Jaw Elevator Research Articles

Surface EMG of jaw elevator muscles: effect of electrode location and inter‐electrode distance

This study addresses methodological issues on surface electromyographic (EMG) signal recording from jaw elevator muscles. The aims were (i) to investigate the sensitivity to electrode displacements of amplitude and spectral surface EMG variables, (ii) to analyse if this sensitivity is affected by the inter-electrode distance of the bipolar recording, and (iii) to investigate the effect of inter-electrode distance on the estimated amplitude and spectral EMG variables. The superficial masseter and anterior temporalis muscles of 13 subjects were investigated by means of a linear electrode array. The percentage difference in EMG variable estimates from signals detected at different locations over the muscle was larger than 100% of the estimated value. Increasing the inter-electrode distance resulted in a significant reduction of the estimation variability because of electrode displacement. A criterion for electrode placement selection is suggested, with which the sensitivity of EMG variables to small electrode displacements was of the order of 2% for spectral and 6% for amplitude variables. Finally, spectral and, in particular, amplitude EMG variables were very sensitive to inter-electrode distance, which thus should be fixed when subjects or muscles are compared in the same or different experimental conditions.

Surface electromyographic response of the neck muscles to maximal voluntary clenching of the teeth

The purpose of this study was to evaluate the activity of digastric, sternocleidomastoid, upper trapezius, lower trapezius and cervical muscles in response to maximum voluntary clenching (MVC) of the teeth. Twenty healthy subjects aged 24-31 years were divided into two groups of 10: a resting group and a clenching group. The resting group simply maintained the mandible at rest with teeth apart for two 1-min periods, whereas the clenching group underwent an initial resting period followed after 1-min by a clenching period during which they were required to perform MVC. The electromyography (EMG) responses of each muscle were recorded during all periods, and means and standard deviations of the root mean square EMG were calculated. Within-group and between-group comparisons of EMG activity were made using, respectively, the Sign two-tailed test for two related samples and the Kolmogorov-Smirnov two-tailed test for two independent samples (significance level 0.05). Data indicated a significant increase of EMG activity of the anterior neck muscles (digastric and sternocleidomastoid) during MVC as compared with EMG recorded during resting periods. If confirmed by further research, our findings suggesting an involvement of anterior neck muscles during isometric contraction of the jaw elevator muscles may have clinical implications.

Effect of conventional TENS on pain and electromyographic activity of masticatory muscles in TMD patients

Temporomandibular disorders (TMD) are characterized by several signs and symptoms, such as pain and changes in the electrical activity of masticatory muscles. Considering that transcutaneous electrical nerve stimulation (TENS) is a resource indicated to promote analgesia, the objective of this study was to evaluate the effect of TENS on pain and electromyographic (EMG) activity of the jaw elevator muscles in TMD patients. This study evaluated 35 female volunteers: 19 TMD patients (mean age = 23.04 +/- 3.5) and 16 normal subjects (mean age = 23.3 +/- 3.0). Transcutaneous electrical nerve stimulation (conventional mode, 150 Hz) was applied once to each group for 45 minutes. Surface electromyography (gain of 100 times and 1 kHz sampling frequency) and the visual analogue scale (VAS) were applied before and immediately after TENS application. Both VAS data and root mean square (RMS) values were analyzed using Student's t-test. The TMD group, compared to the control group, showed higher EMG activity of the jaw elevator muscles at rest. No difference was observed between the groups regarding maximum voluntary clenching (MVC). In TMD patients, TENS reduced both pain and EMG activity of the anterior portion of the temporal muscle, increasing the activity of the masseter muscles during MVC. It is possible to conclude that a single TENS application is effective in pain reduction. However, it does not act homogeneously on the features of the electric activity of the muscles evaluated.

