Effects of orthognathic surgery for class III malocclusion on signs and symptoms of temporomandibular disorders and on pressure pain thresholds of the jaw muscles

The aim of this longitudinal study was to determine the effects of orthognathic surgery on signs and symptoms of temporomandibular disorders (TMD) and on pressure pain thresholds (PPTs) of the jaw muscles. Fourteen consecutive class III patients undergoing pre-surgical orthodontic treatment were treated by combined Le Fort I osteotomy and bilateral sagittal ramus osteotomy. The clinical examination included the assessment of signs and symptoms of TMD and the assessment of PPTs of the masseter and temporalis muscles. Anamnestic, clinical and algometric data were collected during five sessions over a 1-year period. Seven out of 14 patients presented with disc displacement with reduction at baseline, whereas four patients (two of them were new cases) did so at the end of follow up (p>0.05). None of the patients were diagnosed with myofascial pain of the jaw muscles at the beginning or end of follow up. PPTs of the masseter and temporalis muscles did not change significantly from baseline values throughout the whole study period. The occurrence of signs and symptoms of TMD fluctuates with an unpredictable pattern after orthognathic surgery for class III malocclusions.

Skeletal Sagittal and Vertical Facial Types and Electromyographic Activity of the Masticatory Muscle

OBJECTIVE

To investigate the electromyographic activities of the anterior temporal (T) and masseter (M) muscles in different facial skeletal types.

MATERIALS AND METHODS

The samples consisted of 105 subjects (38 males and 67 females; mean age 22.0 +/- 6.7 years) and were classified into six groups according to the values of ANB and SN-GoMe: group 1 for Class I malocclusion and normodivergent type (n = 27), group 2 for Class I and hyperdivergent type (n = 20), group 3 for Class II and normodivergent type (n = 10), group 4 for Class II and hyperdivergent type (n = 23), group 5 for Class III and normodivergent type (n = 12), and group 6 for Class III and hyperdivergent type (n = 13). Temporal muscle activity (TMA), masseter muscle activity (MMA), and T/M ratio were evaluated at resting and clenching status.

RESULTS

Although there was no significant difference in resting MMA among all groups, group 6 showed a higher resting TMA than did other groups and a significant difference in resting T/M ratio compared with groups 1 and 3. There were no significant differences in clenching TMA and MMA among all groups. Although all groups showed a significant increase of TMA and MMA from resting to clenching status, group 6 showed a significant decrease of clenching T/M ratio compared with resting T/M ratio.

CONCLUSIONS

The results suggest that the more Class III and the more hyperdivergent type, the higher resting TMA and the lesser increase of clenching MMA than expressed by other groups. Significant differences existed in TMA and MMA according to sagittal and vertical facial skeletal types.

Serotonin (5-HT) excites rat masticatory muscle afferent fibers through activation of peripheral 5-HT3 receptors

In the present study, we combined immunohistochemical experiments with in vivo single unit recordings to examine whether 5-HT(3) receptors are expressed by masticatory (masseter and temporalis) sensory ganglion neurons and to investigate the effects of intramuscular injection of 5-HT on the excitability and mechanical threshold of rat masticatory muscle afferent fibers. The expression of 5-HT(3) receptors by masticatory ganglion neurons was examined using immunohistochemical techniques. In vivo extracellular single unit recording techniques were used to assess changes in the excitability of individual masticatory muscle afferent fibers. Immunohistochemical experiments detected a relatively high frequency (52%) of 5-HT(3) receptor expression by masticatory ganglion neurons. Injection of 5-HT (10(-4), 10(-3), 10(-2)M) evoked concentration-related increases in the magnitude of afferent discharge, but did not significantly sensitize muscle afferent fibers to mechanical stimuli. No significant sex-related differences in 5-HT-evoked afferent discharge were identified. Afferent discharge evoked by 5-HT was significantly attenuated by co-injection with the selective 5-HT(3) receptor antagonist tropisetron (10(-3)M). Afferent discharge was also evoked by the selective 5-HT(3) receptor agonist 2-methyl-5-HT. Unexpectedly, a significant concentration-related decrease in median blood pressure in response to 5-HT injection was found. This 5-HT-induced decrease in blood pressure was not antagonized by tropisetron or mimicked by 2-methyl-5-HT, indicating that the drop in blood pressure was not 5-HT(3) receptor-mediated. The present results indicate that 5-HT excites slowly conducting masticatory muscle afferent fibers through activation of peripheral 5-HT(3) receptors, and suggest that similar mechanisms may contribute to 5-HT-evoked muscle pain in human subjects.

