Orthodontics and TMJ Disorders

SUMMARY

In American dental schools, dentists are taught that most bite problems should ideally be corrected by orthodontics, but they don't really learn how orthodontics works, and they learn almost nothing about functional orthodontics, which has been popular in Europe for decades and is preferred by most TMJ specialists.  This paper explains and contrasts functional orthodontics with the esthetic orthodontics (braces and invisalign) that is widely practiced in the USA today.  The material will be instructive for both you and your dentist.

ESTHETIC ORTHODONTICS employs braces (metal wires and brackets) or pre-formed plastic shells (invisalign) to align the upper and lower arches of teeth until they form perfectly smooth curves; however there is very little ability to control the location of the lower jawbone at which they fit together - a feature that influences facial growth, TMJ health, and body posture.  

FUNCTIONAL (AND INTERCEPTIVE) ORTHODONTICS  is less concerned with the straightness of the teeth and more concerned with the function of the whole jaw system. It employs removeable oral appliances that contain expansion screws, springs, and inclines which are designed to shape and position the jawbones in a manner that promotes long term jaw system health.  The oral appliances can be removed for brushing and important social functions, but they must be worn most of the time.  The teeth are less precisely aligned, but the jaw positions and facial structural features are improved.  When used in adolescents, it is also called interceptive orthodontics.

Each of these orthodontic techniques, esthetic and functional, has advantages and disadvantages in different situations.   If you are considering orthodontics, the following information should help you and your dentist understand your options.

ESTHETIC ORTHODONTICS

BRACES

In the 1950s, dentists in the United States popularized a system of metal brackets glued to the exact center of each tooth and connected by a series of pre-fabricated arch wires that are placed in the brackets to align all the teeth until they conform to the shape of the arch wires.  Because the technique permits controlling every aspect of torque and angulation, the exact positions of all the teeth can be determined, as long as there is sufficient underlying bone and time available.  The lower teeth are aligned into a smoothly curved arch form, and the upper teeth are aligned into a slightly larger version of the same arch form.  Because the upper and lower arch wires are matched for size, the cusps of the lower teeth end up fitting in the valleys of the upper teeth.  

The orthodontic theory on which this system was based assumed that the shapes and positions of the jawbones are completely determined by genetics, and therefore the goal was to simply straighten the teeth within the existing jawbone structure.  If the jawbones are too small to accomodate all the teeth, some permanent teeth are removed and the rest of the teeth are moved around to fit within the small jaws, or the jawbones are reshaped surgically.  Extraction of permanent premolars in teenagers with small jaws was once practiced by 90% of dental school orthodontic clinics.

However, we now know that the jaw muscles regulate facial growth.  Weak jaw muscles lead to backward facial rotation, failure of the face to widen properly, and excessive height at the front of the face – all common characteristics of TMJ disorder patients.  Studies have shown that, if children with a backwardly rotating facial growth pattern chew exercise gum for a year, their facial growth pattern reverses and rotates forward while chewing the exercise gum; and then it reverts to the previous unhealthy backward rotating facial growth pattern after the exercise gum chewing stops.   

Unfortunately, esthetic orthodontics inhibits jaw muscle development, because the pressure on individual teeth makes them tender, which automatically discourages forceful biting and chewing and thereby denies the jaw muscles the exercise they need to stay healthy and develop fully. In addition, the process of moving the teeth with braces usually requires at least two years, because a tooth can only move within the jawbones as fast as bone can be dissolved in front of it and new bone formed behind it.  After a couple of years, the weak chewing pattern can become permanent.

Furthermore, braces are usually employed when muscle development is most critical – during the pubertal growth spurt. Esthetic orthodontists like to wait for that developmental stage so they can include the 12 year molars in the alignment process. However that is also the time when the jaw muscles are most vulnerable to interrupted development.   

INVISALIGN

Invisalign employs a series of removeable computer generated plastic shells (rather than brackets and wires) to align the upper and lower teeth in a manner very similar to braces but with more convenience and less control. It cannot easily open or close spaces between teeth, and the teeth are less precisely positioned, but the appliance can be removed for brushing, and it is usually worn for a shorter time period.  Its main disadvantage is that it discourages jaw muscle development much like braces do.