Salivary Cortisol Level, Salivary Flow Rate, and Masticatory Muscle Activity in Response to Acute Mental Stress: A Comparison between Aged and Young Women

Background: Age-related effects on physiological stress reactions regarding changes in salivary cortisol concentration, saliva flow rate, and masticatory muscle activity, as well as the corresponding perceived mental stress and performance in response to acute stressors, have not yet been fully described. Objective: Thus, the aim of this study was to assess the age-related variations in these variables in response to minor acute and naturalistic stressors in terms of computer tasks. Methods: 13 aged (60–70 years old) and 13 young (20–30 years old) women with frequent practice and long experience with computer use were recruited by personal contact and flyers. The subjects were healthy and had full dental arches and no orofacial pain. The computer tasks were randomized and comprised a mentally demanding, modified Stroop colour-word test (CWT) and a less demanding reference test, both with a duration of 20 min and with equal physical demand. Visual analogue scales for global assessment of mental stress and perceived task difficulty and performance, measurements of saliva flow rate and cortisol concentration (unstimulated whole saliva), as well as surface electromyography of the temporalis and masseter muscles were used for assessment, and Spearman correlation analysis and Anova with repeated measures were used for statistical evaluation. Results: The perceived task-related stress and task difficulty were significantly higher and the personal satisfaction with the task performance significantly poorer in the aged women. The cortisol concentration, indicating the stress level, showed a small, but significant increase in response to the tasks. Also the saliva flow rate increased. This response was most pronounced in the aged and during the CWT. The average electromyogram varied significantly between age groups and tasks, with higher levels and shorter relative periods with gaps in the aged women and in the CWT. In addition, the peak activity of the jaw elevator muscles at mouse clicking was significantly elevated as a form of co-activation or attention-related activity. Conclusions: The study showed marked differences in the response to mental demands in aged as compared with young women. The mental stress, reflected by increases of salivary cortisol concentration, saliva flow rate, visual analogue scale ratings, and activity level of the jaw elevator muscles, was more pronounced in the aged women in response to the computer tasks.

A prospective, placebo-controlled, randomized, parallel group study of botulinum toxin type a for the treatment of mild to moderate platysmal bands

This double-blind study evaluated the safety and efficacy of botulinum toxin type A for managing mild to moderate platysmal bands in male and female patients ages 25 to 55. Platysmal bands scores had to be > +2 (thin, mild flaccidity) on the Platysmal Band Prominence Scale (PBPS). Patients meeting all eligibility criteria were randomized to receive intramuscular injections of botulinum toxin type A (n = 21) in sterile normal saline (35 U to 75 U total dose) or sterile normal saline placebo (n = 19). The study included a baseline (BL) screening/injection visit and follow up visits at 2, 4, 8, 12, 16, 24, and 32 weeks. The primary efficacy endpoint was the 5-point (0 = non-detectable to 4 = heavy, severe flaccidity, hypertrophy at rest) PBPS rated by the physician observer. Clinical improvement was defined as a shift of ≥1 point from BL to 4 weeks post treatment. Secondary endpoints included additional physician observer scales (horizontal rhytids, jaw elevation), and patient, physician, and objective observer global assessments. Treatment-emergent adverse events (AEs) were monitored throughout. There were no significant differences between the 2 treatment groups in PBPS scores at BL. A significantly higher % of botulinum toxin type A-treated patients had lower PBPS scores from week 2 to week 16 compared to placebo-treated patients (p < .05). The beneficial effect of botulinum toxin type A on this primary efficacy endpoint lasted up to week 16. At week 4, 47.7% of botulinum toxin type A-treated patients and 15.8% of placebo-treated patients achieved clinical success (p < .05). From week 2 to week 16, a higher % of patients in the botulinum toxin type A group showed improvement on the PBPS than did those in the palcebo group. A significantly higher proportion of patients in the botulinum toxin type A group improved compared to placebo-treated patients when scores were categorized as improved versus unchanged or worsened (p < .05). Differences from week 2 to week 16 were also observed between the 2 groups in the secondary efficacy endpoints, although not all were statistically significant. No AEs were reported. These results demonstrate that botulinum toxin type A is effective and safe for the treatment of mild to moderate platysmal bands.