Centric slide, bite force and muscle tenderness changes over 6 months following fixed orthodontic treatment

OBJECTIVE

The postorthodontic change of the masticatory muscles was evaluated using three parameters: maximal voluntary bite force (MVBF), slide in centric (difference between maximal intercuspation and retruded contact position), and muscle sensitivity to palpation.

MATERIALS AND METHODS

MVBF was measured with a custom-made rubber tube bite force device, centric slide with a digital caliper, and sensitivity to palpation of the masseter and temporalis muscles (scale 0-3) during application of standardized digital force (10 N). Data were collected at four time points: T0, before bracket removal; T1, immediately after bracket removal; T2, after 3 months of retention; and T3, after 6 months of retention. Patients (n = 41; 22 females, 19 males; mean age 17.4 +/- 5.4 years) were examined from T0 to T1 and from T1 to T2. Of these, 28 (15 females, 13 males) were followed at T3.

RESULTS

Immediately after bracket removal (T0 to T1), MVBF increased significantly by 15%. Another significant increase (15.5%) was found 3 months posttreatment (T1-T2), and almost no increase (2%) at 6 months (T2-T3). The slide in centric remained within normal values during the three time points. A decline in sensitivity to palpation from T1 to T3 was found for both masseter and temporalis muscles.

CONCLUSIONS

Neuromuscular adaptability begins within several minutes after bracket removal. A second stage of muscular adaptation occurs within 3 months of retention. These findings suggest that muscular adjustment occurs within a short period after orthodontic treatment.

Electromyographic activity of the masticatory and cervical muscles during resisted jaw opening movement

One of the relaxation techniques that have been used by physical therapists when treating patients with temporomandibular disorders, is the agonist contract-antagonist relax technique (AC). When the AC technique is applied, a submaximal resistance to the jaw opening movement is necessary to cause relaxation of the masticatory muscles. No information about the effect of this technique on masticatory or cervical muscles has been found in the literature. Thus, the aim of this study was to evaluate the electromyographic activity of the masseter and anterior temporalis and the upper trapezius and splenius capitis during resisted jaw opening movement. A convenience sample of 30 students was used. Electromyography activity of the superficial masseter, anterior temporalis, upper trapezius and splenius capitis was registered before, during and after the application of this resistive movement to jaw opening. A two-way anova with repeated measures analysis was used to analyse data. The level of significance was at alpha = 0.05. The EMG activity of both the masticatory muscles and the cervical muscles increased during and after the application of resisted jaw opening (P < 0.05). Based on the results obtained from this study, the behaviour of all muscles analysed (masseter, anterior temporalis, splenius capitis and upper trapezius) was similar. All muscles increased their activity when the resistance to the jaw opening movement was applied. Complex muscular interactions of the supra- and infrahyoid muscles (jaw openers), masticatory muscles, and cervical muscles may exist to stabilize the craniomandibular system during resisted jaw opening.

Postural control of the human mandible

This article reviews recent experimental evidence explaining the mechanisms that support the mandible in its rest or postural position when the head is stationary and during locomotion. At rest, and during slow jaw movements, there is alternating activation of the jaw-opening and jaw-closing muscles which arises from a central pattern generator. However, this cannot account for the rest position of the mandible even when the head is stationary. Jaw movements and masticatory muscle activity were measured in subjects who stood, walked and ran on a treadmill. Even during walking, there are no bursts of masseter EMG time-locked to heel-landing. However, when subjects ran, the downward movement of the mandible in each step evokes a burst of EMG in the masseters. This is a stretch reflex in the jaw-closing muscles, which acts to limit the downward movement of the mandible relative to the maxilla during locomotion, and to restore the mandibular position towards its rest position. Thus, when the head is stationary, the low-level activity in the jaw-opening and jaw-closing muscles does not contribute to the rest position. Instead, the mandible is supported by passive viscoelastic forces in perioral soft tissues which limit vertical jaw movements even when the head moves gently up and down during walking. When the head moves more vigorously up and down, stretch reflexes in the jaw-closing muscles limit the movement of the mandible. That is, both passive forces and active reflex responses maintain jaw posture within narrow limits during brisk head movements.

