Adaptation of craniofacial muscles in subjects with craniomandibular disorders

The purpose of this study was to examine muscle function in subjects with muscle pain. Forty-three subjects with pain in the craniomandibular muscles, clinically determined by manual palpation, were studied for alteration in recruitment of temporalis, masseter, and suprahyoid muscles during a series of phasic movements. Seventeen normal subjects were used as controls. The subjects with muscle pain were divided into three subgroups: (1) those with pain in both mandibular and neck muscles; (2) those with pain in these two muscle groups with joint degeneration; and (3) those subjects with pain only in mandibular muscles. Surface electromyographic (EMG) recordings were taken as each subject performed 16 different responses in which mandibular incisor movement was tracked simultaneously. The results show that the subjects with muscle pain use their anterior temporalis muscles with less frequency (i.e., probability) and with less intensity in several responses than normal subjects. These responses include rapid vertical closing movements, retrusion, ipsilateral laterotrusion, and natural as well as contralateral mastication. The masseter muscle is impaired much less in its function, and the recruitment of the suprahyoid muscles is not affected in the patients with muscle pain. Comparison of the bilateral activity in the anterior temporalis muscles during intercuspal clenching shows that the subjects with muscle pain often demonstrate a more severe asymmetrical recruitment of these muscles than the more symmetrical recruitment seen in normal subjects. Similar observations were made for the masseter muscle. These studies demonstrate that subjects with muscle pain in craniomandibular muscles alter the recruitment of their jaw muscles, thus supporting the concept that the neuromuscular system is altered in patients with craniomandibular disorders.     

Medial pterygoid muscle activity during chewing and clenching

Specific activity of the human medial pterygoid muscle in relation to jaw movements and occlusion during mastication and clenching tasks is unclear. EMG activity of the medial pterygoid, masseter, anterior, and posterior temporal muscles was recorded simultaneously with three dimensional incisor point movement of the mandible. Data were sampled and analyzed by an on-line computer system. Patterns of medial pterygoid muscle activity were consistent for ipsilateral chewing and demonstrated activity of the muscle on the chewing side that peaked near the onset of intercuspation. The muscle on the contralateral side was active at the onset of intercuspation for subjects with a chopping stroke and inactive for those with a more lateral stroke guided into intercuspation on cuspal inclines. Activity in the early part of the closing phase was associated with a marked jaw movement toward the chewing side. There was generally less activity in protrusive than intercuspal clenching when occlusion was normal but more activity if marked incisal wear was present or multiple tooth contacts could be attained on protrusion. Intercuspal clenching initiated less activity when force was directed posteriorly and more activity when directed anteriorly than vertical intercuspal clenching.     

Effects of occlusal splints on the asymmetry of masticatory muscle activity during maximal clenching

summary The effects of stabilization splints on the electromyographic activity patterns of the masseter and anterior temporal muscles during maximal clenching in healthy subjects and subjects with different types of stomatognathic dysfunction syndrome were investigated. No marked effect of splints on the asymmetry of muscle activity during bilateral clenching was revealed immediately after splint insertion. During unilateral clenching in the intercuspal position (ICP), the relative asymmetry index (rAI) of the masseter muscles, detecting the imbalance of left and right muscular activity, was significantry increased. The use of splints suppressed the asymmetry of masseter muscle activity during unilateral clenching. This result suggests that the use of a splint is a method of suppressing clench-caused aggravation of stomatognathic dysfunction in the presence of an imbalance between left and right muscle activities.     

The effects of an occlusal splint on the electromyographic activities of the temporal and masseter muscles during maximal clenching in patients with a habit of nocturnal bruxism and signs and symptoms of craniomandibular disorders

The effects of a full arch maxillary plane occlusal splint on the level of electromyographic (EMG) activity in the anterior temporal and masseter muscles during maximal clenching were studied in 31 patients with a habit of nocturnal bruxism and signs and symptoms of craniomandibular disorders, before and after occlusal splint therapy. The results showed, before treatment, that the occlusal splint changed significantly (in 71% of patients) the level of EMG activity during maximal clenching. However, these changes were not consistent and differed between patients and even, in some patients, between muscles. After long-term occlusal splint therapy and improvement of the signs and symptoms of craniomandibular disorders, the number of patients who had an identical level of EMG activity during maximal clenching in the intercuspal position and on the occlusal splint tended to increase. Moreover, in these patients the level of symmetry of action in pairs of muscles during maximal clenching was strong, and the splint did not change this level of symmetry.     

