Co-contraction behaviour of masticatory and neck muscles during tooth grinding.

The objective of this study was to analyse the co-contraction behaviour of jaw and neck muscles during force-controlled experimental grinding in the supine position. Twelve symptom-free subjects were enrolled in the experimental study. Electromyographic (EMG) activity of semispinalis capitis, splenius capitis and levator scapulae muscles was recorded bilaterally with intramuscular fine-wire electrodes, whereas that of sternocleidomastoideus, infrahyoidal, suprahyoidal, masseter and anterior temporalis muscles were registered with surface electrodes. EMG and force measurements were performed during tasks simulating tooth grinding on custom-made intraoral metal splints. The mean EMG activity normalised by maximum voluntary contraction (% MVC) of each of the neck muscles studied during grinding was analysed and compared with previous data from jaw clenching at identical force (100 N) and (supine) position. The occurrence of low-level, long-lasting tonic activation (LLTA) of motor units was also documented. The mean three-dimensional force vector of the grinding forces was 106 ± 74 N. In the frontal plane, the incline to the midsagittal plane ranged between 10° and 15°. In the midsagittal plane, the incline to the frontal plane was negligibly small. Posterior neck muscle activity during grinding ranged between 4.5% and 12% MVC and during clenching with 100 N between 1.8% and 9.9% MVC. Masticatory muscle activity during grinding ranged between 17% and 21% MVC for contralateral masseter and ipsilateral temporalis and between 4% and 6.5% for ipsilateral masseter and contralateral temporalis. LLTA had an average duration of 195 ± 10 seconds. The findings from this study do not support pathophysiological muscle chain theories postulating simple biomechanical coupling of neck and jaw muscles. Co-contractions of neck and masticatory muscles may instead occur as a result of complex neurophysiological interactions.

Realistic kinetic loading of the jaw system during single chewing cycles: a finite element study.

Although knowledge of short-range kinetic interactions between antagonistic teeth during mastication is of essential importance for ensuring interference-free fixed dental reconstructions, little information is available. In this study, the forces on and displacements of the teeth during kinetic molar biting simulating the power stroke of a chewing cycle were investigated by use of a finite-element model that included all the essential components of the human masticatory system, including an elastic food bolus. We hypothesised that the model can approximate the loading characteristics of the dentition found in previous experimental studies. The simulation was a transient analysis, that is, it considered the dynamic behaviour of the jaw. In particular, the reaction forces on the teeth and joints arose from contact, rather than nodal forces or constraints. To compute displacements of the teeth, the periodontal ligament (PDL) was modelled by use of an Ogden material model calibrated on the basis of results obtained in previous experiments. During the initial holding phase of the power stroke, bite forces were aligned with the roots of the molars until substantial deformation of the bolus occurred. The forces tilted the molars in the bucco-lingual and mesio-distal directions, but as the intrusive force increased the teeth returned to their initial configuration. The Ogden material model used for the PDL enabled accurate prediction of the displacements observed in experimental tests. In conclusion, the comprehensive kinetic finite element model reproduced the kinematic and loading characteristics of previous experimental investigations.

Small vertical changes in jaw relation affect motor unit recruitment in the masseter.

Strategies for recruitment of masseter muscle motor units (MUs), provoked by constant bite force, for different vertical jaw relations have not previously been investigated. The objective of this study was to analyse the effect of small changes in vertical jaw relation on MU recruitment behaviour in different regions of the masseter during feedback-controlled submaximum biting tasks. Twenty healthy subjects (mean age: 24·6 ± 2·4 years) were involved in the investigation. Intra-muscular electromyographic (EMG) activity of the right masseter was recorded in different regions of the muscle. MUs were identified by the use of decomposition software, and root-mean-square (RMS) values were calculated for each experimental condition. Six hundred and eleven decomposed MUs with significantly (P < 0·001) different jaw relation-specific recruitment behaviour were organised into localised MU task groups. MUs with different task specificity in seven examined tasks were observed. The RMS EMG values obtained from the different recording sites were also significantly (P < 0·01) different between tasks. Overall MU recruitment was significantly (P < 0·05) greater in the deep masseter than in the superficial muscle. The number of recruited MUs and the RMS EMG values decreased significantly (P < 0·01) with increasing jaw separation. This investigation revealed differential MU recruitment behaviour in discrete subvolumes of the masseter in response to small changes in vertical jaw relations. These fine-motor skills might be responsible for its excellent functional adaptability and might also explain the successful management of temporomandibular disorder patients by somatic intervention, in particular by the use of oral splints.

