Coactivation of jaw muscles: recruitment order and level as a function of bite force direction and magnitude

Effect of passive jaw opening on the electromyographic activity of the temporalis, masseter, digastric, and infrahyoid muscles in healthy adults

OBJECTIVE

To investigate the surface electromyography (EMG) activity of the temporalis, masseter, digastric, and infrahyoid muscles during passive jaw opening in healthy adults.

METHODS

The EMG activity of the masseter, temporalis, digastric anterior belly, and infrahyoid muscles on the right side was recorded during the four jaw-opening tasks: active opening to 20 mm (AO20); active opening to 40 mm (AO40); passive opening to 40 mm with a rubber mouth prop on the right posterior teeth (POR40); and passive opening to 40 mm with a mouth prop on the left posterior teeth (POL40).

RESULTS

The EMG amplitude of the digastric anterior belly and infrahyoid muscles in either POL40 or POR40 was significantly less than that in AO20 or AO40, respectively.

CONCLUSION

Passive jaw opening reduces the EMG activity of the digastric and infrahyoid muscles significantly and could help reduce the load on these muscles during prolonged mouth-opening conditions.

Surface electromyography for evaluating patients with oromandibular dystonia

OBJECTIVE

To investigate myoelectric signals of dystonic activities in oromandibular dystonia (OMD) subjects using surface electromyography (EMG).

METHODS

Twelve OMD subjects were included in this study. Resting myoelectric activities of the superficial masseter, anterior temporalis, and anterior belly of the digastric muscle on both sides were monitored, and dystonic muscle contractions were recorded using surface EMG. Myoelectric signal amplitude, the type of muscle contraction, and contraction rate for phasic activities were evaluated.

RESULTS

Surface EMG revealed that eight subjects had dystonic muscle activities in the phasic contraction pattern, three subjects had a tonic contraction pattern, and one subject had a mixed pattern. Synchronous contraction of dystonic muscles was frequently observed. Many of the monitored muscles showed high resting amplitudes.

CONCLUSION

Surface EMG detects abnormal muscle activities related to oromandibular dystonia. Surface EMG can serve as an objective method for diagnosing oromandibular dystonia.

Masticatory Muscles Activation and TMJ Space During Asymmetrically Loaded Jaw Closing

Masticatory muscle activation and temporomandibular joint (TMJ) load generated during asymmetrically loaded jaw closing are largely unknown. Two different strategies were developed to explain how the central nervous system (CNS) generates muscle activation patterns during motion: minimization of joint load (MJL) vs. minimization of muscle effort (MME). The aim of the present study was to investigate, experimentally, the neuromuscular strategy selected by the CNS to coordinate jaw closing in reaction to the application of an external asymmetric load. Masticatory muscle activation was measured with electromyography (EMG) and the minimum intra-articular distance (MID) was assessed by dynamic stereometry to infer joint loading. Ten healthy subjects performed jaw-closing movements against an asymmetric mandibular load set from 0.0 to 2.0 kg in 0.5-kg steps. Recordings were analyzed by exploratory and graphical statistical tools. Moreover, the observed differences in MID and EMG among the various mandibular loads were tested using non-parametric tests for repeated measures data. The ipsilateral-contralateral differences in MID and EMG of the anterior temporalis showed a significant increase (p < 0.001, p = 0.01) with increasing asymmetrical load with both joints being most heavily loaded at 1 kg. EMG signals of the masseter did not change significantly with increasing load. This study is the first to have analyzed the changes in the TMJ intra-articular space during asymmetrically loaded jaw-closing movements, not only three dimensionally and dynamically, but also combined with EMG. Asymmetrical load affected the TMJ space and masticatory muscle activation patterns, primarily resulting in an increased activation of the anterior temporalis muscle. This might suggest the involvement of a control mechanism to protect the joints from overloading. However, the results do not fully support the hypothesis of MJL nor the MME strategy.

Gape drives regional variation in temporalis architectural dynamics in tufted capuchins

Dynamic changes in jaw movements and bite forces depend on muscle architectural and neural factors that have rarely been compared within the same muscle. Here we investigate how regional muscle architecture dynamics-fascicle rotation, shortening, lengthening and architectural gear ratio (AGR)-vary during chewing across a functionally heterogeneous muscle. We evaluate whether timing in architecture dynamics relates to gape, food material properties and/or muscle activation. We also examine whether static estimates of temporalis fibre architecture track variation in dynamic architecture. Fascicle-level architecture dynamics were measured in three regions of the superficial temporalis of three adult tufted capuchins (Sapajus apella) using biplanar videoradiography and the XROMM workflow. Architecture dynamics data were paired with regional fine-wire electromyography data from four adult tufted capuchins. Gape accounted for most architectural change across the temporalis, but architectural dynamics varied between regions. Mechanically challenging foods were associated with lower AGRs in the anterior region. The timing of most dynamic architectural changes did not vary between regions and differed from regional variation in static architecture. Collectively these findings suggest that, when modelling temporalis muscle force production in extant and fossil primates, it is important to account for the effects of gape, regionalization and food material properties. This article is part of the theme issue 'Food processing and nutritional assimilation in animals'.

Effect of Finishing Protocols on the Surface Roughness and Fatigue Strength of a High-Translucent Zirconia

Purpose

In case of need for esthetical improvement of zirconia restorations, an individualization using extrinsic staining can be applied. This study aimed to evaluate the surface roughness and fatigue strength (survival) of high-translucency zirconia (3Y-TZP, YZ®HT, Vita Zanhfabrik) with extrinsic characterization and/or glaze.