Effects of difference of occlusal splint contacts on masticatory system]

The short-term effects of the difference of occlusal splint contacts on the jaw function were investigated on five healthy subjects. The maxillary stabilization splint (S-type) was fabricated and sectioned into three parts: an anterior section (A-type) and two posterior sections (P-type). These 3 types of splints were used for 10 days for each subject. The EMG activity of the masseter and the anterior and the posterior temporal muscles were measured during the maximum clenching in the intercuspal position and on the wearing splint. Subsequently on the bite force-measuring device with two transducers the bite and the EMG activity were measured during the maximum clenching, and the intercuspal occlusal contacts were recorded. The results were as follows: 1. After wearing the P-type, the total EMG activity during clenching in the intercuspal position was decreased, then increased after removal. 2. After wearing the S-type and the A-type the anteroposterior distribution of the bite force during clenching was changed, then returned after removal. 3. After wearing the A-type, the occlusal contact area of the anterior teeth in the intercuspal position was decreased, then increased after removal, while after wearing the S-type and the P-type that of the posterior teeth was decreased, then increased after removal.     

Longitudinal evaluation of jaw muscle activity and mandibular kinematics in young patients with Class II malocclusion treated with the Teuscher activator

Objectives: A longitudinal study was performed to evaluate the jaw muscle activity and mandibular kinematics after Teuscher activator treatment and at 2 years after orthodontic treatment completion. Material and Methods: Twenty-seven children with Class II division 1 malocclusion were evaluated before treatment (T0; mean: 11.6 years), after functional treatment (T1; mean: 12.8 years), and 2 years after orthodontic treatment (T2; mean: 18 years). Bilateral surface electromyographic activities of the anterior temporalis, posterior temporalis, masseter, and suprahyoid muscle areas were analyzed at rest and during clenching, swallowing, and mastication. Kinematic recordings of the mandibular maximum opening, lateral shift, right and left lateral excursions, and protrusion were evaluated. Results: Compared to T0, the left masseter activity during clenching was decreased at T1 but increased at T2, similar to the other evaluated muscles. The suprahyoid activity during swallowing was increased at T1 but decreased at T2. The masseter activity during mastication was increased at T1 and further increased at T2. The left and right lateral excursions and protrusion did not show significant changes throughout the experiment. Conclusions: Teuscher activator and subsequent fixed orthodontic treatment improved jaw muscle function; however, a long period was needed to attain complete neuromuscular adaptation. Key words:Class II malocclusion, jaw muscles, mandibular kinematics, sEMG, Teuscher activator.     

Electromyography of masticatory muscles in three jaw registration positions

The purpose of this study was to investigate whether anteroposterior changes in mandibular position affect masticatory muscle activity. The electromyographic (EMG) activity of masticatory muscles during full and partial (10%) clenching in three mandibular bite registration positions--retruded contact position (RCP), intercuspal position (IP), and muscular position (MP)--was studied. Three groups of subjects with different ranges of anteroposterior positioning of the condyles were evaluated (normal occlusion, Class II, Division 2 malocclusion, and dual bite malocclusion). A posterior stabilizing splint for each registration position was made. EMG data were obtained from three bilateral muscles (masseter, anterior temporal, and posterior temporal). Clenching in the RCP elicited the lowest masseter muscle activity during full clenching, and the highest anterior temporal and posterior temporal muscle activity during partial clenching. If the relationships of the masticatory muscles are analyzed through a ratio that represents the interaction between biting and positioning muscles (masseter/posterior temporal muscle ratio), the RCP as compared with other positions had the lowest ratio. The RCP required more positioning muscle activity and permitted less biting muscle activity. There was no significant difference in the muscle activity between the IP and MP registrations. Small changes in jaw position (anterior to RCP) are not critical for the masticatory apparatus provided there is good intercuspation. The results of this investigation suggest that intercuspation in RCP is not the optimal position.     

