Etiology of TM Disorders

Altered control of submaximal bite force during bruxism in humans.
Eur J Appl Physiol 79(4):325-30 1999 Mar
The control of bite force during varying submaximal loads was examined in patients suffering from bruxism compared to healthy humans not showing these symptoms. The subjects raised a bar (preload) with their incisor teeth and held it between their upper and lower incisors using the minimal bite force required to keep the bar in a horizontal position. The results indicated that the patients with bruxism used excessively large biting forces for each given submaximal load. This study showed no evidence that the inappropriate control of bite force by patients with bruxism was due to an abnormality in the higher cortical circuits that regulates the function of trigeminal motoneuronsin the brainstem.

A profile of patients with temporomandibular disorders in Singapore--a descriptive study.
Ann Acad Med Singapore 1989 Nov;18(6):675-80
"There was evidence that tension headache reported by TMD sufferers was related to temporalis muscle/tendon dysfunction."

Effect of Parafunctional Clenching on TMD Pain
J Orofac Pain, 12(2):145-52 1998 Spring
The authors conclude that chronic, low-level parafunctional clenching may be a factor in the cause of TMD pain.

Recurrent headaches in relation to temporomandibular joint pain-dysfunction.
Acta Odontol Scand 1978;36(6):333-8
The investigation showed that clenching of teeth was correlated to the severity of headache. The frequency and severity of headache varied also with the severity of mandibular dysfunction. Of the variables included in the dysfunction index, only masticatory musculature painful to palpation was found to have a distinct relationship to headaches.

The temporal/masseter co-contraction: an electromyographic and clinical evaluation of short-term stabilization splint therapy in myogenous CMD patients.
J Oral Rehabil ,22(5):387-9 1995 May
The short-term effect (3-6 weeks) of the use of a stabilization splint was investigated in a group of 35 yogenous craniomandibular disorder patients.  Three groups of patients were then recognized. One group (42%) showed a decrease in temporal muscle activity and symptoms during splint treatment. Another group (45%) did not show any significant change during splint treatment. The third group (11%) showed an increase of temporal muscle activity and symptoms (Ed: i.e., 56% either showed no change or became worse). The results may indicate that the temporal muscle plays an important role in the perception of static pain in the masticatory system.

NIH MAKES RECOMMENDATIONS FOR  TEMPOROMANDIBULAR DISORDERS
NIH Office of Medical Applications of Research
The panel concluded that there are questions about the effectiveness of most treatments now used for TMD (Ed: Nor did the panel comment on the what the cause of TMD is, which may explain the inconsistancy of treatment)

Reported symptoms and clinical findings in a group of subjects with longstanding bruxing behaviour.
J Oral Rehabil 1997 Aug;24(8):581-7
There was a statistically significant correlation between frequent tooth clenching and headache, pain in the neck, back, throat or shoulders, sleep disorders and high scores of the clinical dysfunction index (Di). The frequent clenchers had higher score values than the 'non-clenchers' (Ed: i.e., "grinders") for pain in the face and the jaws; headache; pain in the neck, back, throat or shoulders and the clinical dysfunction index (Di). These findings indicate a causal relationship between frequent tooth clenching and signs and symptoms of CMD, including headache and pain in the neck, back, throat or shoulders and high pathogenicity for frequent clenching.

The influence of altered working-side occlusal guidance on masticatory muscles and related jaw movement.
J Prosthet Dent 1985 Mar;53(3):406-13
Introduction of a hyperbalancing occlusal contact caused significant alterations in muscle activity and coordination during maximal tooth clenching in a lateral mandibular position. A marked shift of temporal muscle EMG activity toward the side of the interference and unchanged bilateral activity of the two masseter muscles were observed. The results suggest that canine-protected occlusions do not significantly alter muscle activity during mastication but significantly reduce muscle activity during parafunctional clenching. They also suggest that non-working side contacts dramatically alter the distribution of muscle activity during parafunctional clenching, and that this redistribution may affect the nature of reaction forces at the temporomandibular joints.

Lack of evidence for malocclusion as a causitive element
(back to top)

Epidemiology of research for temporomandibular disorders.
J Orofac Pain, 9(3):226-34 1995 Summer
The literature on therapy for TMD consists primarily of uncontrolled observations of patients such as uncontrolled clinical trials, case series, case reports, and simple descriptions of techniques. It is generally agreed that such uncontrolled observations, while contributing to knowledge about therapy of TMD, are subject to considerable bias and thus difficult to interpret.

Prevalence of dental occlusal variables and intraarticular temporomandibular disorders: molar relationship, lateral guidance, and nonworking side contacts.
J Prosthet Dent 1999 Oct;82(4):410-5
This study suggests there are no systematic dental occlusal differences that clearly separate symptomatic from asymptomatic patients. Results indicate that it is unclear as to the relationship of the 3 analyzed factors and of intraarticular TMDs.

Effects of Major Class II Occlusal Corrections on Temporomandibular Signs and Symptoms
J Orofac Pain, 12(3):185-92 1998 Summer
This study explored the relationship between malocclusion and signs and symptoms of temporomandibular disorders (TMD) in 124 patients with severe Class II malocclusion, before and 2 years after bilateral sagittal split osteotomy (BSSO).  The magnitude of change in muscular pain was not related to the severity of the pretreatment malocclusion, a finding that suggests that factors other than malocclusion may be respondible for the change in TMD.

Occlusion, orthodontic treatment, and temporomandibular disorders: A review
J Orofac Pain, 9(1):73-90 1995 Winter
A review of the current literature regarding the interaction of morphologic and functional occlusal factors to TMD indicates that there is a relatively low association of occlusal factors in characterizing TMD. (Skeletal anterior open bite, overjets greater than 5 to 7  mm, retruded cuspal position/intercuspal position slides greater than 4 mm, unilateral lingual crossbite, and five or more missing posterior teeth are the five occlusal features that have been associated with specific diagnostic groups of TMD conditions). There is no elevated risk of TMD associated with any particular type of orthodontic mechanics or with extraction protocols. Thus, according to the existing literature, the relationship of TMD to occlusion and orthodontic treatment is minor.

The validity and utility of disease detection methods and of occlusal therapy for temporomandibular disorders
Oral Surg Oral Med Oral Pathol Oral Radiol Endod 1997 Jan;83(1):101-6
The studies we reviewed on the relationship of occlusion to TMD are not convincing, powerful, or practical enough to make any recommendations about a causal association.

Physiological and Theoretical Analysis of K+ Currents Controlling Discharge in Neonatal Rat Mesencephalic Trigeminal Neurons
The Journal of Neurophysiology Vol. 77 No. 2 February 1997, pp. 537-553
Pathologies such as myofacial pain syndromes, tardive dyskinesia, or nocturnal bruxisms are conditions that could be generated by abnormal somatic spike genesis or ectopic discharge

Effect of a full-arch maxillary occlusal splint on parafunctional activity during sleep in patients with nocturnal bruxism and signs and symptoms of craniomandibular disorders.
J Prosthet Dent 69(3):293-7 1993 Mar
The splint does not stop nocturnal bruxism. In 61% of the patients, wear facets on the splint were observed at every visit (2-week intervals) and in 39%, from time to time. The wear facets reappeared in the same location with the same pattern and were caused mainly by grinding. The extension of the facets showed that, during eccentric bruxism, the mandible moved laterally far beyond the edge-to-edge contact relationship of the canines. (Ed: If the occlusion were the cause of muscular parafunction, wear facets would cease to reappear)

Occlusal treatments in temporomandibular disorders: a qualitative systematic review of randomized
controlled trials. Pain, 83(3):549-60 1999 Dec
The use of occlusal splints may be of some benefit in the treatment of TMD. Evidence for the use of
occlusal adjustment is lacking. There is an obvious need for well designed controlled studies to analyse
the current clinical practices.

Psychosocial Influence  (back to top)

Needle electromyographic evaluation of trigger point response to a psychological stressor.
Psychophysiology, 31(3):313-6 1994 May
The results showed increased trigger point electromyographic activity during stress, whereas the adjacent muscle remained electrically silent. These results suggest a mechanism by which emotional factors influence muscle pain. This may have significant implications for the psychophysiology of pain associated with trigger points

A dual-diagnostic approach assesses TMD patients.
J Mass Dent Soc 1995 Winter;44(1):16-9
This article summarizes research describing the development of a psychosocial classification of TMD patients that can be used with the physical axis of the recently proposed research diagnostic criteria for classification of TMD patients. It also presents preliminary evidence supporting the clinical utility of the psychosocial classification.
(Ed: This article also demonstrates that if the actual objective cause can not be found and prevented,
then dentisty assumes the cause must be psychosocial)

Evaluation of the psychological profiles of patients with signs and symptoms of temporomandibular disorders.
J Prosthet Dent 1991 Dec;66(6):810-2
The psychologic profiles of 98 female patients with signs & symptoms of temporomandibular disorders are compared with those of a control group having no signs or symptoms of such disorders. Scores on the Crown Crisp Experimental Index indicate a significant difference in the profiles of somatization and hysteria.
(Ed: When no objective source of TMD can be found, the patient often takes the blame)

Etiological factors and temporomandibular treatment outcomes: the effects of trauma and psychological dysfunction.
Funct Orthod 1997 Aug-Oct;14(4):17-20, 22
Stress and psychological dysfunction  were not significantly related to treatment outcomes. These findings have important implications for practitioners in the field of temporomandibular studies. If it can be confirmed that psychological variables have no impact on treatment outcome, it would be difficult to justify the now frequently employed "dual axis" classifications and major emphasis placed on psychological treatment for temporomandibular patients.