Methods

Sixty (60) zirconia discs (12 × 1.2 mm) were obtained, sintered, and randomly distributed among three groups (n = 20) according to the surface finishing protocol: C (control), C + G (extrinsic characterization followed by a glaze layer), and G (glaze layer). The surface roughness (Ra) was analyzed with a contact profilometer. Subsequently, the specimens were subjected to a fatigue load profile starting at 120 N during 20,000 cycles at 4 Hz frequency, with a 5% increase at each step until failure. The failed specimens were evaluated under a stereomicroscope. Surface roughness analysis was evaluated by using one-way ANOVA and post hoc Tukey tests (95%); while fatigue survival probability was analyzed with Kaplan-Meier and Mantel-Cox (log- rank, 95%).

Results

One-way ANOVA revealed that surface roughness was affected by the finishing protocol, where C + G showed the highest mean value (0.46 ± 0.18 µm)A followed by G (0.30 ± 0.10 µm)B, and C (0.19 ± 0.02 µm)C. While for fatigue strength, the G protocol presented a higher mean value (243.00, and 222.36-263.63)A, followed by C + G (192.75 and 186.61-198.88)B and C (172.50 and 159.43-185.56)C.

Conclusion

Surface finishing protocols modify the surface roughness and fatigue strength of high-translucent zirconia. Regardless of the surface roughness, both glazing protocols improved the ceramic fatigue strength, favoring the restoration's long-term survival.

A reliability study on the cumulative averaging method for estimating effective stimulus time in vibration studies

Finding the reflex circuitry responsible for high-frequency vibration-induced muscle contraction takes work. The main challenge is to determine the effective stimulus time (EST) point at which continuous (sinusoidal) stimulation (i.e., vibration) triggers the reflex response. A novel "cumulated averaging method" has been previously proposed for estimating the EST point. In the current study, we aimed to test the reliability of the cumulated average method. We used five different whole-body vibration (WBV) frequencies in two experiments. The consistency between the EST points estimated from the first and second experiments was analysed with the intraclass correlation (ICC) and technical error of measurement (TEM). The ICC coefficient with 95% CI for the EST point estimation was 0.988 (0.950-0.997). The relative TEM was 1.3%. We concluded that the cumulated average method is highly reliable in estimating the effective stimulus time point for high-frequency continuous sinusoidal signals.

Biomechanical analysis of occlusal modes on the periodontal ligament while orthodontic force applied

OBJECTIVE

The study objective was to investigate four common occlusal modes by using the finite element (FE) method and to conduct a biomechanical analysis of the periodontal ligament (PDL) and surrounding bone when orthodontic force is applied.

MATERIALS AND METHODS

A complete mandibular FE model including teeth and the PDL was established on the basis of cone-beam computed tomography images of an artificial mandible. In the FE model, the left and right mandibular first premolars were not modeled because both canines required distal movement. In addition, four occlusal modes were simulated: incisal clench (INC), intercuspal position (ICP), right unilateral molar clench (RMOL), and right group function (RGF). The effects of these four occlusal modes on the von Mises stress and strain of the canine PDLs and bone were analyzed.

RESULTS

Occlusal mode strongly influenced the distribution and value of von Mises strain in the canine PDLs. The maximum von Mises strain values on the canine PDLs were 0.396, 1.811, 0.398, and 1.121 for INC, ICP, RMOL, and RGF, respectively. The four occlusal modes had smaller effects on strain distribution in the cortical bone, cancellous bone, and miniscrews.

CONCLUSION

Occlusal mode strongly influenced von Mises strain on the canine PDLs when orthodontic force was applied.

CLINICAL RELEVANCE

When an FE model is used to analyze the biomechanical behavior of orthodontic treatments, the effect of muscle forces caused by occlusion must be considered.

Comparison of locking and non-locking reconstruction plate-screw system in lateral mandibular defects by finite element analysis

PURPOSE

The purpose of this study is to analyze and compare stress distribution on bone screws and plate systems in locking and non-locking screw-plates design in lateral mandibular defects.

MATERIAL AND METHODS

Solid mathematical model of mandible was created by three-dimensional finite elements analysis and 25 mm length of lateral resection (L defect) was performed on the model. Models were reconstructed with 2.4 locking and non-locking reconstruction plate system. Each masticator muscles attached to mandible were simulated as direction, attachment area and magnitude on 3D model to compare with reality. The stress formation on bone and hardware system were evaluated.

RESULT

The stress values on the cortical bone, plate and screw system were higher in conventional plate model than the locking system model. The highest stress values were measured in the proximal segment especially in conjunction with conventional screw system. Furthermore, the distribution of stress on the bone surface was more homogenous in the locking system.

CONCLUSION

It is evident that the use of the locking system in 25 mm length lateral mandibular defects provides an additional advantage over conventional systems in reconstruction models.

Functional properties of single motor units in the human medial pterygoid muscle: Thresholds

BACKGROUND

Little is known regarding the functional properties of single motor units (SMUs) in the medial pterygoid muscle (MPt) during jaw movements.

OBJECTIVES

The aims are (a) to report the thresholds of onset of MPt SMUs during 4 goal-directed jaw movement tasks, and (b) to determine whether the threshold of onset of SMU activation varies with the velocity of jaw movement and the location within the muscle.

METHODS

Intra-muscular electrodes were inserted in the right MPt of 18 participants performing ipsilateral (right), contralateral, protrusive and opening-closing jaw movements recorded at 2 velocities. Task phases were as follows: BEFORE, OUT, HOLDING, RETURN and AFTER. SMU onset thresholds were determined from the displacement (mm) of the lower mid-incisor point. Electrode location within 4 arbitrary muscle divisions was determined with computer tomography. Statistical tests: Spearman's correlations, Kruskal-Wallis tests; significance accepted at P < .05.

