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The masticatory muscles of the horse are arranged in an asymmetrical pattern. Four individual muscles on the medial side of the mandible are opposed by one muscle (M. masseter) on the lateral side. However, recent studies on various herbivorous mammals indicate that the masseter muscle features a complex stratigraphic structure that might account for a functional diversity resembling an arrangement of several individual muscles. The functional consideration of the multidirectional equine masticatory movements leads to a similar hypothesis. In order to elucidate the detailed anatomy of the equine masseter muscle, eight cadaveric equine heads were dissected. Additional 29 skull specimens were assessed with regard to the masseteric attachment within the Fossa masseterica. A constant arrangement of nine individual muscular layers within the masseteric muscle was determined. The individual layers were clearly separated by tendon plates and their attachment areas at the masseteric fossa of the mandible were arranged in a constant pattern of bony ridges. With similar consistency, the main trunk of the masseteric nerve was found to run from dorsocaudal to ventrorostral: On that course, the nerve penetrated between the muscular layers from medial to lateral at constant positions. The findings of this study serve as a basis for further studies with the aim of developing biomechanical concepts of equine masticatory movement. [ABSTRACT FROM AUTHOR]

Background: The popularity of noninvasive botulinum toxin type A (BTX‐A) injections for masseter muscle hypertrophy is increasing among Asian individuals with a square‐shaped lower face. Aims: This study aimed to analyze the adverse events (AEs) caused by BTX‐A injections into the masseter muscle. Patients/Methods: This observational study retrospectively evaluated 46 250 patients who underwent BTX‐A injections into the masseter muscle in 2022. The inclusion criteria were the diagnosis of an AE by the physician at the return visit and subsequent follow‐up of progress (n = 223). The patients who were lost to follow‐up (n = 40) were excluded from the study. Results: Among the 223 patients with AEs, the most common AE was paradoxical bulging (88.3%, n = 197/223). The average period from treatment until confirmation of improvement was 159.6 ± 113.6 days (range 13–667 days) for all AEs, all of which were temporary. The period until improvement was 166.1 days in the intervention group (n = 122) and 151.9 days in the observation group (n = 101) (p = 0.24). As the period until improvement of AEs included the period until the patients visited the clinics and the improvements were confirmed by physicians, the actual period was likely to have been shorter. Conclusions: (1) All AEs were temporary. (2) All AEs improved within 22.2 months (within 5.3 ± 3.8 months on average). (3) There was no significant difference between the intervention and observation groups in the period until the improvement of AEs. [ABSTRACT FROM AUTHOR]

Introduction: Botulinum neurotoxin (BoNT) injections are widely used for the treatment of masseter muscle hypertrophy in Southeast Asia. However, there remains a lack of consensus regarding the optimal injection technique. This study aimed to compare the efficacy and patient discomfort associated with single‐entry point injections versus multiple three‐point injections for masseter muscle hypertrophy treatment with BoNT. Materials and Methods: Sixteen participants, comprising both male and female Korean adults aged 22–63, were enrolled in the study. On the left side of the face, single‐entry point injections were administered, followed by multidirectional injections, while on the right side, three‐point injections were given. Pain intensity during the procedure was assessed using visual analogue scale scores. Result: Our results revealed that participants experienced lower levels of pain with single‐entry point injections compared to three‐point injections (average visual analogue scores of 3.31 and 5.19, respectively). Conclusion: These findings highlight the potential benefits of single‐entry point injections in reducing patient discomfort during masseter muscle hypertrophy treatment with BoNT. We advocate for further research to validate these findings and encourage practitioners to consider single‐entry point injections as a viable option for enhancing treatment outcomes in their clinical practice. [ABSTRACT FROM AUTHOR]

This document contains abstracts from the Anatomical Society Winter meeting in Liverpool, UK. The abstracts cover a range of topics related to craniofacial anatomy and development. Some of the studies discussed include the development of an automated craniofacial segmentation tool, postnatal changes in the semicircular canals of mice, the role of the digastric muscle in rabbits during mastication, tissue engineering the human TMJ disc, diet-related morphological changes in the craniofacial skeleton of red squirrels, and the biomechanics of biting in infants. Other studies explore tooth regeneration in mice, the use of anatomical models for remote learning, cranial blunt force trauma, the development of the soft palate in mammals, the morphology of the mandibular symphysis in snakes, and the mechanical behavior of cranial sutures in cases of craniosynostosis. Additionally, there is a study on the presence of bone growths in the external auditory canal of prehistoric human populations and the creation of accurate 3D printed models of juvenile skulls for educational purposes. The integration of ultrasound into the anatomy curriculum and the anatomical covariation between the endocranium and brain are also examined. These studies provide valuable insights into various aspects of craniofacial anatomy and development. [Extracted from the article]

Background: Cancer cachexia is prevalent in head and neck cancer patients. The L3 skeletal muscle index (SMI) is often used to assess sarcopenia and cachexia but is infrequently able to be measured in this population. Masseter muscle thickness (MT) may serve as an alternative predictor of cachexia. Methods: SMI and MT were calculated from 20 trauma (CTRL) and 40 cachectic (CA‐CX) and non‐cachectic (CA‐NCX) head and neck cancer patients. Area Under the Curve of the Receiver Operating Characteristics (AUC‐ROC) analysis was performed for SMI and MT. Results: Both SMI and MT were significantly decreased in CA‐CX patients (vs. CA‐NCX mean difference −19.5 cm2/m2 and −2.06 mm, respectively) and significant predictors of CA‐CX (AUC = 0.985 and 0.805, respectively). When analyzed by sex, the same findings were observed for MT in males and trended toward significance in females. Conclusions: Compared with SMI, MT is a good alternative prognostic biomarker to determine CA‐CX status in HNC patients. [ABSTRACT FROM AUTHOR]

Objective: This study aims to evaluate the effects on bite force and muscle thickness of the botulinum toxin (BoNT) injection for patients with sleep bruxism (SB) by comparing injections into the masseter muscle only and both the masseter and the anterior belly of the digastric muscle (ABDM) in a clinical trial. Methods: Twelve SB patients received BoNT‐A injections using US‐guided techniques into the masseter muscle only (Group A), while the remaining 12 SB patients received injections into both the masseter and ABDM (Group B). Bite force and muscle thickness were measured before injection, as well as 1 and 2 months after injection. Results: The bite force and masseter muscle thickness decreased in both Group A and Group B before injection, and at 1 and 2 months after injection. However, there was no significant difference (p >.05, repeated measures analysis of variance) between the two groups, and there was also no significant difference in ABDM thickness (p >.05, repeated measures analysis of variance). Conclusion: This study is the first to assess the short‐term effects of BoNT injected into ABDM for SB control. Results show no influence on SB reduction, suggesting the need for further research on BoNT's effectiveness in controlling intense ABDM contractions during sleep and assessing suprahyoid muscle potential impact on rhythmic masticatory muscle activity occurrence. [ABSTRACT FROM AUTHOR]

