Temporomandibular disorder (TMD) is the broad term for any dysfunction that affects the Temporomandibular joint (TMJ) and surrounding regions – head, face, neck, and shoulders.
It is believed that TMD has a prevalence of 5% to 15% in the adult population. Throughout the years this dysfunction has been thoroughly studied, concluding that both psychological and environmental components play a key role in this. Therefore, this TMD is under the biopsychosocial model.
Pain, which is the most common symptom of TMD, is commonly associated and aggravated with depression, anxiety, stress and poor sleep.
The pathophysiology of TMD and orofacial pain is still under investigation, but some literature has shown that both peripheral and central nervous responses have a high impact on these disorders. Even though the face sensation and motor functions, such as chewing and clenching, are regulated by the trigeminal nerve, it has been demonstrated that this has a strong relationship with the Autonomous Nervous System. Various correlations have been made between the sympathetic response and the increase of the activity of the muscles of mastication (MoM), namely masseter, temporalis, lateral and medial pterygoid – and what is known by the umbrella term of Bruxism. Bruxism has been thoroughly studied in the past years and a consensus in 2018 has split it into Awake and Sleep Bruxism, that have different implications with TMD. Awake Bruxism (AB) is a masticatory muscle activity during wakefulness that is characterised by repetitive or sustained tooth contact and/or by bracing or thrusting of the mandible. Recent literature has brought interesting outcomes in the correlation of AB with TMD and psychological disorders, reporting a high correlation between all these – the most sustainable explanation for this is that once a high sympathetic response is generated, the trigeminal nerve may also be stimulated, engaging the MoM.
It is understood that this cascade of events does not always increase the risk of an individual to develop TMD, but on the other hand, the affected population has a higher level of correlation between poorer mental health, increased MoM load and higher levels of orofacial pain.
As yet, Sleep Bruxism (SB) has been studied much more in the last years and its physiology well known. This is a masticatory muscle activity during sleep that is characterised as rhythmic or non-rhythmic and once more it has a higher prevalence in individuals with behavioral disorders and poor sleep hygiene.
Some studies done with healthy and pain-free individuals have concluded that SB is a very ordinary process and very uncommonly will lead to a TMD by itself. However, in patients with other comorbidities – physical and/or psychological – SB was shown to, sometimes, have an important role in the exacerbation of orofacial pain, but not as much with TMD. Interestingly, poor sleep either assessed by a sleep study or self-reported, has always been correlated with an increase of SB, in either individual with or without orofacial pain.
At this point, more investigation to guide best practices is still needed but it seems clear that a causal effect is present between all these.
Hugo Dias, physiotherapist.