RETENTION

At the conclusion of esthetic orthodontic treatment, retention is needed to prevent post-treatment relapse due to neutral zone considerations.  The light continuous forces provided by the soft tissues surrounding the teeth together with bite forces create a neutral zone - an area within which the teeth rest in a balance between forces in opposite directions. These forces include pressure inward from the cheeks, outward from the tongue, upward (vertically out of their basal bones and into the bite table) from the tendency of the teeth to keep erupting, and downward (vertically back into their basal bones) from biting - as illustrated below. If a tooth moves outside of its neutral zone, these forces tend to move it back in.  

eruption

Unfortunately, esthetic orthodontics was developed before we recognized the importance of the neutral zone.  As a result, retainers are required to prevent the teeth from shifting toward the neutral zone after treatment.  Bonded retainers are usually placed behind the lower front teeth as a permanent structure, and removeable retainers are made to wear nightly on the upper teeth.  A few esthetic orthodontists also employ myofunctional therapists to assist with patients who have unusual tongue, cheek, or lip habits that seem likely to interfere with treatment goals.

Esthetic orthodontics was also developed before we recognized that facial growth continues during adulthood, at about ten percent of its teenage rate.  The orthodontists who developed braces assumed that, if the teeth are left straight at the end of the second decade when overall growth stops, they will stay straight. However now we know that, unlike other bones, the jawbones were designed to keep growing during adulthood in order to compensate for the tooth wear that also continued during adulthood in our ancestors.  If the jaws didn't keep growing during adulthood in a way that kept bringing the upper and lower teeth toward each other and into the bite table, their chins would keep approaching their noses and their jaw muscles would lose their efficiency. That adult jaw growth still continues today, even though it is rarely needed to compensate for tooth wear.  After esthetic orthodontics has left relatively unworn teeth steeply interdigitated, the bite may not be able to accomodate continued facial growth in adulthood, causing the teeth to shift.  In some important functional features, such as overbite, the relapse after braces averages 50%.  

In summary, esthetic orthodontics is most suitable for people who want very straight teeth and already have well developed jaw muscles, good jawbone structures, and a healthy symmetrical facial growth pattern - especially if there is no history of TMJ disorders in the family. 

FUNCTIONAL ORTHODONTICS

While American orthodontists were developing more efficient methods for straightening the teeth with braces, European orthodontists were developing removeable oral appliances that align the jawbones by moving groups of teeth together with segments of bone.  While the lower jawbone is held in an optimal functional position by bite plates or telescopic mechanics, the teeth are moved into positions that support it there.

Functional orthodontics has a number of advantages over esthetic orthodontics. It can reduce the chances of developing a TMJ disorder later in life by improving the position of the lower jawbone. It can encourage the development of healthy jaw muscles by giving them a bite surface that forms an ideal exercise template. It can also begin early, usually as soon as the 6 year molars have erupted. Stimulating jaw muscle development is more effective if begun early; because the jaw muscles regulate facial growth, and strong jaw muscles can increase the likelihood that normal functional forces will guide the teeth into proper positions.  When used in adolescents, functional orthodontics is often referred to as interceptive orthodontics, because it can intercept developing orthodontic problems.

RETENTION

At the conclusion of functional orthodontics, retention is achieved by stabilizing the bite and continued use of the functional appliance at night rather than by mechanical retainers.  Usually bite stabilizaton is performed at the end of treatment.  Techniques for stabilizing bites are explained in THE ROLE OF THE BITE under TMJ DISORDERS.  

COMMON ORTHODONTIC PROBLEMS 

There are three common orthodontic problems that predispose a child to have TMJ disorders later in life. These are the narrow upper jawbone, the backwardly rotating lower jawbone, and deep overbite.  The following text describes how each of these problems is treated with the different styles of orthodontics.