Background: Oral food processing is an important part of daily food intake. A major part of this process is mastication, which prepares a bolus of food for swallowing by mechanically crushing and grinding ingested food between the teeth using rhythmic movements. Masticatory dysfunction is common in the elderly and in some neurological disorders and can have serious negative health consequences. Objective: This study investigated the effect of restricted mastication, achieved by experimentally reducing the duration of mastication, on masticatory patterns and subsequent swallowing function. Methods: Thirty healthy men (25 ± 3 years old) were instructed to chew gum jelly with a free mastication duration (G100), a half and a quarter duration of G100. Masseter and digastric electromyographic (EMG) activity was recorded to assess mastication and swallowing activity, respectively. In addition, the acceleration of the thyroid cartilage ridge was measured with an accelerometer. The root mean square (RMS) of muscle EMG activity in the masseter and digastric muscles, the number of masseter EMG bursts, time to peak and total duration of each masseter EMG burst, swallowing duration and laryngeal elevation latency were analysed. Results: Restricting masticatory duration reduced the number of mastication cycles (p <.001), prolonged the time to peak (p <.001) and total duration of masseter EMG bursts (p <.001) and resulted in an overall increased RMS score of masseter muscle activity (p =.017). Furthermore, restricted masticatory duration led to a decrease in both swallowing duration (p =.001) and laryngeal elevation latency (p =.012), with a significant increase in the RMS score of digastric muscle activity (p <.001). Conclusions: Under the experimental conditions of restricted mastication, several adaptation features were observed, including changes in masticatory cycle characteristics and swallowing duration. Thus, although the overall masticatory efficiency was reduced, these adaptations allowed healthy individuals to still swallow safely. [ABSTRACT FROM AUTHOR]

Background: A quantitative approach to predict expected muscle activity and mandibular movement from non‐invasive hard tissue assessments remains unexplored. Objectives: This study investigated the predictive potential of normalised muscle activity during various jaw movements combined with temporomandibular joint (TMJ) vibration analyses to predict expected maximum lateral deviation during mouth opening. Method: Sixty‐six participants underwent electrognathography (EGN), surface electromyography (EMG) and joint vibration analyses (JVA). They performed maximum mouth opening, lateral excursion and anterior protrusion as jaw movement activities in a single session. Multiple predictive models were trained from synthetic observations generated from the 66 human observations. Muscle function intensity and activity duration were normalised and a decision support system with branching logic was developed to predict lateral deviation. Performance of the models in predicting temporalis, masseter and digastric muscle activity from hard tissue data was evaluated through root mean squared error (RMSE) and mean absolute error. Results: Temporalis muscle intensity ranged from 0.135 ± 0.056, masseter from 0.111 ± 0.053 and digastric from 0.120 ± 0.051. Muscle activity duration varied with temporalis at 112.23 ± 126.81 ms, masseter at 101.02 ± 121.34 ms and digastric at 168.13 ± 222.82 ms. XGBoost predicted muscle intensity and activity duration and scored an RMSE of 0.03–0.05. Jaw deviations were successfully predicted with a MAE of 0.9 mm. Conclusion: Applying deep learning to EGN, EMG and JVA data can establish a quantifiable relationship between muscles and hard tissue movement within the TMJ complex and can predict jaw deviations. [ABSTRACT FROM AUTHOR]

Background: It remains unclear how the salivary flow and the fat content of food affect bolus formation during mastication. Objectives: We aimed to clarify: (1) how hyposalivation affects jaw‐closing and hyoid‐elevating muscle activities in bolus formation, and (2) if the effect of hyposalivation on muscle activity depends on the fat content of food. Methods: Eighteen healthy male volunteers were instructed to freely ingest four test foods: Plain, Fat without seasoning, Fat with seasoning, and Soft rice crackers. Masseter and suprahyoid electromyographic activities were recorded before and 30 min after the administration of atropine sulfate, a muscarinic receptor antagonist that induces hyposalivation. Results: Hyposalivation extended the masticatory duration significantly in all the test foods except Fat with seasoning. Masticatory cycle time was significantly longer with vs without hyposalivation for the Soft (p =.011). Suprahyoid activity/cycle was significantly greater with vs without hyposalivation (p =.013). Masticatory cycle time was significantly longer at the late stage with vs without hyposalivation for the Soft (p <.001). Suprahyoid activity/cycle was significantly greater at the middle (p =.045) and late stages (p =.002) with vs without hyposalivation for the Soft and greater at the late stage with vs without hyposalivation for the Plain (p =.043). Changes in masticatory cycle time and suprahyoid activity/cycle for these foods had significantly positive relationship (p <.001). Conclusion: Hyposalivation‐induced changes in masticatory behaviours resulted from the middle and late stage suprahyoid activity. Fat content and seasoning compensate for salivary flow inhibition. [ABSTRACT FROM AUTHOR]

Background: Neuroplasticity induced by mandibular advancement appliance (MAD) in patients with obstructive sleep apnoea (OSA) is poorly documented. Objective: This randomised placebo‐controlled crossover mechanistic study assessed the effects of short‐term use of a MAD on corticomotor excitability of the masseter and tongue in patients with OSA. Methods: Adults (n = 28) with mild or moderate OSA were randomly allocated to sleep with a MAD for 2‐weeks with 40% of the maximal protrusion (MAD active position) and without any jaw protrusion (MAD placebo position). The outcomes were assessed at baseline, and after 2 and 6 weeks, with a 2‐week washout period. The primary outcome was the amplitude of motor evoked potential (MEP) assessed on the right masseter, right side of tongue and right first dorsal interosseous with transcranial magnetic stimulation. Corticomotor map volume of the same muscles was also assessed. Repeated‐measures ANOVAs followed by Tukey test were applied to the data (p <.050). Results: There was a significant increase in the MEP amplitude of the masseter and tongue following the MAD active position compared with the baseline and MAD placebo (Tukey: p <.001). There were no significant MEP amplitude differences between the baseline and placebo positions (p >.050). Moreover, there was a significant increase in corticomotor map volume for the masseter and tongue muscles following the MAD active position compared with baseline and MAD placebo (Tukey: p <.003). Conclusion: Excitability of the masseter and tongue motor pathways is, at least transiently, increased in patients with OSA following a short‐term use of MAD. This novel finding of MAD‐induced neuroplasticity in corticomotor pathways may contribute to a further understanding of the mechanisms of oral appliances for treating OSA. [ABSTRACT FROM AUTHOR]

Background: Masticatory muscle training by chewing gum can be performed easily and improve masticatory muscle function and strength. However, increased masticatory muscle activity and function may alter the mandibular shape. Objective: We aimed to investigate the effects of gum chewing training on the occlusal force, masseter muscle thickness (MMT) and mandibular shape in healthy adults. Methods: We conducted a prospective randomised controlled trial from January 2020 to September 2020 at the Yonsei University College of Dentistry. Fifty‐eight participants were randomly assigned to the training and control groups. The training group chewed gum three times a day for 6 months, while the control group received no training. Changes in the maximum occlusal force and MMT were evaluated at baseline and after 1, 3 and 6 months. Changes in the mandibular shape were evaluated at baseline and after 6 months. Results: The mean maximum occlusal force of the training group at 3 months was significantly higher than that at baseline, which was also significantly different from that in the control group (p <.001). As the maximum occlusal force increased, the occlusal contact area also increased (p =.020). There was no statistically significant difference in MMT or mandibular shape compared to the baseline. Conclusion: Mastication training using gum increases maximum occlusal force due to an increase in occlusal contact area but has no effect on MMT or mandibular shape. [ABSTRACT FROM AUTHOR]

