THE CAUSE OF TMJ DISORDERS
The search for the cause of TMJ disorders has left a wake of confusion that still plagues the field, because dental researchers have never understood the central role of the bite in the pathological process. The bite is much more than a tool for chewing food. The contours of the bite table affect the direction in which the face grows, and the way the face grows determines the health of the TMJs and jaw muscles. In most TMJ disorder patients, the bite has prevented healthy harmonious facial growth. As a result, the jaws suffer from ongoing strains.
TMJ disorders were first discovered in denture patients who had lost vertical dimension (the height of the platform between their jawbones), and they are still blamed on loss of vertical dimension by some dentists. Later, when people with good natural teeth and tall bite platforms came to dominate the TMJ disorder population, the search shifted to other bite conditions, condyle position, and muscle imbalances. Then, after dentists were unable to connect the cause to features of the bite and jaw, the search for the cause broadened to include ligament laxity (Ehlers-Danlos), forceps delivery, childhood injuries, whiplash, cervical spine injuries, scoliosis, accumulated microtraumatic episodes, stress, bruxism, personal habits such as fingernail biting, and various systemic conditions. Many still blame nocturnal bruxism, although studies show it is not more frequent in TMJ disorder patients. Others blame old injuries, although TMJs heal rapidly from injuries and do not keep producing symptoms subsequently. The latest fad is to blame a tight lingual frenum and to cut it (frenectomy), although there is no scientific or even common sense support for that explanation or treatment in the vast majority of cases (see MISCONCEPTIONS).
THE MULTI-FACTORIAL THEORY - Looking at the varied types of events that precede the onset of symptoms led some researchers in the 1980's to conclude that the cause must involve some combination of predisposing, initiating, and perpetuating factors. They divided causal factors into physical ones and psychosocial ones, creating a dual axis approach. They changed the name of the condition from a TMJ disorder to a temporomandibular disorder (TMD) to move the focus away from the TMJ itself and onto contributing factors such as stress and coping strategies. They advise patients to not expect to be cured by treatment, but to know that their symptoms can be effectively managed by addressing their multiple causal factors, which unfortunately also creates multiple expenses.
However, other researchers pointed out that the stress and anxiety associated with TMJ disorders are just as likely to be a result of the disorder as a cause of it. In fact, all pain conditions involving skeletal muscles have a biopsychosocial component, because the state of the brain affects muscle tonus. Also the symptoms that are caused by an ongoing strain depend on the ability of the patient to adapt to the strain, which depends on emotional factors; so those emotional factors are correlated with the presence of symptoms; but that does not make the emotional factors causes of the condition. TMJ disorders do not have some unique relationship with psychosocial factors, and effectively treating them does not require eliminating all the multiple potential causal factors, it requires relieving the mechanical strain at the root of the problem, which requires understanding the role of the bite. When dentists are polled about the cause of TMJ problems, about half of them still point to the bite, even though authorities tell them there is no evidence for their belief and that it is simply the result of confirmation bias. The problem is that dentists have not been able to show evidence for the role of the bite, because dental researchers can't collect evidence about something they cannot measure; and they have never been able to measure the functional characteristics of bites. One of the goals of this website is to explain the central role of the bite, in three long files under the FOR DOCTORS tab, so that dentists can treat them effectively and preventively.
THE STRAINED JAWBONE GROWTH PATTERN - The many causes cited to support the multi-factorial theory are actually just triggering events, like the straw that broke the camel's back, because the jaw system was functioning under progressive mechanical strain due to a strained growth pattern, until something pushed the system beyond the point where its adaptive mechanisms could prevent tissue damage. The cause of the strain was not that tough bagel you tried to chew, it was a dysharmony among the components of the jaw system due to a facial growth pattern that continually produces mechanical strains between jawbones that can never achieve a perfect fit, because they lack the regulation that is normally provided by healthy strong jaw muscles, and some component finally gave out when you chewed that bagel. The strained jawbone growth pattern at the root of the problem slows down about ninety percent after the second decade, but then it continues slowly during adulthood. The ongoing misfit it produces between the jawbones and the postural system requires constant adaptation, and symptoms usually come and go depending on the success of that adaptation. The symptoms usually begin to show up after the rapid growth of puberty and then continue to appear until natural adaptation reduces neuromuscular reactivity in middle age and thereby removes one of the components of the symptom generating cycle.1-2
ADULT FACIAL GROWTH - The fact of adult facial growth is an inconvenient truth for orthodontists, and some of them still deny it by calling it remodeling. However, remodeling is adapting to change. It occurs constantly in all bones and joints as they fine tune their shapes to fit small changes in weight bearing and functional forces. In contrast, adult jawbone growth is progressive, - it follows the same general pattern of previous facial growth, with additional increases in vertical height at about the same rate our teeth used to wear down. Adult facial growth was programmed in to reduce airway flow resistance with age as muscles lose strength during adulthood at about 5% per decade so the respiratory muscles don't have to keep increasing their effort with age. Adult facial growth was also needed to maintain the stability of bite table no matter how fast or slow the teeth wore down, therefore it was stimulated by bite forces so that facial height remained stable in proportion to postural height. Our ancestors who used strong bite forces and wore their teeth down quickly produced rapid jawbone growth to compensate for the loss of facial height, while our ancestors who had weaker bite forces and experienced less tooth wear needed and experienced slower jawbone growth to maintain stable facial height.