Force-controlled biting alters postural control in bipedal and unipedal stance.

Human posture is characterised by inherent body sway which forces the sensory and motor systems to counter the destabilising oscillations. Although the potential of biting to increase postural stability has recently been reported, the mechanisms by which the craniomandibular system (CMS) and the motor systems for human postural control are functionally coupled are not yet fully understood. The purpose of our study was, therefore, to investigate the effect of submaximum biting on postural stability and on the kinematics of the trunk and head. Twelve healthy young adults performed force-controlled biting (FB) and non-biting (NB) during bipedal narrow stance and single-leg stance. Postural stability was quantified on the basis of centre of pressure (COP) displacements, detected by use of a force platform. Trunk and head kinematics were investigated by biomechanical motion analysis, and bite forces were measured using a hydrostatic system. The results revealed that FB significantly improved postural control in terms of reduced COP displacements, providing additional evidence for the functional coupling of the CMS and human posture. Our study also showed, for the first time, that reductions in the sway of the COP were accompanied by reduced trunk and head oscillations, which might be attributable to enhanced trunk stiffness during FB. This physiological response to isometric activation of the masticatory muscles raises questions about the potential of oral motor activity as a strategy to reduce the risk of falls among the elderly or among patients with compromised postural control.

Localised task-dependent motor-unit recruitment in the masseter.

Localised motor-unit (MU) recruitment in the masseter was analysed in this study. We investigated whether differential activation behaviour, which has already been reported for distant masseter regions, can also be detected in small muscle subvolumes at the level of single MUs. Two bipolar fine-wire electrodes and an intra-oral 3D bite-force transmitter were used to record intra-muscular electromyograms (EMG) resulting from controlled bite-forces of 10 healthy human subjects (mean age 24.1 ± 1.2 years). Two-hundred and seventeen decomposed MUs were organised into localised MU task groups with different (P < 0.001) force-direction-specific behaviour. Proportions of MUs involved in one, two, three or four examined tasks were 46%, 31%, 18% and 5%, respectively. This study provides evidence of the ability of the neuromuscular system to modify the mechanical output of small masseter subvolumes by differential control of adjacent MUs belonging to distinct task groups. Localised differential activation behaviour of the masseter may be the crucial factor enabling highly flexible and efficient adjustment of the muscle activity in response to complex local biomechanical needs, for example, continually varying bite-forces during the demanding masticatory process.

The effect of experimental balancing interferences on masticatory performance.

Immediate adaptation to experimental-balancing interferences is known to affect jaw kinematics and electromyographic activity (EMG). However, little is known about the influence on masticatory performance parameters. This study hypothesises that balancing-side interferences significantly reduce the performance of the masticatory system. Twenty-one healthy subjects (eleven female, mean age: 24.1 ± 1.2 years) chewed standardised silicone cubes performing 15 masticatory cycles on the right side under three experimental conditions: (i) natural dentition (ND), (ii) splints with structured occlusal profiles (SS) (iii) splints with balancing interferences in the left molar region (OI). The particle size distribution was determined by a validated scanning procedure and curve fitted with the Rosin-Rammler function to determine X(50)-values. The EMG of both temporalis and masseter muscles was recorded simultaneously, and the total muscle work (TMW) was calculated. A jaw-tracking device recorded the incisal movement path (IMP). The functional parameters under the experimental conditions were compared by repeated-measures analysis of variance. The findings confirm our hypothesis. The X(50)-values differed significantly (P < 0.01) between ND and OI (4.34 vs. 4.60 mm), and between SS and OI (4.34 vs. 4.60 mm), respectively. In contrast, no significant differences (P > 0.05) were observed between SS and ND. There was no significant difference in both TMW (1269.0 vs. 1284.9 vs. 1193.9 µV*s) and IMP (720.2 vs. 735.3 vs. 723.1 mm) amongst the three conditions (P > 0.05). These findings confirm the assumption that the disturbance of the habitual chewing cycles by balancing-side interferences significantly reduces the masticatory performance in the short term. Occlusal balancing-side interferences are common technical failures of dental restorations. Simulation of this condition caused deterioration of masticatory performance in healthy young adults. Further studies should be carried out, on whether the observed effect is long-term and whether masticatory performance decreases even more in patients with reduced adaptive capacity.

