The Role of The Bite in TMJ Disorders
BACKGROUND - Dentistry has never understood bites. There is no doubt that bite disturbances can cause TMJ disorders. In provocation studies, a high filling can produce typical TMJ disorder symptoms that persist for months, sometimes even long after the high filling is removed.
The bite affects the TMJs, because it determines the positions of the condyles in the TMJs, and the positions of the bones in joints always affects the health of those joints. In particular, the central bite location, where the jaw system is frequently and most strongly loaded, determines the close packed position of the condyles in their fossae in the TMJs, and the close packed positions of joints gives them stability. Displacement of the bite platform displaces the condyles and thereby triggers remodeling of the condyle and the other joint components in an attempt to fit the new close packed position. Backward displacement of the bite commonly drives the condyle into the back end of the joint, where the disk can no longer fit on top of it, forcing the disk out of place like squeezing out a watermelon seed out from between two fingers.
The bite also affects the jaw muscles, because it forms the platform on which those muscles rest and exercise, - conditions which always affect muscle health in other parts of the body. Instability of the bite platform triggers increased jaw muscle tonus that prevents the postural muscles of the head and neck from relaxing fully. Displacement of the location of the central bite position makes the jaw muscles strain a little every time they have to bring the teeth together.
CURRENT BITE TREATMENT - However, dentists are generally afraid of making any changes to bites, as explained in BITES: CURRENT CONCEPTS under the FOR DOCTORS tab. Most of them will try almost anything else before treating the bite. One of the goals of this website is to explain the role of the bite in TMJ disorders to enable dentists to employ rationale bite treatment that addresses TMJ problems at their source, explained in BITES: ORTHPEDIC ASPECTS under the FOR DOCTORS tab.
RATIONALE CARE - comes from an understanding of the root cause of a disorder. Evidence based care is widely embraced by the medical and dental community, and it represents an important step beyond anecdotal care; but evidence-based care is useless when we cannot collect evidence about something, because we cannot even measure it. Conventional measurements of bite parameters have no relevance to jaw system function or health. Studies on bite treatments have improved jaw system health in some subjects and impaired it in others.
BITES AND FACIAL GROWTH - To understand how bites cause TMJ disorders, it's important to understand how the bite affects facial growth. The central bracing platform is one of the most stable architectural landmarks in the facial growth of all mammals. By design, the upper and lower jawbones shift, expand, and rotate around this platform while maintaining its orientation and height with great stability by continually feeding new tooth structure into it as the old tooth structure is worn away. However, in modern humans, the bite platform is often unstable or displaced. An unstable bite platform prevents the normal healthy development of the jaw muscles, which are needed to regulate facial growth. A displaced bite platform causes the face to grow in the direction of the displacement of the mandible, which frequently causes TMJ disk dislocation on the side of the displacement and produces strains and stresses between the mandible and the spine.
THE CENTRAL BITE PLATFORM - is the part of the bite platform that occupies a small area, within which most or all of the back teeth contact almost simultaneously. This is where you clench, swallow, and chew tough food. It functions as the home base for the jaw system. The jaw muscles automatically bring the lower jawbone closed there, and the back ends of the mandible (the condyles) are fully seated in your disks there, just as the outer cusps of your lower teeth are fully seated in the valleys of your upper teeth there. The easiest way to locate the central bite platform is by tipping your head back and tapping your back teeth together a few times until they make a sharp tapping sound and feel like a solid table.