Occlusal adjustment and myoelectric activity of the jaw elevator muscles in patients with nocturnal bruxism and craniomandibular disorders

The effects of occlusal adjustments on the myoelectric activity of the anterior temporal and masseter muscles, with the mandible at rest and during maximal clenching in the intercuspal position (ICP), were studied in 24 nocturnal bruxism patients. Before the occlusal adjustment, the patients were treated with a flat occlusal splint for chronic craniomandibular disorders. The results revealed that within the short term the occlusal adjustment, in terms of increased number of occlusal contacts and teeth in contact in ICP, did not change the postural activity, whereas, on average, the level of activation of the jaw elevators, in terms of normalized electromyogram, increased during maximal clenching in ICP. The increase of activity was more pronounced in the masseter muscle than in the anterior temporal muscle.     

The immediate effects of a stabilization splint on the muscular symmetry in the masseter and anterior temporal muscles of patients with a craniomandibular disorder

In 36 myogenous craniomandibular disorder patients, the immediate effects of a stabilization splint on the symmetry in the activities of the masseter and anterior temporal muscles during submaximal clenching at five clenching levels were investigated electromyographically. After the adjustment of the splint necessary at the time of delivery, 20 splints remained free from occlusal interferences throughout the treatment period and thus needed no further adjustment. These splints caused an immediate improvement in masseter muscle symmetry at the time of delivery (p less than 0.01). However, 16 splints needed further adjustment for occlusal interferences at the first recall, 2 weeks after delivery of the splint. These splints resulted in a small but statistically significant worsening in masseter muscle symmetry at the 10% clenching level (p less than 0.01). No such response was found for temporal muscle activity. The immediate changes in masseter muscle activity suggest that muscular symmetry is an objective basis in the evaluation of the treatment provided.     

Dental stability and maximal masticatory muscle activity

The electromyographical (EMG) activity of masticatory muscles during full clenching in the retruded contact position (RCP) and intercuspal position (IP) with and without a posterior stabilizing splint was studied. The linear envelope EMG signal from three bilateral muscles was recorded (masseter, anterior temporal and posterior temporal). Thirty-seven subjects were evaluated. Clenching in the RCP without the splint and with the presence of an unstable occlusal contact inhibited the masseter muscle activity and reduced anterior temporal and posterior temporal muscle activity. The masticatory muscle activity returned to normal when clenching in the RCP with a splint that permitted stability in the dentition. EMG activity was the same in the IP with and without the splint. The results indicate that the determinant of maximal masseter isometric muscle contraction is the amount of stability in the dentition rather than the jaw position. If the dentition takes the major role of stabilizing the mandible, i.e., there is good intercuspation, the masseter muscle can exert maximal isometric contraction. If the stability is not provided by the dentition, i.e., there is a premature contact, the jaw muscles must contribute to the stabilization and reduce the magnitude of the maximal contraction to avoid damage to the structures involved in the compensatory stabilization.     

Masticatory muscles. Part IV. The masticatory muscles do not work homogeneously

The temporalis and masseter muscles have a complex architectural design with large attachment areas. As a consequence, each of these muscles is capable of producing a large number of mechanical effects. In addition, within each muscle the muscle fibres and sarcomeres undergo different excursions during jaw movements. This leads to intramuscular differences in the possible excursion range and the capacity to produce force. Depending on the desired motor task, the nervous system is able to vary both the magnitude and direction of muscle force by selective activation of specific muscle portions. The anterior temporalis and the superficial and deep masseter are capable of generating large forces, e.g. during biting and chewing. The posterior temporalis and the anterior and posterior deep masseter are suited to establish a precise adjustment of forces and movements during laterotrusion and protrusion/retrusion.     

The immediate effect of hard and soft splints on the EMG activity of the masseter and temporalis muscles.