Finite element analysis of the principles and loosening force of the conical telescopic crown: a computer-based study.

OBJECTIVES: The primary goal of the present finite element (FE) analysis of a special removable prosthesis, the conical telescopic crown (CTC), was to validate established results based on a rigid model of the CTC and to analyze its characteristic features as a function of the essential material and geometric parameters. Furthermore, the effectiveness of a new element, the composite stop (CS), was investigated. MATERIALS AND METHODS: The study used an axisymmetric FE model containing the inner and outer crown including resin or ceramic veneer, the CS, the cement layer between the interior crown and the tooth, and the upper part of the tooth itself. RESULTS: For a convergence angle (half-cone angle) α = 4 degrees and a moderate chewing force F = 150 N the loosening force decreased from - 50 N without to - 10 N with CS. Increasing α values yielded a decrease of the loosening force. Adherence between the inner (IC) and outer crown (OC) was achieved for all configurations (α = 2 degrees, 4 degrees, and 6 degrees), except for zirconium crowns with α = 6 degrees. In systems without CS, the maximum tensile stress in the veneer increased proportionally to F, but remained limited in those with CS. CONCLUSIONS: The angle a and the coefficient of static friction µ0 emerged as the decisive parameters of the CTC. The computed fitting/loosening behavior agreed well with results of a simple rigid-body model and experiments. The incorporation of a CS allows ceramic veneering of the outer crown. CLINICAL RELEVANCE: The optimal angle α of the CTC is ascribed to a number of customary material combinations for IC and OC. The CS limits the loosening forces of the CTC to values which guarantee non-traumatic removal of the prosthesis.

Accuracy of transfer of bite recording to simulated prosthetic reconstructions.

OBJECTIVES: A key aspect of complex restorative therapy is reconstruction of a new three-dimensional jaw relation. The objective of this study was to test the hypotheses that the initially recorded jaw relation would deviate substantially from the jaw position of the prosthetic reconstruction and that activity ratios of the jaw muscles would be significantly different for each of these jaw positions. MATERIALS AND METHODS: In 41 healthy subjects, 41 examiners incorporated intraoral occlusal devices fabricated with all the technical details and procedures commonly used during prosthetic reconstructions. The jaw positions in centric relation with the incorporated occlusal devices were telemetrically measured in the condylar, first molar and incisal regions, relative to intercuspation. Electromyographic (EMG) activity of the temporalis and masseter muscles was recorded, and activity ratios were calculated for homonymous and heteronymous muscles. RESULTS: The recorded jaw relation differed significantly (p < 0.001) from the jaw position reconstructed with the intraoral occlusal devices. The initially recorded jaw relation was reproduced with the intraoral occlusal device with spatial accuracy of approximately 0.3 mm in the condylar, molar and incisal regions. The EMG ratios between centric relations and the reconstructed positions were significantly different (p < 0.05) for the temporal muscle and the temporalis/masseter ratio. CONCLUSIONS: The findings revealed that three-dimensional jaw-relation recording may be reproduced in a simulated prosthetic reconstruction within the accuracy reported for replicate intraoral bite recordings. CLINICAL RELEVANCE: Centric relation recordings may be reproduced in a prosthetic reconstruction with the spatial accuracy of 0.3 mm.

