Influence of changing occlusal support on jaw-closing muscle electromyographic activity in healthy men and women

Effects of occlusal conditions on masseter and temporalis muscle activity: An electromyographic evaluation

Introduction

Dental occlusion contributes to the development of temporomandibular disorder.

Objective

This case control study examined the influence of different occlusal conditions on the surface electromyography (sEMG) of the superficial part of the masseter muscle (MM) and anterior part of the temporalis muscle (TA) during clenching in the maximum intercuspal position (MIP).

Materials and methods

Twelve healthy subjects had their anterior, right posterior, or left posterior teeth added by composite resin to generate the bilateral posterior, unilateral left, or unilateral right posterior tooth losses, respectively. Muscle activity in the resting stage, MM's and TA's maximum voluntary clenching (MVC; µV) in MIP, each muscle activity's symmetry (%), and ipsilateral MM and TA synergy (%) were measured by sEMG. All parameters were analyzed by SPSS version 23.0, and the significance level was set at p < 0.05.

Results

The MM's and TA's sEMG activity at the resting stage significantly differed from those at the other occlusal conditions (p < 0.05). Both muscles' MVC were highest at the MIP during clenching but lowest during anterior clenching. During unilateral posterior clenching, such MVC was higher at the occluding than at the non-occluding sides. The TA's symmetry during clenching at the anterior and unilateral posterior teeth was lower than that at the MIP during clenching. No significant difference was seen in the ipsilateral MM and TA synergy.

Conclusion

Different occlusal conditions influenced the MM's and TA's sEMG activity. Each masticatory muscle responded differently to the same occlusal conditions.

Skull Biomechanics and Simplified Cephalometric Lines for the Estimation of Muscular Lines of Action

Our study introduces a novel cephalometric analysis aimed at facilitating biomechanical simulations by elucidating the intricate relationship between craniofacial morphology and the size and inclination of the masseter muscle (MM) while incorporating muscle values. Our study analyzes the line of action of the MM drawn between the Gonion (Go) and Orbital (Or) points concerning dental and skeletal references (occlusal and Frankfort planes). A total of 510 pre-treatment lateral cephalometric tracings (217 males, 293 females, aged 6-50 years) and lateral Bolton standard tracings were examined. The key parameters investigated include (a) skeletal-cutaneous class (linear distance between projections of points A' and B' on the occlusal plane), (b) the angle between the perpendicular line to the occlusal plane and the Go-Or line at the molar occlusal point, and (c) the angle between the Go-Or line and the Frankfort plane. The assessment of anterior-posterior jaw discrepancy, measured as the skeletal-cutaneous class, ranged from -14.5 to 15.5 mm. Abnormal values were identified in two adolescents, showing no gender- or age-related patterns. The angle between the MM's line of action (Go-Or) and the normal to the occlusal plane averaged 39.3°, while the angle between Go-Or and Po-Or (Frankfort plane) averaged 41.99°. Age had an impact on these angles, with an average 3° decrease in adults and a 4° increase between ages 6 and 50. A weak relationship was observed between sagittal jaw discrepancy and the angle between Go-Or and the Frankfort plane, with about 20% of the variance explained by the anteroposterior maxillary-mandibular relationship. In conclusion, the study presents a cephalometric analysis of the relationship between craniofacial morphology and masseter muscle parameters. It finds that age influences the angles between key reference points, while the skeletal-cutaneous class does not exhibit age- or gender-specific trends. These findings can contribute to a better understanding of craniofacial biomechanics and aid in clinical orthodontic assessments and treatment planning.

Impact of post-orthodontic dental occlusion on masticatory performance and chewing efficiency

BACKGROUND

Whether precise orthodontic detailing of occlusion impacts masticatory function is unknown. In this study, we aimed to assess the impact of post-orthodontic dental occlusion on masticatory performance and chewing efficiency.

MATERIALS AND METHODS

Fifty-four adults who completed orthodontic treatment were categorized into two groups using the American Board of Orthodontics (ABO) model grading system: one meeting ABO standards (ABO, N = 29), the other failing to meet them (non-ABO, N = 25). The electromyographic (EMG) signals of the anterior temporalis (AT) and superficial masseter muscles were recorded bilaterally during static (clenching) and dynamic (gum chewing) tests. Chewing efficiency was measured by calculating the median particle size (MPS) and broadness of particle distribution (BPD) after five chewing trials of experimental silicone food at a standardized chewing rate.

RESULTS

Participants of the ABO group had a slightly more symmetric activation of the AT muscles during clenching (P = 0.016) and chewed a gum at a slower rate (P = 0.030). During the standardized chewing test with silicone food, ABO subjects had slightly greater EMG potentials at all muscle locations than non-ABO individuals (all P < 0.05). MPS and BDP did not differ significantly between groups (all P > 0.05).

