CAUSES OF TMJ DISORDERS

The search for the cause of TMJ disorders has been a wild ride with sharp twists and turns.  They 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, accumulated microtraumatic episodes, stress, bruxism, habits such as fingernail biting, and various systemic conditions. 

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 promoted the "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 to expect instead that their symptoms can be effectively managed by addressing their multiple causal factors.

However, these "multiple causal factors" that often preceed the symptoms are usually just triggering events.  They function like the straw that broke the camel's back, because the jaw system was under progressive mechanical stress due to its growth pattern, and all it took was a triggering event to push 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 treating the multiple factors that seem to affect them but simply understanding and treating the root cause of the symptoms.

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 dysstrophic growth pattern becomes set with the maturation of the jaw muscles at puberty and then continues during adulthood, with some additional progressive increases in vertical height at about the same rate our teeth used to wear down.  Although adult craniofacial growth represents a big inconvenient truth for orthodontists, it has been confirmed in numerous studies.1-2

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 facial growth and redirect it 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.   

 behrents.jpg

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.  We didn't learn that, in natural human dentitions, the front teeth do not contact until the mandible is shifted anteriorly; and that the role of overbite was to align the dental arches in childhood and prevent the early fast forward growth of the mandible from pushing the lower teeth past the upper teeth.  Later, the mandibular anterior teeth gradually ride up the palatal surfaces of the maxillary anterior teeth onto a stable anterior bracing platform for the mandible.  Today, our bites and our soft food diets inhibit these horizontal growth patterns.  When horizontal growth is restricted, it is redirected down and back.

Although the root cause of the symptoms may be a strained growth pattern, effective treatment of the symptoms does not require intervening to alter the growth pattern.  Short term relief just requires eliminating the causal factors that have been generating the symptoms.  The most common of these causal factors and their roles in producing symptoms are described in CAUSAL FACTORS under the tab TMJ DISORDERS.  Long term relief requires understanding how any orthopedic mechanics introduced will affect subsequent adult facial growth.  

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