Despite the extensive use of oral splints in the treatment of temporomandibular disorders (TMD) and bruxism, their mechanisms of action remain controversial. Various hypotheses have been proposed to explain their apparent efficacy (i.e., true therapeutic value), including the repositioning of the 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. However, the results of a controlled clinical trial lend support to the effectiveness (i.e., the patient's appreciation of the positive changes which are perceived to have occurred during the trial) of the stabilizing splint in the control of myofascial pain. In light of the data supporting their effectiveness but not their efficacy, oral splints should be used as an adjunct for pain management rather than a definitive treatment. For sleep bruxism, it is prudent to limit their use as a habit management aid and to prevent/limit dental damage potentially induced by the disorder. Future research should study the natural history and etiologies of TMD and bruxism, so that specific treatments for these disorders can be developed.Key words. Oral splints, temporomandibular disorders, bruxism, myofascial pain, disc displacement disorders. (I) IntroductionAmong the treatments provided for temporomandibular disorders (TMD), intra-oral dental appliances, whether with full or partial occlusal coverage, and referred to in this paper as oral splints, have been repeatedly reported as being the most widely adopted choice. Introduced by Karolyi in 1901 (see Ramfjord and Ash, 1994) for the treatment of bruxism, it is striking to see the versatility of their current applications. Other than their use in the prevention of dental injuries and oral soft-tissue trauma potentially induced by bruxism, sports, cheek biting (Walker and Rogers, 1992), and electroconvulsive therapy (Minneman, 1995), oral splints of various designs have been prescribed in the management of diverse disorders including: (a) motor disorders such as Parkinson's disease (Durham et al., 1993) and oral tardive dyskinesia (Kai et al., 1994); (b) sleep disorders such as snoring (George, 1993) and sleep apnea (George, 1993;Athanasiou et al., 1994;Lowe, 1994;Yoshida, 1994;Osseiran, 1995); (c) sensitive teeth related to chronic sinusitis (Dawson, 1974); (d) various headaches, from the tension-type to migraine (Ouayle et al., 1990;Lamey and Steele, 1996); and (e) all subgroups of TMD, e.g., myofascial pain, disc displacement disorders, and the arthritides (Table 1).It is also surprising to see the wide acceptance of oral splints and their "multi-purpose usage", while little is known about the mechanisms by which they exert their effect. For the TMD, a survey of 10,000 members of...

1. Mastication was produced by stimulation of the right motor-sensory cortex of urethan-anesthetized rabbits with 15-s trains of shocks (1-ms duration) at 50 Hz. Movements of the lower jaw and jaw muscle electromyograms (EMGs) were recorded on magnetic tape for later computer analysis. 2. The stimulus site was chosen, and stimulus intensity adjusted, so that stereotyped movements were produced that included a wide swing of the mandible to the left side during jaw closure. 3. Control trials were alternated with trials in which a steel ball (2 mm diam) was thrust between the anterior molar teeth on the left side and left in place for several seconds. 4. When the obstruction was first introduced, a jaw opening reflex was sometimes evoked if the ball struck the buccal surface of the advancing mandibular molar teeth. Thereafter, when the ball was crushed between the occlusal surfaces of the teeth, no jaw opening reflex was seen. 5. Instead, the amplitude and duration of all the jaw closing EMGs increased, beginning at least 12 ms after contact with the ball. This caused a prolongation of the slow closing (SC) phase of the cycle that, coupled with a delay in the start of activity in the digastric muscle (jaw opener), prolonged the cycle by more than 60 ms. 6. During the SC phase of the obstructed trials, the medially directed grinding stroke was exaggerated because of an increase in the contraction of the contralateral zygomaticomandibular and anterior temporal muscles. 7. After collecting data, the sensory nerves to the maxillary and mandibular anterior molar teeth were cut to eliminate feedback from the periodontal pressoreceptors. Control and obstructed trials were repeated. 8. Following denervation, the obstructed cycles were of shorter duration. The mandible still moved to the right during SC in some animals, but the increase in closer muscle EMG activity was much reduced. 9. We conclude that periodontal receptors provide positive feedback to the jaw closing muscles during mastication. This is supplemented by input from other receptors, probably muscle spindles. In addition, an increase in periodontal feedback prolongs the SC phase and the early phases of the opening stroke.