Trigger Points as a Cause of Orofacial Pain

SUMMARYOrofacial pain is a common symptom experienced by approximately one-quarter of the adult population and is most often caused by myofascial pain. In contrast to other postural body muscles, trigger points are not often found in the jaw elevator muscles. Therefore, the term myofascial pain used in this article refers to a muscle pain condition characterized by the presence of muscle tenderness.The masticatory myofascial pain, that may be triggered by oral parafunction during the awake state, often fluctuates with pain episodes followed by pain-free intervals. Especially non-chronic light to mild masticatory myofascial pain often resolves spontaneously. In each case non-chronic pain can be easily treated by a therapeutic plan that includes patient education, self-care, physiotherapy, pharmacotherapy, and eventually an occlusal appliance. Only in a small percent of patients does the masticatory myofascial pain become chronic. These patients need a multimodal therapeutic approach that addresses both the...

Alteration in mandibular posture by vibration of posterior fibres of the temporalis muscle

Introduction: Vibration causes reflex muscle shortening because of excitation of muscle spindle primary afferents. Although this is subordinate to voluntary control, responses have been demonstrated in jaw elevator and (unexpectedly) in jaw depressor muscles. Forward posturing of the mandible is a common habit. The only part of the perioral musculoskeletal system that actively retrudes the mandible is the posterior part of the temporalis muscle. This study set out to investigate the effect of vibration of this area on the horizontal position of the mandible during a jaw registration procedure. Method: In a randomized cross‐over study, the posterior fibres of both left and right temporales of 17 subjects were vibrated synchronously at 180 Hz. Interocclusal wax jaw registrations were recorded for each subject with and without vibration applied. Position of the mandible was assessed by mounting pairs of casts using these registrations and measuring their position relative to intercuspal position. Results: In 17 subjects, the effects of vibration on mandibular posture were: no change in one; protrusion in two; and retrusion in 14. A sign test returned a value of P &lt; 0·01. Discussion: Reasons for the protrusion seen in two subjects are discussed. These may be anatomical or methodological. However, vibration of the posterior part of the temporalis muscle retruded the mandible in a significant majority of cases.

A functional analysis of jaw suspension in elasmobranchs

The morphology of the jaw suspension and upper jaw are quantified and related to upper jaw protrusion in five elasmobranchs possessing four jaw suspension types: broadnose sevengill, Notorynchus cepedianus, orbitostylic – amphistylic; spiny dogfish, Squalus acanthias, orbitostylic; bonnethead, Sphyrna tiburo, and lemon, Negaprion brevirostris, hyostylic; and Atlantic guitarfish, Rhinobatos lentiginosus, euhyostylic. The results indicate that jaw suspension type is not a good predictor of jaw mobility as previously thought. Instead, the best morphological predictor of jaw mobility appears to be either a long ethmopalatine ligament or no ligament, both of which allow the upper jaw to project further from the cranium. The morphology of the palatoquadrate–cranial articulation is key in determining the mobility of the jaws indicating that it should be included in descriptions of jaw suspension states. One advantage of upper jaw protrusion appears to be a reduction in the time to jaw closure by closing the mouth dorsally by upper jaw protrusion as well as ventrally by lower jaw elevation. A table of revised jaw suspension types is presented to illustrate the phylogenetic differences among gnathostome groups. Jaw suspension types mapped onto a gnathostome phylogeny support the evolution of holostyly and hyostyly from an autodiastylic ancestral condition. © 2002 The Linnean Society of London, Biological Journal of the Linnean Society, 2002, 75, 483–502.

Electromyographic Activity of Masticatory Muscle in Sheep.

The masseter muscle of the Australian Merino sheep consists of three layers, superficial (MS), middle (MM) and deep (MD). We studied the EMG activities of jaw elevator muscles including the three layers of the masseter muscle during mastication. The EMGs were recorded with a bipolar electrodes of stainless wire which was coated with enamel, and from ten muscles as follows, temporal muscle (T), MS, MM, MD and medial pterygoid muscle superficial fiber (MP) on both sides. Masticatory movements were induced by feeding. Three types of feed were given ; grasses as a fibrous type, oats as an elastic type and sheep nuts as a solid type. In T, MD and MM, the discharge volumes of the EMGs on the working side were larger than those on the balancing side, but in MS and MP, the durations on the working side were smaller than those on the balancing side. The starting times of activities of these muscles in mastication were in sequence, MD, MM and MS on the working side, MD, MS and MM on the balancing side. The starting time of MD on the balancing side in mastication arose earliest. Therefore, the activity patterns of these muscles reveal which is the working side and which is not. It is recognized that the activities of the three layers of masseter muscle differed from one another in mastication.