1. THE NARROW UPPER JAWBONE

The size of the teeth and the shape of the lower jawbone are determined by genetics, but the size of the upper jawbone is determined largely by the activity of the jaw muscles.  Strong jaw muscles and vigorous chewing created wide upper jawbones in our ancestors. Modern children generally have relatively weak jaw muscles and narrow upper jawbones.  One result is that the upper teeth often come in crooked, because there is not enough room for them.  A second result is that the lower jawbone may not be able to grow forward normally, because it must fit within the upper dental arch, leading to bite strain and TMJ disk dislocation.  A third result is that the lower back teeth may tip inward to fit within the narrowed upper dental arch and thereby impinge on tongue posture.   

CROSSBITE occurs when the lower jawbone shifts to one side in order to achieve intercuspation, resulting in asymmetrical facial growth. The teeth on that side interdigitate in a reversed relationship called crossbite while the teeth on the other side achieve normal relationship.  Chewing can still be effective, but the asymmetrical bracing position of the lower jawbone often produces an asymmetrical pattern of subsequent facial growth.

MOUTHBREATHING is the most important negative consequence of a narrow upper jawbone.  The roof of the mouth (the palate) is the floor of the nose.  When the roof of the mouth is too narrow, the nasal cavity may be too narrow to allow normal resting nasal airway flow, and the mouth must stay open for additional air.  Because this air that comes in through the mouth is not filtered, warmed, and moistened by the nose before it hits the back of the throat; it promotes upper respiratory infections.

The way a small upper jaw is traditionally treated with braces is to extract four permanent premolars and then move the rest of the teeth around to fill the spaces.  The upper jaw is left narrow and simply made to house fewer teeth.  

The way a small upper jaw is treated with functional orthodontics is by widening it, using either a palatal expansion screw or bent wire appliance. Any of these removeable appliances can also carry components such as inclined ramps or telescopic components to position the lower jawbone, springs to straighten individual teeth, and various airway controlling components.

Expansion screw wideners (such as Schwartz, Hyrax, and Haas appliances) employ a turnbuckle located in the middle of the appliance, and the patient is given a key that turns it to adjust it.  With each turn of the screw, the two sides of the palate are driven slightly further apart. The patient uses the key to maintain light steady pressure on the upper teeth.  

Bent wire wideners, (such as quad helix appliances, bionators, and crozats), employ a framework of wires and springs that are soldered to thin metal bands which are usually glued to the teeth.  They can be easy to wear, but their main disadvantage is that they tend to do more tipping of teeth and less expansion of the underlying basal bones, such as the pillars of the nasal cavity.  ALF (advanced lightwire functional) appliances are bent wire appliances that they apply very low forces for a long period of time.  Because their forces are gentle, they are promoted as being compatible with cranial osteopathy, which is discussed in CRANIAL AND CRANIOSACRAL under TREATMENTS. 

widening

 2. THE BACKWARDLY ROTATING LOWER JAWBONE

A second orthodontic problem that has become common in modern children and is also correlated with weak jaw muscles is a backwardly rotating lower jawbone. The front of the face grows vertically longer than the back of the face, making the angle between the lower jawbone and the rest of the face obtuse, as seen on the left side below, instead of the "square" jaw representing the opposite type of facial growth as seen on the right side below.

                             BACKWARD ROTATING FACIAL GROWTH                                                          FORWARD ROTATING FACE WITH DEEP OVERBITE

strange skulls

The biggest problem with this type of facial growth is that the lower jawbone rotates backwards into the space needed for airway passage in the throat and thereby causes a number of health problems including difficulty swallowing, snoring, sleep apnea, and forward head posture. The mechanism by which backward lower jawbone posture causes forward head posture is summarized in THE ROLE OF BODY POSTURE under TMJ DISORDERS, and it is explained in detail in technical language in OCCLUSION AND POSTURE under FOR DOCTORS.

The midface and upper jawbone follow the backward rotation of the lower jawbone. The upper front teeth and the surrounding bone and gums may erupt excessively and create a framework of bones and teeth that is too long for the lips to comfortably cover them, - resulting in difficulty maintaining a lip seal, a gummy smile, and/or habitual mouthbreathing.  In other cases, the tongue intervenes by filling in the space created behind the backwardly rotating lower jawbone and prevents the upper jawbone from following the lower jawbone, - creating an open bite that makes it impossible to bite things off. 

