By Karen Raphael
For at least 50 years, most dentists have told their TMJ patients that grinding or clenching of their teeth is a major factor in the cause and persistence of their facial pain. Consequently, many patients accept this to be true. If a sleep partner tells you that they heard you grind your teeth at night (“sleep bruxism”), it seems to provide even stronger proof that sleep bruxism is the cause of your facial pain. This belief system forms the theoretical justification for many treatments, including bite plates or oral splints, devices usually worn at night which are often thought to interfere with bruxism.
In 2012, a large study 1 published in the Journal of the American Dental Association found that this belief was simply not supported by evidence. Comparing a large sample of TMJ patients to a large sample of controls observed in a state-of-the-art sleep laboratory in which chewing muscle activity was recorded in combination with audio and visual recording, both groups were scored for frequency of sleep bruxism activity. Surprisingly, clinically significant sleep bruxism as defined by stringent research standards 2 was detected in only about 10% of both TMJ patients and control subjects. However, occasional grinding sounds during sleep (2 or more times per night) were much more common, found in about 60% of TMJ patients but even more frequently in control subjects (78%). On average, both groups engaged in tooth grinding for only about one minute per night. Of note, TMJ patients with the highest levels of average pain were actually less likely to brux at night than TMJ patients with lower pain levels.
So how do we explain the common sensation of waking up with a particularly sore jaw in the morning? Many patients and their doctors have viewed this experience as the best evidence of sleep bruxism, when a clinical sleep laboratory study is not practical.
In a reanalysis of our data 3, we looked at the level of ‘background’ activity occurring during sleep in the sample of TMJ patients and controls who participated in the laboratory sleep study. Muscle always maintains a certain level of electrical (‘electromyographic’) activity, even when it is not actively engaged in a specific activity. Think of this background activity as the resting muscle activity in the chewing muscles when nothing specific is going on — no grinding or clenching, no sleep talking, no yawning, etc. Our analyses found that the level of background electrical activity in the chewing muscles was significantly higher in TMJ patients than controls. The average level of electrical activity in the chewing muscles of TMJ patients was higher than the level found in nearly three quarters of the controls. Although the actual electrical activity at any single second in time in TMJ patients was not dramatically different than the level in controls, it appears that these small increases in activity accumulate over the course of the night to lead to chewing muscle fatigue.
It turns out that TMJ patients who reported particularly elevated pain on waking in the morning were most likely to be the same patients who had high levels of background chewing muscle activity, not the patients who were grinding or clenching at night. So, if you have more facial muscle pain in the morning when you wake, it is probably due to your overall level of background muscle activity during the night being subtly higher than average, over long periods of sleep.
Despite these strong findings, not every TMJ patient had higher levels of sleep chewing muscle background activity than every control subject. Thus, as many researchers have long believed, TMJ problems are undoubtedly caused by multiple factors and their interaction. Activity of chewing muscles during sleep is only one of several risk factors. In any case, sleep bruxism is not a risk factor, but background facial muscle activity is.
So, the next time your dentist wants to treat your pain by trying to treat underlying sleep bruxism, you can confidently point him or her to studies showing that some long-held beliefs about the cause of TMJ pain are wrong. You are likely to waste time, energy, and money unnecessarily by trying to treat sleep bruxism to reduce your facial pain.
1. Raphael KG, Sirois DA, Janal MN, et al. Sleep bruxism and myofascial temporomandibular disorders: A laboratory-based polysomnographic investigation. J Am Dent Assoc 2012;143(11):1223-31.
2. Lavigne GJ, Rompre PH, Montplaisir JY. Sleep bruxism: validity of clinical research diagnostic criteria in a controlled polysomnographic study. J Dent Res 1996;75(1):546-52.
3. Raphael KG, Janal MN, Sirois DA, et al. Masticatory muscle sleep background electromyographic activity is elevated in myofascial temporomandibular disorder patients. J Oral Rehabil 2013;40(12):883-91.