A clinical and physiological evaluation of prosthodontic treatment by removable partial dentures with free-end saddles--a 3.5-year follow-up study

The aim of this study was to evaluate the longitudinal clinical and physiological changes in jaw elevator muscle activity of partially edentulous patients, for approximately 3.5 years after treatment by removable partial dentures with free-end saddles. Seven patients (4 males and 3 females, aged 38-68 years) who visited a university dental hospital, and were found to have no signs and symptoms of TMJ disorders, participated in the follow-up study. The functional impression method with wax was used in the treatment process of these patients. At the follow-up times of 0.5, 1, 2, 3, and 3.5 years after denture deriverly, EMG activities using bipolar surface electrodes were recorded from masseter and anterior temporal muscle on both sides during maximal voluntary clenching and tapping, and at the same time the maximal bite force was also recorded using the Dental Prescale System. The integrated EMG, the correlation coefficient between muscle activities of both sides, bite force, and the location of the occlusal load center, were analyzed using repeated measured ANOVA. A comparison between each measuring session was carried out using Bonferroni's method. Results were as follows: i) The muscle function of partially edentulous patients was clearly activated by wearing a removable partial denture, and the activation was kept at the same level until 3.5 years after denture deriverly. ii) The activation of jaw elevator muscles by wearing a denture was greater in masseter muscle than in anterior temporal muscle. This finding reflected the effect of the recovered occlusal contact. iii) The occlusal load center of the recovered dentition by a removable partial denture shifted antero-posteriorly close to the end abutment teeth.

The Role of the Postorbital Ligament in the Zebra Finch (Taeniopygia Guttata)

Cranial kinesis in birds has been the subject of many functional morphological investigations. One of the mechanisms proposed for avian jaw movement is a mechanical coupling of the upper and the lower jaw by the postorbital ligament. Upper jaw elevation is believed to enable lower jaw movement, which is blocked by the limited extensibility of the ligament. In a previous study it was shown that in the Zebra Finch the elastic force of the postorbital ligament is very small for low and medium amplitude gapes (< 7 mm) and does not block lower jaw depression in this species. However, these experiments were not suited to detect very small ligament forces and the jaw positions were imposed externally, which may affect the kinematics of the skull and thus the stretching of the postorbital ligament. In this study, we investigated in vivo whether the postorbital ligament plays a role during the husking of red millet seeds (diameter 2 mm; max gape 5 mm) in Zebra Finches (Taeniopygia guttata). We measured seed husking time, number of mandibulations, and percentage of rejected seeds before and after surgically removing part of the left and right postorbital ligament. Although no effect was expected removal of the ligament did change husking performance. Individual birds react differently to removal of the postorbital ligament. The data suggest that there is an inverse relationship between husking time and percentage of rejected seeds. Birds that reject more seeds after cutting the postorbital ligament have an average husking time that is similar or even lower than before surgery. When birds have the same rejection rate as before the treatment, husking time is significantly higher in the post-operative condition. Although the postorbital ligament primarily serves to limit the maximal extension of the lower jaw, it is speculated that small amplitude stretching of the ligament may play a role in the fine control of jaw opening during dorsoventral husking movements in estrildids.

Normal side-to-side variation of the exteroceptive suppression of masseter muscle in young dentate adults.

Previous studies have shown that various stimuli applied in the orofacial region, evoke bilateral inhibitory responses in the jaw elevator muscles. The exteroceptive supression (ES) of the masseter muscle after electrical stimulation of the mental nerve, often appears as a double phase of inhibition interrupting the voluntary sustained contraction of the muscle. The aim of the present study was to investigate the normal bilateral variation of the masseteric ES in a control group of 20 healthy dentate adults, and to determine the range of its boundaries. The reflex was elicited by electrical stimulation of the mental nerves, during maximum intercuspal clenching. Generally, the overall mean values for the latencies and durations were in line with those reported in previous investigations, while no significant differences were found between the left and the right masseter muscles, regardless of the side of stimulation. However, in the intra-individual analysis, significant bilateral variation was occasionally recorded, particularly in the latency and duration of the late exteroceptive suppression. It was concluded that the exteroceptive suppression of the masseter muscle is a repeatable and clear bilateral reflex response, although an increased number of observations is recommended in normal subjects, before its use could be extended for diagnostic purposes.