Cervical spinal pain is frequently found in conjunction with idiopathic cervical dystonia (ICD), a focal dystonia characterized by sustained deviation of the head. Since the perception of noxious stimuli has never been studied in ICD, we performed a controlled study to obtain more insight into the psychophysics of dystonia-related muscle pain by evaluating pressure-induced pain levels. In nine ICD patients and five gender- and age-matched asymptomatic control subjects, pain-pressure thresholds (PPTs) were determined in the sternocleidomastoid and upper trapezius muscles, both at resting activity and at maximal voluntary contraction (MVC). The masseter muscles served as non-pathological control regions. To determine the accuracy of PPT values, pain intensity and unpleasantness were rated at threshold on 100-mm visual analogue scales. Four replication measurements were obtained. The data were analyzed by multilevel procedures. For all muscles under investigation, average PPTs of the ICD patients were about two times lower than those of the control subjects (P < 0.001-0.0005) and showed a smaller intra-subject variance. Further, average PPTs at MVC were about two times higher than those at resting activity (P < 0.005). These results provide psychophysical evidence to suggest that, at controlled levels of muscle contraction, the threshold of pain perception is decreased in ICD. In addition, ICD patients seem to be better able to establish their own PPTs than control subjects, which might be due to a different setting of the discriminative aspect of pain in ICD. Surprisingly, lower intensity and unpleasantness scores were found in ICD patients with coinciding painful and deviated sides than in ICD patients for whom the painful side was opposite to the deviated one (P < 0.05). This finding might be of clinical importance for defining functional disability and predicting treatment outcome.

Objective: The interactions between sleep, neck muscle activity, and cervical spinal pain were examined in a controlled study with nine patients suffering from idiopathic cervical dystonia (ICD; also referred to as spasmodic torticollis), and nine gender-and age-matched controls. Methods: From each participant, two all-night polysomnograms with additional electromyographic recordings from the sternocleidomastoid and upper trapezius muscles were obtained. The first night was for habituation to the laboratory environment; the second night for experimental data collection. Visual analogue scales were used to collect intensity and unpleasantness ratings of cervical spinal pain before and after the second sleep recording. Results: None of the standard sleep variables showed statistically significant differences between average values of both groups of participants. However, a significantly larger variance in sleep latency was obtained for the ICD patients. In general, abnormal cervical muscle activity decreased immediately when lying down without the intention to go to sleep. Subsequently, abnormal muscle contractions were gradually abolished in all ICD patients during the transition from relaxed wakefulness to light NREM sleep. Following this transition phase, no more abnormal EMG activity was found in any of our patients. Finally, cervical spinal pain intensity and unpleasantness were reduced by about 50% overnight. Conclusions: Both supine position and sleep can be associated with an improvement of symptoms of ICD, and this disorder does not induce any sleep perturbations. RESUME: Interrelation entre sommeil, activity musculaire et douleur dans le cas de dystonie cervicale. Objectif: Le but de cette 6tude 6tait de mettre en evidence les liens possibles entre le sommeil, l'activite musculaire (trapeze et sterno-cl6ido-mastoi'dien) et la douleur cervicale chez neuf patients prdsentant une dystonie cervicale idiopathique (torticolis spasmodique) par rapport a neuf sujets tfimoins, appari6s pour l'age et le sexe. Methode: L'activite polysomnographique et musculaire ainsi que la douleur ont e;t6 dtudi6es. Seules les donnees de la deuxieme nuit sont prSsentdes, la premiere nuit servant d'adaptation aux conditions expdrimentales. Resultats: Aucune des variables de sommeil n'a permis de differencier les deux groupes. Toutefois, la variance de la variable "latence du sommeil" fitait significativement plus importante chez les patients ayant une dystonie cervicale idiopathique. Ces memes patients ont d6montr6 une chute de l'activite' 61ectromyographique cervicale anormale, ce en position couchee sans intention de dormir. Cette activity diminuait graduellement au d£but de l'endormissement et aucune activity anormale ne subsistait lorsque la premiere peYiode de sommeil "lent lfiger" a dtd atteinte. Enfin, il est a noter qu'une diminution d'environ 50% de la douleur a 6t6 rapport6e, sur une dchelle visuelle analogue, en relation avec la baisse d'activit6 musculaire en cours de sommeil. Conclusions: La position couchee...

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