RESULTS

A significant inverse relation occurred between velocity and threshold for the RETURN of the ipsilateral movement (n = 62 SMU thresholds), while a significant positive relation occurred for the OUT of the contralateral movement (n = 208); there were no significant associations for the protrusive (n = 131) and opening-closing (n = 58) tasks. Significant threshold differences occurred across the 4 muscle divisions only during the OUT of the contralateral and protrusive movements. Some evidence was provided for gender differences in MPt SMU properties.

CONCLUSIONS

The absence of a significant inverse relation between velocity and SMU threshold for most recorded movements suggests the MPt acts as a stabilizer of the jaw in horizontal and opening-closing jaw movements.

Over-the-counter bite splints: A randomized controlled trial of compliance and efficacy

Background

Occlusal splints are often used to curb the impacts of sleep bruxism (SB) on the dentition, and over‐the‐counter (OCT) options are becoming increasingly popular. OTC splints are usually fabricated at home by patients, but not routinely evaluated by dental professionals. It is unclear how OCT splints compare with more traditional splints that receive dental oversight.

Objectives

The present randomized controlled study tested how an OTC splint compared with a gold standard bite splint in terms of patient compliance (primary outcome) and efficacy (secondary outcomes).

Methods

Sixty‐seven subjects were randomly assigned to receive either the OTC (SOVA, N = 35) splint or the gold standard “Michigan” bite splint (MI, N = 32), with 61 completing the study (SOVA, N = 30; MI, N = 31). OTC‐splint subjects were required to fabricate their splints to clinically acceptable standards. Both groups wore the splints nightly for four months. Compliance was measured via daily diary. Efficacy outcomes evaluated stability, retention, periodontal health, night‐time rhythmic masticatory muscle activity (RMMA), and material wear.

Results

OTC‐splint subjects had difficulty fabricating splints to clinically acceptable standards. The number of night‐time RMMA bursts was significantly greater for the OTC splint group. Compliance and all other efficacy measurements were not significantly different between‐groups.

Conclusions

The results support the potential use of OTC splints for curbing the impacts of SB. However, the results strongly suggest that dentists should be actively engaged in overseeing patients' use of self‐fabricated appliances. This clinical trial is registered at ClinicalTrials.gov, Identifier number NCT02340663.

Evaluating the triplet hypothesis during rhythmic mastication in primates

Mammalian mastication involves precise jaw movements including transverse movement of the mandible during the power stroke. Jaw elevation and transverse movement are driven by asymmetrical jaw elevator muscle activity, which is thought to include a phylogenetically primitive and conserved triplet motor pattern consisting of: triplet I (balancing side: superficial masseter and medial pterygoid; working side: posterior temporalis), which reaches onset, peak and offset first; and triplet II (working side: superficial masseter and medial pterygoid; balancing side: posterior temporalis), which is active second. Although the presence of a triplet motor pattern has been confirmed in several primate species, the prevalence of this motor pattern - i.e. the proportion of masticatory cycles that display it - has not been evaluated in primates. The present study quantifies the presence and prevalence of the triplet motor pattern in five different primate species, Eulemur fulvus, Propithecus verreauxi, Papio anubis, Macacafuscata and Pan troglodytes, using mean onset, peak and offset time relative to working superficial masseter. In all five of the species studied, the mean triplet motor pattern was observed at peak muscle activation, and in four out of the five species the triplet motor pattern occurred more frequently than expected at random at peak muscle activation and offset. Non-triplet motor patterns were observed in varying proportions at different time points in the masticatory cycle, suggesting that the presence or absence of the triplet motor pattern is not a binomial trait. Instead, the primate masticatory motor pattern is malleable within individual cycles, within individual animals and therefore within species.

Fiber-type Composition of the Human Jaw Muscles—(Part 1) Origin and Functional Significance of Fiber-type Diversity

This is the first of two articles on the fiber-type composition of the human jaw muscles. The present article discusses the origin of fiber-type composition and its consequences. This discussion is presented in the context of the requirements for functional performance and adaptation that are imposed upon the jaw muscles. The human masticatory system must perform a much larger variety of motor tasks than the average limb or trunk motor system. An important advantage of fiber-type diversity, as observed in the jaw muscles, is that it optimizes the required function while minimizing energy use. The capacity for adaptation is reflected by the large variability in fiber-type composition among muscle groups, individual muscles, and muscle regions. Adaptive changes are related, for example, to the amount of daily activation and/or stretch of fibers. Generally, the number of slow, fatigue-resistant fibers is relatively large in muscles and muscle regions that are subjected to considerable activity and/or stretch.

Functional Activity of Superior Head of Human Lateral Pterygoid Muscle during Isometric Force

There is controversy as to the jaw tasks for which the superior head of the human lateral pterygoid muscle (SHLP) becomes active. The aim was to describe the functional activities of SHLP single motor units (SMUs) during horizontal isometric force tasks. In 11 subjects, 48 SMUs were recorded from computer-tomography-verified SHLP sites during generation of horizontal isometric force in the contralateral (CL), protrusive (P), and ipsilateral (IL) directions and intermediate directions (CL-P, IL-P). In eight subjects, SHLP SMUs were active in CL, CL-P, and P. Qualitatively, SHLP EMG activity increased with increased isometric force. Forty-two SMUs were active in directions other than IL; 6 exhibited activity at IL and other directions. The similarity of these data to previous human lateral pterygoid (IHLP) data supports the notion that SHLP and IHLP should be regarded as a single muscle, with activities shaded according to the biomechanical demands of the task.