Psychological factors and temporomandibular outcomes.
Cranio 1998 Apr;16(2):72-7
Treatment outcomes appeared to be unrelated to the initial psychosocial symptom severity and physical symptoms outcomes and psychosocial outcomes appeared to be significantly related.
(Ed: That is, when the patient's physical symptoms improve, so do their psychosocial symptoms)

Temporomandibular disorders: a review of current understanding.
Oral Surg Oral Med Oral Pathol Oral Radiol Endod 1999 Oct;88(4):379-85
Advances in basic and clinical science have resulted in important changes in the understanding and management of temporomandibular disorders. The present science-based understanding of a biopsychosocial disorder is important in properly and responsibly dealing with patients with temporomandibular disorders. (Ed: Translation: Current treatment methodolgy  failure is blamed on biopsychosocial disorders, rather than misguided treatment.)

Personality traits in a group of subjects with long-standing bruxing behaviour.
J Oral Rehabil 1997 Aug;24(8):588-93
A strong correlation was found between high values in the muscular tension scale and headache; aching neck, back, throat or shoulders; tooth clenching; number of muscles tender at palpation and the clinical dysfunction index (Di). The results of this study indicate a possible aetiological relationship between personality, tooth clenching and craniomandibular dysfunction (CMD). (Ed:  Would long-standing pain from clenching alter one's personality?)

Efficacy of Traditional Therapy(back to top)

Nocturnal electromyographic evaluation of myofascial pain dysfunction in patients undergoing occlusal splint therapy.
J Am Dent Assoc, 99(4):607-11 1979 Oct
The level of nocturnal activity of the masseter muscle was monitored as were symptoms before, during, and after occlusal splint therapy. A decreased nocturnal EMG level during treatment was noted for 52% of the patients. A return to pretreatment EMG levels after removal of the splint was noticed in 92% of the patients; in 28% no change was shown and in 20%, an increase was shown in nocturnal EMG levels. The splint was most likely to reduce nocturnal EMG levels in patients with least severe symptoms.  (Ed: 48% show no change, or get worse)

Effect of muscle relaxation splint therapy on the electromyographic activities of masseter and anterior temporalis muscles.
Oral Surg Oral Med Oral Pathol Oral Radiol Endod, 85(6):674-9 1998 Jun
The results of the study were as follows: (1) the electromyographic activity of the two muscles during maximal biting was not markedly changed after the muscle relaxation splint was used; and (2) the changes observed in electromyographic activity of the involved and noninvolved sides were insignificant as well.

Effect of muscle relaxation splint therapy on the electromyographic activities of masseter and anterior temporalis muscles.
Oral Surg Oral Med Oral Pathol Oral Radiol Endod 1998  Jun;85(6):674-9
The electromyographic activity of the two muscles during maximal biting was not markedly changed after the muscle relaxation splint was used; and (2) the changes observed in electromyographic activity of the involved and noninvolved sides were insignificant as well. (Ed: full coverage splints are sometimes referrred to as "muscle relaxation" splints)

Treatment-seeking patterns of facial pain patients: many possibilities, limited satisfaction.
J Orofac Pain, 12(1):61-6 1998 Winter
Patients with persistent facial pain see a large number of different providers, and that nonmedical/nondental treatment approaches are common. The results showed that on average 4.88 providers from 44 different categories were consulted. A general dentist or a dental specialist was seen by about 70% of patients.

EMG response to alteration of tooth contacts on occlusal splints during maximal clenching.
J Prosthet Dent 1984 Mar;51(3):394-6
Maximum clenching on an equilibrated occlusal splint yielded an increase of 17% in overall muscle activity over that of maximum intercuspation contributed mainly by masseter muscles. Maximum clenching on an anterior occlusal splint yielded a decrease of 13% in overall muscle activity compared with that of an equilibrated occlusal splint. When maximum clenching was performed with six left-sided teeth removed from contact while the left second molar remained in contact, there was no significant change in muscle activity when compared with that of an equilibrated occlusal splint. Changes in the position of the tooth contacts altered the overall muscle activity during maximum clenching. Changes in occlusal contact symmetry did not cause changes in symmetry of muscle pairs during maximum clenching. Unilateral support produced the subjective response of pressure on the contralateral TMJ during maximum clenching.

Nocturnal electromyographic evaluation of myofascial pain dysfunction in patients undergoing occlusal splint therapy
JADA, Vol. 99, 1979
The level of nocturnal muscle activity is 25 patients with myofascial pain dysfunction was monitored before, during and after therapy with occlusal splints.  Correlations were made between the severity of symptoms before treatment and the effectiveness of the splint in reducing nocturnal activity of muscles. (Ed: The more severe the symptoms, the less likely the patient experienced relief)

Effect of a full-arch maxillary occlusal splint on parafunctional activity during sleep in patients with nocturnal bruxism and signs and symptoms of craniomandibular disorders.
J Prosthet Dent 69(3):293-7 1993 Mar
The splint does not stop nocturnal bruxism. In 61% of the patients, wear facets on the splint were observed at every visit (2-week intervals) and in 39%, from time to time. The wear facets reappeared in the same location with the same pattern and were caused mainly by grinding. The extension of the facets showed that, during eccentric bruxism, the mandible moved laterally far beyond the edge-to-edge contact relationship of the canines.

Influence of stabilization occlusal splint on craniocervical relationships. Part II: Electromyographic analysis.
Cranio,12(4):227-33 1994 Oct
A full-arch maxillary stabilization occlusal splint was made for each of the 15 subjects. In the sternocleidomastoid muscle, tonic and saliva swallowing EMG activity decreased significantly with the splint, whereas maximal clenching activity did not change. In the trapezius muscle, no significant changes were observed with the occlusal splint.

The temporal/masseter co-contraction: an electromyographic and clinical evaluation of short-term stabilization splint therapy in myogenous CMD patients.
J Oral Rehabil ,22(5):387-9 1995 May
The short-term effect (3-6 weeks) of the use of a stabilization splint was investigated in a group of 35 myogenous craniomandibular disorder patients.  Three groups of patients were then recognized. One group (42%) showed a decrease in temporal muscle activity and symptoms during splint treatment. Another group (45%) did not show any significant change during splint treatment. The third group (11%) showed an increase of temporal muscle activity and symptoms (Ed: i.e., 56% either showed no change or became worse). The results may indicate that the temporal muscle (i.e., the tempooralis) plays an important role in the perception of static pain in the masticatory system.

Oral splints: the crutches for temporomandibular disorders and bruxism?
Crit Rev Oral Biol Med, 9(3):345-61 1998
Various hypotheses have been proposed to explain their apparent efficacy (i.e., true therapeutic value), including the repositioning of condyle and/or the articular disc, reduction in the electromyographic activity of the masticatory muscles, modification of the patient's "harmful" oral behavior, and changes in the patient's occlusion. Following a comprehensive review of the literature, it is concluded that any of these theories is either poor or inconsistent, while the issue of true efficacy for oral splints remains unsettled. Future research should study the natural history and etiologies of TMD and bruxism, so that specific treatments for these disorders can be developed.

NIH MAKES RECOMMENDATIONS FOR  TEMPOROMANDIBULAR DISORDERS
NIH Office of Medical Applications of Research
The panel concluded that there are questions about the effectiveness of most treatments now used for TMD
(Ed: Nor did the panel comment on the what the cause of TMD is, which may explain the inconsistancy of treatment)

Epidemiology of research for temporomandibular disorders
J Orofac Pain, 9(3):226-34 1995 Summer
A systematic review was performed in response to a request the National Institute of Dental Research to evaluate in broad terms the strength of evidence regarding therapy for temporomandibular disorders (TMD). The literature on therapy for TMD consists primarily of uncontrolled observations of patients such as uncontrolled clinical trials, case series, case reports, and simple descriptions of techniques. If treatment of TMD is going to follow the trend in medicine to base patient-care decisions on evidence rather than expert opinion or pathophsiologic rationales, then more rigorously controlled clinical trials of most therapies will be necessary.

Effect of a prefabricated anterior bite stop on electromyographic activity of masticatory muscles.
J Prosthet Dent, 82(1):22-6 1999 Jul
The anterior bite stop had a significant effect in decreasing electromyographic activity for both clenching and grinding for all the tested muscles, except the anterior digastric. CONCLUSIONS: For this patient population, the ready-made anterior bite stop reduced electromyographic muscle activity for the anterior and posterior temporalis and the masseter muscles during both clenching and grinding.

Characterization of 86 bruxing patients with long-term study of their management with occlusal devices and other forms of therapy.
J Orofacial Pain , 7(1):54-60 1993 Winter
Most of the bruxing patients had a chief complaint that related to pain, and 89.6% of the patients had a craniomandibular disorder. The patients were initially managed with an anterior deprogrammer and were later managed with other occlusal devices as signs and symptoms dictated. Definitive treatment was determined by the patient's maxillomandibular relationship.  (Ed.:  Why a traditional anterior deprogrammer can not be used for management)

Therapeutic Motion of the Joint :“TMJ”
Submitted to the Journal of Pain Management, June 2000
Therapeutic Motion of the Joint (“TMJ”) has been an underused treatment for Temporomandibular Disorders, due to potential strain in excursive movement allowed by traditional full-coverage and anterior bite plane therapy.  Previously, unstrained Therapeutic Motion of the Joint (“TMJ”) was available only through Continuous Passive Motion (CPM) machines, used primarily post surgically.  Now the benefits of Therapeutic Motion of the Joint (“TMJ”) is presented as a logical inclusion to the treatment regime of patients, by using the same AMPS appliance used for the treatment of their muscular pains.