[Electromyographic activity of the temporal and masseter muscles at different occlusal positions]

INTRODUCTION

Normal occlusion or eugnathia, is a morphologically and functionally balanced bite. Differences in the form and function of orofacial structures cause morphological and functional deviations of the orofacial system. Irregular occlusal postitions may consequently change the electromyographic activity of the muscles of orofacial region. The aim of the research was an analysis and a comparative analysis of the bioelectrical activity of masticatory muscles in normal occlusion and distocclusion, in rest position, in the position of the central occlusion of the mandible and at the maximum voluntary muscle contraction.

MATERIAL AND METHODS

The metodology of the research is based on electromyographic recording of the action potentials of the examined muscles in different mandibular positions. Registration of action potentials of the masticatory muscles was done using, facial, intramuscular, and coaxial electrodes, (Greenfield scheme) and measurements were performed on both sides. The research was carried out on a sample of 60 patients with an average age of 14,25 years. 30 subjects had normal occlusion and 30 had distal occlusion.

RESULTS

The results were expressed in microvolts, as average cumulative amplitude voltages of action potentials, by means of which changes in the masticatory muscles were established and the degree of correlation between the electromyographic activity of the masticatory muscles and the occlusal type was analyzed.

CONCLUSION

The results of the research show that the bioelectrical activity of the examined masticatory muscles falls in the group of subjects with distal occlusion, which is the result of a decreased number of active muscular tissues, or a decreased number of impulses coming into the muscle, or the combination of both.

The use of a bruxchecker in the evaluation of different grinding patterns during sleep bruxism

A variety of problems involving the masticatory system can be partially attributed to parafunctional habits such as bruxism. These include occlusal trauma, abfractions, tooth migration, as well as temporomandibular dysfunction. Since bruxism is considered a contributing factor to the above-mentioned dental problems, it is essential to consider parafunctional habits in the diagnosis and treatment planning before doing any occlusal reconstruction. However, the problem lies in the lack or absence of a simple device or gauge useful to be able to diagnose and evaluate the occlusal schemes in the patient's grinding pattern. In this study, we have developed a very simple device (BruxChecker) for evaluating the grinding patterns in sleep bruxism. Using the BruxChecker, it was possible to visualize real or actual interferences during sleep bruxism. Therefore, examination of the grinding pattern using this device is necessary and crucial for making the proper treatment plan for each patient.

Influence of jaw gape on EMG of jaw muscles and jaw-stretch reflexes

The influence of jaw gapes on jaw-stretch reflexes and jaw muscles activity was studied in order to test the sensitivity of human muscle spindle afferents in various jaw muscles. Twelve healthy men (mean age+/-S.E.M.: 25.0+/-1.2yr) participated in the study. Short-latency excitatory reflex responses were evoked by a custom-made stretch device with the subjects biting on a jaw-bar with their front teeth. Surface electromyographic (sEMG) recordings from right masseter (MAR), and right temporalis (TAR), intramuscular EMG (imEMG) recordings from right lateral pterygoid (LPR) and right anterior digastric (ADR) muscles were made. The reflex at different jaw gapes of 6, 10, 14, 18, 22, 26, 30, 34, and 38mm were examined in random order with standard stretch conditions of 1mm displacement and 10ms ramp time. Twenty sweeps of the reflex were recorded at each level with at least 5s interval between each sweep with online monitoring of the visual feed back at 15% of maximum voluntary contraction (MVC) of each jaw gape from MAR. The results showed that the peak-to-peak amplitude of the jaw-stretch reflex in MAR was significantly higher at 14mm compared to 30, 34, and 38mm (P<0.038), whereas the reflex amplitude in TAR increased with jaw gape until a maximum at 34mm. There was no significant effect of jaw gape in LPR muscles (P=0.825) and no obvious stretch reflex was observed in ADR. When the amplitude was normalised to the pre-stimulus EMG at each jaw gape, the highest normalised amplitude was observed at 14mm jaw gape in MAR, however there was no significant effect of jaw gape on the normalised amplitude in TAR and LPR. In addition, masseter EMG at MVC significantly decreased with the increase of the gapes, i.e. biting at 6, 14, and 18mm gapes had a significantly higher MVC compared to 26, 30, 34, and 38mm (ANOVA: P<0.013). It is concluded that the jaw gapes influence the sensitivity of the human muscle spindle afferents in jaw-closing muscles with a distinct peak, which is within normal jaw gapes during function.