LIMITATIONS

The severity of the initial malocclusion of the study participants was not in the statistical model as a potential confounder on the outcome measures.

CONCLUSIONS

Meeting ABO standards contributes to a slightly more balanced activation of the temporalis muscles during clenching and more efficient muscle recruitment during chewing but does not improve chewing efficiency.

Masseter response to long-term experimentally induced anterior crossbite in Sprague-Dawley rats

OBJECTIVE

To detect the long-term response to unilateral anterior crossbite (UAC) in masticatory muscles and in molecular biomarkers of peripheral blood leukocytes.

DESIGN

Fifty-six six-week-old Sprague-Dawley rats were used. The gene-fold changes in peripheral blood leukocytes were detected by the microarray analysis to compare the rats that received 20-week UAC treatment with age-matched controls (n = 4). Muscle atrophy-related gene Fbxo32 was selected based on the data of the microarray analysis verified by using real-time PCR. The remaining 36 rats were randomly separated in the UAC and control groups at 12 and 20 weeks (n = 12). The protein expression of Fbxo32 and the muscle injury and myogenesis-related markers, αB-crystallin and desmin, were detected in the masseter and lateral pterygoid muscles by western blot assay.

RESULTS

In the 20-week UAC group, the masseter muscle weight was lower than that in the age-matched control group, and the expression level of Fbxo32 gene in peripheral blood leukocytes was increased according to the microarray analysis confirmed by real-time PCR detection. The increased protein expression levels of Fbxo32 were detected in the masseter in the 20-week UAC group, and the protein expression levels of desmin and αB-crystallin were decreased at this time point. No similar changes were detected in the lateral pterygoid muscle.

CONCLUSIONS

Masseter atrophy is induced by long-term stimulation of UAC. The increased expression of the Fbxo32 gene in peripheral blood leukocytes may be a candidate biological marker of masseter atrophy.

Interferential effect of the over-erupted third molar on chewing movement

OBJECTIVE

To clarify whether over-eruption of the mandibular third molar can disturb chewing movement.

METHODS

Eighteen patients with a unilateral mandibular over-erupted third molar confirmed by both study cast observation and T-scan occlusal detection were selected from a sample of patients with complaints of temporomandibular disorder (TMD) symptoms. A unilateral gum-chewing trace was recorded separately for left and right side chewing by an electrognathography system. The average chewing pattern (ACP) was created based on segments from the recorded chewing trace to represent the chewing movement characteristics of each individual. Two factors, the TMD symptomatic side and the over-eruption side, were analysed for their effects on values of difference in the parameters (Δvalue) regarding the ACP between chewing with right and left side. Three-dimensional amplitudes of ACP and the cross point value of ACP with the vertical axis (termed the cross zero point value) which described the turning point of the chewing cycle from the balancing side to the working side, were compared between sides.

RESULTS

The over-eruption side had an effect on the Δvalue of the medial amplitude, the lateral amplitude, and the cross zero point (P<0.05), but the symptomatic side didn't (P>0.05). When chewing on the over-eruption side, the medial amplitude was shorter, the lateral amplitude was larger, and the cross zero point value was smaller than those when chewing on the other side (P<0.05).

CONCLUSION

The present data indicate an effect of the over-erupted mandibular third molar on the chewing pattern while that from the symptom(s) is limited.

Contribution of each masticatory muscle to the bite force determined by MRI using a novel metal-free bite force gauge and an index of total muscle activity

PURPOSE

To develop a metal-free bite force gauge that can monitor the bite force in a strong magnetic field and to analyze the correlations between bite-force and total T2 shift of the mastication muscles.

MATERIALS AND METHODS

The gauge used a micro-pressure sensor made of optical fiber. Ten subjects performed a 60-s isometric bite task at 40% of maximum clenching in various occlusal support conditions (intact dentition, right molar loss, or left molar loss). Spin-echo images were taken with a 1.5 Tesla scanner before and immediately after the task to correlate the bite force with the mean voxel count, mean shift in transverse relaxation time (ΔT2), and total T2 shift of each masticatory muscle.

RESULTS

Measurements of total T2 shift identified significant correlations between the bite force and activities of the superficial layer of the bilateral masseter muscle, regardless of the occlusion condition (intact dentition: left, P = 0.007 and right, P < 0.001; right molar loss: left, P = 0.02 and right, P = 0.021; and left molar loss: left, P = 0.022 and right, P = 0.049). In contrast, significant correlations were not detected between the bite force and mean ΔT2 (intact dentition: left, P = 0.102 and right, P = 0.053; right molar loss: left, P = 0.393 and right, P = 0.868; and left molar loss: left, P = 0.531 and right, P = 0.92).

CONCLUSION

Measurement of total T2 shift using a metal-free bite force gauge is a more sensitive index of individual muscle activity than mean ΔT2 during a bite task. J. MAGN. RESON. IMAGING 2016;44:804-813.