The effect of a partial bite raising splint on the occlusal face height. An x-ray cephalometric study in human adults.
Acta Odontol Scand 1982;40(1):17-24
20 patients...were treated...by means of a (permanently cemented) partial chrome-cobalt splint covering the palatal surfaces of the six upper front teeth.  Continuous use of the splint caused intrusion of the front teeth and eruption of the others in all patients.  (Ed: posterior supraeruption requires continual lack of functional stimulation of the posterior teeth)

New Therapies

The use of botulinum toxin for the treatment of temporomandibular disorders: preliminary findings.
J Oral Maxillofac Surg, 57(8):916-20; discussion 920-1 1999 Aug
Both masseter muscles received 50 units each under eletromyographic (EMG) guidance. Similarly, both temporalis muscles were injected with 25 units each.  RESULTS: All mean outcome measures, with the exception of bite force, showed a significant (P = .05) difference between the preinjection assessment and the four follow-up assessments. No side effects were reported. CONCLUSIONS: BTX-A injections  produced a statistically significant improvement in four of five measured outcomes,  specifically pain, function, mouth opening, and tenderness. No statistically significant changes were found in mean maximum voluntary contraction or in paired correlation of  factors such as age, sex, diagnosis, depression index, or time of onset.

Taming Destructive Forces Using a Simple Tension Suppression Device
Postgraduate Dentistry, vol.7, num 3, 2000
ABSTRACT: "Bruxism" historically has been casually defined as "the clenching and/or grinding of the teeth". Since there can be no teeth grinding without the jaws first being clenched, a re-definition of bruxism is presented: "Jaw clenching, with or without forcible excursive movement, where the intensity of  the clenching dictates the severity of teeth grinding". Traditional inter-occlusal splint methods of treating bruxism have been unpredictable because  their specific design addresses lateral movement (grinding), when it is the degree of intensity of vertical movement (clenching) which dictates severity of symptoms. A new method and device (a simple modification of pre-existing concepts) which suppresses clenching intensity by exploiting the nociceptive trigeminal inhibition reflex while preventing canine and posterior tooth occluding, is presented.

The effect of incisal bite force on condylar seating
Angle Orthod 1994;64(1):53-61
Therefore, when taking a centric relation record, a technique involving an anterior stop and sufficient biting force should seat the condyles more fully.

Signs and Symptoms (back to top)

Joint Strain  (back to top)

The relationship between parafunctional masticatory activity and arthroscopically diagnosed temporomandibular joint pathology
J Oral Maxillofac Surg, 57(9):1034-9 1999 Sep
It was concluded that parafunctional masticatory activity and its influence on joint loading contribute to osteoarthritis of the temporomandibular joint. Because abnormal joint loading is a major causative factor in cartilage degradation, biochemical and biomechanical abnormalities,  and intraarticular temporomandibular pathology, clinicians must identify and address parafunctional masticatory activity during nonsurgical, surgical, and postsurgical treatment regimens.

Loading on the temporomandibular joints with five occlusal conditions.
J Prosthet Dent  56(4):478-84 1986 Oct
(From conclusions: "Biting on an anterior splint was an effective method for guiding the condyles to a superior position, which when combined with a proper anterior-posterior relationship, is often desireable.) (graphic)

The influence of altered working-side occlusal guidance on masticatory muscles and related jaw movement.
J Prosthet Dent 1985 Mar;53(3):406-13
Introduction of a hyperbalancing occlusal contact caused significant alterations in muscle activity and coordination during maximal tooth clenching in a lateral mandibular position. A marked shift of temporal muscle EMG activity toward the side of the interference and unchanged bilateral activity of the two masseter muscles were observed.  The results suggest that canine-protected occlusions do not significantly alter muscle activity during mastication but significantly reduce muscle activity during parafunctional clenching (for the masseter,  but not the temporalis). They also suggest that non-working side contacts dramatically alter the distribution of muscle activity during parafunctional clenching (of the temporalis'), and that this redistribution may affect the  nature of reaction forces at the temporomandibular joints.

Interactions between jaw-muscle recruitment and jaw-joint forces in Canis familiaris
J Anat, 164(-HD-):101-21 1989 Jun
During mastication, balancing-side temporalis electromyographic activity was much less than that of the working side while masseter muscle electromyographic activities were of similar amplitude. Working-side muscle activity produced bone strain that correlated with a compressive joint loading, while balancing-side muscle activity, with an occlusal fulcrum at the carnassial teeth, produced bone strain indicative of an anteroventral movement of the working-side mandibular condyle which eventually ruptured the joint capsule.

Condyle and mandibular bending deformation due to bite force.
Kokubyo Gakkai Zasshi 59(1):142-59 1992 Mar
The purpose of this study was to investigate the influence of the difference of the biting pivot positions, vertical dimensions and mandibular positions on the condylar displacement during clenching. When clenching on the unilateral 2nd-molar, the mandible on the non-pivot side had an inward and upward bending deformation and the arch width decreased. It can be inferred that the actual idling condylar displacement was more inward and upward than that measured by the Pantograph.

A three-dimensional investigation of temporomandibular joint loading.
J Biomech 20(10):997-1002 1987
The results show that the reaction forces are in approximately a 2:1 ratio with the balancing side condyle carrying the greater load.

The effect of different condylar positions on masticatory muscle electromyographic activity in humans
Oral Surg Oral Med Oral Pathol Oral Radiol Endod, 85(1):18-23 1998 Jan
The result of any therapeutic position should be an improvement in muscle function.

Influence of experimental occlusal discrepancy on masticatory muscle activity during clenching.
J Oral Rehabil, 23(1):55-60 1996 Jan
Clenching on the experimental interferences resulted in distinct patterns in the jaw elevator muscles, and the most characteristic change was observed when clenching effort was exerted on the experimental non-working side interference. Resultant bilateral activity in the anterior and posterior temporal muscles is thought to cause a superior movement of the working side condyle and an inferior movement of the non-working side condyle. (Ed: i.e., strain)

Relationship between mandibular position and the coordination of masseter muscle activity during sleep in humans.
J Oral Rehabil 25(12):902-7 1998 Dec
During sleep grinding, EMG bursts of masseter muscle were observed mainly with mediotrusive mandibular movement from the canine edge-to-edge position. From the results of the present study, it is suggested that muscular dynamics during sleep are unique compared to that during voluntary clenching, and exert a greater mechanical load to the balancing side temporomandibular joint.

The role of passive muscle tensions in a three-dimensional dynamic model of the human jaw.
Arch Oral Biol, 44(7):557-73 1999 Jul
Both states revealed condylar loading in the opening and closing phases of mastication. During unilateral chewing, compressive force on the working-side condyle exceeded that on the balancing side. In contrast, during the "chopping" cycle, loading (strain) on the balancing side was greater than that on the working side

The association among occlusal contacts, clenching effort, and bite force distribution in man.
J Dent Res, 76(6):1316-25 1997 Jun
The contact area during habitual biting can vary according to the activity of the jaw musculature. Forceful masticatory muscle activity may also induce deformations of the dento-alveolar tissues and the supporting skeleton, yielding various tooth loads despite an apparently even distribution of tooth contacts. Forces in the anterior region (especially at the canine) significantly increased (up to 10 times) when clenching took place on unilateral contacts only (type U) as compared with fully balanced ones (type F).  Bite force distribution thus changed with biting strength and the location of occlusal contacts. Increased
force in the canine region during unilateral clenching seems related to the pattern of jaw muscle co-activation and the physical properties of the craniomandibular and dental supporting tissues which induce complex deformations of the lower jaw.
(Ed: A premier example of the strain caused by "Excursive Clenching")

Relationship between occlusal contacts and jaw-closing muscle activity during tooth clenching
J Prosthet Dent, 52(5):718-28 1984 Nov
Vertical clenching efforts in the natural or simulated intercuspal position generally showed the highest muscle activities for all the muscles recorded. When the contact point moved posteriorly along the arch from incisors to molars, the activity in the ipsilateral temporal muscles was seen to increase, while the activity in the ipsilateral medial pterygoid and the masseter muscles bilaterally was seen to decrease during vertical clenching tasks.  The ipsilateral temporal and contralateral pterygoid muscles showed the most activity during maximal clenches in lateral direction with little contribution from the other muscles.
(Ed: Confirming the activity of "Excursive Clenching", allowing the contralateral ptyergoid to strain
the contralateral TMJ)

The association among occlusal contacts, clenching effort, and bite force distribution in man.
J Dent Res 1997 Jun;76(6):1316-25
The contact area during habitual biting can vary according to the activity of the jaw musculature.  Forceful masticatory muscle activity may also induce deformations of the dento-alveolar tissues and the supporting skeleton, yielding various tooth loads despite an apparently even distribution of tooth contacts.  Forces in the anterior region (especially at the canine) significantly increased (up to 10 times) when clenching took place on unilateral contacts only (type U) as compared with fully balanced ones (type F).Bite force distribution thus changed with biting strength and the location of occlusal contacts. Increased force in the canine region during unilateral clenching seems related to the pattern of jaw muscle co-activation and the physical properties of the craniomandibular and dental supporting tissues which induce complex deformations of the lower jaw.

Condylar position recorded using leaf gauges and specific closure forces.
Int J Prosthodont 1993 Jul-Aug;6(4):402-8
Retruded interocclusal records were made for 40 subjects after deprogramming using leaf gauges and controlled incisal forces, which were exerted on a specially constructed occlusal force sensor. These records were used to assess the resulting displacements of the mandibular condyles from their positions in centric occlusion. The leaf gauges were found not to position the condyles inferiorly and posteriorly as has been previously reported.

The effect of different condylar positions on masticatory muscle electromyographic activity in humans
Oral Surg Oral Med Oral Pathol Oral Radiol Endod 1998  Jan;85(1):18-23
When mandibular condyles were placed anteroinferiorly in a neuromuscular position, total masticatory muscle recruitment was the greatest. In a bimanually manipulated or a leaf gauge position, mandibular condyles were positioned superoposteriorly, producing the least amount of muscle recruitment. CONCLUSIONS: The result of any therapeutic position should be an improvement in muscle function. With respect to balance and activation, a neuromuscular condylar position proved to be the position capable of recruiting the greatest motor unit activity when compared with a bimanually manipulated position, a leaf gauge position, and a neuromuscular position.