Morphology and Physiology of Masticatory Muscle Motor Units

Motor unit territories in masticatory muscles appear to be smaller than territories in limb muscles, and this would suggest a more localized organization of motor control in masticatory muscles. Motor unit cross-sectional areas show a wide range of values, which explains the large variability of motor unit force output. The proportion of motor unit muscle fibers containing more than one myosin heavy-chain (MHC) isoform is considerably larger in masticatory muscles than in limb and trunk muscles. This explains the continuous range of contraction speeds found in masticatory muscle motor units. Hence, in masticatory muscles, a finer gradation of force and contraction speeds is possible than in limb and in trunk muscles. The proportion of slow-type motor units is relatively large in deep and anterior masticatory muscle regions, whereas more fast-type units are more common in the superficial and posterior muscle regions. Muscle portions with a high proportion of slow-type motor units are better equipped for a finer control of muscle force and a larger resistance to fatigue during chewing and biting than muscle portions with a high proportion of fast units. For the force modulation, masticatory muscles rely mostly on recruitment gradation at low force levels and on rate gradation at high force levels. Henneman's principle of an orderly recruitment of motor units has also been reported for various masticatory muscles. The presence of localized motor unit territories and task-specific motor unit activity facilitates differential control of separate muscle portions. This gives the masticatory muscles the capacity of producing a large diversity of mechanical actions. In this review, the properties of masticatory muscle motor units are discussed.

Jaw Clenching: Muscle and Joint Forces, Optimization Strategies

Realistic masticatory muscle and temporomandibular joint forces generated during bilateral jaw clenching are largely unknown. To determine which clenching directions load masticatory muscles and temporomandibular joints most heavily, we investigated muscle and joint forces based on feedback-controlled electromyograms of all jaw muscles, lines of action, geometrical data from the skull, and physiological cross-sectional areas acquired from the same individuals. To identify possible motor control strategies, we applied objective functions. The medial pterygoid turned out to be the most heavily loaded muscle for all bite directions. Biting with accentuated horizontal force components provoked the highest loading within the medial and lateral pterygoids. The largest joint forces were also found for these bite directions. Conversely, the lowest joint forces were detected during vertical biting. Additionally, joint forces with a clear posterior orientation were found. Optimization strategies with the elastic energy as objective function revealed the best fit with the calculated results.

Review: the Use of Electromyography on Food Texture Assessment

Sensory evaluation (SE) involves evoking, measuring and interpreting human responses to the properties of foods. Among these properties texture is an important one for food acceptability. Texture is mainly perceived through mastication, a process that changes food characteristics throughout time by comminuting and salivation. Electromyography (EMG) has emerged as a new tool in sensory evaluation mainly for assessing texture characteristics. Thus, it is interesting to analyze the knowledge so far generated and the procedures employed. Bipolar surface electrodes are placed on the four main masticatory muscles (masseter right-left and temporalis right-left) and their electric activity recorded during mastication. The signals need to be amplified and filtered, and afterwards their acquisition by a computer allows conditioning and analysis. The output is a series of bursts corresponding to the chews during mastication. EMG parameters include the area under the curve (linked to masticatory work) and voltage (linked to the force exerted) at different moments of the process. Many other parameters are derived from the recording and related to food texture characteristics. EMG allows the study of changes throughout the masticatory process, these changes in EMG parameters permit a better assessment of sensory characteristics than mechanical measurements. Nevertheless, close-fitting correlations have been found between sensorymechanical-EMG measurements. In the literature, texture assessment by using EMG includes many products like carrots, biscuits, cooked meat, cooked rice, cheese, etc., as well as different texture characteristics like tenderness, hardness, crunchiness, juiciness, among others. Nowadays, texture studies involving EMG are mainly linked to describing changes or differences in the signals, good descriptors of perceived differences. Nevertheless for quantifying purposes, in order to allow comparisons among authors, a first step should be undertaken to standardize EMG parameters definition and procedures used in SE.

Isotonic resistance jaw exercise alters jaw muscle coordination during jaw movements

The aim was to investigate the effects of isotonic resistance exercise on the electro-myographic (EMG) activity of the jaw muscles during standardised jaw movements. In 12 asymptomatic adults surface EMG activity was recorded from the anterior temporalis and masseter muscles bilaterally and the right anterior digastric muscle during right lateral jaw movements that tracked a target. Participants were randomly assigned to a Control group or an Exercise group. Jaw movement and EMG activity were collected (i) at baseline, before the exercise task (pre-exercise); (ii) immediately after the exercise task (isotonic resistance at 60% MVC against right lateral jaw movements); (iii) after 4 weeks of a home-based exercise programme; and, (iv) at 8-weeks follow-up. There were no significant within-subject or between-group differences in the velocity and amplitude of the right lateral jaw movements either within or between data collection sessions (P > 0.05). However, over the 8 weeks of the study, three of the tested EMG variables (EMG Duration, Time to Peak EMG from EMG Onset, and Time to Peak EMG activity relative to Movement Onset) showed significant (P < 0.05) differences in the five tested muscles. Many of the significant changes occurred in the Control group, while the Exercise group tended to maintain the majority of the tested variables at pre-exercise baseline values. The data suggest a level of variability between recording sessions in the recruitment patterns of some of the muscles of mastication for the production of the same right lateral jaw movement and that isotonic resistance exercise may reduce this variability.

Afferent sensory mechanisms involved in jaw gape-related muscle activation in unilateral biting

OBJECTIVES

In unilateral biting or chewing, the working/balancing-side ratio (W/B-ratio) of masseter activities is inversely proportional to the jaw gape which was interpreted as a neuromuscular strategy to protect occlusion. This suggests that jaw separation is afferently perceived, raising the question how this perception might work. In related studies, isometric biting was exerted on rubber pieces that slightly yielded similar to compressed food in chewing. We hypothesized that minor jaw movements associated with this yielding are necessary to elicit a jaw gape-related control of relative activation in isometric biting.