The aim of this study was to compare the effects of hard and soft splints on the activity of the anterior temporalis and masseter muscles. Surface EMG recordings were made from these muscles during clenching at 10% of maximum, 50% of maximum and at maximum clench, both before and after insertion of a hard splint. This sequence was then repeated with a soft splint. The relative level of activity in the anterior temporalis and masseter muscles at all three activity levels was quantified by means of an Activity Index, which provides a measure of the balance of activity in the masseter relative to the activity in the anterior temporalis muscle. It was found that hard splints led to a decrease in EMG activity in relation to activity with no splint in both muscles at maximum clench and particularly the anterior temporalis. Soft splints produced a slight increase in activity of both muscles, but particularly the masseter muscle. The Activity Index indicated a shift in the balance of activity away from the anterior temporalis muscles with both splints, particularly at 10% of the maximum clenching level. It is possible that the decrease in activity of the temporalis muscles relative to the masseter muscles may be a factor in the therapeutic effect of both a hard and a soft splint, although the decrease is clearly greater with the hard splint.     

Short-term effect of a stabilization splint on the asymmetry of submaximal masticatory muscle activity

Ten healthy subjects continuously wore equilibrated maxillary full-arch stabilization splints in the retruded position for 7 days. The muscular activity balance of the masseter muscles during submaximal isometric clenching at 10% and 50% of the maximum voluntary contraction (MVC) did not change immediately on insertion of the splint, but was improved at the 50% level after 7 days (P less than 0.05). While the muscular balance of the anterior temporal muscles was not affected, either immediately on splint insertion, or after wearing it for 7 days, temporal muscle activity at 10% of the MVC was greater on the side to which the mandible moved from the retruded contact position (RCP) to the inter-cuspal position (ICP), both before (P less than 0.025) and after (P less than 0.01) wearing the splint. Splint removal after 7 days resulted in increased awareness of interferences in the ICP and increases in masseter muscle asymmetry (10%, P less than 0.025; 50%, P less than 0.05) when the electromyograms in the ICP after splint removal were compared with those on the stabilization splint before removal. After wearing the splint, the masseter muscle activity at the 10% level was greater on the side where premature contacts were present in the RCP (P less than 0.01). The use of masticatory muscle asymmetry indices in the evaluation of splint treatment for craniomandibular dysfunction is indicated since submaximal masticatory muscle activity is related to occlusal stability, premature contacts in the RCP and the direction of lateral slides from the RCP to the ICP.     

Effect of occlusal splint thickness on electrical masticatory muscle activity during rest and clenching

The extent of separation between the maxillary and mandibular teeth in the fabrication of interocclusal splints designed to achieve efficiency and muscle relaxation is controversial and undefined in the literature. Based on this premise, the aim of this study was to evaluate the effect of interocclusal splint thicknesses of 3 and 6 millimeters on the electrical activity of the anterior temporal and masseter muscles during rest and dental clenching. Twenty asymptomatic individuals (10 males and 10 females) were selected using the Research Diagnostic Criteria (RDC). Electromyography (EMG) was performed both with and without the 3- and 6-mm splints using the Bio EMG software package, which recorded values given in microvolts (μV). The results, which were assessed using analysis of variance (ANOVA) to a 5% significance level (p < 0.05), showed increased electrical activity of the masticatory muscles during dental clenching compared with at rest, with greater activity in the masseter muscle. The electrical activity did not differ according to the thickness of the splints or between males and females. We can conclude that both splint thicknesses are effective in treating muscle hyperactivity given their similar clinical behavior for asymptomatic individuals.     

[牙合]垫对牙齿重度磨耗患者口颌功能影响的临床研究

目的：研究殆垫对牙齿重度磨耗患者口颌功能的影响。方法：用殆垫恢复10例牙齿重度磨耗患者的垂直距离,分别检测治疗前、治疗后1个月、3个月、6个月的咬合平衡性、咬肌及颞肌前束的肌电幅值、颌位及殆的稳定性以及边缘运动的平滑度及对称性的变化。结果：①咬合平衡性、颌位及胎的稳定性以及边缘运动轨迹的平滑度及对称性在治疗后呈逐渐改善趋势,治疗6个月后80％的患者恢复正常;②治疗后各组的息止位肌电幅值均较治疗前显著降低（P〈0．05）,正中颌位紧咬时肌电幅值在治疗后3个月组和6个月组较治疗前显著增加（P〈O．05）。结论：通过6个月的治疗和观察,聆垫对重度磨耗患者的口颌功能的影响呈现持续改善趋势。     