[Clinical management of masticatory muscle pain: an update of the recommendations]. / Therapie bei Schmerzen der Kaumuskulatur: Aktualisierung der Empfehlungen.

Besides the different forms of odontalgia, myalgias of the masticatory muscles are the most frequent noninfectious complaints in the orofacial region. After summarizing the recommendations set forth by the Interdisciplinary Working Group of Orofacial Pain within the German Chapter of the IASP (German Pain Society), we present the current state of knowledge regarding the etiology, diagnosis, and therapeutic options, based on an extensive literature search. A systematic literature search was carried out in PubMed, the Cochrane Library, and the database of the Deutsche Zahnärztliche Zeitschrift. The results corroborate our previous recommendations that for the majority of patients pain reduction or pain relief can be achieved with noninvasive, reversible methods. Short- and long-term trials have added more evidence to the fact that different therapeutic measures have similar efficacy. In patients with chronic myalgias of the jaw muscles, involvement of a psychotherapist is crucial. Patient education, oral appliances, physiotherapeutic exercises, and acupuncture are recommended therapeutic measures, while physical therapy, pharmacotherapy, and psychological therapy received a limited recommendation.

Surface EMG of the masticatory muscles (part 2): fatigue testing, mastication analysis and influence of different factors.

The second part of this review of the literature on the clinical significance of surface electromyography (EMG) of the masticatory muscles systematically examines the results of clinical studies in patients with temporomandibular disorders (TMD), preferably randomized controlled trials, investigating relevant aspects of EMG activity during prolonged chewing activity (fatigue effects), during the mastication process, and under the influence of different factors. Studies on the influence of factors such as gender, age, tooth status, orofacial morphology and (acute) pain, the significance of different occlusal relationships during static and dynamic occlusion, and the impact of changes in static occlusion on EMG activity of the masticatory muscles were included in the review.

Surface EMG of the masticatory muscles (Part 3): Impact of changes to the dynamic occlusion.

The third part of this literature review on the clinical relevance of surface electromyography (EMG) of the masticatory muscles summarizes the results of clinical studies in patients with temporomandibular disorders (TMD), preferably randomized controlled trials, examining the impact of changes to the dynamic occlusion. Clenching in left and right laterotrusive positions results in decrease in EMG activity of masseter and temporalis muscles on both working and non-working side. Masseter muscle exhibits largely uniform bilateral activity in laterotrusive positions, independent of canine guidance or group function with and without non-working side contacts. There is a dominance of temporalis muscle activity on the working side and, in case of posterior contacts and balancing contacts, temporalis muscle activity increases and changes from an unilateral to a symmetrical pattern.

Comparison of particle-size distributions determined by optical scanning and by sieving in the assessment of masticatory performance.

Aim of this study was to introduce a feasible and valid technique for the assessment of masticatory performance that is comparable to the standard sieving method. Twenty-one chewing samples (Optosil) comminuted by healthy dentate adults were analysed with a sieving and scanning method. Scanning was performed using a conventional flatbed scanner (1200dpi). All scanned images underwent image analysis (ImageJ), which yielded descriptive parameters such as area, best-fitting ellipse for each particle. Of the 2D-image, a volume was estimated for each particle, which was converted into a weight. To receive a discrete distribution of particle sizes comparable to sieving, five chewing samples were used to calculate a size-dependent area-volume-conversion factor. The sieving procedure was carried out with a stack of 10 sieves, and the retained particles per sieve were weighed. The cumulated weights yielded by either method were curve-fitted with the Rosin-Rammler distribution to determine the median particle size x(50) . The Rosin-Rammler distributions for sieving and scanning resemble each other. The distributions show a high correlation (0·919-1·0, n= 21, P<0·01, Pearson's correlation coefficient). The median particle sizes vary between 3·83 and 4·77mm (mean: 4·31) for scanning and 3·53 and 4·55mm (mean: 4·21) for sieving. On average, scanning overestimates the x(50) values by 2·4%. A modified Bland-Altman plot reveals that 95% of the x(50) values fall within 10% of the average x(50) . The scanning method is a valid, simple and feasible method to determine masticatory performance.