Today the growth patterns that cause most TMJ disorders are the long narrow midface and the backwardly displaced mandible. These growth patterns were never seen in human skeletal remains before the last couple of centuries, and they have arisen at the same time as TMJ disorders, because they are caused by weak jaw muscles that fail to stimulate horizontal jawbone growth and by restrictive bites that inhibit horizontal jawbone growth. When horizontal growth of the jawbones is restricted, that growth gets redirected vertically. Usually the mandible rotates down and back, crowding the tongue against the pharyngeal wall and restricting the space available for airway passage in the oropharynx. Very similar growth patterns have been induced experimentally in animals simply by softening their diets or damaging their jaw muscles, and they are seen in humans with muscle disease or jaw muscle damage. In humans with relatively stronger muscles today, the jaw closing muscles are able to prevent the rotation down and back, but the mandible usually stays locked back posteriorly behind a steep anterior overbite, restricting the space available for airway passage in the nasopharynx.
THE FEMALE JAWBONE GROWTH PATTERN - The link between jaw muscle weakness and restricted horizontal jawbone growth explains why females after puberty are the primary victims of TMJ disorders. Their jaw muscles do not develop as rapidly as male jaw muscles during and after the post-pubertal growth spurt, causing their mandibles to rotate down and back (clockwise), while male mandibles rotate counter-clockwise. This difference in the growth pattern of the male and female mandibles can be seen in Behrents' comparison of the average male and female adult growth patterns, seen below. Solid line is earlier, dotted line is later. The mandible rotates counterclockwise in adult males and clockwise in adult females. The Y-axis (N-S-Gn) decreases in males and does not change in females. The mandibular plane angle (S-N/Go-Gn) decreases in males and increases in females. Males also had greater posterior vertical development of the mandible than females.
THE TROUBLE WITH OVERBITE - is that it can restrict horizontal facial growth, which is needed for airway preservation; but dentists do not recognize that restriction, because dental schools teach that overbite is a feature of a normal bite. We even learned that the front teeth should contact when the back teeth contact in CR, which is not a feature of natural bites and simply reflects the fact that dentistry has never understood the bite. In natural human dentitions that have achieved functional capacity, the front teeth do not contact in overbite until the mandible shifts anteriorly, like a ball in a socket. The role of the anterior overbite is to keep the dental arches in close proximity during childhood to prevent the early fast forward growth of the mandible from pushing the lower teeth past the upper teeth. Then, in natural denitions, the overbite gradually disappears as mandibular advancement drives the mandibular anterior teeth gradually up the palatal surfaces of the maxillary anterior teeth and onto their incisal edges, where they form a stable anterior bracing platform for the mandible that is generally continuous with the stable posterior bracing platform (rather than a dual bite). The loss of overbite with age in adulthood is obvious from examining collections of skeletal remains, but it cannot be proven, because we don't have a way to determine the age of skeletal remains after the teeth have all erupted.
Today, our overbites persist throughout adulthood despite the continual slow growth of the basal bones. The pressure on the overbite from mandibular advancement in the presence of strong jaw muscles can splay the upper front teeth or crowd the lower front teeth. In the presence of weak jaw muscles, it is more likely to redirect mandibular growth down and back, carrying the mandibular corpus into the space needed for the pharyngeal airway and reflexively producing forward head posture to compensate for the mandibular intrusion by using the posterior neck muscles to tip the head back, which pulls the mandible forward relative to the pharynx and thereby preserves the airway space.
1. Behrents RG. Growth in the aging craniofacial skeleton. Ann Arbor: University of Michigan center for Human Growth and Development, 1985.
2. Al-Taai N, Persson M, Ransjo M, et al. Craniofacial changes from 13 to 62 years of age. Eur J Orthod March 2022