Background: Electromyographic activity (EMG) of masticatory muscles during wakefulness is understudied. It is unclear if single channel ambulatory EMG devices are sensitive enough to detect masticatory muscle activity (MMA) during wakefulness. Objectives: To compare the MMA of various oral tasks recorded with a single channel EMG device ((Grindcare4‐datalogger Prototype device) (GC4‐β)) and a conventional EMG (cEMG) device. Methods: EMG activity of 30 standardised oral tasks was recorded unilaterally from the masseter and anterior temporalis muscle in 24 healthy volunteers using GC4‐β and a cEMG device. To compare the EMG data, signal‐to‐noise ratios (SNR) were calculated as a way to normalise EMG activity across tasks. Analysis of variance was used to compare the SNR between the devices, muscles and oral tasks. Results: SNR measured from GC4‐β was overall significantly higher than the cEMG device (p =.001). The SNR for maximum voluntary contraction (MVC) was significantly higher than all other tasks (p <.001). SNR for temporalis with GC4‐β was significantly higher for MVC, hard food, soft food, gum chewing (dominant side), rhythmic clenching and upper lip biting compared to the cEMG device (p <.021). The SNR for masseter with GC4‐β was significantly higher for hard food and gum chewing (dominant side), rhythmic clenching, rhythmic biting of an object and yawning compared to the cEMG device (p <.022). Conclusions: This study provides novel insight into the EMG patterns of numerous oral tasks enhancing knowledge of physiological differences between the masticatory muscles. Further, single channel EMG devices can effectively measure the EMG activity of various oral tasks during wakefulness. [ABSTRACT FROM AUTHOR]

Objective: There is a strong relationship between masticatory muscle atrophy and condyle degeneration. Although electrical stimulation (ES) is an effective treatment for muscle atrophy, its influence on the underlying condyle is unclear. This study aimed to investigate whether ES can prevent condyle degradation during the stage of masseter muscle atrophy. Materials and Methods: Six‐week‐old rats were randomly divided into the control, botulinum toxin (BTX), or BTX + ES group. BTX was injected into the bilateral masseters of rats to induce masseter atrophy. The left‐side masseters without ES treatment were served as BTX group, and the right‐side masseters received ES with different parameters (5 mA/10 Hz, 5 mA/50 Hz, 6 mA/10 Hz, 6 mA/50 Hz, 7 mA/10 Hz, and 7 mA/50 Hz) were served as BTX + ES groups. After 4 weeks, micro‐CT and qualitative or quantitative analysis of osteogenesis, chondrogenesis, and angiogenesis‐related genes in condyles were conducted. Results: ES, especially at 7 mA/50 Hz, significantly attenuated masseter atrophy, condyle degeneration, and subchondral bone loss. Moreover, the upregulation of related proteins, including collagen 1, osteocalcin, bone morphogenetic protein 2, collagen 2a, and vascular endothelial growth factor were observed. Conclusion: ES partly rescued condylar degeneration and subchondral bone loss following masseter atrophy. [ABSTRACT FROM AUTHOR]

The purpose of this study is to investigate the biomechanical effect of bimaxillary osteotomies on patients with mandibular retraction. Mandibular retraction, as a typical maxillofacial deformity, and has great adverse effects on TMJ. Bimaxillary osteotomies are widely used to correct symptoms of mandibular retraction. It is necessary to understand the effect of surgery on temporomandibular disc (TMJ). Five patients with mandibular retraction and 10 asymptomatic subjects were involved in this study. Finite element models of preoperative, postoperative and control group were constructed based on the CT data. Nine sets of concentrated forces were used to simulate the muscle effect and contact method was used to simulate the interaction within the TMJs and dentitions. The results showed that bimaxillary osteotomies could effectively improve the maxillofacial morphology and alleviate the overload in TMJ. The facial asymmetry and right chewing side preference could cause imbalanced stress distributions in the TMJs and interfere the surgical treatment. [ABSTRACT FROM AUTHOR]

Background: Botulinum toxin type A (BTX‐A) is increasingly used to manage painful temporomandibular disorders (TMD). However, the effect of BTX‐A on muscular TMD remains unclear. Objective: To assess the efficacy, safety and optimal dose of BTX‐A for treating TMD. Methods: We conducted systematic literature searches in MEDLINE, Embase, Web of Science, ClinicalTrials.gov and Cochrane Library until March 2023. We extracted data from randomized controlled trials (RCTs) that evaluated the efficacy and safety of BTX‐A in treating muscular TMD. We performed a meta‐analysis using a random‐effects model. Results: Fifteen RCTs involving 504 participants met the inclusion criteria. BTX‐A was significantly more effective than placebo in reducing pain intensity, as measured on a 0–10 scale, at 1 month (MD [95% CI] = −1.92 [−2.87, −0.98], p <.0001) and 6 months (MD [95% CI] −2.08, [−3.19 to −0.98]; p =.0002). A higher dosage of BTX‐A (60–100 U bilaterally) was associated with a greater reduction in pain at 6 months (MD [95% CI] = −2.98 [−3.52, −2.44]; p <.001). BTX‐A also resulted in decreased masseter muscle intensity (μV) (MD [95% CI] = −44.43 [−71.33, −17.53]; p =.001) at 1 month and occlusal force (kg) at 3 months (MD [95% CI] = −30.29 [−48.22 to −12.37]; p =.0009). There was no significant difference in adverse events between BTX‐A and placebo. Conclusions: BTX‐A is a safe and effective treatment for reducing pain and improving temporomandibular muscle and joint function in muscular TMD patients. A bilateral dose of 60–100 U might be an optimal choice for treating muscular TMD pain. [ABSTRACT FROM AUTHOR]

Aims: Rett syndrome (RTT) is a neurodevelopmental disorder caused by a mutation in the methyl‐CpG‐binding protein 2 (MECP2) gene. It is characterized by profound and severe motor disability. Patients with RTT grind their teeth, causing damage to their permanent teeth. This article aims to describe the case of an 8‐year‐old child with RTT and the methods of botulinum toxin‐based treatment for teeth grinding. Methods and Results: Surface electromyography (sEMG) was used to monitor the effects of treatment. The masseter and temporal muscles, responsible for the elevation of the mandible, were injected with botulinum toxin. The results of the sEMG examination performed 1 week, 1 month and 4 months after toxin administration were compared to the sEMG conducted before treatment. The research demonstrates a significant reduction in both grinding frequency and tension of the masseter and temporal muscles. Conclusion: Possible adverse effects after chronic use of botulinum toxin were discussed. The article also emphasizes the need for masticatory muscle rehabilitation during botulinum toxin therapy. However, it is necessary to repeat this procedure on a larger group of children with RTT to establish our efforts' efficacy. [ABSTRACT FROM AUTHOR]

Objective: Quantitative sensory testing is widely used in clinical and research settings to assess the sensory functions of healthy subjects and patients. It is of importance to establish normative values in a healthy population to provide reference for studies involving patients. Given the absence of normative values for pain thresholds in Taiwan, the aim of this study was to report the normative values for future reference in the Taiwanese population and compare the differences between male and female participants. Methods: Healthy adults without any chronic or acute pain condition were recruited. The pain thresholds were assessed over the cephalic (supraorbital area and masseter muscle) and extracephalic (medio‐volar forearm and thenar eminence) areas. The heat, cold, mechanical punctate, and pressure pain thresholds were measured with a standardized protocol. Comparisons between male and female participants were performed. Results: One hundred and thirty healthy participants (55 males: 30.4 ± 7.4 years; 75 females: 30.5 ± 8.1 years) finished the assessments. Male participants were less sensitive to mechanical stimuli, including pressure over masseter muscle (male vs. female: 178.5 ± 56.7 vs. 156.6 ± 58.4 kPa, p =.034) and punctate over medio‐volar forearm (male vs. female: 116.4 ± 45.2 vs. 98.7 ± 65.4 g, p =.011), compared to female participants. However, female participants were less sensitive to cold stimuli, indicated by lower cold pain thresholds over the supraorbital area (male vs. female: 18.6 ± 8.4 vs. 13.6 ± 9.3°C, p =.004), compared to male participants. No significant differences were found between sexes in other pain threshold parameters. Conclusions: We provided the normative values of healthy male and female adults in Taiwan. This information is crucial for comparison in future pain‐related studies to identify potential hypoalgesia or hyperalgesia of tested subjects. [ABSTRACT FROM AUTHOR]