Condylar displacement and mandibular bending deformation due to bite force
Kokubyo Gakkai Zasshi, 59(1):142-59 1992 Mar
The direction and magnitude of the condylar displacement changed with the biting pivot position.  The displacement of the idling condyle was multi-directional when clenching on the habitual closure whereas it was unidirectional and of a smaller magnitude when clenching on the most retruded closure.  When clenching on the unilateral 2nd-molar, the mandible on the non-pivot side had an inward and upward bending deformation and the arch width decreased. It can be inferred that the actual idling condylar displacement was more inward and upward than that measured by the Pantograph.

The variability of condylar point pathways in open-close jaw movements.
J Prosthet Dent, 77(4):394-404 1997 Apr
The trajectory of each condylar point, whether average value or radiographically determined, was different in form and dimension from any other condylar point within a subject for the same open-close jaw movement. CONCLUSIONS:  Depending on the point chosen in the vicinity of the condyle, quite different interpretations of condylar movement within a subject could be made. The data underscore the caution that must be exercised when interpreting condylar movement from the movement of a single condylar point.

Condylar movement and mandibular rotation during jaw opening.
Am J Orthod Dentofacial Orthop 1995 Jun;107(6):573-7
All of the subjects demonstrated both translation and rotation of the condyle during initiation of jaw opening, and none had a center of mandibular rotation located at the condylar head. The findings support the theory of a constantly moving, instantaneous center of jaw rotation during opening that is different in every person. There were also differences in movement within the subjects between experimental trials. The uncertainty of predicting    mandibular rotation for a given patient should be considered when planning surgical treatment and fabricating orthodontic appliances.

Bruxism / Neuromuscular Sleep Disorder / Parafunction
(back to top)

Reported symptoms and clinical findings in a group of subjects with longstanding bruxing behaviour.
J Oral Rehabil 1997 Aug;24(8):581-7
There was a statistically significant correlation between frequent tooth clenching and headache, pain in the neck, back, throat or shoulders, sleep disorders and high scores of the clinical dysfunction index (Di). The frequent clenchers had higher score values than the 'non-clenchers' (Ed: i.e., "grinders") for pain in the face and the jaws; headache; pain in the neck, back, throat or shoulders and the clinical dysfunction index (Di). These findings indicate a causal relationship between frequent tooth clenching and signs and symptoms of CMD, including headache and pain in the neck, back, throat or shoulders and high pathogenicity for frequent clenching.  (Ed:Confirming the significance of differentiation between "clenching" and "grinding".  Supports the definition of "Bruxism" as: Parafunctional clenching, with or without forceful excursive  movement.)

Bruxing patterns in man during sleep
J Oral Rehabil, 11(2):123-7 1984 Mar
Nocturnal clenching was monitored using a dedicated microprocessor, appropriate EMG amplification and digitisation. The hardware was located at the subject's bedside and the software provided for the real time recording of clenching bruxism, duration of the episode and the severity in electronic values. Forced clenches before retiring and on arousing provided maximal baseline data against which to compare the severity of sleeping clenches. All ten subjects tested were found to brux and two used intensities of effort while asleep that exceeded their maximal conscious clenches. (Ed: All humans clench during sleep, some more intensely than others.  The frequency, duration, intensity, and position of the mandible dictates resultant signs and/or symptoms)

Relationship between mandibular position and the coordination of masseter muscle activity during sleep in humans.     Oral Rehabil 25(12):902-7 1998 Dec
During sleep grinding, EMG bursts of masseter muscle were observed mainly with mediotrusive mandibular movement from the canine edge-to-edge position. From the results of the present study, it is suggested that muscular dynamics during sleep are unique compared to that during voluntary clenching, and exert a greater mechanical load to the balancing side temporomandibular joint.

Influence of bruxism during sleep on stomatognathic system
Kokubyo Gakkai Zasshi 66(1):76-87 1999 Mar
The purpose of this study was to investigate the influence of bruxism on the stomatognathic system. Clenching pattern during "Mixed movement" was most frequently observed for all three subjects, and EMG activities during clenching were stronger than those during grinding. These findings suggested that tooth clenching during sleep may be harmful to the stomatognathic system, rather than other mandibular movements (Ed: i.e., grinding)

Effect of a full-arch maxillary occlusal splint on parafunctional activity during sleep in patients with nocturnal bruxism and signs and symptoms of craniomandibular disorders.
J Prosthet Dent 69(3):293-7 1993 Mar
The splint does not stop nocturnal bruxism. In 61% of the patients, wear facets on the splint were observed at every visit (2-week intervals) and in 39%, from time to time. The wear facets reappeared in the same location with the same pattern and were caused mainly by grinding. The extension of the facets showed that, during eccentric bruxism, the mandible moved laterally far beyond the edge-to-edge contact relationship of the canines.

Digital assessment of occlusal wear patterns on occlusal stabilization splints: a pilot study.
J Prosthet Dent 80(2):209-13 1998 Aug
Splint wear was asymmetric between sides and uneven between dental locations. CONCLUSIONS: For full coverage occlusal splints, the appliance wear phenomenon can be site specific and, if left undisturbed, may yield two extremes of high wear and a zone of low wear in-between.

Descriptive physiological data on a sleep bruxism population.
Sleep 20(11):982-90 1997 Nov
24 bruxers (23-67 years old), 65% reported frequent headaches in the morning (Ed: Most likely due to clenchng rather than grinding) . Deep sleep and rapid eye movement (REM) were delayed. An average of 167 orofacial episodes developed during the night. The mean number of masseter bursts strictly defined as bruxism was 79, the mean delay for the first occurrence after sleep onset 18 minutes. The majority of bruxism occurred in stage 2 sleep and REM sleep.  (Ed:  Sleep studies typically record masseter grinding activity to objectively report "bruxism", and relate it to the subjective symptom of headache (a clenching symptoms), further emphasizing the need to differentiate the two)

The incidence of parasomnias in child bruxers versus nonbruxers.
Pediatr Dent 18(7):456-60 1996 Nov-Dec
Bruxism in children has been reported to occur in association with certain parasomnias (i.e., sleep talking, bed wetting). One-hundred fifty-two subjects (77 bruxers and 75 controls) revealed that five of the 54 factors (nocturnal muscle cramps, bed wetting, colic, drooling while sleeping, and sleep talking) showed significant differences between bruxers and controls (odds ratios ranged from 3.11 to 1.95). These findings strongly suggest the possibility of a common sleep disturbance underlying these nonsleep-stage specific parasomnias.

Myofascial Tenderness  (back to top)

Evaluation of pericranial tenderness and oral function in patients with common migraine, muscle contraction headache and 'combination headache'.
Pain, 12(4):385-93 1982 Apr
Clenching and grinding teeth and tongue pressure were all significantly  more common in headache patients. Tenderness of pericranial muscles was present in all headache patients with severity increasing in the order Common Migraine, Tension-type Headache, Mixed Headache (common migraine + tension-type); it was absent in all the controls.

Tenderness on palpation and occlusal abnormalities in temporomandibular dysfunction.
J Prosthet Dent 1992 Jun;67(6):839-45
Two hundred ten patients were examined; 96% had tenderness and 80% of cases of tenderness were diagnosed as occlusally related.  Tenderness was observed most frequently in the lateral pterygoid muscle, followed by the insertion of temporal muscle.

Recurrent headaches in relation to temporomandibular joint pain-dysfunction.
Acta Odontol Scand 1978;36(6):333-8
The investigation showed that clenching of teeth was correlated to the severity of headache. The frequency and severity of headache varied also with the severity of mandibular dysfunction. Of the variables included in the dysfunction index, only masticatory musculature painful to palpation was found to have a distinct relationship to headaches.

Prevalence of signs and symptoms of craniomandibular disorders and orofacial parafunction in 4-6-year-old African-American and Caucasian children.
J Oral Rehabil 1995 Feb;22(2):87-93
Seventeen per cent had recurrent headache. Pain or tiredness in the jaws during chewing was reported by 25%. Thirteen per cent of the children had problems in opening the mouth.

Pericranial muscle tenderness and pressure-pain threshold in the temporal region during common migraine
Pain, 35(1):65-70 1988 Oct
Twenty-six patients were examined during attacks of common migraine as well as during headache-free interval. Pericranial tenderness was scored blindly by a systematic manual palpation on both occasions by the same observer. Pressure-pain threshold (PPT) in a fixed location over the temporal muscle was determined by the use of a pressure algometer. A 28% increase in total tenderness score was observed during attacks (P less than 0.01). During unilateral attacks, tenderness scores were significantly higher on the ipsilateral side as compared to the contralateral.

`Cervical Involvement  (back to top)

The effect of jaw clenching on the electromyographic activities of 2 neck and 2 trunk muscles
J Orofac Pain , 13(2):115-20 1999 Spring
Jaw clenching resulted in increases in neck muscle activity ranging from 7.6 to 33 times resting muscle activity; for the trunk muscles, the increases ranged from 1.4 to 3.3 times resting activity. CONCLUSION: These results add further information to the concept of the interrelatedness of jaw, neck, and trunk muscle activity.

Co-activation of sternocleidomastoid muscles during maximum clenching
J Dent Res 72(11):1499-502 1993 Nov
All subjects demonstrated co-activation of the SCM during strong abrupt clenching efforts. Fifty percent of masseter activity was required to achieve 5% activity of the SCM bilaterally, and there was a progressive development of the SCM co-activation which paralleled the masseter activation

The effect of vertical dimension and mandibular position on isometric strength of the cervical flexors.
Cranio 17(2):85-92 1999 Apr
The results suggest that when biting, individuals with deep bite may be functioning at about 60% of their potential cervical flexor, isometric strength. The interaction between occlusal position, vertical dimension and cervical muscle function suggests a craniomandibular-cervical masticatory system.

Studies on the relationship between functional disturbances of stomatognathic system and chronic suboccipital headaches
Protet Stomatol 1990 May-Jun;40(3):120-5
 The obtained results confirmed the relationship between certain symptoms of functional stomatognathic system disturbances and chronic suboccipital headaches in these patients.