Electromyographic examination of jaw muscles in relation to symptoms and occlusion of patients with temporomandibular joint disorders.

Clinical/occlusal scores and jaw-muscle EMGs were recorded in 24 TMD symptomatic (group S) and 20 normal (group N) subjects to evaluate the significance of EMG parameters and their clinical associations. Results indicated: (1) integrated EMG activity (IEMG) was larger at the rest position (RP) in anterior temporalis (Ta) but smaller at maximal voluntary clenching (MVC) in masseter (Ma) and Ta, and the ratios of IEMG at 70%MVC to the corresponding bite force (70%BF) were greater in group S; (2) mean power frequency (MPF) were almost the same in both groups but its shift was more rapid in group S; (3) silent period duration (SPD) was longer in group S; (4) asymmetry indices for SPD and silent period latency (SPL) were larger in group S; (5) muscle pain was associated negatively with IEMG at MVC and 70%BF but positively with IEMGs at RP and 70%MVC, and impaired jaw movements were associated negatively with the above EMG values; (6) muscle pain was positively associated with SPD in Ma, while joint pain and sound showed positive and negative associations with SPD, respectively; (7) associations between occlusion and EMG parameters were found more in group N. These findings verify: (1) jaw elevators in TMD may have hyper-tonic activities and a weak functional efficiency; (2) jaw muscles in TMD may become easily fatigued following a functional effort, and less relaxed following a muscle twitch; (3) the severity of pain could not be reflected in EMG activities, but impaired jaw movement may increase tonic activity and decrease functional effort; (4) TMD symptoms may alter the functional adaptation of jaw-muscle activities and occlusion.

Muscle afferent block for the treatment of oromandibular dystonia.

Oromandibular dystonia is a focal dystonia involving the masticatory and tongue muscles, causing difficulties in speech or mastication. We treated 13 patients with this condition by injecting diluted lidocaine and alcohol intramuscularly. This method is aimed at reducing muscle spindle afferent activity. The symptoms had been resistant to other therapies such as pharmacotherapy or dental treatment. All patients showed clinical improvement after this therapy with reduced EMG activities in the affected muscles, whereas control injection of normal saline gave no changes in EMG activities. The overall subjective improvement was 57.7 +/- 25.1% (mean +/- standard deviation) in a self-rating scale. The mean response of the jaw elevator muscles (70 +/- 13.1%) was significantly higher (p < 0.02, t test) than that of the depressor muscles (38 +/- 28.4%). Despite the precise mechanism being unknown, this difference might be related to the smaller number of muscle spindles in the depressor than the elevator muscles. This therapy is useful for the treatment of drug-resistant oromandibular dystonia.

Functional properties of jaw and tongue muscles in rats fed a liquid diet after being weaned.