Background: Masseter muscle activity during wakefulness may be associated with temporomandibular disorder (TMD)‐related symptoms, psychosocial status and pain‐related disability; however, this relationship is unclear. Objectives: This study aimed to determine the relationship between masseter muscle electromyography (EMG) burst/duration during wakefulness and TMD‐related symptoms, psychosocial status and pain‐related disability. Methods: Sixty participants were assessed masseter muscle activity during wakefulness using a data‐logger‐type ultraminiature EMG system and TMD‐related symptoms, psychosocial status and pain‐related disability through Axis I and II of the diagnostic criteria for TMD (DC/TMD). EMG bursts lasting longer than 0.25 s but less than 2.0 s and those lasting longer than 2.0 s were classified as phasic and tonic bursts, respectively. Results: Participants with palpation‐related pain in the temporalis and masseter muscles, as assessed through the DC/TMD examination form in Axis I, had more bursts (number/h) (p =.035 and p =.009, respectively) and longer duration (time/h) (p =.013 and p =.004, respectively) of tonic bursts of the masseter muscle during wakefulness. Participants with palpation‐related pain in the masseter muscles had higher oral behaviour scores during wakefulness using Axis II (p =.001), which affected the number and duration of tonic bursts of the masseter muscle activity during wakefulness (p =.011 and p =.007, respectively). Conclusion: As tonic bursts mainly reflect clenching, individuals with pain in the masseter muscles by palpation may have a high frequency and longer duration of clenching, as well as a high frequency of oral behaviours during wakefulness. [ABSTRACT FROM AUTHOR]

Background: Lingual pressure (LP) generation is cooperatively controlled not only by the intrinsic tongue (I‐ton) muscles but also by hyoid muscle activation. However, the measurement of endurance and fatigue properties of I‐ton muscles is difficult due to the instability of electrodes. Objective: The purpose of this study was to apply suction electrodes to measure electromyograms (EMGs) of I‐ton muscle and to evaluate integrated EMG amplitude (iEMG) and mean power frequency (MPF) of EMG in the I‐ton and hyoid muscles performing continuous LP. Methods: Twenty healthy adult volunteers (10 males, 10 females, mean age 28.8 years) were instructed to perform 10‐s LP generation tasks at 25%, 50%, 75% and 100% of maximum LP in randomised order with visual feedback. During each task, EMGs of the I‐ton, suprahyoid (S‐hyo), infrahyoid (I‐hyo) and masseter (Mass) muscles were simultaneously recorded. The iEMG and MPF of EMG burst during 10‐s LP tasks were compared. The recording period was divided into three substages to analyse temporal changes with the Friedman test. Results: During the 10‐s task, the iEMG significantly increased as the LP strength increased (p <.001). There was no time‐dependent change in the I‐ton iEMG; however, the MPF of the I‐ton EMG burst decreased in all tasks (p <.05). The S‐hyo and I‐hyo iEMGs gradually increased, especially with strong LP (p <.01). Conclusion: While I‐ton muscles may easily fatigue during 10‐s LP generation, S‐hyo and I‐hyo muscles may help compensate for the weakened I‐ton muscle activity by increasing their activity to maintain LP. [ABSTRACT FROM AUTHOR]

During mastication, food undergoes state and texture changes influenced by various mechanical properties, including compression and fracturing of the molar teeth, mixing with saliva, and oral temperature. Prior studies have explored mastication simulators, however, no studies have assessed the forces and duration applied to the molars by the food during bolus formation. In this study, we developed a novel system that integrates a masticatory simulator and analysis method to evaluate mechanical properties. We developed ORAL‐MAPS which is equipped with 6‐axis force sensor, pneumatic pressure control mechanism, vertical movement, molar‐like module, artificial saliva injection unit, and temperature control apparatus. A gap exists between the upper and lower unit at the closest point, allowing the sensor to measure vertical upward force and duration from food, while compressed air provides constant downward pressure. We hypothesized a correlation between the total integrated muscle activity ratio obtained from the human masseter muscle electromyography (iEMG). We compared the normalized impulse obtained from ORAL‐MAPS with the normalized total iEMG obtained from human studies with four different types of gummy candies. As a result, the normalized total impulse of gummy candies A, B, C, and D were 1.00 ± 0.00, 1.29 ± 0.06, 0.95 ± 0.00, and 0.39 ± 0.0, respectively. The normalized total iEMG of the same gummy candies were 1.00 ± 0.00, 1.23 ± 0.15, 0.98 ± 0.09, and 0.45 ± 0.07, respectively. Thus, no significant difference was observed between the normalized total impulse obtained in vitro and the normalized total iEMG values for masticating the gummy candies B, C, and D (p >.05). [ABSTRACT FROM AUTHOR]

Objective: In the esthetic field, the masseter muscle is commonly targeted by botulinum neurotoxin for facial contouring. However, multiple botulinum neurotoxin injections have been reported to cause muscle fibrosis. Ultrasonography can be useful for clinical consideration in such cases. Materials and Methods: This study presents nine cases of masseteric fibrosis caused by repeated botulinum neurotoxin injections with ultrasonographic analysis of full and partial masseteric fibrosis. Results: Repetitive botulinum neurotoxin injections resulted in reduced masseter muscle volume, which frequently appeared hyperechoic on ultrasonography. The hyperechoic region was mostly located in the deep and posterior portions; however, in some cases, it was observed throughout the muscle, including the superficial, deep, or both areas. Conclusion: The fibrotic masseter muscles appear hyperechoic, and ultrasonography is necessary to analyze the degree and location of fibrosis. Predictions can be made for cases in which botulinum neurotoxin injections may have less of an effect after ultrasonography. Because muscle fibrosis can be localized, it is necessary to confirm the degree and location of fibrosis before determining the effective area of injection. In clinical practice, muscle fibrosis may be visible in a specific area where blind injections are administered. [ABSTRACT FROM AUTHOR]

This special issue of Experimental Physiology features 13 manuscripts based on presentations from a meeting on "Mechanotransduction, Muscle Spindles, and Proprioception" held in Munich in July 2022. The meeting brought together over 30 speakers from various countries, including Taiwan, Canada, Australia, the USA, Israel, and Europe. The issue covers topics such as the molecular basis of muscle spindles, their role in motor control and posture, and their implications in diseases and skeletal health. The meeting aimed to foster collaboration and knowledge exchange among experts in the field. The text discusses various aspects of proprioception and muscle spindle function, including the role of different sensory structures, the relationship between muscle spindles and musculoskeletal pain, and the effects of age and tendon compliance on proprioceptive ability. Computational models are also explored as a tool for understanding muscle spindle function. The text emphasizes the importance of collaboration and further research in the field of proprioception. [Extracted from the article]