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.

Precision of jaw-closing movements for different jaw gaps

Jaw-closing movements are basic components of physiological motor actions precisely achieving intercuspation without significant interference. The main purpose of this study was to test the hypothesis that, despite an imperfect intercuspal position, the precision of jaw-closing movements fluctuates within the range of physiological closing movements indispensable for meeting intercuspation without significant interference. For 35 healthy subjects, condylar and incisal point positions for fast and slow jaw-closing, interrupted at different jaw gaps by the use of frontal occlusal plateaus, were compared with uninterrupted physiological jaw closing, with identical jaw gaps, using a telemetric system for measuring jaw position. Examiner-guided centric relation served as a clinically relevant reference position. For jaw gaps ≤4 mm, no significant horizontal or vertical displacement differences were observed for the incisal or condylar points among physiological, fast, and slow jaw-closing. However, the jaw positions under these three closing conditions differed significantly from guided centric relation for nearly all experimental jaw gaps. The findings provide evidence of stringent neuromuscular control of jaw-closing movements in the vicinity of intercuspation. These results might be of clinical relevance to occlusal intervention with different objectives.

Effect of experimental jaw muscle pain on EMG activity and bite force distribution at different level of clenching

Bite force at different levels of clenching and the corresponding electromyographic (EMG) activity in jaw-closing muscles were recorded in 16 healthy women before, during and after painful stimulation of the left masseter muscle. Experimental pain was induced by infusion of 5·8% hypertonic saline (HS), and 0·9% isotonic saline (IS) was infused as a control. EMG activity was recorded bilaterally from the masseter and temporalis muscles, and static bite force was assessed by pressure-sensitive films (Dental Pre-scale) at 5, 50 and 100% of maximal voluntary contraction (MVC) during each session. Visual feedback was applied by showing EMG activity to help the subject perform clenching at 5, 50 and 100% MVC, respectively. EMG activity at 100% MVC in left and right masseter decreased significantly during painful HS infusion (1·7-44·6%; P < 0·05). EMG activity at 5% and 50% MVC was decreased during HS infusion in the painful masseter muscle (4·8-18·6%; P < 0·05); however, EMG activity in the other muscles increased significantly (18·5-128·3%; P < 0·05). There was a significant increase in bite force in the molar regions at 50% MVC during HS infusion and in the post-infusion condition (P < 0·05). However, there were no significant differences in the distribution of forces at 100% MVC. In conclusion, experimental pain in the masseter muscle has an inhibitory effect on jaw muscle activity at maximal voluntary contraction, and compensatory mechanisms may influence the recruitment pattern at submaximal efforts.

Electromyographic activity during awake tooth grinding tasks at different jaw posture in the sagittal plane

OBJECTIVE

The goal of this study was to evaluate the electromyographic (EMG) activity of the anterior temporalis, suprahyoid, infrahyoid and trapezius muscles during tooth grinding at different jaw posture tasks.

MATERIALS AND METHODS

Participants were 30 healthy subjects with natural dentition, bilateral molar support and incisive protrusive guidance. Bipolar surface electrodes were located on the right side of the subject. EMG recordings were performed in the following tasks: (A) Eccentric grinding from intercuspal position to protrusive edge-to-edge contact position; (B) concentric grinding from protrusive edge-to-edge contact position to intercuspal position; (C) eccentric grinding from intercuspal position to the maximum voluntary retrusive position; and (D) concentric grinding from the maximum voluntary retrusive position to intercuspal position. The results were analyzed statistically by Friedman test and Wilcoxon signed rank-sum test.

RESULTS

EMG activity in the anterior temporalis and infrahyoid muscles was significantly higher during task C than the other tasks. EMG activity in the suprahyoid muscles was significantly higher during task C than task D. EMG activity in the trapezius muscle was significantly higher during task C than tasks A and B.

CONCLUSIONS

The higher EMG activity recorded in task C could become important when its frequency, duration and magnitude are enough to exceed the adaptation capability of the individual.

Stable tooth contacts in intercuspal occlusion makes for utilities of the jaw elevators during maximal voluntary clenching

Data are inconsistent concerning whether the level of the surface electromyographic (SEMG) activity of jaw-closing muscles increases when biting forces elevated during maximal voluntary clenching (MVC). In this study, T-Scan III system and BioEMG III system were used to record bite force, occlusal contacts and SEMG activity of the anterior temporalis (TA) and of the masseter muscles (MM) simultaneously. Recordings were obtained from 16 healthy young adult males during different conditions: (i) a fast MVC from resting position to intercuspal position (ICP); (ii) mandibular movements from ICP to protrusive or lateral edge-to-edge positions with teeth in contact with biting; (iii) a fast MVC in protrusive and lateral edge-to-edge positions. A higher level of SEMG activity was associated with a higher bite force during occluding movements (P < 0.05). However, during fast MVC from rest to ICP, the largest number of occlusal contacts was achieved and distributed more symmetrically, the highest level of biting force was obtained, but the SEMG activity of the jaw elevator muscles was reduced compared with its maximum level (P < 0.05). This phenomenon was not observed during the fast MVC in protrusive or lateral edge-to-edge positions. The present results that a lower SEMG activity was associated with the largest number of occlusal contacts and the highest level of bite force during centric MVC demonstrated a complex integration of jaw-closing muscles when a stable occlusion is present.