Effect of occlusal splints on the electromyographic activities of masseter and temporal muscles during maximum clenching

OBJECTIVES

Despite the broad use of occlusal splints in the treatment of temporomandibular disorders, the efficacy of splints is a source of controversy. In this study, the effectiveness of occlusal splints on the electromyographic activity of the masseter and temporal muscles in healthy individuals was investigated.

METHOD AND MATERIALS

An occlusal splint was made for 25 healthy individuals. Surface electromyographic recordings were done during maximum clenching without the splint and immediately after application of the splint. The relative level of activity in the masseter and temporal muscles was quantified by means of activity index. Paired t test and Wilcoxon signed-rank tests were used for data analysis.

RESULTS

The mean electromyographic activities of masseter and temporal muscles (in mV/s) were 0.7712 and 1.0488 without the splint and 0.8396 and 0.9276 immediately after application of the splint. There was no significant difference between the electromyographic activities of both muscles before and after insertion of the splints. The activity index increased after application of the occlusal splints. This increase was also insignificant.

CONCLUSION

Immediate application of occlusal splints has no significant effect on the activity of masseter and temporal muscles. It could decrease the relative activity of the temporal to the masseter muscle (increasing the activity index).

Masticatory muscle thickness, bite force, and occlusal contacts in young children with unilateral posterior crossbite

Few investigations have evaluated the characteristics of functional and structural malocclusion in young children. Thus, the aim of this study was to assess the ultrasonographic thickness of the masseter and anterior portion of the temporalis muscles, the maximum bite force, and the number of occlusal contacts in children with normal occlusion and unilateral crossbite, in the primary and early mixed dentition. Forty-nine children (26 males and 23 females) was divided into four groups: primary-normal occlusion (PNO), mean (PNO) age 58.67 months; primary-crossbite (PCB), mean age 60.50 months; mixed-normal occlusion (MNO), mean age 72.85 months; and mixed-crossbite (MCB), mean age 71.91 months. Thickness was evaluated with the muscles at rest and during maximal clenching, and comparison was made between the right and left side (normal occlusion), and between the normal and crossbite side (crossbite). The results were analysed using Pearson's correlation, paired and unpaired t-test, and Mann-Whitney ranked sum test. The anterior temporalis thickness at rest was statistically thicker for the crossbite side than the normal side in the MCB group (P = 0.0106). A statistical difference in bite force and the number of occlusal contacts was observed between the MNO and MCB groups, with greater values for the MNO subjects (P < 0.05). Masseter muscle thickness showed a positive correlation with bite force, but the anterior temporalis thickness in the PCB and MCB groups was not related to bite force. Masticatory muscle thickness and bite force did not present a significant correlation with occlusal contacts, weight, or height. It was concluded that functional and anatomical variables differ in the early mixed dentition in the presence of a malocclusion and early diagnosis and treatment planning should be considered.

Electromyographic evaluation of masseter and anterior temporalis muscles in resting position and during maximum tooth clenching of edentulous patients before and after new complete dentures

The aim of this study was to assess the electrical activity of the masseter and anterior temporal muscles in patients with severe bone resorption, with complete dentures worn for over ten years, and five months after having new dentures put in place. The RDC questionnaire was applied to twelve asymptomatic patients, before and five months after new dentures were put in place. The electrical activity recordings were made in the mandibular position at rest, and during maximum tooth clenching. The electrical activity of the masseter and anterior temporal muscles in the position at rest presented no statistically significant difference after five months of wearing the new complete dentures. Electrical activity during tooth clenching exhibited a statistically significant reduction only in the right temporal muscle. A period longer than five months of wearing the new complete dentures is required for adaptation and the acquisition of functional capacity.