Tinnitus(back to top)

Continuous, high-frequency objective tinnitus caused by middle ear myoclonus
Ear Nose Throat J, 77(10):814-8 1998 Oct
Myoclonus of the middle ear is characterized by abnormal repetitive muscle contractions of the tympanic cavity. (Ed.:  Innervated by the same branch of the trigeminal nerve to the medial pterygoid) Administration of curare for anesthesia causes complete disappearance of the tinnitus. Sectioning of the stapedius and tensor tympani tendon renders the patient asymptomatic and confirms the diagnosis of middle ear myoclonus.

The Relationship between Tinnitus and Temporomandibular Disorder (TMD) Therapy.
Int Tinnitus J 1997;3(1):55-61
Forty TMD patients rating their tinnitus as moderate or severe, were asked questions and participated in clinical tests. Upon completion of TMD therapy:  53% tinnitus resolved, 30% significant improvement,  17% unchanged.

Tinnitus and vertigo in patients with temporomandibular disorder.
Arch Otolaryngol Head Neck Surg 1992 Aug;118(8):817-21
Tinnitus and vertigo symptoms were significantly more prevalent in the TMD group than in either of the control groups. The mechanism of the association of TMD and otologic symptoms is unknown.

Tinnitus, vertigo, and temporomandibular disorders.
Am J Orthod Dentofacial Orthop 1995 Feb;107(2):153-8
The results revealed that tinnitus and vertigo were significantly more prevalent in the TMD group than in either control group. Reasons for the association of TMD and these otologic symptoms have been proposed and they are discussed.

Trigeminally innervated muscles of the inner ear and palate
James P. Boyd, DDS, website
The tensor tympani (which dampens and stabilizes inner ear to vibration) and the tensor veli palatini (which tenses the palate and facilitates opening and closing of the eustacian tube), are both innervated by the trigeminal nerve, which also innervates the muscle medical pteyrgoid of the TM system.

Trigeminal Pharyngioplasty: Treatment of the Forgotten Accessory Muscles of Mastication  Which Are Associated With Orofacial Pain and Ear Symptomology
Submittted to the Journal of Pain Management, June 2000
Diagnostic importance to the dental/orofacial pain clinician is the fact that trigeminal pharyngioplasty treatments have shown that a significant portion of patients thought to be suffering from pain of temporomandibular joint, facial, or upper quadrant origins, actually may only be suffering from dysfunction of the two forgotten accessory muscles of mastication, the tensor veli palatini and tensor tympani muscles.

Headache and Migraine(back to top)Survey of Migraineurs

Headache Etiology (back to top)

Evaluation of pericranial tenderness and oral function in patients with common migraine, muscle contraction headache and 'combination headache'.
Pain, 12(4):385-93 1982 Apr
Clenching and grinding teeth and tongue pressure were all significantly  more common in headache patients. Tenderness of pericranial muscles was present in all headache patients with severity increasing in the order Common Migraine, Tension-type Headache, Mixed Headache (common migraine + tension-type); it was absent in all the controls.

Myofascial trigger points show spontaneous needle EMG activity.
Spine, 18(13):1803-7 1993 Oct 1
Monopolar needle electromyogram (EMG) was recorded simultaneously from trapezius myofascial trigger points (TrPs) and adjacent nontender fibers (non-TrPs) of the same muscle in normal subjects and in two patient groups, tension headache and fibromyalgia. Sustained spontaneous EMG activity was found in the 1-2 mm nidus of all TrPs, and was absent in non-TrPs. Mean EMG amplitude in the patient groups was significantly greater than in normals. The authors hypothesize that TrPs are caused by sympathetically activated intrafusal contractions (of the spindle fibers).

Needle electromyographic evaluation of trigger point response to a psychological stressor.
Psychophysiology, 31(3):313-6 1994 May
The results showed increased trigger point electromyographic (within the sympathetically innervated intrafusal fibers of the spindle) activity during stress, whereas the adjacent muscle remained electrically silent. These results suggest a mechanism by which emotional factors influence muscle pain. This may have significant implications for the psychophysiology of pain associated with trigger points

Electromyography of pericranial muscles during treatment of spontaneous common migraine attacks.
Pain 1982 Oct;14(2):137-47
During the attack of migraine, activity in the anterior temporal muscles significantly exceeded the patient's own baseline recordings and all muscles were activated more strongly than in the control sample.
Following treatment the activity of the temporal and sternocleidomastoid muscles decreased in 5 (of 7) patients at the same time as the pain and nausea to the level of the controls. (No reference of clenching the jaw is made, which would be the result if the skeletal muscle fibers of the anterior temporalis were firing.  The researches may have been recording the activity of the spindle fibers.)

Recurrent headaches in relation to temporomandibular joint pain-dysfunction.
Acta Odontol Scand 1978;36(6):333-8
The investigation showed that clenching of teeth was correlated to the severity of headache. The frequency and severity of headache varied also with the severity of mandibular dysfunction. Of the variables included in the dysfunction index, only masticatory musculature painful to palpation was found to have a distinct relationship to headaches.

Automatic regulation of sinus rhythm in patients with migraine
Neurol Neurochir Pol 1995 Nov-Dec;29(6):889-900
The clinical symptoms of migraine point to autonomic disturbances, especially to disrupted regulation of  the circulatory system and autonomic balance. The autonomic balance is shifted to the parasympathetic innervation side in patients with migraine. (Ed: Would a reflexive over-compensation of the sympathetic allow "spasm" of the intrafusal fibers of the spindles?)

Initiating mechanisms of experimentally induced tension-type headache.
Cephalalgia, 16(3):175-82; discussion 138-9 1996 May
To elucidate possible myofascial mechanisms of tension-type headache, the effect of 30 min of sustained tooth clenching (10% of maximal EMG-signal) was studied in 58 patients with tension-type headache and in 30 age- and sex-matched controls. Pericranial tenderness, mechanical and thermal pain detection and tolerance thresholds and EMG levels were recorded before and after the clenching procedure. Within 24 h, 69% of patients and 17% of controls developed a tension-type headache.   A peripheral mechanism of tension-type headache is therefore possible. Researchers commented: "The exact degree of clenching seems to be of minor importance.  Approximately the same percentage of subjects developed headache with 10% maximal contraction in the present study and with 5% or 30% of maximal contraction in the preveious migraine study".  (Ed: The authors arbitrarily choose to evaluate 10% of voluntary maximum, while nocturnal tooth clenching often exceeds voluntary maximum)

Experimental toothclenching in common migraine
Cephalalgia 5(4):245-51 1985 Dec
The effect of 30 min voluntary toothclenching was studied in 48 patients with common migraine, randomized in two groups. Group 1 performed low-level tension at 5% and group 2, high-level tension at 30% of the individual maximum, (Ed: sustained for 30 minutes (with two rest breaks)), as judged by surface EMG from the temporal muscle.  Pericranial muscle tenderness was evaluated by manual palpation and a four-point verbal scale. Headache, nausea, and soreness of the chewing muscles were scored on visual analogue scales. Although surface EMG, soreness, blood pressure, heart rate and difficulty in completing the toothclenching session all showed that group 2 patients were subjected to significantly higher levels of muscle tension than group 1 patients, headache developed equally often in both groups (63%).  Migraine frequency was not increased.  (Ed:  Researchers commented on the subjects' curious lack of requiring rest periods.  Possibly indicates that 30% of maximal clenching may be far below the sufferers' usual parafucntional clenching intensity.  See: "Waking and sleeping EMG levels in tenison-type headache patients", where clenching during sleep is 14x greater that controls, and Bruxing Patterns in Man During Sleep)

Muscular factors are of importance in tension-type headache.
Headache 1998 Jan;38(1):10-7
Muscular factors may, therefore, be of major importance for the conversion of episodic into chronic tension-type headache.  (Ed: As frequency of intense nocturnal clenching increases and becomes habitual, so would episodic headache become chronic)

Pericranial muscle tenderness and pressure-pain threshold in the temporal region during common migraine
Pain, 35(1):65-70 1988 Oct
Twenty-six patients were examined during attacks of common migraine as well as during headache-free interval. Pericranial tenderness was scored blindly by a systematic manual palpation on both occasions by the same observer. Pressure-pain threshold (PPT) in a fixed location over the temporal muscle was determined by the use of a pressure algometer. A 28% increase in total tenderness score was observed during attacks (P less than 0.01). During unilateral attacks, tenderness scores were significantly higher on the ipsilateral side as compared to the contralateral.
(Ed: The fixed location measuring PPT was not necessarily a dysfunctional spindle fiber / trigger point)

Soft occlusal splint therapy in the treatment of migraine and other headaches.
J Dent, 18(3):123-9 1990 Jun
Fifty-seven patients suffering from migraine, tension headache or tension vascular headache were prescribed a soft occlusal splint for night-time wear.  Most patients suffering from tension headache failed to benefit from splint therapy.  (Ed:  A full coverage splint does not reduce clenching intensity)

Occlusal abnormalities, pericranial muscle and joint tenderness and tooth wear in a group of migraine patients.
J Oral Rehabil <18(5):453-8 1991 Sep
Seventy-two migraine sufferers, whose attacks normally begin during or soon after waking from sleep, were compared with 37 age- and sex-matched controls to establish whether signs of mandibular dysfunction, occlusal discrepancies and known clenching or grinding habits were any more frequent among the former group.  Evidence was found to support an aetiological role for nocturnal tooth clenching or grinding in migraine characterized by attacks that start predominantly during sleep or soon after waking, but no evidence of a link with occlusal factors was found in these patients.  (Ed.: It's not what the patient has, it's what they *do* with what they have, supporting the necessity to differentiate temporalis clenching and masseter grinding)

Chronic paroxysmal hemicrania presenting as toothache.
J Orofac Pain 1993 Summer;7(3):300-6
A set of symptoms that defines chronic paroxysmal hemicrania is presented.  The attacks usually produce pain in the frontotemporal region and two cases in which the presenting symptom was toothache are reported.