Background: Sleep bruxism (SB) occurring during No‐REM (nREM) sleep and increase in microarousals per hour have been described in adults, but not in children. Objective: To assess the correlation between sleep architecture and masseter muscle activity related to sleep bruxism (SB/MMA) in children. Materials and methods: Forty‐three children aged 7–12 years (mean age: 9.4 ± 1.3) with confirmed SB underwent a two‐night polysomnographic (PSG) study in a sleep laboratory, for accommodation (first night) and data collection (second night). Data on sleep architecture (total sleep duration (TSD), sleep efficiency (SE), sleep onset latency (SOL), REM and nREM sleep duration and proportion and microarousals/hour during REM and nREM sleep) and episodes/hour of SB/MMA were recorded. Single and multiple‐variable linear regression analyses were performed to assess the correlation between data on sleep architecture (predictors) and SB/MMA (dependent variable). Results: Shorter TSD, REM and nREM stage 1 sleep duration, longer SOL and more microarousals/hour during REM and nREM sleep were found to be positive predictors of SB/MMA in children in the multiple‐variable regression analysis (R2 = 0.511). Conclusion: Within the limitations of this study, it can be concluded that SB/MMA is correlated with altered sleep architecture in children (shorter total sleep duration (TSD), shorter nREM and REM sleep and higher microarousals during REM and nREM sleep). Nevertheless, the clinical significance of these findings need to be demonstrated in future studies. [ABSTRACT FROM AUTHOR]

Background: There is limited knowledge of the possible side‐effects of clear aligners on jaw function. Objectives: To determine the short‐term effect of passive clear aligners (PCAs) on masticatory muscle activity (MMA), occlusal discomfort (OD) and temporomandibular disorder (TMD) symptoms in adults with different levels of self‐reported oral parafunction. Materials and Methods: Participants were screened for oral parafunctional behaviours using the oral behavioural checklist. Respondents in ≥85th and ≤15th percentiles were invited to participate and allocated to a high (HPF: N = 15) or low (lower parafunction [LPF]: N = 16) parafunction group. Participants underwent a TMD clinical examination; somatisation and somatosensory amplification were assessed by questionnaires; OD and stress were assessed by visual analogue scales. While wearing PCAs, awake‐time MMA was assessed three times over 9 days using a wearable electromyography device, along with OD, stress and TMD symptoms. Results: The wearing of PCAs was associated with a significant decrease in mean contraction episode amplitude in both groups (p = 0.003). OD levels increased and remained raised in all participants after insertion of the PCAs (p < 0.001), more so in the HPF group (p = 0.048). The HPF group had higher somatisation scores (p = 0.006) and reported more TMD symptoms at all time points (p ≤ 0.004). No significant changes in stress or TMD symptoms were found in either group during the study period. Conclusions: PCAs were associated with a decrease in MMA in all participants. HPF individuals had greater somatisation and reported greater discomfort when wearing PCAs than LPF individuals. [ABSTRACT FROM AUTHOR]

Fossa masseterica, to which the musculus masseter is attached, is the pit on the lateral surface of the ramus mandibulae. Processus coronoideus is a protrusion on the upper part of fossa masseterica. Carnivores have a more developed fossa masseterica and a wider processus coronoideus than other species, due to their well‐developed jaw muscles. However, there is limited information about the differences of these two structures among carnivorous species. In this study, it was investigated whether fossa masseterica and processus coronoideus differ in shape in domestic cats and domestic dogs. For this purpose, 22 dogs and 20 cats were examined by 3D geometric morphometry. Eighty‐one landmarks were used on the fossa masseterica and processus coronoideus. The difference in centroid sizes and shape for cats and dogs was statistically significant (p < 0.0001). PC1 explained 26.47% of the total variance. It was seen that cats and dogs were completely separated according to PC1 results. In cats with a high PC1 value, the processus coronoideus was narrower than in dogs. Also, feline processus coronoideus was more curved than domestic canine processus coronoideus. In addition, the caudal inclination of processus coronoideus was deeper in dogs than in cats. Dogs had a negative PC1 value except for one sample (German Shepherd). The sample with the lowest PC1 value was the French Bulldog (female, 7 years, and 13 kg). Discriminant analysis results showed that the domestic cats and domestic dogs used in the study were completely separated from each other, and this difference was statistically significant. The results of this study showed that dogs with stronger jaw muscles had a deeper fossa masseterica and a wider processus coronoideus than cats. [ABSTRACT FROM AUTHOR]

Objective: This study aimed to investigate whether the sex steroid precursor hormone dehydroepiandrosterone sulphate (DHEA‐S), sex hormone‐binding globulin (SHBG) and testosterone (TT) are associated with temporomandibular (TM) pain on palpation in male adolescents. Methods: Out of the LIFE Child study dataset containing 1022 children and adolescents aged 10–18 years (496 males, 48.5%), we used a subsample of 273 male adolescents (mean age: 13.8 ± 2.3 years) in advanced pubertal development (PD) to analyse the association between hormones and TM pain. The Tanner scale was applied to describe the stage of PD. Pain on palpation of the temporalis and masseter muscles and the TM joints (palpation pain) was assessed using the Diagnostic Criteria for Temporomandibular Disorders (DC/TMD). Serum levels of sex hormones (DHEA‐S, SHBG and TT) were determined using standardised laboratory analyses. Free TT was estimated from the ratio between TT and SHBG (free androgen index[FAI]). We calculated the risk of perceived positive palpation pain for male participants as a function of hormone levels (DHEA‐S, FAI) taking into account age and body mass index (BMI). Results: Among more developed (Tanner stage 4–5) male adolescents, 22.7% (n = 62) reported palpation pain in the TM region. In these participants, FAI levels were approximately half that of individuals without such pain (p <.01). DHEA‐S levels were about 30% lower in the pain group (p <.01). In multivariable regression analyses, the odds ratio (OR) for pain on palpation decreased to 0.75 (95% confidence interval [CI]: 0.57–0.98) per 10 units of FAI level compared to those without pain, after controlling for the effects of age and adjusted BMI. We observed the same effect for this subgroup per unit of DHEA‐S serum level (OR = 0.71; 95% CI: 0.53–0.94). Conclusion: At subclinical lower levels of serum free TT and DHEA‐S, male adolescents are more likely to report pain on standardised palpation of the masticatory muscles and/or TM joints. This finding supports the hypothesis that sex hormones may influence pain reporting. [ABSTRACT FROM AUTHOR]

Spatial growth constraints in the head region can lead to coordinated patterns of morphological variation that pleiotropically modify genetically defined phenotypes as the tissues compete for space. Here we test for such architectural modifications during rhesus macaque (Macaca mulatta) postnatal ontogeny. We captured cranium and brain shape from 153 MRI datasets spanning 13 to 1090 postnatal days and tested for patterns of covariation with measurements of relative brain, eyeball, and masseter muscle size as well as callosal tract length. We find that the shape of the infant (<365 days) macaque cranium was most closely aligned to masseter muscle and brain size measured relative to face size. Infant brain and juvenile (365–1090 days) cranium shape were more closely linked with brain size relative to basicranium and face size. Meanwhile, the juvenile macaque brain shape was dominated by the size of the brain relative to that of the basicranium. Associations with relative eyeball size and commissural tract lengths were weaker. Our results are consistent with a spatial‐packing regime operating during postnatal macaque ontogeny, in which relative growth of the masseter, face and basicranium have a greater influence than brain growth on the overall shape of the cranium and brain. [ABSTRACT FROM AUTHOR]