MATERIALS AND METHODS

Surface electromyograms of masseter muscles were recorded bilaterally in 20 males during (a) unilateral chewing, (b) isometric biting on rubber pieces inducing jaw gapes of 5, 3, 2, 1, and 0.5 mm, and (c) isometric biting with teeth embedded in rigid splints causing gapes of 5 and 1 mm.

RESULTS

With rubber, the masseter W/B-ratio increased from 100 % (5 mm) to 166 % (1 mm) (p = 0.0003) whereas with the splint it increased just slightly to 112 % (p = 0.005). With 1 mm gape, W/B-ratios in splint biting were significantly smaller than in rubber biting or in chewing (p = 0.01).

CONCLUSIONS

We conclude that minor jaw motion preceding peak force in unilateral biting is necessary to create afferent sensory information that could elicit jaw gape-related activation of masseter muscles.

CLINICAL RELEVANCE

Demonstrating a condition under which jaw gape-related activation can lose its occlusion protecting effect, these findings might contribute to disclose the causes of craniomandibular disorders.

Finite Element Analysis: A Maxillofacial Surgeon's Perspective

The science of finite element analysis (FEA) is purely a mathematical way of solving complex problems in the universe. In medical field, this is an innovation in biomedical research and development, as it gives easier mathematical solution to biological problems. This article deals with the understanding of various basic material properties of bone like Young's modulus, yield strength, Bulk modulus, shear modulus, Poisson's ratio and density from a maxillofacial surgeon's perspective. Basic concepts in FEA, its application, advantages, disadvantages, and limitations in the field of maxillofacial surgery have been discussed. The importance of surgical fraternity to be in coordination with evolving technologies has been emphasized for the future of evidence based practice of oral and maxillofacial surgery.

Reliability of electromyographic activity vs. bite-force from human masticatory muscles

The reproducibility of electromyographic (EMG) activity in relation to static bite-force from masticatory muscles for a given biting situation is largely unknown. Our aim was to evaluate the reliability of EMG activity in relation to static bite-force in humans. Eighty-four subjects produced five unilateral static bites of different forces at different biting positions on molars and incisors, at two separate sessions, and the surface EMG activities were recorded from temporalis, masseter, and suprahyoid muscles bilaterally. Intraclass correlation coefficients (ICCs) were determined, and an ICC of ≥ 0.60 indicated good reliability of these slopes. The ICCs for jaw-closing muscles during molar biting were: temporalis muscles, ipsilateral 0.58-0.93 and contralateral 0.88-0.91; and masseter muscles, ipsilateral 0.75-0.86 and contralateral 0.69-0.88. The ICCs for jaw-closing muscles during incisor biting were: temporalis muscles, ipsilateral 0.56-0.81 and contralateral 0.34-0.86; and masseter muscles, ipsilateral 0.65-0.78 and contralateral 0.59-0.80. For the suprahyoid muscles the 95% CIs were mostly wide and most included zero. The slopes of the EMG activity vs. bite-force for a given biting situation were reliable for temporalis and masseter muscles. These results support the use of these outcome measurements for the estimation and validation of mechanical models of the masticatory system.

Indications for jaw gape-related control of relative muscle activation in sequent chewing strokes

Jaw muscle activity ratios in unilateral isometric biting differ from ratios of unilateral chewing but approach the latter if the jaw gape in biting is made as small as the minimum interocclusal distance (MID) of chewing. Especially, the masseter working/balancing side ratio (W/B-ratio) becomes as asymmetric as in chewing, because of reduction in balancing side (BS) masseter activity. This behaviour of ratios might reflect a 'chewing-specific' motor strategy induced when isometric biting is performed with a 'chewing-like' gape. If this hypothesis applies, activity ratios should be associated with MIDs of sequent chewing strokes in a similar manner as with incremented jaw gapes in isometric biting. To test this prediction, bilateral surface electromyograms of masseter and anterior temporalis muscles and incisor movements were recorded during unilateral chewing in 52 subjects. W/B-ratios of masseter and temporalis activities and temporalis/masseter-ratios on both sides were calculated. The ratios were related to MIDs of consecutive chewing cycles. Three of the four ratios were associated with masticatory MID in the same manner as with jaw gape in isometric biting. In particular with decreasing MID, the masseter W/B-ratio increased from 1.5 to 2.2 (P < 0.01). This increase in asymmetry was attributed to a stronger decrease in masseter activity on the BS than on the working side. We conclude that relative jaw muscle activation is associated with interocclusal distance in a similar way in isometric biting and in chewing. This analogy supports the idea of a common jaw gape-related neuromuscular strategy facilitated by afferent signalling of interocclusal distance.

Synergist coactivation and substitution pattern of the human masseter and temporalis muscles during sustained static contractions

OBJECTIVE

Previous reports indicated that between-muscle substitution of active motor unit pools can be found in a variety of synergist muscles, including shoulder and leg muscles, but little information is available for the masticatory muscles. We hypothesized that, during a prolonged clenching effort performed at low- to moderate-bite force levels, a substitution pattern of activity can be found also in the masseter and anterior temporal muscles.

METHODS

Ten healthy volunteers were recruited and were asked to clench unilaterally on a force transducer for 10min at 10%, 15%, and 20% of the maximum bite force. During each session, bite force, perceived muscle pain and electromyographic activity were continuously assessed. Data analyses were performed by means of cross-correlation and periodogram analyses.

RESULTS

During sustained static contractions, different contraction patterns of jaw elevator muscles could be identified. These included a coactivation pattern, a substitution pattern, and several intermediate situations between coactivation and substitution.