Odontogenic (concomitant) etiology of headache
Wien Med Wochenschr 1997;147(15):365-8
Our results once more underlined the multifactorial etiology of headache, that is opposed to a monocausal oriented headache diagnosis (as the IHS-nomenclature tries to impose). Still it has considered to be relevant that a good diagnostic examination in the field of tooth-, jaw- and mouth medicine should be conducted in every headache patient, even in "typical" migraine patients. (Ed: Most important of which is assessment of temporalis spindular dysfunction / trigger point presense)

An immunocytochemical and autoradiographic investigation of the serotoninergic innervation of trigeminal mesencephalic and motor nuclei in the rabbit.
Neuroscience 1993 Apr;53(4):1113-26
The findings suggest that release of serotonin from fibres in close proximity to trigeminal primary afferent somata could modify the transmission of action potentials from muscle spindle receptors during mastication through an action on serotonin2 receptors.

The comparison of patients suffering from temporomandibular disorders and a general headache population.
Headache 1993 Apr;33(4):210-3
Results indicate that patients with temporomandibular disorders exhibit significantly more jaw dysfunction and pericranial muscle tenderness than migraine and tension headache patients.  (Ed: TMD patients are usually identified by their symptoms resulting from "grinding",a lateral pterygoid and/or massester activity, i.e., "jaw dysfunction")  Migraine and tension headache patients were found to have similar amounts of pericranial muscle tenderness (Ed: Temporalis clenching does not obligate lateral pterygoids or massesters, therefore the less frequent "jaw dysfunction")

Migraine:  What Is Migraine Headache?
American Medical Association website
The exact cause of migraine is uncertain, although various theories are being studied. One theory favored by many researchers is that migraine is due to a vulnerability of the nervous system to sudden changes in either your body or the environment around you (Ed: i.e., responses evoked by the sympathetic nervous system).  There is no medical test (Ed: objective) that can specifically diagnose migraine. Migraine can only be diagnosed by effectively communicating your symptoms to your physician
(Ed: subjective)

Exteroceptive suppression of the masseter, temporalis and trapezius muscles produced by mental nerve stimulation in patients with chronic headaches.
Cephalalgia 1991 Feb;11(1):23-8
A low degree of exteroceptive suppression may play a role not only in chronic tension-type headache associated with a disorder of the pericranial muscles, but also in migraine without aura.

Effects of tizanidine administration on exteroceptive suppression of the temporalis muscle in patients with chronic tension-type headache.
Headache 1994 Sep;34(8):455-7
The aim of this study was to clarify the changes of inhibitory interneuronal activity in patients with chronic tension-type headache with disorder of pericranial muscle after treatment, and the pharmacological mechanisms of tizanidine--an alpha 2 adrenergic (sympathetic) agonist. ES2 produced by four times the sensory threshold was lengthened after tizanidine administration. This fact suggests that tizanidine improves the inhibitory function in the central nervous system, and  then relieves headache.

Amitriptyline, a combined serotonin and noradrenaline re-uptake inhibitor,  reduces exteroceptive suppression of temporal muscle activity in patients with chronic tension-type headache.
Electroencephalogr Clin Neurophysiol 1996 Oct;101(5):418-22
Although reflexes in human jaw muscles have been extensively studied, the neurotransmitters involved in the regulation of these reflexes are not well known.  The aim of the present study was to investigate whether amitriptyline, a combined serotonin and noradrenaline re-uptake inhibitor, modulates the late exteroceptive suppression period (ES2) of temporal muscle activity in chronic tension-type headache.  Our results demonstrate that amitriptyline reduces ES2 and indicate that ES2 is modulated by serotonergic as well as  noradrenergic neuronal pathways.

Exteroceptive suppression of activity of the temporal muscle. Principles and applications
Nervenarzt 1996 Oct;67(10):846-59
The exteroceptive suppression period (ES) of the temporalis muscle activity is a trigemino-trigeminal brain stem reflex. However, above all the ES nowadays attracts most attention as a tool to analyse different pain syndromes. A large number of results have been obtained showing that chronic pain syndromes such as chronic tension-type headache and migraine cause changes within the normal ES recording pattern.

Pericranial muscle tenderness and exteroceptive suppression of temporalis muscle activity: a blind study of chronic tension-type headache.
Headache 1997 Jun;37(6):368-76
Consistent with our previous findings, abnormalities in pericranial muscle tenderness, but not in the second exteroceptive suppression period distinguished chronic tension-type headache sufferers from controls. The chronic tension headache sufferers exhibited the highest pericranial muscle tenderness and the control group exhibited the lowest tenderness (P < .001).  Pericranial muscle tenderness was quite successful in distinguishing recurrent headache sufferers  from controls, but failed to distinguish chronic tension-type headache sufferers from migraineurs.  Our findings raise the possibility that pericranial muscle tenderness is present early in the development of chronic tension-type headache and migraine without aura, and thus might contribute to the etiology of headache disorders.

Suppression of voluntary temporalis muscle activity by peripheral limb stimulations in healthy volunteers, migraineurs and tension-type headache sufferes.
Funct Neurol 1996 Nov-Dec;11(6):307-15
We studied the inhibition of voluntary temporalis muscle activity after stimulations of extracephalic cutaneous or mixed nerves in 23 healthy volunteers, 29 patients suffering from migraine without aura, 24 from episodic and 42 from chronic tension-type headache. Index-"ES2" occurred significantly more often in migraineurs (83%) than in controls (48%) suggesting that the spino-bulbar pathways involved in index-/median-"ES2" are hyperexcitable in migraine.

Mandibular tori, migraine and temporomandibular disorders.
Br Dent J 1996 May 25;180(10):382-4
Parafunction in the form of tooth clenching or grinding has been associated with temporomandibular disorders (TMD) and recently migraine. The results support an association with parafunction in the aetiology of mandibular tori and suggest that tori are a useful marker of past or present parafunction in some patients.

Evaluation of the status of the stomatognathic system in patients with idiopathic headaches
Neurol Neurochir Pol 1991 Mar-Apr;25(2):181-8
The stomatognathic system was assessed in 114 subjects with various types of idiopathic headaches.  Stomatognathic system dysfunctions were recognized most frequently in patients with atypical facial pain and headache, and with mixed headache . Much less frequently these dysfunctions were present in patients with migraine (Ed: primary/centric clenching does not obligate the muscles which present as facial pain) or cluster headaches. The obtained results suggest that (certain) stomatognathic system dysfunctions should be considered in the pathogenesis of certain headaches.  (Ed: While other stomatognathic symtem dysfunctions, i.e., temporalis clenching, should be considered in other headaches)

Pathogenesis of Migraine
New Treatment Options in Migraine [Neurology Treatment Updates - © 2000 Medscape, Inc.]
Pathogenesis of Migraine: The pathogenesis of migraine involves changes in vascular dilation, neural activation, and muscle spasm. Currently, it is believed that migraine occurs as a result of a 4-step process... Step 3: Dilatation of the blood vessels triggers the trigeminal system. Signals are returned from the trigeminal nucleus back along the same nerve fibers to the dilated blood vessels, causing them to dilate further. The trigeminal system activates the hypothalamus, causing cravings, photophobia, and phonophobia; signals also go to the upper part of the spinal cord, creating tightness and spasms of the muscles in the back of the head and neck. Step 4: Signals go up to the thalamus and the cortex, and a headache occurs.  (submitted by Roy Brown)

The neurobiology of vascular head pain.
Ann Neurol, 16(2):157-68 1984 Aug
Nervous connections between the trigeminal ganglia and cerebral blood vessels have recently been identified in experimental animals and have been termed the trigeminovascular system. The relationship of trigeminovascular fibers to the pathogenesis of vascular head pain sheds light on possible mechanisms of migraine and other central nervous system conditions associated with headache and inflammation.
(Ed: Although it has yet to be proven, the asumption here remains that headache pain is vascular, rather than spindular)

Migraine and masticatory muscle volume, bite force, and craniofacial morphology
Headache 2001 Jan;41(1):49-56
There was a significant difference in the volume of both masseter and medial pterygoid muscles between the two subject groups (P<.0001), with the muscles of the migraineurs nearly 70% larger. The migraineurs recorded significantly higher maximal bite forces (P<.0001) than did the controls. No significant differences for any craniofacial morphological measurement were demonstrated between the two groups. It was concluded that the migraineurs had larger masseter and medial pterygoid muscle volumes, and greater bite forces than the controls, which could not be explained by any change in craniofacial morphology.
(Ed: Muscles get bigger and stronger when they are vigorously excercised, as in nocturnal clenching)

Masseter and temporalis muscle EMG levels and bite force in migraineurs
Headache 2000 Nov-Dec;40(10):813-7
The group with migraine had higher levels of absolute EMG activity during anterior and posterior maximum voluntary contractions. Furthermore, the group with migraine demonstrated higher levels of anterior and posterior bite force.  (Ed: Migraine patients practice more)

Headache Treatment  (back to top)

Migraine and Tension-type Headache Reduction Through Pericranial Muscular Suppression:
 A Preliminary Report
Shankland W, Cranio 2001 Oct; vol 19, num 4:269-278
The NTI-tss device was compared to a full-coverage occlusal splint to observe the effect on patients who had been medically diagnosed with migraine.  82% of migraineurs who wore the NTI-tss nightly had a 77% average reduction of migraine events within the first 8 weeks.  Overall, there was a 62% reduction in migraine events with the NTI-tss and a 38% reduction in migraine events with the full-coverage control splint. (report to the FDA)

Migraine: the effect of acrylic appliance design on clinical response.
Br Dent J 1996 Feb 24;180(4):137-40
 Two appliance designs were employed, one covering the maxillary occlusal surfaces of the dentition and another which contacted the palatal mucosa only and was free of the occlusion. Treatment outcome was expressed as the number of attacks of migraine per week per patient. The occlusal cover appliance reduced the number of attacks on average to about 40% of that normally experienced. The improvement was most marked in those who had frequent attacks of migraine ie two attacks per week on a regular basis. (Ed: Demonstrating an alteration in occlusing pattern)

Clinical effectiveness of occlusal splint therapy in patients with classical migraine.
Scott Med J 1987 Feb;32(1):11-2
Nineteen patients with migraine symptoms were provided with acrylic occlusal splints for nocturnal wear. A good clinical response with considerable reduction in frequency and severity of pain attacks was achieved.

