Epidemiologic surveys of temporomandibular disorders (TMD) have demonstrated that a considerable proportion of the population--up to 5% or 6%--will experience persistent pain severe enough to seek treatment. Unfortunately, the current diagnostic classification of craniofacial muscle pain is based on descriptions of signs and symptoms rather than on knowledge of pain mechanisms. Furthermore, the pathophysiology and etiology of craniofacial muscle pain are not known in sufficient detail to allow causal treatment. Many hypotheses have been proposed to explain cause-effect relationships; however, it is still uncertain what may be the cause of muscle pain and what is the effect of muscle pain. This article reviews the literature in which craniofacial muscle pain has been induced by experimental techniques in animals and human volunteers and in which the effects on somatosensory and motor function have been assessed under standardized conditions. This information is compared to the clinical correlates, which can be derived from the numerous cross-sectional studies in patients with craniofacial muscle pain. The experimental literature clearly indicates that muscle pain has significant effects on both somatosensory and craniofacial motor function. Typical somatosensory manifestations of experimental muscle pain are referred pain and increased sensitivity of homotopic areas. The craniofacial motor function is inhibited mainly during experimental muscle pain, but phase-dependent excitation is also found during mastication to reduce the amplitude and velocity of jaw movements. The underlying neurobiologic mechanisms probably involve varying combinations of sensitization of peripheral afferents, hyperexcitability of central neurons, and imbalance in descending pain modulatory systems. Reflex circuits in the brain stem seem important for the adjustment of sensorimotor function in the presence of craniofacial pain. Changes in somatosensory and motor function may therefore be viewed as consequences of pain and not factors leading to pain. Implications for the diagnosis and management of persistent muscle pain are discussed from this perspective.

Craniofacial muscle pain: review of mechanisms and clinical manifestations.

https://www.scielo.br/pdf/clin/v64n1/a11v64n1.pdf

The relationship between the stomatognathic system and body posture.

A relationship between particular characteristics of dental occlusion and craniomandibular disorders (CMD) has been reported, while less attention has been focused on the possible effect of dysfunction of the masticatory system on head posture or cervicovertebral and craniofacial morphology. Natural head position roentgen-cephalograms of 16 young adults with complete dentition taken before and after stomatognathic treatment displayed an extended head posture, smaller size of the uppermost cervical vertebrae, decreased posterior to anterior face height ratio, and a flattened cranial base as compared with age- and sex-matched healthy controls. The lordosis of the cervical spine straightened after stomatognathic treatment. The results are an indication of the close interrelationship between the masticatory muscle system and the muscles supporting the head, and lead to speculation on the principles of treating craniomandibular disorders.

Head posture and cervicovertebral and craniofacial morphology in patients with craniomandibular dysfunction

In spite of differences in embryologic origin, central nervous organization, and muscle fiber distribution, the physiology and action of mandibular elevator muscles are comparable to those of skeletal muscles of the limbs, back, and shoulder. They also share the same age-, sex-, and activity-related variations of muscular strength. With respect to pathogenesis, the type of muscular performance associated with the development of fatigue, discomfort, and pain in mandibular elevators seems to be influenced by the dental occlusion. Clinical research comparing the extent of occlusal contact in patients and controls as well as epidemiologic studies have shown reduced occlusal support to be a risk factor in the development of craniomandibular disorders. In healthy subjects with full natural dentition, occlusal support in the intercuspal position generally amounts to 12-14 pairs of contacting teeth, with predominance of contact on first and second molars. The extent of occlusal contact clearly affects electric muscle activity, bite force, jaw movements, and masticatory efficiency. Neurophysiologic evidence of receptor activity and reflex interaction with the basic motor programs of craniomandibular muscles tends to indicate that the peripheral occlusal control of the elevator muscles is provided by feedback from periodontal pressoreceptors. With stable intercuspal support, especially from posterior teeth, elevator muscles are activated strongly during biting and chewing with a high degree of force and masticatory efficiency, and with relatively short contractions, allowing for pauses. These variables of muscle contraction seem, in general, to strengthen the muscles and prevent discomfort. Therefore, occlusal stability keeps the muscles fit, and enables the masticatory system to meet its functional demands.

Mandibular elevator muscles: physiology, action, and effect of dental occlusion

Electromyographic activity of anterior temporal and masseter muscles was measured in 92 young healthy men and women with sound dentitions during rest position, contact in centric occlusion and clench. Male and female mean potentials were similar except in clench, where males had higher electromyographic levels. Mean pooled electromyographic potentials were 1.9 microV (TA) and 1.4 microV (MM) during rest position, 6.5 microV (TA) and 2.8 microV (MM) during contact in centric occlusion. Mean maximum voluntary clench potentials were 181.9 microV (TA) and 216.2 microV (MM) in men, 161.7 microV (TA) and 156.8 microV (MM) in women. Examined muscles were more asymmetric at low electromyographic activity (rest and centric occlusion) with the temporal muscle less asymmetrical than the masseter. In females temporal muscle activity tended to dominate at every contraction level, while in males masseter activity was stronger in clench, and temporal activity in centric occlusion and in rest position.

Electromyographic activity of human masticatory muscles in normal young people. Statistical evaluation of reference values for clinical applications.

— With the mandible at rest, electromyographic recordings were obtained from the anterior temporal, lateral pterygoid, and digastric muscles with the subjects (A) sitting upright with the head unsupported, (B) inclining backwards with the back-rest at an angle of 45°, and (C) supine. Two minutes after a shift in posture the electrical activity in the anterior temporal muscles was strongest in the upright position, it decreased slightly when the back-rest was at 45°, and was least in the supine position. The digastric muscles showed a pattern similar to that of the temporal. The activity in the lateral pterygoid muscles was strongest when the subject was inclined backwards at 45°. The influence, therefore, of these muscles on the posterior border position of the mandible varies with posture. In all the muscles studied there was least activity when the subject was in the supine position. This would suggest that the supine position is suitable for recording the most retruded position of the mandible. In the intermediate position the lateral pterygoid muscles appear to counteract retrusion.

Postural activity in the muscles of mastication with the subject upright, inclined, and supine.

– During swallowing, the electrical activity in the anterior temporal, lateral pterygoid, and digastric muscles was recorded in subjects (A) sitting upright with the head unsupported, (B) inclining backwards with the back-rest at 45°, and (C) supine. The degree of activity (maximal mean voltage) was not influenced by posture and the time course was the same with the subjects inclined 45° and supine. In the anterior temporal muscle, swallowing in the inclined and supine positions lasted 100 msec less than during sitting upright. Because of the change from upright to inclined or supine positions the action of the lateral pterygoid and digastric muscles was advanced 30–100 msec in relation to the temporal muscle. It is suggested that the time pattern of the activity varied with posture because of a shortening of the oral phase when swallowing was performed in inclined and supine positions. The variation induced by altering posture was confined to changes of 5–10 % in the time course of the activity.

T. Nishiyama

Papers

Postural activity in the muscles of mastication with the subject upright, inclined, and supine.

Swallowing in upright, inclined, and supine positions: action of the temporal, lateral pterygoid, and digastric muscles