Clinical relevance of surface EMG of the masticatory muscles. (Part 1): Resting activity, maximal and submaximal voluntary contraction, symmetry of EMG activity.

Based on a comprehensive computerized literature search supplemented by a specific manual search of the literature, the present review article focuses on concrete aspects of the application of surface electromyography (EMG) for evaluation of the masticatory muscles in general and of the masseter and anterior temporal muscles in particular, and presents the current base of knowledge on the clinical relevance of surface EMG in dental applications. In the first stage of the review, publications from the year 2000 or later reporting the results of controlled clinical trials (randomized as far as available) of patients with craniomandibular or temporomandibular disorders (TMD) were analyzed. Data from the selected publications were systematically compiled and divided into subject areas as follows: Resting activity, maximal and sub-maximal voluntary contraction, symmetry of EMG activity, and fatigue effects; EMG activity during mastication, factors (including pain) that affect EMG activity, and the impact of adjusting static and dynamic occlusal relationships; Effects of occlusal splints and other occlusal treatments. Surface electromyography is in principle a suitable tool for neuromuscular function analysis in the field of dentistry. If used according to the specific recommendations and in conjunction with a thorough and conscientious clinical history and physical examination, surface EMG measurements can provide objective, documentable, valid, and reproducible data on the functional condition of the masticatory muscles of an individual patient.

The effect of various jaw motor tasks on body sway.

Alterations of body sway caused by isometric contractions of the jaw muscles have been reported previously. The objective of this study was to test if motor tasks of the masticatory system with different control demands affect body posture differently during quiet stance. Position and sway displacements of the center of foot pressure (COP) were measured for 20 healthy subjects who either kept the mandible at rest or performed unilateral and bilateral maximum voluntary teeth clenching, feedback-controlled biting tasks at submaximum bite forces, or unilateral chewing. Two weeks later the measurements were repeated. Compared with quiet stance, the COP results revealed significant changes during the feedback-controlled biting tasks. Robust sway reduction and anterior displacement of the COP were observed under these conditions. Body oscillations were not significantly affected by maximum bites or by unilateral chewing. For most of the variables investigated there were no significant differences between unilateral and bilateral biting. Robust sway reduction during feedback-controlled biting tasks in healthy subjects involved a stiffening phenomenon that was attributed to the common physiological repertoire of posture control, and might optimize the stability of posture under these conditions.

Long-term training effects on masticatory muscles.

Neuromuscular adaptations during skill acquisition have been extensively investigated for skeletal muscles. Motor rehabilitation is the main target for application of motor training. Such measures are also relevant for the musculature of the jaw, but few data are available for motor adaptation of the masticatory system. The objective of this study was to evaluate and compare long-term training effects of different motor tasks on masseter and temporal muscles. In 20 healthy subjects, the electromyographic response to unilateral and bilateral maximum voluntary tooth clenching, balancing the mandible on a hydrostatic system under force-feedback-controlled conditions, and unilateral chewing was investigated in an initial session and then in two follow-up sessions separated by 2 and 10 weeks from baseline. Motor tasks were repeated three times for chewing, nine times for maximum biting (MB) and 24 times for the coordination tasks (CT). The sequences of the various motor tasks were applied once in the first session and twice in the second and third sessions. No effects of training were observed for MB tasks except for MB in intercuspation, for which significant yet transient avoidance behaviour occurred in the second session. No significant effects were found for chewing tests. For the CT, however, a robust significant long-term training effect was detected which reduced the electric muscle activity in session 2 by approximately 20% and in session 3 by approximately 40% compared with the initial measurements. The study showed that the masticatory muscles are remarkably prone to motor adaptation if demanding CT must be accomplished.

Influence of neck rotation and neck lateroflexion on mandibular equilibrium.