BITE STABILITY - A stable central bite platform is critical for the long term health of the jaw system. Bite stability protects the teeth, gives the muscles a healthy platform to rest on and excercise against, and gives the bony remodeling in the TMJs a steady target. People with TMJ disorders have less stable bites than other people. The three main causes of destabilizing bites are described below:
1) TMJ DAMAGE due to disk displacement can destabilize a bite suddenly by removing a cap from the top of a condyle and thereby effectively shortening that condyle, much like removing a shim from under a table leg. The resulting slight change in the cant of the mandible usually causes the teeth of that affected side to hit before the teeth of the other side. Arthritic degeneration in a TMJ can destabilize a bite by gradually shortening the condyle due to regressive remodeling of the bones. In some acute TMJ disorders, such as those resulting from a bout of juvenile arthritis, the loss of bone is so dramatic that it causes the whole lower jawbone to rotate around the rearmost molars and thereby separate all the other teeth, creating an open bite that makes chewing difficult. If the tongue then responds by acquiring a resting posture that fills in the space between the teeth to stabilize the lower jawbone, it can prevent the open bite from ever closing again.
2) TMJ inflammation can destabilize a bite in the opposite direction by effectively lengthening the affected condyle, because the swelling pushes it down and away from the cranium like inflating a bag of water under a table leg. The back teeth on that side separate, and trying to squeeze them together produces immediate pain, because it forces the condyle into the swollen area.
A continuously shifting bite is frequently reported by patients with chronic TMJ disorders, because mandible shifts back and forth between these conditions in response to small fluctuations in the state of inflammation and the arthritic degeneration of one or both TMJs. Patients with this condition often report feeling like they have many different bites or no bite at all.
3) DENTAL CHANGES cause bite destabilization when teeth tip into spaces opened up by cavities between them or shift position in response to gum disease reshaping their sockets. Wearing an upper oral appliance full time can shift all the lower teeth until they only fit the surface of the appliance; then, when the appliance is removed, there is no natural bite. Wearing a lower nightguard, especially during a time of increased stress, can destabilize a bite by confining the powerful compressive forces of clenching and grinding (bruxism) to the back teeth and thereby causing those teeth to shorten by sinking (intruding) into the jawbones while the front teeth lengthen by elongating (extruding) out of the jawbones - thus increasing the overbite. The bite becomes unstable when the overbite prevents you from being able to bite the back teeth forcefully together without the front teeth also hitting forcefully. The bite becomes very unstable when you can’t even touch the back teeth. Contact on a steep overbite can wedge the mandible backward toward the ears, which damages the TMJ. Dental work can also destabilize a bite when it is left too high, or more frequently when it is left too low, because our technology cannot make it just right, and leaving it high causes immediate problems.
BITES CAN RESTABILIZE on their own. The same adaptability that originally produced a stable bite can also recreate one later in life. Teeth were designed to keep erupting throughout life in order to maintain a stable bite in spite of constant tooth wear, and natural eruption of teeth that have become too low can bring them back up into a good bite at any age. However, some modern bites may be unable to restabilize naturally because of dental features such as irregular alignment of the teeth or tight contacts between adjacent teeth. Balancing side interferences occur when the mandible slides to one side, but the teeth contact on the other side. Such a contact pattern is anti-orthopedic, because joints are wired to receive sensory feedback in the direction of movement; however, once it is established, it protects itself by causing the neuromuscular system to stop firing the jaw closing muscles as soon as it contacts.
BITE STRAIN - Even if your bite is stable, it may still be strained. Even if your bite forms a stable platform on which all of your back teeth can hit at the same time and make a sharp tapping sound, that platform may still be located too far back and/or off to one side. The jaw muscles are arranged in bilateral slings that converge down onto the mandible from origins spread widely around the skull. When the jaw muscles are at rest, the mandible should hang from these slings in a central position. Then, when the jaw muscles fire, they should swing the mandible closed along a central trajectory that is also determined by the anatomy of the slings. At the top of that trajectory should be a central bite platform created by simultaneous contacts between most or all the back teeth. In many TMJ disorder patients, the central bite platform is not located at the top of an unstrained mandibular closing trajectory. Instead, the jaw muscles must shift the mandible during closing horizontally away from its unstrained mandibular closing trajectory and into an adaptive mandibular closing trajectory aimed at the central bite position. As a result, the condyles get displaced within the sockets of the TMJs, and the jaw muscles are forced to do all of their exercise in a strained position - like riding a bicycle with the seat too low or lifting weights with your feet planted off to the side. However, even if a bite is extremely strained, you are unlikely to become aware of the strain, because the teeth are protected by a vast network of neuromuscular reflexes that continuously program the jaw muscles to only close the lower jawbone into the central bite position, wherever it is located. They can make the jaw muscles habituate an intricate dance (engram) to avoid crashing into misplaced teeth with every closure, so that even a very strained bite can feel perfectly natural.