Clenching and Grinding: Effect on Masseter and Sternocleidomastoid Electromyographic Activity in Healthy Subjects

Abstract

This study compares the effect of clenching and grinding on masseter and sternocleidomastoid electromyographic (EMG) activity during different jaw posture tasks in the sagittal plane. The study included 34 healthy subjects with natural dentition, Class I bilateral molar Angle relationship, and absence of posterior occlusal contacts during mandibular protrusion. An inclusion criterion was that subjects had to be free of signs and symptoms of any dysfunction of the masticatory system. Bipolar surface electrodes were located on the right masseter and sternocleidomastoid muscles. EMG activity was recorded while the subjects were in standing position, during the following jaw posture tasks: A. maximal clenching in the intercuspal position; B. grinding from intercuspal position to edge-to-edge protrusive contact position; C. maximal clenching in the edge-to-edge protrusive contact position; D. grinding from edge-to-edge protrusive contact position to intercuspal position; E. grinding from retrusive contact position to intercuspal position. EMG activities in tasks B, C, D, and E were significantly lower than in task A in both muscles (mixed model with unstructured covariance matrix). EMG activity among tasks B, C, D, and E did not show significant differences in both muscles, except between tasks D and E in the masseter muscle. A higher effect was observed on the masseter than on the sternocleidomastoid muscle to avoid excessive muscular activity during clenching and grinding. The EMG patterns observed could be of clinical importance in the presence of parafunctional habits, i.e., clenching and/or grinding.     

An evaluation of anterior temporal and masseter muscle activity in appliance therapy

Needle electromyography (NEMG) was used to compare the effects of the Hawley bite plane (HBP) and superior repositioning splint (SRS) on the ipsilateral masseter and anterior temporal muscles at rest and in maximum closure. Twenty adult subjects were randomly divided into two groups of 10. One group wore the HBP for 8 hours per day and the other group wore SRS for the same time period. After 2 weeks of use, NEMG was conducted on the ipsilateral masseter and temporal muscles with and without the device in place. Each group then wore the other appliance for an identical period and underwent NEMG. Both appliance designs produced decreases in electromyographic responses during use. Individuals in the HBP group showed a 48.6% decrease in anterior temporal activity and a 34.1% decrease in masseter muscle activity. The SRS group demonstrated 23.5% and 16.2% decreases in anterior temporal and masseter muscles activities, respectively. This is the first report of the use of NEMG as a mode of assessment of muscle activity in human or animal subjects.     

Bite force and electromyograpy during maximum unilateral and bilateral clenching

Maximum voluntary bite force has often been studied as an indicator of the functional state of the masticatory system. Bilateral, as well as unilateral, methods have been used to determine bite force. Only a few studies have compared the outcomes of both methods. The aim of this study was to measure bite force and jaw-muscle activity during bilateral as well as unilateral maximum clenching in a large number of healthy subjects, so that the results could be compared. In a group of 81 dentate subjects we observed an average bilateral bite force of 569 N. The average unilateral bite force was significantly lower, being 430 N (right) and 429 N (left). Masseter and anterior temporal muscle activities were also significantly lower during unilateral clenching as compared with bilateral clenching. The masseter muscles showed no difference in activity between the ipsilateral side and the contralateral side during unilateral clenching. In contrast, the activity of the anterior temporal muscle on the ipsilateral side was significantly higher than on the contralateral side. Thus, the change in the forces acting on the jaw during unilateral clenching compared with bilateral clenching leads to a different response in the temporal muscles than in the masseter muscles.     

EMG response to alteration of tooth contacts on occlusal splints during maximal clenching

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. When left-sided muscles were compared with right-sided muscles for the same situation, there was no significant change. When maximum clenching was performed with all left-sided teeth removed from contact, there was a decrease of 21% in overall muscle activity compared with that of an equilibrated occlusal splint. When left-sided muscles were compared with right-sided muscles for the same situation, there was no significant change. Changes in the number of tooth contacts did not cause changes in the overall muscle activity during maximum clenching. 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.