A possible biomechanical role of occlusal cusp-fossa contact relationships

Biomechanical features of occlusal contacts are important in understanding the role of the occlusion contributing to masticatory function. Cusp-fossa contact is the typical pattern of occlusion between upper and lower teeth. This includes static relations, such as that during clenching, and dynamic relations when mandibular teeth contact in function along the maxillary occlusal pathways, as during mastication. During clenching in the maximum intercuspal position (ICP), cuspal inclines may take the role of distributing the occlusal forces in multi-directions thus preventing excessive point pressures on the individual tooth involved. During chewing movement on the functional side, the mandible moves slightly from buccal through the maximum ICP to the contralateral side. The part of the chewing cycle where occlusal contacts occur and the pathways taken by the mandible with teeth in occlusal contacts are determined by the morphology of the teeth. The degree of contact is associated with the activity of the jaw muscles. To obtain repeatable static and dynamic occlusal contact information provided by the morphology of the teeth, maximum voluntary clenching and chewing movements with maximum range are needed. In conclusion, in addition to the standard occlusal concepts of centric relation/centric occlusion and group function/cuspid protection relation, biomechanics in static and dynamic cusp-fossa relationships should be included to develop an understanding of occlusal harmony which includes no interfering or deflective contacts in functional occlusal contact.

Postural loads during walking after an imbalance of occlusion created with unilateral cotton rolls

Background

It was showed that stomatognathic functions correlate with alterations in locomotion, that are detectable through the analysis of loading during walking. For example, subjects with symptoms of Temporomandibular disorders (TMDs) showed a significant higher load pressure on the two feet, respect to health subjects, when cotton rolls were inserted. This previous study appeared to suggest that the alteration of postural loads associated to a particular alteration of stomatognathic condition (in this case, the cotton rolls inserted between the two dental arches) is detectable only in TMD's subjects, while it resulted not detectable in health subjects, because in that study, health subjects did not show any significant alteration of postural loads related to the different stomatognathic tested conditions. In other words, in that previous study, in the group of health subjects, no significant difference in postural loads was observed among the different test conditions; while TMD subjects showed a significant higher load pressure on the two feet when cotton rolls were inserted, respect to all the other tested conditions. Thus, the aim of this study was to better investigate these correlations in health subjects without TMD's symptoms, testing other different intra-oral conditions, and to verifywhether an experimentally induced imbalance of occlusion, obtained putting an unilateral cotton roll, could cause an alteration of postural loading on feet during walking.

Findings

In a sample of thirty Caucasian adult females (mean age 28.5 ± 4.5), asymptomatic for TMDs, when a cotton roll was positioned on the left or the right sides of dental arches, so causing a lateral shift of the mandible, the percentage of loading and the loading surface of the ipsi-lateral foot, left or right, were found to be significantly lower than in habitual occlusion (p < 0.05). Males were not included because of their different postural attitude respect to females. Further studies in a sample of males will be presented.

Conclusions

This study showed that in health subjects without TMD's symptoms, an experimentally induced imbalance of the occlusion, obtained through an unilateral cotton roll, is associated to detectable alterations in the distribution of loading on feet surface, during walking.

Mandibular condylar cartilage response to moving 2 molars in rats

INTRODUCTION

The purpose of this study was to investigate the responses of mandibular condylar cartilage to moving 2 molars in different combinations.

METHODS

Rats were assigned to male and female control and experimental groups (each, n = 5). Elastic rubber bands were used to move medially the maxillary left and the mandibular right first molars in experimental group I. The same method was used to distally move the maxillary left and the mandibular right third molars, 2 mandibular third molars, and 2 maxillary third molars in experimental groups II, III, and IV, respectively. At the end of the eighth week, all condyles were examined histologically. The areas of histologic change as a percentage of total cartilage area were compared by using the Mann-Whitney U test.

RESULTS

Cartilage degenerative remodeling was observed in experimental groups II, III, and IV. The percentage areas of degenerative remodeling were higher in female experimental groups II and III than in the female control group, and in female experimental group II than in female experimental group IV and male experimental group II (all, P <0.05).

CONCLUSIONS

The mandibular condylar cartilage of female rats responded variously to different combinations of molar movement; the most obvious remodeling was observed in groups in which the maxillary left and mandibular right third molars were moved.