CONCLUSIONS

The findings support the concept that the masticatory muscles are functionally heterogeneous and provide evidence that the neuromuscular strategies used by the masticatory system to perform sustained static contractions differ between individuals.

SIGNIFICANCE

Individual neuromuscular strategies might play a role in the development of masticatory muscle pain conditions.

A complete finite element model of a mandibular implant-retained overdenture with two implants: Comparison between rigid and resilient attachment configurations

PURPOSE

The aim of this study was to evaluate the influence of the retention mechanism on the behavior of a mandibular implant-retained overdenture (IRO) during the simulation of mastication. Therefore, a complete three-dimensional finite element model of a mandible with its IRO was developed.

MATERIALS AND METHODS

The geometry of the edentulous mandible and overdenture was generated from computed tomography. Two MKIII implants (Nobel Biocare) with ball abutments and Dalbo Plus (Cendres et Métaux) attachments were placed in the canine areas. Three foodstuff positions were analyzed for two retention mechanisms, "resilient" or "rigid". Special attention was given to the modeling of the mandibular environment and of the existing contact between the different components. A probable muscular action was determined following the minimal work principle.

RESULTS

The food-crushing force was provided by masseters with a two-third/one-third ratio between working and non-working sides. The "resilient" configuration provided a wider contact area between the mucosa of the denture bearing area and the prosthesis. An increase of the mastication force transiting through the mucosa was also noted and lower stresses were observed in the bone surrounding implants.

CONCLUSION

Resilient attachments allowed for an increase of the mastication load transiting through denture bearing surface. Furthermore, this study proposed an accurate model of the mandibular IRO, including its environment and faithful behavior reproduction.

Relationship Between the Morphologies of the Masseter Muscle and the Ramus and Occlusal Force in Patients With Mandibular Prognathism

PURPOSE

The purpose of this study was to examine the relationship between the morphologies of the masseter muscle and the ramus and occlusal force in patients with mandibular prognathism.

PATIENTS AND METHODS

The study group consisted of 71 patients with mandibular prognathism. They were divided into 2 groups, consisting of prognathism with or without symmetry, determined by frontal cephalogram analysis. All patients underwent 3-dimensional (3D) computed tomography (CT) and occlusal force was recorded with pressure-sensitive sheets.

RESULTS

In the cross-sectional area of masseter muscle, there were no significant differences between the right and left sides in the symmetry and asymmetry groups. In occlusal force, there was no significant difference between the symmetry and asymmetry groups. Occlusal force was not significantly correlated to the cross-sectional area of the ramus, but it was significantly positively correlated to the cross-sectional area of the masseter muscle (P < .05).

CONCLUSION

Occlusal force was associated with the ipsilateral cross-sectional area of masseter muscle in patients with prognathism; however, it was not associated significantly with the degree of mandibular deviation.

Functional properties and regional differences of human masseter motor units related to three-dimensional bite force

The aim of this study was to estimate numerically the properties of masseter motor units (MUs) in relation to bite force magnitude and direction three-dimensionally and to confirm the hypothesis that the properties differ between different parts of the muscle by means of simultaneous recording of MU activity along with the MU location and three-dimensional (3D) bite force. The MU activity of the right masseter of four healthy men was recorded using a monopolar needle electrode in combination with a surface reference electrode. The location of the needle electrode was estimated stereotactically with the aid of magnetic resonance images and a reference plate. The magnitude and direction of the bite force was recorded with a custom-made 3D bite force transducer. The recorded bite force was displayed on a signal processor, which enabled the participant to adjust the direction and magnitude of the force. The activities of 65 masseter MUs were recorded. Each MU had specific ranges of bite force magnitude and direction (firing range: FR) and an optimum direction for recruitment (minimum firing threshold: MFT). There was a significant negative correlation between MFT and FR width. There were functional differences in MU properties between the superficial and deep masseter and between the superficial layer and deep layer in the superficial masseter. These results indicate that the contribution of human masseter motor units to bite force production is heterogeneous within the muscle.

A working-side change to lateral tooth guidance increases lateral pterygoid muscle activity

The inferior head of lateral pterygoid (IHLP) is thought to play a critical role in the generation and control of lateral jaw movements.

AIM

The aim was to test the hypothesis that a change to the lateral tooth guidance (working-side occlusal alteration, OA) results in a significant change in the electromyographic (EMG) activity of the IHLP during standardised lateral jaw movements (laterotrusion) tracked by a jaw-tracking system.

METHODS

Ten trials of right laterotrusion were repeated under: control 1 (before occlusal alteration), OA (after occlusal alteration placement), and control 2 (after occlusal alteration removal) conditions in 14 subjects while recording left IHLP, bilateral anterior and posterior temporalis, masseter and submandibular muscles.

RESULTS

IHLP activity was significantly (p<0.05) increased with the occlusal alteration during the outgoing (movement from intercuspal position to approximately 5mm right) and return phases of laterotrusion. The other muscles demonstrated no change or a significant decrease in activity.

CONCLUSIONS

These findings suggest that a change to the occlusion on the working-side in the form of a steeper guidance necessitates an increase in IHLP activity to move the mandible down the steeper guidance. It must be emphasised that these data cannot be used as justification for occlusal therapy.

