The Misunderstood Role of The Bite in TMJ Disorders
BACKGROUND - The primary source of the confusion around TMJ disorders is the role of the bite. Neuromuscular reflexes force the jaw muscles to always close and brace the mandible wherever the teeth form the most stable bite, even if bringing the mandible to that location strains the muscles or damages the TMJs. However, there has never been any agreement about the location of the mandible at which to establish a stable bite platform.
There is no doubt that bite disturbances can cause TMJ disorders. In provocation studies, experimental fillings only 1/2 mm high produce typical TMJ disorder symptoms, which persist long after the high filling is removed. Most websites include misaligned bites as one of the causes of TMJ disorders, and most dentists polled recently still believe that bites are involved in the cause of TMJ disorders.
However, dental authorities can find no evidence for treating misaligned bites or any other bite characteristics. Every bite parameter they can measure is found about evenly in TMJ disorder patients and "normals", so they have concluded that improper bites must not cause TMJ disorders, and there should be no role for the treatment of bites in TMJ disorders. Although most dentists still believe that bites have a causal role in TMJ disorders, these authorities have concluded that the beliefs of the majority of dentists in the role of the bite are simply due to confirmation bias. These authorities warn dentists to avoid bite treatment, because it is irreversible.
The problem with such reasoning is that the lack of evidence of a connection is not evidence of lack of a connection. The lack of evidence connecting bites and TMJ disorders is due to the fact that dentists cannot collect evidence about something they cannot measure, and they have never had a way to measure the functional aspects of bites, because their working understanding of bites is based on a conceptual model that grew out of labwork techniques for making dentures on a hinge articulator a century ago. Using that conceptual model to evaluate bites has led dentistry to measure bites using parameters that are unrelated to health. The bite treatments previously studied for evidence of effectiveness were all based on centric relation equilibration, which probably improved jaw system health in some subjects and impaired it in others.
RATIONALE CARE - When there is no evidence on which to build a model of evidence-based care, we need to rely on understanding the root cause of a disorder and applying common sense, which is what I call rationale care. One of the goals of this website is to explain the role of the bite in TMJ disorders to enable dentists to apply common sense and employ rationale bite treatments that address TMJ problems at their source. Studies of rationale bite treatments will show positive results.
The position of the mandible in its most stable bite affects the TMJs, because that determines the close packed positions of the condyles in the TMJs, and the close packed positions of the bones in joints always affects the health of those joints. Displacement of the position of the mandible in its most stable bite also displaces the close packed positions of the condyles in the TMJs and thereby triggers remodeling of the condyles and the other TMJ components in an attempt to re-establish stable close packed TMJs.
The position of the mandible in its most stable bite also affects the jaw muscles, because the location of the platform on which muscles rest and exercise always affects their health. Displacement of the location of the mandible in its most stable bite makes the jaw muscles strain every time they have to bring the teeth together. It impairs their exercise like riding a bicycle with a seat too low or lifting weights off to one side. Normally, when people spend hours exercising a muscle, it gets strong; but many people spend hours every night grinding or clenching, and still their jaw muscles tire easily when trying to chew something tough; because they have been exercsing against a displaced exercise platform.
The contours of the bite platform also affect the jaw muscles like the contours of an exercise platform. Instability of any joint reflexively increases tonus in the muscles which cross that joint, and the bite functions as a joint between the jawbones, so an unstable bite triggers increased jaw muscle tonus.
BITES AND FACIAL GROWTH - To understand how these alterations of the bite platform can cause TMJ disorders, it's important to understand how bites affect facial growth. The bite 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 by continually feeding new tooth structure into it as the old tooth structure is worn away from it. However, in modern humans, the teeth don't just feed tooth structure into the bite platform; they can displace or destabilize it.A displaced bite platform can cause the mandible to lead the whole face in the direction of the displacement and drive the condyle on the side of the displacementtoo far backward, thereby dislocating the articular disk from that TMJ.
THE BITE PLATFORM - occupies a small area, where most or all of the back teeth contact almost simultaneously. This is where you clench, swallow, and chew tough food; and it functions as a home base for the jaw system. The jaw muscles automatically bring the mandible closed there, and the back ends of the mandible (the condyles) are fully seated 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 your bite platform is by tapping your back teeth together a few times until they make a sharp tapping sound.
BITE STABILITY - A stable bite platform is critical for the long term health of the jaw system; because it 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 main causes of unstable bites are described below:
1) DISK DISLOCATION - can destabilize a bite by removing a cap (the disk) from the top of a condyle and thereby effectively shortening that condyle, much like removing a shim from under a table leg. The resulting change in the cant of the mandible usually causes the teeth of that affected side to hit before the teeth of the other side. Subsequently, arthritic degeneration can continue to gradually shorten the condyle due to regressive remodeling of the bones of the TMJ. In some acute TMJ disorders, the loss of bone from the condyles is so fast that the whole mandible rotates around the rearmost molars, separating all the other teeth and creating an open bite that makes chewing difficult; and treatment requires restoring acceptable face height by shortening the rearmost molars.