Finite element analysis of stresses in the maxillary and mandibular dental arches and TMJ articular discs during clenching into maximum intercuspation, anterior and unilateral posterior occlusion

The objective of this study was to investigate distribution of stresses in the human TMJ discs, generated during clenching into various occlusal positions. The work presents a biomechanical finite element model of interaction of mandibular and maxillary dental arches and the TMJ discs of a particular person, based on real geometrical data obtained from spiral computed tomography two-dimensional images. 3D contour coordinates - point clouds were collected from these images and solid model was created. The system under investigation consisted of eight basic parts: two rigid structures representing the mandibular and maxillary dental arches, two mandibular condyles, two mandibular fossae of temporal bone, and solid models of two articular discs. The model of maxillary dental arch was fixed in space. The model of the mandibular dental arch was able to move in space synchronically with the mandibular condyles under action of applied forces, which were considered as prescribed and known at insertion points of masticatory muscles. The motion of the mandible was constrained by interdental contact interactions and contact interaction with articular discs, which were situated in between mandibular condyles and mandibular fossae of temporal bone. The model was implemented by using LS-DYNA finite element software. The obtained results presented a 3D view of stresses exhibited in the articular discs, as well as the real contact points of dental interactions at given masticatory geometry of a particular subject and the values of interaction forces. The expected practical value of the developed model is the facilitation of biomechanical evaluations of the influence of tolerances of teeth shapes and occlusal areas together with the supporting areas on the final stress distribution in the dental arches and articular discs.

Functional significance of strain distribution in the human mandible under masticatory load: numerical predictions

A common feature of studies of mandibular morphology is the assumption that there is some functional relation between the form of the lower jaw and masticatory stress. It was noted that the local variation in cortical bone thickness in the mandibular corpus appears to be stereotypical among anthropoids. This occurs at sections under the molars, where the lingual cortical plate is thinner than buccal one. In this study we investigate and contrast the strain pattern along buccal and lingual surfaces of the mandibular corpus during mastication using a numerical model of a human mandible. We show that strain distribution differs in alveolar and mid-corpus segments of the mandible and that the latter develops an alternate pattern between the buccal and lingual aspects of the working and balancing sides of the jaw. We then relate the magnitude of these strains to Frost's mechanostat. Our results suggest that the cortical asymmetry of the human mandible is in fact not related to strain patterns generated during mastication.

Randomized controlled clinical trial for verifying the effect of silicone-based resilient denture liner on the masticatory function of complete denture wearers

PURPOSE

The purpose of this study was to investigate whether application of permanent silicone-based resilient denture liner (SR) to mandibular complete dentures significantly improves patients' masticatory ability compared to conventional heat-activated acrylic resin (AR).

MATERIALS AND METHODS

Twenty-eight edentulous patients were randomly placed into 1 of 2 crossover groups (AR-SR/SR-AR) by using a random permuted block within strata method. The AR-SR group received AR denture treatments followed by SR denture treatments. The SR-AR group received treatments in the reverse sequence. The outcomes were classified by matiscatory performance, mandibular movement, electromyographic activity, and maximum occlusal force.

RESULTS

No significant differences were observed in any of the baseline characteristic measurements between groups. SR denture wearers exhibited significantly higher masticatory performance than AR denture wearers. SR denture wearers exhibited a longer early-stage occluding period than AR denture wearers. There were no differences in electromyographic activity between the AR and SR groups. There were no significant differences in maximum occlusal force between the AR and SR groups.

CONCLUSION

This study showed that the application of SR to mandibular complete dentures resulted in significant improvements to the patients' masticatory ability compared to AR.

Influence of different condylar and incisal guidance ratios to the activity of anterior and posterior temporal muscle

OBJECT

Few studies quantify reactions of masticatory muscles during clenching at different occlusal positions in individuals with different anterior and posterior guidance relationship and different number of working-side occlusal contacts. The hypothesis that altered incisal guidance (IG) and a different number of working-side occlusal contacts in complete denture wearers (CDW) change a pattern of temporal muscle activity and loadings to the mandible during clenching in incisal and lateral positions was tested.

DESIGN

EMG activity during clenching in incisal (IP) and left (LOP) and right lateral occlusal (ROP) positions was compared between dentate subjects (DS) with steeper IG than condylar guidance (PG) and canine or canine+first premolar guidance in laterotrusive movements with CDWs who had steeper PG than IG and group function. EMG values were expressed as percentages of maximum voluntary clenching in maximum intercuspation (ICP) in each individual.

RESULTS

The CDWs exhibited significantly higher posterior temporal muscle (TP) activity in IP and during lateral biting on mediotrusive side than DSs. Their coronoid process had to be pulled backward by TP fibers to rotate condyle in a counter-clockwise direction (PG>IG); contrary DSs had to rotate condyle in a clockwise direction to compensate for vertical overlap (IG>PG). Group function allowed more working-side contacts in CDWs and significantly higher anterior temporalis (TA) activity.