Decreased masticatory demands due to liquid or soft diets cause a reduction in the growth of craniofacial bones and in the development of feeding musculature, but the effects on masticatory function and jaw/tongue muscle activities are unclear. The present study was undertaken to test the hypotheses that a liquid diet feeding after weaning affects the critical-period programming of mastication and the motor performances of jaw and tongue muscles. Thirty-six male Wistar rats were divided into two equals groups at weaning and fed either a solid (solid-diet group) or a liquid (liquid-diet group) diet until they reached 50 days of age. Electromyograms (EMG) of the masseter, medial pterygoid, temporalis, anterior digastric, styloglossus, and genioglossus were recorded while animals were naturally ingesting ordinary pellets, apple cubes, and a liquid diet. It was found that: (1) a more irregular chewing rhythm, a shorter chewing sequence, and a longer chewing cycle were found in the liquid-diet group, but there were no differences observed during lapping/licking between the two groups; (2) during the chewing cycles, the EMG onset of each muscle in relation to that of the masseter in the liquid-diet group was similar to that in the lapping/licking cycles in both groups; (3) the activities of jaw elevators (masseter, medial pterygoid, and temporalis) during the chewing cycles were significantly higher in the liquid-diet group; and (4) the increase in the EMG activities of jaw elevators during pellet chewing compared with apple cube chewing was significantly weaker in the liquid-diet group, whereas such an enhancement was found simultaneously in the styloglossus in the solid-diet group, and in the anterior digastric in the liquid-diet group. These findings verify that: (1) the motor output of jaw and tongue muscles may be altered in rats fed a liquid diet after being weaned; (2) the feeding of a liquid diet to rats after being weaned may obstruct the functional transition from suckling to mastication; and (3) jaw elevators that develop without motor learning of mastication are inefficiency when performing functionally.

Influence of the location of the bite point on the electrical efficiency of human jaw elevator muscles.

Bite force decreases as bite points change posteroanteriorly along the dentition; this raises questions about how the efficiency of the jaw elevator muscles is affected. The objective of this study was to determine the relation between bite force and the EMG/Force characteristics of the jaw elevator muscles as the bite point is changed posteroanteriorly during maximal (MVBF) and submaximal (30%MVBF) isometric clenching efforts. Seven fully dentate subjects without any history of craniomandibular dysfunction, mean age 30.8±2.4 volunteered for this study. Bilateral surface bipolar EMG activity from the superficial masseter (SM) and anterior temporal (AT) muscles was recorded simultaneously to alternate antagonistic vertical bite force measurements from the right and left first molar, first premolar, canine and anterior incisor teeth. The subjects clenched three times for 2 seconds each time to a requested level (MVC; 30%MVBF) on a force transducer. EMG/Force ratio (E/F) values for working (W) and balancing (B) muscles were calculated and normalized for each task. Statistical differences among unilateral bite points were analyzed with Two-way ANOVA. The magnitude of bite forces were similar to those proposed by other authors. Changing posteroanteriorly the bite point induced a gradual and significant (MVBF: p<0.01; 30%MVBF: p<0.001) increase of the corresponding working and balancing SM E/F values, with the latter being either the same or higher than the working ones. The E/F registered at the MVBF and 30%MVBF molar and anterior bite points were different in a proportion of 1: 2 in all subjects. At MVBF the AT's E/Fs also increased corresponding to the anteroposterior shift of the bite point (p<0.01) ; however, the 1: 2 proportion of the molar vs the anterior bite point was not observed; at 30%MVBF there was not any evident pattern, and the E/F were not significantly different among bite points. From these results it can be concluded that the EMG/Force ratio of the working and balancing superficial masseter and anterior temporal muscles is a posteroanterior increasing function and therefore has an inverse relationship with bite force, condition which may induce a negative effect in the electrical efficiency of the jaw elevator muscles.The results of this study have been previously presented at the following meetings:74th General Session of the International Association for Dental Research.20th Annual Conference of the European Prosthodontic Society.96th Meeting of the Japan Prosthodontic Society.44th Annual Meeting of the Japanese Association for Dental Research.75th General Session of the International Association for Dental Research.Annual Meeting of the Niigata Dental Association, July 1997.

Jaw reflexes in healthy old people.

to investigate variations in the masseteric myotatic reflex (jaw-jerk) and the silent period from the 5th to the 9th decades of life. electromyographic data were recorded from the masseter muscle of the preferred chewing side by surface electrodes, using a computerized recording and analysis system. Chin taps were applied with a neurologist's hammer during mandibular rest and at 40% intercuspal clenching in 30 healthy people aged from 49 to 87 years. The influence of age, gender and silent period type were analysed by multiple regression analysis (P < or = 0.05). even in the very old subjects all reflexes were elicited, at least once. However, with increasing age the overall occurrence of the jaw-jerk reflex at rest (%) and its amplitude, at rest and at clench, were reduced, while its latency at rest was significantly increased (P < or = 0.05). No age effects were recorded in most parameters of the jaw-jerk reflex at clench and in the silent period. Women showed a tendency for reduced latencies of the jaw-jerk and the early silent period and increased silent period duration (P < or = 0.05). They also had a steeper decline in myotatic reflex activity, particularly at rest. simple masseteric reflex activity is maintained until very old age, particularly when elicited during contraction of the jaw elevators.

