CAUSES OF TMJ DISORDERS
The search for the cause of TMJ disorders has been a wild ride, jumping around between a number of anatomic and physiologic conditions. 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, then ligament laxity (Ehlers-Danlos), forceps delivery, childhood injuries, whiplash, cervical spine injuries, scoliosis, accumulated microtraumatic episodes, stress, bruxism, habits such as fingernail biting, and various systemic conditions. The latest fad is to blame a tight lingual frenum or "tethered oral tissues" and to treat it by myofunctional therapy. Making muscles healthier helps all the affected tissues, but it does not in any way address the root of the problem.
THE MULTI-FACTORIAL THEORY - Looking at the events that precede the onset of symptoms led researchers in the 1980's to conclude that the cause must involve some combination of predisposing, initiating, and perpetuating factors. They used the name temporomandibular disorder (TMD) in order to change the focus away from the TMJs themselves, and they promoted a biopsychosocial model of TMD to de-emphasize the role of mechanical factors such as the bite and the TMJs, and instead address a whole spectrum of other causal factors such as stress, coping strategies, and social influences. They advised dentists to warn their patients not to expect their symptoms will be eliminated by treatment, but they can be effectively managed by addressing their multiple causal factors. That treatment quickly becomes expensive. Now it's understood that those supposed causal factors are actually just triggering events, functioning like the straw that broke the camel's back; because the jaw system was under progressive mechanical strain due to its growth pattern until a triggering event pushed the system beyond the point where its adaptive mechanisms could prevent tissue damage. Effectively treating the symptoms in both the long-run and short-run does not require eliminating all the potential triggering factors but addressing the mechanical strain at the root of the problem.
THE STRAINED JAWBONE GROWTH PATTERN - The ultimate cause of nearly all modern TMJ disorders is a dysharmony among the components of the jaw system due to a dystrophic jawbone growth pattern that continually produces mechanical strains between structural components that can never achieve a perfect fit, because they lack the regulation that is normally provided by healthy strong jaw muscles. The dystrophic growth pattern becomes set with the maturation of the jaw muscles at puberty and then continues slowly during adulthood.1-2
ADULT FACIAL GROWTH - Continuing facial growth during adulthood is an important but inconvenient truth for orthodontists, because the techniques used to align the teeth often leaves them steeply interdigitated in an occlusion that may be unable to undergo the small positional shifts that are required to accommodate the diverse growth patterns of the upper and lower dentitions during adulthood. To prevent that growth from making the teeth visibly crooked again by buckling the lower front teeth as they get slowly driven forward from growth behind them, orthodontists now use forever retainers on mandibular anterior teeth.
Some orthodontists still try to deny the fact of adult facial growth by calling it remodeling. Normal remodeling occurs over time in all bones and joints as they adapt their surface shapes and even internal shapes to different conditions of weight bearing and functional forces. In contrast, adult jawbone growth is progressive, - it keeps occuring in directions that maintain 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. Remodeling is adapting to change. Adult jawbone growth is the change. The rest of the craniofacial complex remodels to adapt to the changes brought about by adult jawbone growth.
The growth patterns that cause most TMJ disorders are the long narrow midface and the backwardly rotating or 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 and restrictive bites that inhibit horizontal jawbone growth. Usually that growth gets redirected down and back. Very similar growth patterns have been induced experimentally in animals simply by softening their diets or damaging their jaw muscles. The way jaw muscle weakness and the associated bite changes have caused TMJ disorders to become endemic in modern societies is explained in detail in ETIOLOGY.
THE FEMALE JAWBONE GROWTH PATTERN - The link between jaw muscle weakness and restricted 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 more than males. 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 female growth pattern after puberty is more typical of a weak muscle growth pattern, and the male growth pattern after puberty is more typical of a strong muscle growth pattern.
THE TROUBLE WITH OVERBITE - is important to understand and not generally recognized, because dentists are taught in dental school that overbite is normal. We even learned that the front teeth should contact lightly when the back teeth contact in CR, which is nonsense. In natural human dentitions, the front teeth do not contact until the mandible is shifted anteriorly, and the role of overbite is to align the dental arches in childhood to prevent the early fast forward growth of the mandible from pushing the lower teeth past the upper teeth. During adulthood, overbite diminishes as the mandibular anterior teeth gradually ride up onto the palatal surfaces of the maxillary anterior teeth, where they form a stable anterior bracing platform for the mandible. In contrast, in modern human dentitions, overbites persist throughout adulthood and inhibit the natural advancement of the mandible and teh slower expansion of the maxilla. When horizontal growth is restricted, it usually gets redirected down and back, producing the forward head posture and crowded tongue posture that cause symptoms.
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