CONCLUSION

Alteration of IG-PG ratio and a number of occlusal contacts during lateral clenching change a pattern of TA and TP activity in CDWs and a direction of mandibular loadings, although age related changes might also be responsible.

Reproducibility of surface EMG variables in isometric sub-maximal contractions of jaw elevator muscles

The aims of this study were: (1) to develop and assess reproducibility of a new method for measuring masticatory force in the intercuspal position; (2) to test the reproducibility of surface EMG signal amplitude and spectral variables in constant force contractions of jaw elevator muscles and its dependency on inter-electrode distance. The study was performed on the masseter and temporalis anterior muscles of both sides of nine healthy volunteers. An intraoral compressive-force sensor was used to measure maximal voluntary contraction forces in the intercuspal position and to provide a visual feedback on sub-maximal forces to the subject. Three experimental sessions were performed in three days. In each session, three isometric contractions at 80% of the maximal force were sustained by the subjects for 30s. The intra-class correlation coefficient (ICC) of the maximal force measure was 71.9%. ICC of average rectified value and mean power spectral frequency of the EMG signal increased with inter-electrode distance, with values larger than 70% with 30 mm inter-electrode distance. It was concluded that surface EMG variables measured in isometric contractions of the jaw elevator muscles with the proposed force recording system show good reproducibility for clinical applications when a 30 mm inter-electrode distance is considered.

[A preliminary study on the relationship between the wearing time of full denture and EMG in edentulous patients]

PURPOSE

This study was aimed to analyze the relationship between the periods of wearing time of full denture and function of mastication in edentulous patients.

METHODS

Electromyogram (EMG) was done at an initial time of wearing full denture for 30 patients (Group A) and at an interval of wearing full denture for more than 3 months for 32 patients (Group B). EMG activities were recorded for 2 pairs of muscles (anterior temporalis, masseter) by means of Medelec Synergy Electromyography. Student's t test was used for statistical analysis.

RESULTS

EMG amplitude of Group A was significantly lower than that of Group B (left anterior temporalis P < 0.05, bilateral masseter P < 0.05). Group A has larger area than Group B (P < 0.05); No significant difference was found in asymmetry index between Group A and Group B (P > 0.05).

CONCLUSION

The function of mastication in patients who wore full denture for short time has not been achieved as expected.

The effect of age on the function of the masticatory system – an electromyographical analysis

BACKGROUND

Old age is the last stage of human life and, unfortunately, the ageing rhythm of the oral cavity and masticatory system seems to be rather accelerated. Hence, there is a reduction in the amount of food ingested, establishing the imbalance of nutritional phenomena.

OBJECTIVE

To assure a better quality of life for the elderly, it is necessary to understand their masticatory system.

MATERIALS AND METHODS

An electromyographical analysis of the masticatory system in 10 individuals aged between 60-75 years (group 1) and 10 between 23-30 years old (group 2) was carried out. The analysis was performed using a MyoSystem-Br1 electromyographer with differential active electrodes. The test was registered during positioning and functional conditions, such as chewing, and the muscles assessed were the temporalis and masseter. Data were normalised by maximum voluntary contraction (MVC), and the results were analysed statistically using independent t-test during the comparison between groups.

RESULTS

Statistical significance (p < or = 0.05) was found in the analysis of various activities, in which the elderly showed greater electromyographical activity values relative to their MVC during the different mandibular positions, such as left laterality. While the elderly subjects revealed values of 30% relative to their MVC, the young revealed 10%. However, during masticatory activities, the elderly revealed values of up to 79%, and the young of 108%, relative to their MVC, such as when they are chewing peanuts.

CONCLUSION

It could be concluded that the elderly show hyperactivity of masticatory musculature during posture maintenance and a slight hypoactivity of this musculature during chewing when analysed side by side with young individuals.

Passive resistance increases differentially in various jaw displacement directions

OBJECTIVES

In the present study, the passive resistance of the human jaw system was quantified in relation to the three-dimensional jaw displacement and the Posselt-envelope, using both in vivo measurements and computer simulation.

METHODS

In eight subjects, the jaw was passively displaced with a step-wise increasing force in three orthogonal directions. Muscle relaxation was monitored using electromyography (EMG) with visual feedback. A biomechanical model of an average human system was used to examine the contributions of the jaw muscles.