Alterations of temporalis muscle contractile force and histological content from the myostatin and Mdx deficient mouse

Myostatin (GDF8) and dystrophin are critical molecules for muscle organisation. Myostatin is involved in regulating muscle growth and development, whereas dystrophin is part of the dystrophin-glycoprotein complex (DGC), which anchors the cytoskeleton to the sarcolemma. We examined temporalis muscle morphology and function in myostatin deficient and dystrophin deficient (Mdx) mice in order to determine how myostatin and dystrophin affect bite force and muscle fibre composition. Bite forces from 4-month-old myostatin-/-, dystrophin deficient (Mdx) and normal control mice were measured by load cell and field stimulation of the temporalis muscle. Tissue sections were stained with haemotoxylin and eosin (H&E) and the periodic acid-Schiff reaction (PAS) to assess morphology and fibre type differences. A positive relationship between bite force and muscle mass for both genetic models was observed. Both Mstn-/- and Mdx mice showed significant elevation in bite force and muscle mass. Histological examination revealed greater muscle fibre cross-sectional area variability in Mdx mice (ANOVA, F=5.6, P<0.01). Surprisingly, the Mstn-/- mice demonstrated a disproportionate increase in bite force at higher stimulation frequencies with comparison of regression lines for force-frequency data (ANOVA, F=3.46, P<0.07). Muscle fibre typing using a PAS staining technique revealed significantly more type IIx/b glycolytic muscle fibres in the Mstn-/- mice. Our results suggest that histopathologies associated with Mdx mice did not diminish gross temporalis structure or function, whilst the force-frequency changes associated with Mstn-/- mice were reflected in an elevation of type IIx/b fibres.

Activity patterns of the masticatory muscles during feedback-controlled simulated clenching activities

In 10 normal subjects, the electromyographic (EMG) activities of the masseter, anterior and posterior temporalis, medial pterygoid, inferior lateral pterygoid, as well as the anterior digastric, were bilaterally recorded during clenching with various resulting force vectors. The intraoral force transfer was achieved with a three-component force transducer. The direction and magnitude of the force vectors were controlled by visual feedback. The goal of the study was to investigate how various clenching directions at constant magnitude of force influence the EMG activity in the masticatory muscles during gradual increase of the horizontal force component. Depending on the force direction and the individual muscle, an increase or decrease of activity was observed during clenching with increasing horizontal force components at constant magnitude of force. The inferior lateral pterygoid exhibited the highest activation (about 80% of maximal voluntary clenching) of all masticatory muscles. The medial pterygoid showed the greatest range of variation in activation behavior, and it was the most active muscle in relation to all clenching directions. In conclusion, the results show that with growing horizontal force components at constant magnitude of force, all muscles demonstrated an increase or decrease of activity in several clenching directions.

Modeling of Temporomandibular Joint Function Using MRI and Jaw-Tracking Technologies – Mechanics

The study of mechanics of the temporomandibular joint (TMJ) is important because its dysfunction and breakdown could be, at least partially, of mechanical origin. The incongruity of the articular surfaces of the TMJ is compensated by a fibrocartilaginous articular disc. Its dislocation and failure seem to be closely related to the development of osteoarthritis of the TMJ. The analysis of mandibular kinematics permits the detection and assessment of irregularities of TMJ function due to internal obstacles such as a displaced articular disc. Furthermore, the measurement of the dynamic relationship between the articular surfaces of the TMJ is useful to determine the strains undergone by the disc that if too high might compromise its integrity. The development of our research in TMJ mechanics has evolved from the acquisition of the traces of single mandibular points to an accurate and compact description of mandibular motion, in which the mechanical advantage of jaw muscles, and forces and torques acting on the jaw are considered as well. The combination of three-dimensional software models of TMJ anatomies obtained from MRI and jaw tracking with six degrees of freedom permits a subject-specific dynamic analysis of the intra-articular space, providing insight into individual disc deformation during function and TMJ loading. Studies performed with this system indicate that both TMJs are loaded during chewing, the balancing more so than the working joint. In fact, during chewing, the intra-articular distance is smaller for hard than for soft food, on closing than on opening, on the balancing than on the working side. This last finding is confirmed by static biting experiments, in which the condyle-fossa distance decreases more on the side contralateral to the bite force, depending on its magnitude. Also studies on the dynamics of compression areas indicate that plowing can occur through the disc during function, especially mediolaterally, due to stress field translation. This effect might contribute to cartilage wear and fatigue also because the disc is weaker mediolaterally. Further data indicate that the lateral area of the disc is mostly exposed to a higher mechanical energy density. This will be more intensively investigated using finite element method.

Jaw movement alters the reaction of human jaw muscles to incisor stimulation

The changes in the minimum time to consciously react (reaction time) and the order of jaw muscle recruitment to precisely controlled axial stimulation of the incisors during controlled jaw movements are not known. To this end, ten subjects were recruited to investigate the reaction time of bilateral temporalis and masseter muscles and bite force. Stimuli were delivered axially to the upper central incisors during active jaw closing and opening, and under static conditions. The results showed that the reaction time was increased an average of 35% during both jaw opening and closing movements when compared with static jaw conditions. The left temporalis was recruited approximately 10 ms before the right temporalis, whereas no significant side differences were found between the masseter muscles. The masseter muscles were recruited an average of 20 ms before the temporalis muscles during jaw closing, but no difference existed during opening. Under static conditions the reaction time in the bite force was approximately 16 ms longer than the left temporalis, but was not significantly different from the reaction time of any of the other muscles, indicating that, under the static conditions tested, the left temporalis was more often responsible for initiation of the mechanical reactions in the jaw. Because of active compensation, no force measurements were made during jaw movement. This study is a prerequisite for investigations into the modulation of reflexes during jaw movement, because a response to a stimulus commencing after the minimum reaction time may not be entirely reflex in origin.

Activity of inferior head of human lateral pterygoid muscle during standardized lateral jaw movements

OBJECTIVE

(a) To describe the changes in electromyographic (EMG) activity from selected jaw muscles during a standardized lateral jaw movement with the teeth together, and (b) to investigate the effects on jaw muscle activity of changes in both the rate of lateral jaw movement and the relative magnitude of jaw-closing force.