Diagnosing bite strain is most easily accomplished by deprogramming the jaw muscles by wearing a front flat plate appliance during sleep to stop the jaw muscles from always closing the mandible into the central bite position. When you wake up with this appliance every morning, your deprogrammed jaw muscles will open and close along a trajectory that is determined by the jaw muscles without the influence of the teeth. Then, when you first remove the appliance in the morning and bite down along the deprogrammed jaw trajectory, you can see where your back teeth should fit together. If some teeth hit first and slide your mandible away from this deprogrammed position and into your old central bite platform, your central bite position is strained, and those prematurely contacting teeth are likely the primary cause of your bite strain.
THE ROLE OF BODY POSTURE - One factor that can influence the natural opening and closing trajectory of the mandible is body posture. If your muscles are fully relaxed and you tip your head back while you bite, you'll hit just the back teeth. If you tip your head forward while you bite, you'll hit just your front teeth. If you turn your head to one side while you bite, you'll hit your teeth on the other side. This natural variation in jaw closing trajectories requires a central bite platform with enough freedom of movement along a small area. However, postural influences on the bite are normally override by neuromuscular reflexes that were designed to protect our teeth. That connection is summarized in THE ROLE OF BODY POSTURE under the tab TMJ DISORDERS, and it is described in detail in BITES AND BODY POSTURE under the tab FOR DOCTORS.
ADJUSTING BITES - There are many ways to eliminate bite strain. Teeth can be reshaped and/or moved orthodontically. People with excessively long faces can benefit from orthodontically intruding or shortening teeth; and people with excessively short faces can benefit from building up teeth.
REDUCING high areas requires first accurately locating them and then reducing them with a high speed drill just enough to hit together with the rest of the teeth. This process (called equilibration) does not harm teeth or make them more susceptible to developing cavities, and it can help them periodontally by reducing their crown/root ratio. Therefore, equilibration can be a simple and inexpensive way to improve jaw muscle health, especially in people who have long faces and steeply interdigitating tooth inclines which restrict the range of motion of the lower jawbone.
BUILDING UP low areas, to stabilize a bite, is easily accomplished on a provisional basis by bonding tooth colored filling material (composite resin) directly to the tops of the teeth. The material is versatile, and it can reinforce teeth that already have fillings, because it bonds to enamel and to itself, but it can wear down rapidly in people with strong jaw muscles and vigorous tooth grinding habits, – especially on the molars which receive most of the bite force. After it proves successful, it can be replaced by more permanent materials before it wears away.
WEAKENED TEETH due to cavities or previous fillings will require gold or porcelain onlays or crowns, which can be used to rebuild their bite surfaces as part of the process to recreate a healthy bite. Filling materials prevent food from collecting in a hole, but they do little to reinforce the tooth structurally. As teeth become more brittle with age, they tend to crack or chip from around the filling, and they will eventually need to be covered by a crown or onlay so they cannot break apart.
GOLD is the ideal material for reconstructing bite surfaces. It is extremely accurate and biocompatible, and it has a hardness very similar to natural tooth structure – which minimizes wear. In addition gold is strong even when thin, requiring minimal drilling of the teeth in the preparation process.
PORCELAIN has excellent esthetics, however it also has disadvantages. Porcelain needs bulk for strength, so the tooth preparation process requires significant reduction of tooth structure, which increases the chance of the tooth later needing a root canal. Also, porcelain is so much harder than natural teeth that, instead of wearing down itself, it tends to wear away the opposing natural teeth.