Objective: Fundamentally, this review addresses the following question: In partially or fully edentulous patients, do implant‐supported dental prostheses preserve orofacial tissues when compared to conventional prostheses or no therapy? Materials and Methods: This study was conducted according to the 2020 PRISMA guidelines for systematic reviews. Electronic searches were conducted at PubMed and Embase databases followed by manual search. Clinical studies comparing the effect of implant‐supported prostheses with conventional rehabilitation or no treatment on alveolar bone resorption, remaining teeth, and jaw muscle thickness were considered for inclusion. A qualitative synthesis was conducted with all included studies, and data from selected studies were pooled quantitatively to perform a meta‐analysis. Results: A total of 14 studies were selected for analysis. Six studies reported on the effect of implant therapy on alveolar bone resorption (n = 453), six on the remaining teeth (n = 1014), while four studies evaluated masseter muscle thickness (n = 158). The results of the meta‐analyses assessing alveolar bone resorption in the posterior mandible and in the anterior area of the maxilla, both fixed and random effects models, yielded no benefit of rehabilitation with implant‐supported prostheses when compared to conventional prostheses. For masseter bone thickness, however, a significant benefit for implant‐supported prosthesis was observed. Conclusions: This systematic review and meta‐analysis were unable to unequivocally answer the focus question. There are some indicators of the benefit of implant‐supported prostheses over conventional prostheses or no therapy in preserving orofacial tissues, particularly for masseter muscle thickness. However, the evidence is still insufficient to confirm such perception. [ABSTRACT FROM AUTHOR]

Background: Clinical use of botulinum neurotoxins (BoNTs) in masticatory muscles is usually bilateral, but most studies on the functional consequences of BoNT treatment have used unilaterally treated animals. Objectives: To test the hypothesis that bilateral BoNT treatment of the rabbit masseter hampers mastication and to assess its effects on bone density of the mandibular condyles. Methods: Ten 5‐month‐old female rabbits received injections of BoNT into both masseter muscles and nine sham animals received saline. Body weight, incisor bite force during masseter tetany, and surface and fine‐wire electromyography (EMG) of the masseter and medial pterygoid muscles were assessed at regular intervals. Half the sample was terminated after 4 weeks and the remainder after 12 weeks. Muscles were weighed and mandibular condyles were scanned with microCT to analyse bone density. Results: BoNT rabbits lost weight and required a soft‐food diet. Incisor occlusal force plummeted after BoNT injection and remained lower than the shams. The duration of masticatory cycles was increased in the BoNT rabbits for 5 weeks, with most of the increase due to the adductor burst. Masseteric EMG amplitude began to improve at Week 5, but remained low on the working side throughout the experiment. At the 12‐week endpoint, masseter muscles were smaller in the BoNT rabbits. Medial pterygoid muscles did not compensate. Condylar bone density was reduced. Conclusion: Bilateral treatment of the rabbit masseter by BoNT severely affected chewing performance. Even after a 3‐month recovery period, deficits remained in bite force, muscle size and condylar bone density. [ABSTRACT FROM AUTHOR]

The conditions of stress contagion are induced in bystanders without direct experiences of stressful events. This study determined the effects of stress contagion on masseter muscle nociception in mice. Stress contagion was developed in the bystanders after cohabitating with a conspecific mouse subjected to social defeat stress for 10 days. On Day 11, stress contagion increased anxiety‐ and orofacial inflammatory pain‐like behaviors. The c‐Fos and FosB immunoreactivities evoked by masseter muscle stimulation were increased in the upper cervical spinal cord, while c‐Fos expressions were increased in the rostral ventromedial medulla, including the lateral paragigantocellular reticular nucleus and nucleus raphe magnus in stress contagion mice. The level of serotonin in the rostral ventromedial medulla was increased under stress contagion, while the number of serotonin positive cells was increased in the lateral paragigantocellular reticular nucleus. Stress contagion increased c‐Fos and FosB expressions in the anterior cingulate cortex and insular cortex, both of which were positively correlated with orofacial inflammatory pain‐like behaviors. The level of brain‐derived neurotrophic factor was increased in the insular cortex under stress contagion. These results indicate that stress contagion can cause neural changes in the brain, resulting in increased masseter muscle nociception, as seen in social defeat stress mice. [ABSTRACT FROM AUTHOR]

Summary: Masticatory muscle activity during sleep has been considered independent of variations in the structural characteristics of the dental occlusion. However, scientific evidence contradicting an occlusal causal role is missing. The purpose of this study was to test the null hypothesis that sleep bruxism (SB) is independent of the presence of occlusal interferences. A total of 17 healthy female subjects (mean [SD] age 24.9 [4.1] years) presenting with SB, and randomly divided into two groups, were evaluated after receiving either elimination of occlusal interferences (Test group) or elimination of sharp margins without change in occlusal contacts (Control group). Audio‐video polysomnography (PSG) recordings were undertaken before and after treatment (mean [SD] duration 14.4 [3.8] months). Two subjects in each group (< 2 rhythmic masticatory muscle activity [RMMA]/h) were excluded for statistical analysis. During the total sleep time (TST), the Test group exhibited a higher reduction in frequency of episodes per hour than the Control group (p < 0.05). The reduction in duration of episodes was also higher in the Test group during the TST. The Test group presented an increase in sleep stage N3 (p < 0.05) at the final PSG when compared with the initial PSG, and a higher percentage of N3 (p < 0.05) at the final PSG when compared to the Control group. Elimination of occlusal interferences resulted in a significant reduction of masseter and temporal muscle activity during sleep in females presenting with SB. Based on these results, the null hypothesis is rejected. The hypothesis of occlusal interferences as a risk factor for SB is still patent. [ABSTRACT FROM AUTHOR]

Summary: Sleep bruxism (SB) is a repetitive jaw‐muscle activity characterised by clenching or grinding of the teeth and/or by bracing or thrusting of the mandible. Sleep bruxism has been linked with insomnia symptoms. Moreover, it has been suggested that there is a positive association between distress and the occurrence of sleep bruxism. However, the occurrence of sleep bruxism and its association with distress have not been studied in patients with insomnia. Therefore, we hypothesised that: (1) the occurrence of sleep bruxism is higher in patients with insomnia than in healthy controls; and (2) the occurrence of sleep bruxism in insomnia patients with moderate to high distress (IMHD) is higher than that in insomnia patients with slight distress (ISD). A total of 44 controls (34 females, 10 males, mean ± SD age = 46.8 ± 14.4 years) and 42 participants with insomnia (35 females, 7 males, mean ± SD age = 51.3 ± 12.1 years) were enrolled in this study. Among 42 participants with insomnia, 20 participants were subtyped as IMHD, 17 participants as ISD. Another five participants were not subtyped due to insufficient information. Group differences in rhythmic masticatory muscle activity (RMMA), a biomarker of sleep bruxism, were evaluated with Mann–Whitney U tests. The medians and interquartile ranges of the RMMA indices were 0.8|1.8|3.3 in controls, 1.1|1.6|2.3 in IMHD and 1.2|1.9|2.9 in ISD. There was no significant difference in the RMMA index, neither between participants with insomnia and controls (P = 0.514) nor between IMHD versus ISD (P = 0.270). The occurrence of RMMA indicators of possible sleep bruxism is not significantly different between individuals with insomnia and controls, nor between IMHD versus ISD. [ABSTRACT FROM AUTHOR]