2) TMJ INFLAMMATION - can destabilize a bite in the opposite direction by effectively lengthening the affected condyle, because the swelling of the joint capsule pushes the condyle 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. Patients feel like they have many different bites or no bite at all.
3) DENTAL CHANGES can destabilize a bite when teeth tip into spaces opened up by cavities or extractions, or when they shift position in response to gum disease, which reshapes their sockets.
4) DENTAL WORK can destabilize a bite by leaving a crown or filling too low. We don't yet have the technology to make bites fit perfectly, and leaving a crown or filling too high causes problems almost immediately; so most crowns are made to fit a little low, which removes some support for the mandible. The low teeth usually rise up enough to fill in the tiny gap quickly, but the loss of support can have consequences for some TMJs. Also, some crowns contain a balancing side interference, which creates a bite contact opposite the chewing side and thereby disturbs neuromusculature function, because joints are wired to receive sensory feedback in the direction of movement. In addition, once a balancing side interference is established, it protects itself from wear by causing the neuromuscular system to stop firing the jaw closing muscles as soon as it contacts.
5) IMPROPER ORAL APPLIANCES can also destabilize a bite. Wearing a lower oral appliance full time can shift all the upper teeth until they fit the surface of the appliance; then, when the appliance is removed, the lower teeth no longer fit the upper teeth. Similarly, lower nightguards can destabilize a bite by confining the powerful compressive forces of nocturnal 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, because the front teeth are in the way and contact first. Biting forcefully on a steep overbite can wedge the mandible backward toward the ears, which damages the back ends of the TMJs, where most TMJs show bruising in anatomical studies.
BITES CAN RESTABILIZE on their own. The same adaptability that creates stable bites when the teeth first erupt can also recreate bite stability later. 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 at any age. However, some bites may be unable to restabilize naturally because of dental features such as irregular alignment or tight contacts between adjacent teeth.
BITE STRAIN - Even if your bite is stable, it may still be strained. Even if your bite forms a solid 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 and the rest of the postural muscles. At the top of that trajectory should be a stable bite platform created by simultaneous contacts between most or all the back teeth.
In many TMJ disorder patients, the stable bite platform is not located at the top of an unstrained mandibular closing trajectory, but is displaced backwards and/or sideways, which forces the jaw muscles to shift the mandible during closing horizontally away from its unstrained mandibular closing trajectory and into an adaptive mandibular closing trajectory aimed at the displaced 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 requires 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 your mandible along a trajectory that is determined only by your postural system, without the influence of the teeth. Then, when you remove the appliance and bite down along the deprogrammed jaw trajectory until you contact teeth, that is where all your back teeth should fit together. If some teeth hit first and slide your mandible away from this deprogrammed mandibular position and into your old central bite platform, your old central bite platform 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, tipping your head forward will cause your front teeth to contact, tipping your head back will cause your back teeth to contact, and turning your head to either side causes contact on the opposite side. However, postural influences on the bite are normally overriden by neuromuscular reflexes that were designed to protect the teeth. That connection is summarized in THE ROLE OF BODY POSTURE under the tab TMJ DISORDERS and described in detail in BITES AND BODY POSTURE under the tab FOR DOCTORS.
ADJUSTING BITES - to eliminate bite strain can be accomplished by reshaping teeth and/or moving them orthodontically. People with excessively long faces can benefit from shortening teeth by reducing high areas; and people with excessively short faces can benefit from lengthening (building up) teeth.
REDUCING high areas requires first accurately locating them and then drilling them down until they 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 mandibular range of motion.
BUILDING UP low areas, to stabilize a bite, can be accomplished 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; however, it can wear down rapidly in people with strong jaw muscles and vigorous tooth grinding habits, – especially on the molars, so it may need to be replaced by more permanent materials.
WEAKENED TEETH due to cavities or previous fillings will eventually require onlays or crowns. Filling materials prevent food from collecting in a hole, but they do little to reinforce the tooth structurally. As teeth become brittle with age, they tend to crack or chip from around the filling, so they will eventually need to be covered by a crown or onlay. Typically, when dentists make crowns or onlays, they simply duplicate the existing bite. When several crown or onlays are needed, they can be used to recreate a healthy bite. Therefore, if you are planning to get many crowns or onlays, that could be a an opportunity to shift or stabilize your bite table.
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 drilling away at least 1.5 mm from all around the tooth, which can traumatize a tooth, causing it to eventually die and need 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.