RESULTS

The largest excursion was found for the vertical direction. Protrusive and lateral directions were more restricted. In protrusive and lateral directions, the jaw could generally move beyond the Posselt-envelope. The stiffness of the jaw increased with proceeding jaw displacement in all directions. The stiffness was larger in the protrusive direction than in the vertical and lateral directions. The model's predictions of stiffness were comparable to the in vivo measurements. However, in protrusive direction, the maximum jaw displacement was larger than in vivo. The estimated passive muscle forces showed that vertical displacement was mainly restricted by the complete group of closing muscles, while protrusive and lateral jaw displacement was restricted by selective individual muscles.

CONCLUSIONS

The human jaw system has larger motion range in the protrusive and lateral directions than can be exploited by active muscle use. Stiffness of jaw displacement is higher in the protrusive direction compared to the vertical and lateral directions.

Masticatory function with mandibular implant-supported overdentures fitted with different attachment types

Oral Rehabilitation by means of implant-retained mandibular overdentures is known to improve the oral function. The improvement may depend on the degree of retention and stability of the denture and thus on the type of attachment. In this within-patient cross-over clinical trial, we examined the hypothesis that more retention and stability of the overdenture improves the masticatory function. Eighteen patients received two permucosal implants, a new overdenture, and, successively, three different suprastructure modalities: a magnet, a ball, and a bar-clip attachment. We quantified aspects of the oral function by measuring the electromyographic activity of the jaw muscles and the jaw movement during chewing. The muscle activity was significantly lower for the unsupported new mandibular denture compared with values for the supported new denture. No significant differences in muscle activity were observed among the three attachment types. Furthermore, we did not observe significant differences in jaw muscle activity between the old unsupported denture and the new supported denture, despite significant differences in masticatory performance. Measuring muscle activity during chewing will thus not provide adequate information about masticatory function. Thus, subjects chewed more efficiently after implant treatment. No changes in cycle duration or in jaw movement parameters were observed among the various measurement moments.

Effect of Texture of Plastic and Elastic Model Foods on the Parameters of Mastication

Mastication is continually modified throughout the chewing sequence in response to the texture of the food. The aim of this work was to compare the effects of an increase in hardness of two model food types, presenting either elastic or plastic rheological properties, on mastication. Each model food type consisted of four products of different hardness. Sensory testing experiments conducted with one group of 14 subjects showed significant perceived differences between products in terms of their increasing hardness. Fifteen other volunteers were asked to chew three replicates of each elastic and plastic product during two sessions. EMGs of masseter and temporalis muscles were recorded simultaneously with jaw movement during chewing. Numerous variables were analyzed from these masticatory recordings. Multiple linear regression analyses were used to assess the respective effects of food hardness and rheological properties on variables characterizing either the whole masticatory sequence or different stages of the sequence. Muscle activities were significantly affected by an increase in hardness regardless of the food type, whereas the shape of the cycles depended on the rheological properties. The masticatory frequency was affected by hardness at the initial stage of the sequence but overall frequency adaptation was better explained by a change in rheological behavior, with plastic products being chewed at a slower frequency. A dual hypothesis was proposed, implicating first a cortical–brain stem preprogrammed mechanism to adapt the shape of the jaw movements to the rheological properties of the food, and second, a brain stem mechanism with mainly sensory feedback from the mouth to adapt muscle force to the food hardness.

Sensitivity of rat temporalis muscle afferent fibers to peripheral N-methyl-D-aspartate receptor activation

The temporalis muscle is a common source of pain in headache and chronic craniofacial pain conditions such as temporomandibular disorders, which have an increased prevalence in women. The characteristics of slowly conducting temporalis afferent fibers have not been investigated. Therefore, the aim of the present study was to examine the characteristics of slowly conducting temporalis muscle afferent fibers and to determine whether these fibers are excited by activation of peripheral N-methyl-D-aspartate receptors. The response properties of a total of 117 temporalis afferent fibers were assessed in male and female rats. A majority of these fibers had high mechanical thresholds and slow conduction velocities (<10 m/s). The mechanical threshold of the temporalis afferent fibers was inversely correlated with afferent conduction velocity, however, no sex-related differences in mechanical threshold were identified. There were also no sex-related differences in N-methyl-D-aspartate-evoked afferent discharge. Indeed, injection of a high concentration (1600 mM) of N-methyl-D-aspartate into the temporalis muscle was necessary to evoke significant afferent discharge. Thirty minutes after the initial injection of N-methyl-D-aspartate into the temporalis muscle, a second injection of N-methyl-D-aspartate produced a response only about 50% as large as the initial injection. Co-injection of ketamine (20 mM) with the second injection of N-methyl-D-aspartate significantly decreased N-methyl-D-aspartate-evoked afferent discharge in both sexes. This concentration of ketamine is greater than that needed to attenuate afferent discharge evoked by injection of glutamate into the masseter muscle. These results suggest that unlike masseter afferent fibers, temporalis afferent fibers are relatively insensitive to peripheral N-methyl-D-aspartate receptor activation.