The only tool that esthetic orthodontics can use to limit excessive vertical elongation in the front of the face is high-pull headgear. However, even the highest pull headgear still forces the midface backward and can thereby restrict the nasal airway passage.  Also, using mechanical devices to move bones without also strengthening the muscles that will be needed to hold them there invites relapse.  Because esthetic orthodontics tends to weaken rather than strengthen the jaw muscles, it has difficulty dealing with the excessive vertical growth at the front of the face that accompanies backward rotation.

Functional orthodontics treats backward rotation by employing bite plates that stimulate jaw muscle activity.  The ability of jaw muscle strength to control backward rotation was shown by one study in which children with backwardly rotating faces underwent a change to forward rotation while chewing exercise gum.  Some of the early functional orthodontic appliances were called activators, because they employed a loose fit to activate the jaw muscles in order to limit vertical facial growth.  Modern functional orthodontic appliances for treating backward facial rotation can employ tall interlocking bite plates (dual block appliance) or telescopic components (Herbst appliance) while also using the passive stretch of the jaw muscles to intrude the front teeth and reverse vertical elongation at the front of the face.  

Backward lower jawbone rotation and excessive vertical growth at the front of the face should be treated at early ages, because they are difficult to reverse after they occur.  To reverse excessive lengthening of the midface usually requires orthognathic surgery in which a wedge of bone is cut out between the nose and the upper teeth and the whole upper jawbone is rotated upward into the space left by the removal of the wedge.  To reverse the effects of backward rotation of the lower jawbone requires a sliding osteotomy in which the lower jawbone is split so its front half can be repositioned relative to its back half.  Functional orthodontics can begin correcting these problems by redirecting facial growth as early as age 6.

3. DEEP OVERBITE

A third orthodontic problem that has become common today is deep overbite. 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. 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 and the lower front teeth may impact the soft tissues of the palate just behind the upper front teeth.  In other people, the overbite may be relatively shallow but can cause problems because it is very steep.

overbite

Overbite is often confused with overjet, which is the horizontal distance between the front teeth.  Large overjets are common at puberty and subsequently diminish due to elongation of the lower jawbone which brings the lower front teeth forward as part of normal facial growth.

The trouble with a deep or steep overbite is that it can inhibit the normal forward growth of the lower jawbone. In normal healthy facial growth, the lower jawbone grows forward faster and farther than the upper jaw.  Thus the face flattens in profile with age.  However, if the lower jawbone is locked back behind a steep wall of tooth structure in overbite, the forward growth of the lower jawbone may be blocked.  Then, instead of pushing the lower teeth forward, the continual elongation of the lower jawbone may drive the condyles backward into the sensitive retrodiskal tissues of the TMJs and cause disk displacement, or it may redirect growth vertically at the front of the face and thereby cause backward facial rotation, as described in the previous section.

Braces can eliminate deep overbite at least temporarily by arch leveling. Usually the patient is left with only a small overbite at the conclusion of the treatment. However, shortly after treatment, much of the overbite usually recurs, because the back teeth are still mobile from the orthodontic process, and clenching can intrude them into the jawbones while allowing the front teeth to super-erupt out of the jawbones.  One consequence of clenching after treatment is a rapid increase in overbite and a retrusion of the lower jawbone that frequently dislocates a TMJ disk.  In fact, TMJ disk dislocations were first discovered in teenagers who developed clicking within a year after their braces were removed.

Functional orthodontics reduces overbite by incorporating an upper front flat bite plate that redirects the forces of nocturnal grinding and clenching upward on the upper front teeth and downward on the lower front teeth. The use of the upper front flat bite plate to reduce overbite is explained in THE FRONT FLAT PLATE APPLIANCE under TREATMENTS. If the upper front flat plate is worn just at night, the reduction of overbite will be very gradual – usually less than a millimeter per year.  However that rate is more than adequate to accomodate the slow facial growth that occurs during adulthood.  

COSTS OF FUNCTIONAL ORTHODONTICS

The cost of single stage treatment involving one removeable appliance is $900 to $1800, depending on complexity. A second stage is often added for more fine tuning of tooth positions.  Either first or second stage appliance can later serve as a post-treatment retainer, or a separate retainer can be made incorporating invisalign type mechanics for additional minor tooth movement.