Background: Learning and memory deficits and pathologic changes in the hippocampus caused by toothlessness and soft diet feeding are related to reduced masseter muscle (MM) function. Objective: Myosin heavy chain (MyHC) isoform expression in the MM also changes under different chewing conditions. The neurotransmitter calcitonin gene‐related peptide (CGRP) and vascular endothelial growth factor A (VEGF‐A) are involved in MM formation. However, the relationship between CGRP, VEGF‐A and MyHC isoforms in the MM in the senescence‐accelerated mouse prone 8 (SAMP8) strain, a model of learning and memory deficits, remains unclear. Methods: Changes in CGRP, VEGF‐A, vasculogenesis marker and MyHC isoform mRNA expression in the MMs of ageing SAMP8 and senescence‐accelerated mouse resistant 1 (SAMR1) mice was investigated through quantitative real‐time polymerase chain reaction (qRT–PCR) and in situ hybridization. Results: qRT–PCR revealed obviously high CGRP levels in the SAMP8 mouse MM (p <.001). MyHC‐IId/x mRNA expression in the MM was higher in 24‐week‐old SAMP8 mice than 24‐week‐old SAMR1 mice (p <.001) but lower in slow‐MyHC SAMP8 mice than SAMR1 mice (p <.001). CGRP mRNA was observed on the muscle fibres of the SAMP8 mouse MM but not the SAMR1 mouse MM through in situ hybridization. Principal component analysis (PCA) revealed strong positive contributions of SAMP8‐MyHC‐IId/x, SAMP8‐CGRP, SAMR1‐MyHC‐emb, SAMR1‐CGRP, SAMR1‐VEGF‐A, SAMR1‐CD31, SAMP8‐VEGF‐A, and SAMP8‐CD31 in the MM at 12 and 24 weeks. Conclusion: Calcitonin gene‐related peptide is also key for the MyHC‐IId/x and slow‐MyHC patterns in the MMs of SAMP8 mice. [ABSTRACT FROM AUTHOR]

Summary: A descriptive case report of the management of a miniature pony presenting to a UK equine veterinary hospital with marked trismus and increased masseter muscle size. Diagnosis of masseter myodegeneration (MM) was aided by the use of computed tomography and excisional biopsy. Hyperlipaemia was diagnosed as a sequelae of MM. Treatment included nonsteroidal anti‐inflammatories, parenteral nutrition and fluid therapy in addition to supportive care. The gelding was discharged from the hospital 30 days after admission, having made a full recovery. MM in the horse is a rare condition that appears to be physiologically distinct from canine masticatory muscle myositis which is primarily immune mediated. [ABSTRACT FROM AUTHOR]

Summary: Regional and systemic disorders of myopathic or neurogenic cause affecting the masticatory muscles have been reported or suspected in horses. The primary masticatory muscles include the masseter, temporalis, lateral and medial pterygoid muscles. The masseter muscle is the largest of the primary masticatory muscles and the most affected based on sporadic reports. This is a brief review of disorders involving the masseter muscle. [ABSTRACT FROM AUTHOR]

Background: Swallowing function is formed by the synergistic, coordinated, and complex movements of approximately 30 muscles. However, There is no study on which one(s) of these muscles can predict the possibility of dyspfagia. Objectives: The purpose of this study was to investigate whether there is a relation between dysphagia and ultrasonographic measurements of swallowing muscles mass, and to determine which muscle mass would predict the possibility of dysphagia. Methods: This prospective and controlled study was performed on 87 older people (aged >65 years). Clinical (Eating assessment tool‐10 and The Gugging Swallowing Screen test), endoscopic, electrophysiological (dysphagia limit and swallowing intervals), and ultrasonographic (masseter [MM], genioglossus [GGM], geniohyoid [GHM], myohyoid [MH] and anterior digastric muscles [ADM]) evaluations were performed on all patients. The relationship between ultrasound measurements and demographic/swallowing characteristics and presence of dysphagia were investigated. Results: The muscle mass measurement which was associated with dysphagia was contracted masseter muscle. In terms of diagnostic performance, the sensitivity was the highest in GHM (92.3%) and the specificity was the highest in contracted MM thickness (90.3%). The probability of dysphagia increases from 54% to 80–89% with a decrease in contracted MM thickness, and the probability of dysphagia increased from 48% to 72% with a decrease in contracted GGM thickness. In addition, the highest AUC level was detected in contracted MM and contracted GGM muscles. Moreover, it was determined that the risk of dysphagia increased from 9.6 to 14.1 times with one unit decrease in muscle thickness in contracted MM muscles, and the risk of dysphagia increased 9.1 times with one unit decrease in contracted GGM muscles. Conclusion: Decreased swallowing muscle mass in older adults are independent risk factors for dysphagia. Especially the decrease in the thickness of the contracted MM increases the possibility of dysphagia at high rate. [ABSTRACT FROM AUTHOR]

Background: Previous study showed that in individuals with obstructive sleep apnea (OSA), the contractions of masseter muscles after respiratory events can be nonspecific motor phenomena, dependent on the duration of respiratory arousals rather than the occurrence of the respiratory events. However, the role of intermittent hypoxia in the occurrence of jaw‐closing muscle activities (JCMAs) was not taken into consideration. An exposure to intermittent hypoxia has been shown to initiate a series of activities, including muscular sympathetic activity in patients with OSA. Objective: To determine the effects of mandibular advancement appliance (MAA) therapy on JCMA time‐related to oxygen desaturation with and without arousal in individuals with OSA. Methods: Eighteen individuals with OSA (age: 49.4 ± 9.8 years, apnea‐hypopnea index (AHI): 10.0|18.4|30.3, JCMA index: 1.7|4.3|5.6), participated in a randomised controlled crossover clinical trial, in which two ambulatory polysomnographic recordings were performed: one with MAA in situ and the other without MAA in situ. JCMAs were recorded bilaterally from both masseter and temporalis muscles. Results: There was no significant effect of the MAA on the overall JCMA index (Z = −1.372, p =.170). With the MAA in situ, JCMA index time‐related to oxygen desaturation with arousal significantly decreased (Z = −2.657, p =.008), while there was no significant effect of the MAA on the JCMA index time‐related to oxygen desaturation without arousal (Z = −0.680, p =.496). Conclusion: Effective mandibular advancement appliance therapy significantly reduces jaw‐closing muscle activities time‐related to oxygen desaturation with arousal in individuals with OSA. [ABSTRACT FROM AUTHOR]

Objectives: The purpose of this study was to elucidate how masticatory muscles are involved in the generation of occlusal force. Materials and Methods: The experiment was conducted by fabricating an experimental apparatus for a unilateral occlusion model with the masticatory muscles imparted. The experimental apparatus was fabricated by enlarging the lateral photograph of a dried adult skull specimen 3.5 times larger than that of a standard adult and drawing the outlines of the maxilla and mandible, canines and molars of the upper and lower jaws, and temporomandibular joint on a wooden board. The masticatory muscles used in the experiment were the masseter muscle, the temporalis muscle (anterior and posterior muscle bundles), and the lateral pterygoid muscle. For the measurement of the contractile force of the masticatory muscle, we used the spring scale. For the food, we used cut plastic cylinders. Results: The results of the experiment revealed the following: First, the occlusal force was generated under the condition that the contraction forces of all the masticatory muscles were balanced. Second, when the occlusal force was applied to food, the occlusal planes of the upper and lower jaws were parallel. Third, the occlusal force occurred perpendicular to the occlusal plane. Fourth, the occlusal force was generated with a force greater than the contraction force of the individual masticatory muscles. And finally, even if occlusal force was applied to the food, the occlusal force did not load the temporomandibular joint. Conclusion: Occlusal force is not generated by the action of a single masticatory muscle but under the balanced contractile force of all masticatory muscles. The occlusal force then emerges with a force greater than the contraction force of all the masticatory muscles, and its direction occurs perpendicular to the occlusal plane. [ABSTRACT FROM AUTHOR]