DESIGN

In 16 healthy volunteers, recordings were made using a jaw-tracking system, of mid-incisor point (MIPT) movements, as well as EMG activity from the contralateral inferior head of the lateral pterygoid muscle (IHLP), and bilateral anterior and posterior temporalis, masseter and submandibular muscles, during lateral jaw movement tasks at two speeds and two closing force levels with the teeth together.

RESULTS

The IHLP was the only muscle to show a consistent increase in activity in association with the outgoing phase of the task and a decrease during the return phase. Under high closing force at slow speed, the EMG activities of the IHLP and bilateral anterior temporalis and masseter muscles were significantly (p < 0.05) higher than those under a low closing force, while there was no significant change (p > 0.05) in bilateral posterior temporalis and submandibular muscles. The change from slow to fast lateral movement at low force did not significantly (p > 0.05) alter the mean activity except for the IHLP (increase in activity) and the contralateral anterior temporalis (decrease in activity).

CONCLUSIONS

The data suggest that the IHLP is one of the principal jaw muscles involved in a lateral jaw movement with the teeth together while the other jaw muscles may play a contributory or facilitatory role.

Exteroceptive reflexes in jaw-closing muscle EMG during rhythmic jaw closing and clenching in man

Exteroceptive jaw reflexes might play a role in normal functions of the mouth such as mastication. Until now these reflexes have only been studied under isometric conditions. The aim of this study was to compare exteroceptive reflexes in jaw muscle EMG during the closing phase of rhythmic open-close movements and clenching, at the same jaw gape and with similar muscle EMG. Reflexes consisting of successive waves of decreased and increased muscle activity (the Q, R, S and T waves of the post-stimulus electromyographic complex (PSEC)), evoked by light noxious electrical stimulation of the vermillion border of the lower lip, were recorded from the jaw closing muscles of 17 subjects. Differences between the two tasks occurred in two phases of the PSEC: (1) in an early phase, around the R wave, there was significantly less EMG during jaw closing (mean EMG ratio between jaw-closing and clenching 0.71), and (2) in a late phase, around the transition between the S to the T wave, there was significantly more EMG during jaw closing (mean EMG ratio: 1.40). The decrease in EMG activity around the R wave during jaw closing may be due to a change in reflex sensitivity at an interneuron level. The increase in EMG activity around the transition between the S and T waves during jaw closing might, at least in part, be due to a proprioceptive stretch reflex. This reflex is mediated by muscles spindles that are activated by the deceleration of the jaw evoked by the lip stimulus. The finding of inhibitory reflex mechanisms that predominate more during rhythmic jaw movements than during clenching in an early phase of the PSEC might be related to protecting oral tissues from trauma when the jaw is closing with potentially a large muscle force. In contrast, when food is held between the teeth, a possible inhibitory influence of light noxious stimuli is diminished.

Effects of increased muscle mass on mouse sagittal suture morphology and mechanics

The purpose of this study is to test predicted form-function relationships between cranial suture complexity and masticatory muscle mass and biomechanics in a mouse model. Specifically, to test the hypothesis that increased masticatory muscle mass increases sagittal suture complexity, we measured the fractal dimension (FD), temporalis mass, and temporalis bite force in myostatin-deficient (GDF8(-/-)) mice and wild-type CD-1 mice (all male, 6 months old). Myostatin is a negative regulator of muscle mass, and myostatin-deficient mice show a marked increase in muscle mass compared to normal mice. We predicted that increased sagittal suture complexity would decrease suture stiffness. The data presented here demonstrate that increased suture complexity (measured as FD) was observed in a hypermuscular mouse model (GDF8(-/-)) with significantly increased temporalis muscle mass and bite forces. Hypermuscular mice were also found to possess suture connective tissue that was less stiff (i.e., underwent more displacement before failure occurred) when loaded in tension. By decreasing stiffness, suture complexity apparently helps to dissipate mechanical loads within the cranium that are related to chewing. These results suggest that cranial suture connective tissue locally adapts to functional demands of the biomechanical suture environment. As such, cranial sutures provide a novel model for studies in connective tissue mechanotransduction.

Neuromuscular objectives of the human masticatory apparatus during static biting

OBJECTIVE

The central nervous system controls the muscles of mastication and may dictate muscle outputs according to a biologically important objective. This study tested the hypotheses that (a) the effective sagittal TMJ eminence morphology, and (b) the outputs of the masticatory muscles during static biting, are consistent with minimisation of joint loads or minimisation of muscle effort.

DESIGN

Numerical modelling predicted effective eminence morphology (from sagittal plane directions of TMJ force for centred loading over a range from molar to incisor biting) and TMJ and muscle forces during static unilateral biting in seven subjects. In vivo effective eminence morphology was measured from jaw tracking recorded from each subject. Muscle activities during biting tasks on first molar and incisor teeth were measured by electromyography using surface or indwelling electrodes.

RESULTS

Subject-specific predicted effective eminence morphology correlated with in vivo data (0.85< or =R2< or =0.99). Mixed and random coefficient analysis of covariance indicated good agreement between predicted and measured muscle outputs for all muscles of mastication investigated. Individual linear regression analysis showed that modelled muscle outputs accurately predicted EMG data, with average errors of 8% for molar and 15% for incisor biting.

CONCLUSIONS

Effective sagittal eminence morphology was consistent with minimisation of joint loads for all subjects. Masticatory muscle outputs during unilateral biting were consistent with minimisation of joint loads or minimisation of muscle effort, or both, depending on the subject. These results are believed to be the first to test model predictions of muscle output during biting for all muscles of mastication.