBITE
WEAK CHIN LONG FACE STRONG CHIN SHORT FACE
The downward and backward rotation pattern, seen on the left side above, can be contrasted with the upward and forward rotation pattern seen on the right side above. The difference is seen most dramatically in the jaw angle, which is obtuse on the left side and acute on the right side. However, in the case pictured on the right, the mandible is locked back (posteriorly displaced) behind a deep overbite; which is a different pathological facial growth process that is discussed in the next section.
The downwardly and backwardly rotating facial growth pattern can certainly be caused by weak jaw muscles. People with jaw muscle injury or disease show a remarkable downward and backward rotating growth pattern, as seen in the X-rays of a patient with muscular dystrophy below. The picture on the right below is a longitudinal study, and the picture on the left below compares the myotonic dystrophy face at late adolescence with a normal adolescent face, seen as a white line. In the myotonic dystrophy patient, the upper teeth have acquired a form that fits around the tongue, which has found a resting posture that maintains a pharyngeal airway, despite the front portion of the mandible (the corpus containing the teeth) rotating downward and backward around it.
Conversely, the pattern of backward rotation can be reversed by making the jaw muscles stronger and healthier. A study of children with downwardly and backwardly rotating facial growth patterns found that chewing exercise gum for a year reversed their facial growth to a forward rotating pattern, which reverted back to the backward rotating pattern again after the gum chewing stopped.
In most people with backwardly rotating mandibles, the upper dental arch follows the lower dental arch down and back. The resulting vertical redirection of growth in the upper jawbone prevents it from expanding normally. This can exacerbate the narrow upper jawbone problem described just before this one.
In many people, the downward and backward rotation of the mandible makes the framework of bones and teeth at the front of the face grow too long for the curtain of soft tissues hanging down from the front of the head to comfortably cover them, producing a characteristic gummy smile and visible strain in the muscles surrounding the mouth when they have to hold the lips closed to maintain a lip seal. The opening of the lips that occurs when the peri-oral muscles relax often results in habitual mouth breathing, because the open lips create an easy path for airway flow, although it is not a healthy one.
In some people with weak jaw muscles and downwardly and backwardly rotating lower jawbones, the tongue intervenes to prevent the upper jawbone from following the front of the lower jawbone down and back. Instead, the tongue holds the upper jawbone upward and forward to maintain an adequate airway passage, as can be seen above in the growth of the muscular dystrophy patient. This frequently results in an anterior open bite that makes it impossible to incise. For decades, orthodontists assumed that these anterior open bites were caused by a retained infantile swallowing pattern which included a brief tongue thrust. However, studies using force sensors showed that it is the resting posture of the tongue, not its functional activity during swallowing, that determines tooth positions. Here again, it is the light steady types of forces, such as those produced by postural muscle tonus, that determine the shapes of bones. Tongue posture is steady, because it maintains the airway, so it shapes the bones. Intermittent functional forces, like swallowing, primarily determine the internal architecture of the bones, enabling them to withstand the forces they will receive.
EXCESSIVE VERTICAL FACIAL GROWTH - In some people with weak jaw muscles, the back of the mandible grows as far downward as the front of the mandible, making the face long without the lower jawbone rotating backward. The gummy smiles and narrow nasal airway passages are still frequent problems in these people.
TREATMENT
Excessive vertical facial growth has always been a difficult problem for esthetic orthodontics, because the only tool they have had to limit it is high-pull headgear. However, even the highest pull headgear is still retractive in nature. It forces the upper jawbone partly backward into the area needed for nasal airway passage. In addition, by the time the second molars have erupted at 12 years, much of the vertical excess has already occurred, and reversing it would require maxillary impaction surgery to remove a wedge of bone from under the middle of the nose so the upper jawbone can be rotated upward into the space left by the wedge, or intruding multiple teeth using springs or elastics attached to temporary anchorage devices (TADs) that have been temporarily implanted in bones around the face.
Functional orthodontics treats excessive vertical growth by redirecting further growth horizontally. The upper jawbone is expanded, and the lower jawbone is advanced.
3. DEEP OR STEEP OVERBITE
A third common orthodontic problem that causes TMJ disorders is an overbite that is deep or steep enough to lock the mandible back and thereby prevent it from advancing normally. The overbite is the vertical overlap of the front teeth. When you bite all the way down on your back teeth, your upper front teeth should only cover a small portion of your lower front teeth, as shown on the right below, and you should be able to slide your bottom teeth easily forward onto your top teeth. If your lower front teeth are mostly covered by your upper front teeth, you have a large overbite, as shown on the left below. If your lower front teeth are completely covered by your upper front teeth, you have a 100% overbite. In some people, the overbite is even deeper than that, and the lower front teeth may impact the soft tissues of the palate just behind the upper front teeth. In other people, the overbite is steep rather than deep, making it difficult or impossible to slide your lower front teeth up onto the backs of your upper front teeth.
The trouble with a deep or steep overbite is that it can prevent the mandible from advancing normally with age, which is needed to reduce airway resistance as muscles weaken.
TREATMENT of steep or deep overbite can be accomplished with braces or functional orthodontics. Braces can eliminate the overbite by arch leveling, especially if assisted by using TADs (temporary anchorage devices) to intrude super-erupted teeth. Functional orthodontics can eliminate the overbite by wearing a front flat bite plate at night for many years, and the process can be accelerated by using finger springs to tip out some of the front teeth or orthodontic elastics on little buttons temporarily glued to the back teeth.
COSTS OF FUNCTIONAL ORTHODONTICS
The cost of single stage treatment involving a removable appliance is about $2000. A second stage is often added at about half that cost for fine tuning of tooth positions and to later function as a retainer and TMJ protective oral appliance. Either first or second stage appliance can also incorporate mechanics such as springs or inclines for further minor tooth movements.