Electric versus magnetic transcranial stimulation of the trigeminal system in healthy subjects. Clinical applications in gnathology.

This investigation is focused on the analysis of the masseter responses evoked by means of magnetic and electric stimulation of the cortex and root of the trigeminal system of 20 healthy subjects. Moreover, in order to determine jaw elevation in centric occlusion, the analysis also focused on the motor response of the trigeminal bilateral roots evoked simultaneously using two stimulators. The masseter responses show a high level of symmetry in both latency and amplitude. The difference between the hinge axis in natural centric occlusion (CO) and in centric relation (CR) as determined by electric and magnetic transcranial stimulation is lesser than 2.5 mm. This is due to two main factors: (i) the response is evoked from the trigeminal motor neural pool involving all the masticatory muscles and hence simulating the natural 'muscular coactivation'; (ii) there is no manual manipulation to guide the jaw into the centric relation (CR). This method could be of some help for diagnosis in patients suffering from TMJ dysfunction who have lost stereognostic control over the jaw and display obvious signs of deviation during closure of the mandible.

Electric versus magnetic transcranial stimulation of the trigeminal system in healthy subjects. Clinical applications in gnathology

summary This investigation is focused on the analysis of the masseter responses evoked by means of magnetic and electric stimulation of the cortex and root of the trigeminal system of 20 healthy subjects. Moreover, in order to determine jaw elevation in centric occlusion, the analysis also focused on the motor response of the trigeminal bilateral roots evoked simultaneously using two stimulators. The masseter responses show a high level of symmetry in both latency and amplitude. The difference between the hinge axis in natural centric occlusion (CO) and in centric relation (CR) as determined by electric and magnetic transcranial stimulation is lesser than 2.5 mm. This is due to two main factors: (i) the response is evoked from the trigeminal motor neural pool involving all the masticatory muscles and hence simulating the natural ‘muscular coactivation’; (ii) there is no manual manipulation to guide the jaw into the centric relation (CR). This method could be of some help for diagnosis in patients suffering from TMJ dysfunction who have lost stereognostic control over the jaw and display obvious signs of deviation during closure of the mandible.

Relationships between the orientation and moment arms of the human jaw muscles and normal craniofacial morphology.

It has been suggested that subjects with increased vertical craniofacial dimensions have relatively oblique orientated jaw muscles with a reduced possibility to restrain the vertical component of craniofacial growth. To test this hypothesis, relationships were investigated between the spatial orientation of the jaw muscles and the craniofacial morphology. Computer reconstructions of the external shape of the jaw muscles of 30 adult males with a normal skull were made with the use of serial magnetic resonance imaging (MRI) scans. The orientation of the jaw muscles was defined by a regression line through the centroids of the serial cross-sections. Sagittal and frontal projections of the moment arms of the muscles were measured with respect to the centre of the ipsilateral condyle. Craniofacial morphology was analysed three-dimensionally using lateral head films and coronal MRI scans. The cephalometric data were analysed statistically using regression and factor analyses. Six cephalometric factors with Eigen values higher than 1 were correlated with jaw muscle orientation and moment arm data, using a multiple regression analysis. The anterior face height factor was significantly correlated with the orientation of the jaw opening muscles in the sagittal plane but was not significantly correlated with the orientation of the mandibular elevators. The sagittal moment arms of the mandibular elevators showed significant correlations with the factors describing the gonial angle and the posterior face height. It was concluded that the variation of spatial orientation of the human jaw closing muscles is predominantly associated with the variation of mandibular morphology (expressed by the gonial angle) and the posterior face height. The orientation of the jaw opening muscles shows significant relationships with anterior vertical craniofacial dimensions. The hypothesis that persons with an increased anterior face height have relatively oblique orientated jaw elevators was rejected.