Neuromuscular interaction between neck and jaw muscles has been reported in several studies. However, the influence of experimentally modified posture of the neck on jaw muscle activity during isometric biting was not investigated so far. The aim of the present study was to test by the aid of simultaneous electromyographic and intraoral bite force measurements whether neck rotation and lateroflexion, in contrast to a straightforward neck position, change the isometric cocontraction patterns of masticatory muscles under identical submaximum bite forces of 50-200 N. Electric muscle activity of all masticatory muscles and changes of the reduction point (RP) of the resultant bite force vectors were examined. An anteroposterior displacement of the RPs could be observed for the rotated and lateroflexed neck position in comparison with the straightforward position. On the other hand, the results revealed no significant differences between bilateral muscle activation under the different test conditions. These findings suggest a force transmission between the neck and the masticatory system, but no essential activity changes in the masticatory muscles under short time posture modification of the neck.

Small unilateral jaw gap variations: equilibrium changes, co-contractions and joint forces.

After complex prosthetic reconstructions, small differences in vertical distances between the left and right side of the jaw may occur during jaw closing, nevertheless providing bilateral tooth contacts in intercuspation by small deformations of the mandible. Their effects on the co-contraction of the masticatory muscles, the temporomandibular joint reaction forces, and the point of application of the resultant bite force vector in the maxillary occlusion plane - the so-called reduction point - have not been investigated, thus far simultaneously in one sample. The main goal of this study was to investigate variations of these measures in an experimental intercuspation simulated by one anterior and two posterior force transmission points.

[Myalgia of the masticatory muscles]. / Myalgie der Kiefermuskulatur.

Masticatory muscle pain can be regarded as a regional manifestation of musculoskeletal disorders similar to those observed in other body regions. Along with temporomandibular joint pain and some painless disturbances related to mandibular mobility they are subsumed under the term temporomandibular disorders (TMD). Masticatory muscle pain is assumed to be associated with a variety of biophysiological risk factors. Valid diagnostic instruments make it possible to differentiate between the various TMD subgroups. In most cases, masticatory muscle pain can be treated/managed successfully. In a considerable number of patients, however, the pain persists over a long period of time despite therapeutic interventions. Understanding of the underlying neurobiological background of acute and chronic pain may help in therapeutic decision-making and evaluation of the therapeutic effects.

[Suggestion for a classification of odontalgias]. / Vorschlag einer Klassifikation der Odontalgien.

BACKGROUND: Toothache prevalence in the overall population is considerable. However, for clinical purposes, the classification schemes available do not appear to be sufficiently sophisticated. Moreover, not all known forms of dental pain are considered. A refined classification that meets current standards is therefore introduced. To facilitate diagnosis, the characteristic features of the various types of odontalgia are summarized. RESULTS: The new classification differentiates among seven different origins of pain: 1. dentinal pain (originating from the pulpal tissues), 2. pulpal pain (originating from the pulpal tissues), 3. periodontal pain, 4. alveolar-osseous pain, 5. atypical odontalgia, 6. heterotopic dental pain, 7. odontalgia associated with primary psychosocial factors. CONCLUSIONS: In our opinion, the proposed classification differentiates among the different forms of odontalgia more precisely than all previous ones. However, its viability and advantages over other available classification schemes still need to be verified in daily practice.

Surface electromyography of the masticatory muscles for application in dental practice. Current evidence and future developments.

On the basis of well-founded clinical evaluations and taking into account special methodological recommendations, surface electromyography (EMG) enables the acquisition of valid and reliable quantitative data on the functional condition of the masticatory muscles as a "neuromuscular functional analysis". On the basis of a current review of the literature, surface EMG appears to be able to deliver additional diagnostic and therapy-relevant information. Meaningful EMG data can be gained regarding resting activity, maximum muscle activation, frequency spectrum under various loading conditions, as well as bilateral symmetry of the contraction behavior of the jaw muscles. With these parameters--completed by kinematic information on movements of the mandible--it appears to be possible under clinical conditions to assess (a) the neuromuscular chewing performance after prosthetic reconstructions, (b) to record the influence of pain on the neuromuscular system, and (c) to document the individual efffect of therapeutic interventions on temporomandibular disorders.