Immediate effects of plantar inputs on the upper half muscles and upright posture: a preliminary study

This purpose of this study was to investigate the immediate effects of plantar inputs on both the upper half muscle activity (anterior temporal, masseter, digastric, sternocleidomastoid, upper and lower trapezius, cervical) and the body posture, by means of electromyography (EMG) and vertical force platform, respectively. Twenty four (24) healthy adults, between the ages of 24 and 31 years (25.3 +/- 1.9), with no history of craniomandibular disorder or systemic musculoskeletal dysfunction, were randomly divided into two groups: test group (fourteen subjects) and control group (ten subjects). A first recording session (TO) measured the baseline EMG and postural patterns of both groups. After this session, the test group wore test shoes with insoles that stimulated the plantar surfaces, while the control group wore placebo shoes. After one hour, a second set of measurements (T1) were performed. Significant differences between the groups at baseline were observed in the left anterior temporal, left cervical, and left upper trapezius, as well as at T1 in the left anterior temporal and right upper trapezius (p < 0.05). Within-test group analysis showed a significant increase of the right upper trapezius activity (p < 0.05), whereas no changes were found by within-control group analysis. Lower risk of asymmetric muscle patterns and postural blindness in the test group compared to the control group was observed. Further studies are warranted to investigate the short and long-term effects of this type of insole, in patients with both craniomandibular-cervical and lower extremity disorders.

Occlusion, sternocleidomastoid muscle activity, and body sway: a pilot study in male astronauts

The modifications induced by microgravity on the coordinated patterns of movement of the head, trunk, and limbs are reported on extensively. However, apparently there is little data on the masticatory muscles. In normal gravitational conditions, information from the neck and stomatognathic apparatus play a role in maintaining the body's balance and equilibrium. The current pilot study used normal gravity conditions to investigate the hypothesis of a functional coupling between occlusion and neck muscles and body postural oscillations. The immediate effect of modified occlusal surfaces on the contraction pattern of the sternocleidomastoid muscles during maximum voluntary clenching and on the oscillation of the center of foot pressure was analyzed in 11 male astronauts (aged 31-54 yrs). All subjects were healthy and free from pathologies of the neck and stomatognathic apparatus. Occlusal splints were prepared using impressions of their dental arches. The splints were modeled on the mandibular arch, had only posterior contacts, and were modified to obtain a more symmetric, standardized contraction of the masseter and temporalis muscles during teeth clenching. Surface EMG activity of the sternocleidomastoid muscles was recorded during a maximal voluntary clench with and without the splint. Sternocleidomastoid potentials were standardized as percent of the mean potentials recorded during a maximum contralateral rotation of the head, and the symmetry of the EMG waves of left- and right-side muscles was measured. Body sway was assessed with and without the splint, either with eyes open or closed. The variations of the center of foot pressure were analyzed through bivariate analysis, and the area of the 90% standard ellipse was computed. Within each visual condition (eyes open or closed), the difference between the areas of oscillation measured with and without the splint was computed. Muscular activity was more symmetric with the splint. The area of oscillation of the center of foot pressure was larger without the splint than with the splint, both with eyes open and eyes closed. The modifications, induced by the occlusal splint in the sternocleidomastoid muscles' symmetry, and center of foot pressure differential area with closed eyes, were significantly related (p < 0.05): the larger the increment in muscular symmetry, the smaller the area of oscillation with the splint as compared to without the splint. A functionally more symmetric maxillo-mandibular position resulted in a more symmetric sternocleidomastoid muscle contraction pattern and less body sway. Modifications in the contraction of the masticatory muscles may therefore affect the whole body.