Advances in the pharmacotherapy of migraine. How knowledge of pathophysiology is guiding drug development.
Drugs R D 1999 Dec;2(6):361-74
This century has seen the shift from the vascular theory of migraine pathophysiology (i.e. that cranial vessels were the prime movers in the disorder and thus vasoconstriction would be the prime treatment) to a more integrated neurovascular theory. The neurovascular theory takes the view that vascular change is secondary to neural activation, so changes such as release of trigeminal neuropeptides is predicted, has been demonstrated and suggests a possible new treatment. Similarly, it may be possible to block trigeminal nociceptive transmission in the trigeminal nucleus which might avoid the vascular adverse effects that, while small, plague current treatments. The future for migraine and cluster headache, the neurovascular headaches, is bright as we unravel their biology and this leads to further therapeutic advances. (Ed: As in suppressing the intensity of chronic contraction of the trigeminally innervated musculature, i.e., the temporalis, with an NTI-tss, thereby reducing the dysfunctional environment in which the sympathetically innervated spindle fibers reside)

Changes in headache after treatment of mandibular dysfunction.
Cephalalgia 1985 Dec;5(4):229-36
The frequency of headache was reduced in 79% and the intensity in 53% of patients suffering from muscle contraction headache or combination headache in whom the adjustment of the dental occlusion had been successfully accomplished.

Craniomandibular disorders and headaches.
J Prosthet Dent 1983 May;49(5):702-5
Patients were questioned regarding the number of headaches per week they had before and after occlusal splint therapy. The following results were observed: 1. Twenty-one (63.6%) patients showed a decrease in the frequency of their headaches. 2. Ten (30.3%) patients showed complete remission of headaches.  3. No patient showed an increase in the frequency of headaches

Treatment of functional disorders of the masticatory system in patients with headaches
Neurol Neurochir Pol 1991 Sep-Oct;25(5):634-9
In 114 cases of spontaneous headaches treated at the Migraine Centre at the Department of Neurology, Medical Academy in Lodz, various types of stomatognathic system dysfunctions were found, and 24 were qualified for stomatological treatment of these dysfunctions. The treatment was causative and included rehabilitation of the stomatognathic system and denture reconstruction (Ed: Thereby reducing the effect of occlusal abnormalities during excursive clenching?). This treatment gave particularly good results in atypical pains in the face and head. Less good effects were obtained in chronic tension headaches and mixed headaches. The discussed method gave no improvement in migraine and cluster headaches.  (Ed: Improving the occlusal scheme has no effect on centric/primary clenching).

Proprioception / Nociception of Teeth(back to top)

Electromyographic parameters related to clenching level and jaw-jerk reflex in patients with a simple type of myogenous cranio-mandibular disorder
J Oral Rehabil, 19(5):495-511 1992 Sep
In all subject samples, the activity of the anterior temporal muscles decreased with respect to the masseter muscles when the bite-fork was inserted (P less than 0.05-0.001). The therapeutic effect of a relaxation splint may, in part, be related to a relief of the temporal muscles.

Reflex response of temporal muscle induced by mechanical stimulation to periodontal ligament--in the lateral jaw movement during mastication
Kokubyo Gakkai Zasshi 1994 Mar;61(1):82-97
The tonic response of the contralateral temporalis muscle (CTM) to tonic mechanical stimulation of the maxillary canine on the working side was observed. The CTM responded much more clearly than the ipsilateral temporal muscle did.

Encoding of tooth loads by human periodontal afferents and their role in jaw motor control.
Prog Neurobiol, 49(3):267-84 1996 Jun
Most afferents are tuned broadly to direction of force application, and about half respond to forces applied to teeth adjacent to the one to which the afferent distributes. Populations of periodontal afferents, nevertheless, reliably encode information about both the teeth stimulated and the direction of forces applied to the individual teeth.  These afferents efficiently encode food contact during biting and continuously discharge while food is held between the incisors.

Modulation of an inhibitory reflex in single motor units in human masseter at different joint angles.
Neurosci Lett 1989 May 22;100(1-3):157-63
"Some modulation of the inhibitory response at different vertical jaw positions (i.e. temporomandibular joint angle) was observed in most masseter units, with the majority of the modulated units being inhibited less when the teeth were closer together."

Patterns of jaw reflexes induced by incisal and molar pressure stimulation in relation to background levels of jaw-clenching force in humans.
Jpn J Physiol 1993;43(1):87-102
In ten adults at low background clenching force (BCF), excitatory reflexes were elicited from the jaw-closing muscles and jaw-clenching force.  For two of the ten during high background clenching force, the magnitude of excitatory reflexes decreased with as BCF increased. (Ed: In 20% of this random population, a dysfunction
of temporalis clenching was observed)

Reflex responses of motor units in human masseter muscle to mechanical stimulation of a tooth.
Exp Brain Res 1994;100(2):307-15
The reflex responses evoked by controlled mechanical stimulation of an upper central incisor tooth in single motor units in the human masseter muscle were examined. Inhibition of the jaw-closing muscles will tend to protect the teeth and soft tissues when one bites unexpectedly on a hard object while chewing

Mechanoreceptors around the tooth evoke inhibitory and excitatory reflexes in the human masseter muscle.
J Physiol (Lond) 1993 May;464:711-23
The reflex responses evoked in the human masseter muscle by controlled mechanical stimulation of an incisor tooth were examined electromyographically. The reflex responses to 3 N pushes and 2 N taps were abolished when the receptors around the tooth were blocked with local anaesthetic, indicating that the response was elicited from receptors located within the periodontal area.  The inhibitory response to taps is essentially a protective reflex which probably serves to reduce the activity of the jaw-closing muscles when one bites unexpectedly on hard objects...or may act as a load compensation reflex to control chewing force

Effects of remote noxious stimulation on exteroceptive reflexes in human jaw-closing muscles.
Brain Res 1996 Jul 8;726(1-2):189-97
Reflexes evoked by applying non-painful taps to an incisor tooth were recorded from the jaw closing masseter and temporal muscles of 21 human subjects.  The results suggest that... the effects are mediated by mechanisms acting directly at the brainstem level and are not secondary to pain or autonomic responses.

Electrophysiological study of size and fibre distribution of motor units in the human masseter and temporal muscles.
Arch Oral Biol 1986;31(8):521-7
Probably related to their unique function, human jaw muscles have a specialized muscle-fibre composition different from that of limb and trunk muscles.  The temporal muscle differed from the masseter in having significantly-higher fibre density, which may be explained by the higher frequency of small type-II fibres in the temporal causing closer packing of the type-I, low-threshold motor units.

Jaw reflexes evoked by mechanical stimulation of teeth in humans.
J Neurophysiol 1999 May;81(5):2156-63
The averaged bite force records showed that when the stimulus force was developing rapidly, the periodontal reflex could reduce the bite force and hence protect the teeth and supporting tissues from damaging forces. It also can increase the bite force; this might help keep food between the teeth if the change in force rate is slow, especially when the initial bite force is low.

The roles of periodontal ligament mechanoreceptors in the reflex control of human jaw-closing muscles.
Brain Res 1996 Aug 26;731(1-2):63-71
These findings provide evidence that mechanoreceptors in the periodontal ligament contribute to the control of human jaw-closing muscles, notably to short-latency reflex responses. It may be concluded that the additional reflex responses produced by tapping stimuli result from the activation of receptors elsewhere due to vibration.

Role of periodontal mechanoreceptors in evoking reflexes in the jaw-closing muscles of the cat.
J Physiol (Lond) 1993 Jun;465:581-94
The results confirm that when forces are applied to a tooth, periodontal mechanoreceptors are stimulated which evoke reflex inhibitions to motor units in the jaw-closing muscles. However, there is evidence that mechanoreceptors situated distant to the periodontium can also evoke such reflexes

Muscle Functions / Innervation(back to top)

The Significance of Canine Contact to the Temporalis
Data compared from two studies, demonstrating that bilateral canine contact allows for (near) maximal canine
contraction (clenching)

Rotation of synergistic activity during isometric jaw closing muscle contraction in man
Acta Physiol Scand 1983 Jul;118(3):203-7
5 healthy subjects were studied during 10-15 min of isometric jaw elevator contraction above fatigue threshold level. When the test was repeated, however, great relief from the pain in the fatigued masseter was sometimes experienced and the temporalis took over the load. All subjects experienced this "switch" phenomenon after a varying number of tests, one of them already during his first test. The mechanism seemed to be completely out of voluntary control and showed facilitation at repeated tests.
(This may explain why "primary clenchers" have temporal pain, but not masseter pain)

Immediate electromyographic response in masseter and temporal muscles to bite plates and stabilization splints.
Scand J Dent Res, 97(6):533-8 1989 Dec
Activity during maximal biting on (full coverage) stabilization splints was not different from that without the appliance while (anterior) bite plates caused a decrease in activity in both muscles in both groups. The reduced maximal activity was probably due to the smaller number and exclusively anterior positioned occlusal contacts on the (anterior) bite plate.

Bite force on single as opposed to all maxillary front teeth.
Scand J Dent Res 1994 Dec;102(6):372-5
A highly significant difference between maximal bite forces was seen in comparing biting between a single central incisor tooth (lessor) and with a splint covering only the anterior maxillar incisors from canine to canine (greater).

Rotation of synergistic activity during isometric jaw closing muscle contraction in man
Acta Physiol Scand, 118(3):203-7 1983 Jul
5 healthy subjects were studied during 10-15 min of isometric jaw elevator contraction above fatigue threshold level (ed: clenching).  Bite force was measured between upper and lower front teeth and electromyographic (EMG) activity recorded from the right temporal and masseter muscles. Of the two muscles only the masseter was active at the start of the test and usually during the whole test. When the test was repeated, however, great relief from the pain in the fatigued masseter was sometimes experienced and the temporalis took over the load. All subjects experienced this "switch" phenomenon after a varying number of tests, one of them already during his first test. The mechanism seemed to be completely out of voluntary control and showed facilitation at repeated tests.
(ed: the massester shuts down upon fatigue, while the temporalis can, and will, continue to contact)

Difference in central projection of primary afferents innervating facial and intraoral structures in the rat.
Exp Neurol 1991 Mar;111(3):324-31
These observations agree with previously reported data that the central projection of trigeminal nerve is organized in different manners for the facial and intraoral structures. The central mechanism of trigeminal nociception is discussed with particular respect to its difference between the facial and intraoral structures.

Effect of canine guidance of maxillary occlusal splint on level of activation of masticatory muscles.
Swed Dent J  17(6):235-41 1993
The effect of canine guidance of a full-arch maxillary flat occlusal splint on the level of activation of the anterior and posterior, temporal, masseter and suprahyoid muscles during maximal clenching, were studied in 14 subjects without craniomandibular disorders.  The results revealed that, the level of electromyographic activity of anterior and posterior temporal and suprahyoid muscles during maximal clenching on the occlusal splint in habitual closure was unchanged.  The level of activation of the ipsilateral massester decreased significantly during maximal clenching on the cuspid ramp of the splint, while not as pronounced in the ipsilateral temporalis. (Ed: Canine guidance (thus allowing for posterior disclusion of the posterior teeth in excursive movement) reduces masseter contraction intensity, while having less effect on the temporalis, further demonstrating the association between the temporalis and canine teeth.)

A review of masticatory muscle function.   (graphic)
J Prosthet Dent 1987 Feb;57(2):222-32
Current reports of the actions of the major muscles of mastication are reviewed for clenching tasks in centric occlusion and eccentric jaw positions, mandibular opening, and unilateral chewing. The elevator muscles demonstrate maximum activity  when even bilateral occlusal contacts occur during clenching in the intercuspal position.  Increasing the number of eccentric tooth contacts increases the muscle activity during both chewing and clenching.   The inferior head of the lateral pterygoid muscle has a reciprocal role with the medial pterygoid muscle during chewing and contributes to forward and lateral bracing of the condyle of the mandible. Although the superior head of the lateral pterygoid muscle appears to be active during mandibular
closing, the significance of this finding is not fully understood.

Influence of variation in anteroposterior occlusal contacts on electromyographic activity.
J Prosthet Dent 1989 May;61(5):617-23
The electromyographic activity from the elevator muscles with the anterior blocks was significantly less than with the intermediate and posterior occlusal blocks.

An electromyographic study of the inferior head of the lateral pterygoid muscle and the anterior belly of the digastric muscle during jaw-opening
Nippon Hotetsu Shika Gakkai Zasshi, 34(3):559-72 1990 Jun
The LPt was found to slide the condyle along the articular tubercles (translation) and the Da was moved the incisor region downward-backward for condylar rotation (hinging).

Electromyographic activity of the human lateral pterygoid muscle during contralateral and protrusive jaw movements.
Arch Oral Biol, 44(3):269-85 1999 Mar
These observations support the notion that the lateral pterygoid provides the principal driving force for moving the jaw forwards or laterally in protrusive or lateral excursive condylar movements. Further, the data suggest that the muscle plays a part in the fine control of jaw movements.

Fundamental properties of the human lateral pterygoid muscle activity and quantitative observation in relation to vertical dimension and bite force
Nippon Hotetsu Shika Gakkai Zasshi 1990 Jun;34(3):545-58
The purpose of this study is to investigate the fundamental property of the superior head of the lateral pterygoid muscle (LPT) of the human beings. From these findings it was concluded that superior head of LPT was qualitatively similar to but quantitatively distinct from jaw closing muscles such as masseter and temporal muscle.
(Ed.: Relevance: The superior lateral pterygoid contacts (stabilizes the disc)  during clenching (i.e., simultaneously with the temporalis), thereby possibly allowing straining and  when occuring druing translation on a non-working side interference..)

Development of trigeminal mesencephalic and motor nuclei in relation to masseter muscle innervation in mice.
Brain Res Dev Brain Res 1998 Jun 15;108(1-2):1-11
Cells located in motV innervate muscles of mastication while the mesV nucleus contains populations of primary afferent cells that innervate muscle spindles in jaw closing muscles and periodontal mechanoreceptors around the roots of teeth.

Mastication-related neurons in the orofacial first somatosensory cortex of awake cats.
Somatosens Mot Res 1997;14(2):126-37
The activities of perioral rhythmical-mastication-related neurons (MRNs), mandibular transient-MRNs, tongue rhythmical-MRNs and periodontal transient-MRNs were correlated with food texture, while perioral rhythmical-MRNs, perioral sustained-MRNs and tongue sustained-MRMs were not.

Relationship between occlusal contacts and jaw-closing muscle activity during tooth clenching: Part I
J Prosthet Dent 1984 Nov;52(5):718-28
Vertical clenching efforts in the natural or simulated intercuspal position generally showed the highest muscle
activities for all the muscles recorded. When the contact point moved posteriorly along the arch from incisors to
molars, the activity in the ipsilateral temporal muscles was seen to increase, while the activity in the ipsilateral medial pterygoid and the masseter muscles bilaterally was seen to decrease during vertical clenching tasks. Eccentric efforts on specific contact points generally  resulted in lower activity than the corresponding vertical effort. This was usually seen in all muscles, but not all values were significant. The ipsilateral temporal and contralateral pterygoid muscles showed the most activity during maximal clenches in lateral direction with little contribution from the other muscles. The temporal muscles showed the most activity in retrusive clenching, with activity in the other muscles nearly nonexistent. The medial pterygoid and masseter muscles were found to be the most active muscles during protrusive and incisal clenching, while the temporal muscle activity was low. The findings of this electromyographic study on change of the contact point, size of contact point, and the direction of effort applied on a contact point confirm their specific associations with the activity of muscle groups. Significant data have also been made available for a biomechanic approach of the investigation of degenerative joint changes. (Ed: i.e., "Degenerative Joint Disease" as a function of chronic muscular strain)

Effect of jaw opening on the direction and magnitude of human incisal bite forces.
J Dent Res, 76(1):561-7 1997 Jan
The results showed that: (1) the average % maximum bite force increased as the jaw was opened, reached a plateau between 14 and 28 mm of incisal separation...the activity of the masseter muscles declined and that of the temporalis muscles was largely unchanged, resulting in an increase of the ratio between the activity in temporalis and masseter muscles (T/M). (Ed: To grasp an object with the canine teeth, the mouth must open, thereby allowing canine teeth to engage the object while the temporalis muscles contract)

The feasibility of palpating the lateral pterygoid muscle
J Prosthet Dent 1980 Sep;44(3):318-23
CONCLUSION: Although dysfunction of the lateral pterygoid muscle could contribute to the pain associated with TMJ disorders, it has been demonstrated through the use of dissections and lateral head radiographs that it is not possible to palpate the lateral pterygoid muscle directly by conventional clinical techniques without applying pressure through the overlying superficial head of the medial pterygoid muscle. The possibility of confusing temporal muscle hypersensitivity with that of another muscle in this region is suggested.

Anatomical and electromyographic studies of the lateral pterygoid muscle.
J Oral Rehabil, 14(5):429-46 1987 Sep
 Strong to very strong activity was consistently observed in the superior head during clenching and tooth gnashing. The inferior heads were silent or had negligible to slight activity most of the time during ipsilateral movements or clenching, but were co-activated bilaterally, with strong to very strong activity during jaw opening, protrusion, swallowing, tooth gnashing and during passive retrusion. They showed marked activity unilaterally during contralateral movements.

The macroscopic and microscopic study of the human lateral pterygoid muscle
Tsurumi Shigaku, 15(1):1-26 1989 Jan
1) The lateral pterygoid muscles observed in this study were classified into three types based on the number of their heads; 65% of the macroscopic specimens had two heads (superior and inferior), 20% three heads (superior, inferior and medial), and
15% a single head.  2) The fascicular architecture of the single-headed specimens was quite simple and so the orientation of their fasciculi was parallel or radial to the line of pull. In their course the fasciculi of the two and three-headed specimens showed a relatively complicated architecture, having a large number of crossover fibers between each head.
(Ed: could complicated architecture facilitate disk displacement?)

Direction of a bite force determines the pattern of activity in jaw-closing muscles
J Dent Res 1994 May;73(5):1112-20
The direction of a bite force, not its magnitude, determines the pattern of activity of jaw-closing muscles. The shared patterns of muscle activation may be the result of a subconscious optimization of jaw muscle forces to    improve efficiency. (Ed: The orientation of the mandible, plus the intensity, frequency and duration of muscle contraction dictates degree of signs and symptoms)

Bilateral reflex fracture of the coronoid process of the mandible. A case report.
Int J Oral Maxillofac Surg, 28(3):195-6 1999 Jun
Bilateral fractures of the coronoid process of the mandible occurred following a blow to the left temporal region in an assault. There was no evidence of direct trauma and the zygoma and other facial bones were intact. The probable cause was acute reflex contraction of the temporalis muscles leading to bilateral stress coronoid fractures.  (Ed: Moral of the story: Don't mess with temporalis')