Background: Mouth breathing (MB) can affect morphological changes in the craniofacial structures, electromyography is widely used for quantitative analysis of muscle function. Objective: The aim was to evaluate the electromyographic (EMG) activities of the anterior temporalis (TA), masseter muscle (MM), orbicularis oris superior (OOS) and mentalis muscle (MT) in children with different vertical skeletal patterns and breathing modes during rest and various functional mandibular movements. Methods: BioEMG III was used to measure the variations in EMG activities of TA, MM, OOS, and MT in 185 subjects aged 6–12 years during continuous clenching, rest, maximal intercuspation, lips closed lightly and swallowing. Results: The results of logistic regression analysis showed that the model with vertical skeletal patterns as the dependent variable was ineffective (p =.106), while the model with breathing modes as the dependent variable was effective (p =.000). When considering both vertical skeletal patterns and breathing modes, the following significant differences were found. (1) In the normal‐angle group, the EMG ratio in OOS with lips closed lightly of MB was significantly higher than NB (p =.005). (2) In the low‐angle group, EMG ratios in TA and MM during the swallowing of MB were significantly lower than NB (p =.020, p =.040, respectively). (3) In the high‐angle group, EMG ratios of MB were significantly higher in MT during continuous clenching, rest, lips closed lightly and swallowing (p =.038, p =.036, p =.005, p =.028, respectively), and OOS with lips closed lightly compared to NB (p =.005). Conclusion: Breathing modes and vertical skeletal patterns interacted to alter maxillofacial EMG activities, with breathing modes having a greater effect. Breathing modes and vertical skeletal patterns interacted to alter maxillofacial EMG activities, with breathing modes having a greater effect on the maxillofacial EMG. [ABSTRACT FROM AUTHOR]

Background: Previous studies concerning the effect of botulinum toxin in masseter muscle have mainly reported effects observed through inspection of facial features or differences in pain levels. One systematic review of studies utilizing objective measurements reported that long‐term muscular effect of botulinum neurotoxin injections into masseter muscle was inconclusive. Objective: To evaluate the duration of reduced maximal voluntary bite force (MVBF) after botulinum toxin intervention. Methods: The intervention group was comprised of individuals seeking aesthetic treatment for masseter reduction (n = 20), the reference group (n = 12) comprised of individuals with no intervention. Intervention through 25 units of Xeomin® (Merz Pharma GmbH & Co KGaA, Frankfurt am Main, Germany) botulinum neurotoxin type A injected into the masseter muscles bilaterally (totalling 50 units). A reference group did not receive any intervention. MVBF was measured in Newtons using a strain gauge meter at the incisors and first molars. MVBF was measured at baseline, at 4 weeks, 3 months, 6 months, and after 1 year. Results: Both groups were similar in terms of bite force, sex and age at baseline. MVBF remained similar compared to baseline in the reference group. At 3 months, a significant reduction at all measurement points was observed in the intervention group; at 6 months, this reduction was no longer significant. Conclusion: A single intervention of 50 units of botulinum neurotoxin results in a reversible MVBF reduction of at least 3 months, although a visually discernable reduction may be more long‐lasting. [ABSTRACT FROM AUTHOR]

Introduction: Occlusal overload is considered to be one of the causes of late implant failure. However, it is unclear whether the magnitude of the patient's occlusal force is a risk factor for late implant failure. Purpose: This case–control study aimed to clarify the association between the cross‐sectional area (CSA) of the masseter muscle and late implant failure. Methods: This case–control study was limited to implant‐supported fixed prostheses. We compared cases with at least one late implant failure (n = 25 patients) to controls (n = 82 patients) without implant failure. Patients were matched by age, sex, year of surgery, jaw and tooth type, and bone graft. Log‐rank and Cox proportional hazard regression analyses were used to identify possible risk factors for late implant failure. Results: The incidence of late implant failure was significantly associated with masseter muscle CSA ≥504.5 mm2 (hazard ratio: 4.43; 95% CI: 1.82–10.79; p < 0.01). Conclusion: Higher masseter muscle CSA increases the risk of late implant failure. [ABSTRACT FROM AUTHOR]

There are few documented cases of facial abscesses due to masseter abscess formation in South American camelids. Facial swellings in South American camelids have been reported and are frequently due to apical tooth root infections. Masseter abscess has been described in people. However, this lesion has not previously been described in South American camelids. This case report describes the clinical signs, diagnostic procedures and pathological findings in adult male Huacaya alpaca suffering from a masseter abscess caused by Fusobacterium necrophorum and Actinomyces spp. The mass was delimited by computed tomography and removed under general anaesthesia. The histopathology of the mass revealed severe focally extensive suppurative inflammation, which infiltrated and effaced the skeletal muscle. Bacterial cultures revealed that the abscess was caused by Fusobacterium necrophorum and Actinomyces spp. Once removed, the alpaca was started on antimicrobial therapy and did fully recover at the farm. [ABSTRACT FROM AUTHOR]

Background: Although several studies have investigated effective treatments for masticatory muscle pain (MMP), no unified conclusion has been drawn regarding the effectiveness of these treatments. Objectives: This study aimed to define quantitative indicators for predicting the outcome of MMP treatment. Materials and Methods: In total, patients aged 20-70 years were recruited and divided into the MMP (n = 24) and control (n = 36) groups, based on the presence of MMP according to the Diagnostic Criteria for Temporomandibular Disorders. At pretreatment, the MMP group was assessed using quantitative indicators such as subjective pain levels, pain duration, graded chronic pain scale (GCPS), and perceived stress scale (PSS). Salivary alpha-amylase (sAA) and interleukin-6 (IL-6) levels were analyzed. The masticatory muscle palpation score and the range of mouth opening were measured. At posttreatment, subjective pain levels, mouth opening, and treatment/medication duration were examined. The PSS and sAA levels were assessed in the control group. Results: sAA levels in the MMP group were significantly higher than those in the control group (p < .05). The masseter muscle palpation score (MPS) showed a positive correlation with IL-6 levels (= 0.503, p < .05) and a negative correlation with nonsteroidal anti-inflammatory drug (NSAID) treatment period (= -0.462, p < .05). The temporalis muscle palpation score (TPS) was positively correlated with pain duration and GCPS grade (= 0.483, p < .05, and = 0.445, p < .05, respectively). Conclusions: Treatment with NSAIDs was effective in the MMP group with high MPS and IL-6 levels, but not in the MMP group with high TPS, pain duration, and GCPS grade. [ABSTRACT FROM AUTHOR]

Objectives: Main aim of the study was to explore the association between genetic polymorphisms in ACTN3 and bruxism. Secondary objectives included masseter muscle phenotypes assessment between bruxers and non‐bruxers and according to genetic polymorphisms in ACTN3. Materials and Methods: Fifty‐four patients undergoing orthognathic surgery for correction of their malocclusion were enrolled. Self‐reported bruxism and temporomandibular disorders status were preoperatively recorded. Saliva samples were used for ACTN3 genotyping. Masseter muscle samples were collected bilaterally at the time of orthognathic surgery to explore the muscle fiber characteristics. Results: There were significant differences in genotypes for rs1815739 (R577X nonsense) (p = 0.001), rs1671064 (Q523R missense) (p = 0.005), and rs678397 (intronic variant) (p = 0.001) between bruxers and non‐bruxers. Patients with self‐reported bruxism presented a larger mean fiber area for types IIA (p = 0.035). The mean fiber areas in individuals with the wild‐type CC genotype for rs1815739 (R577X) were significantly larger for type IIA fibers (1394.33 μm2 [572.77 μm2]) than in those with the TC and TT genotypes (832.61 μm2 [602.43 μm2] and 526.58 μm2 [432.21 μm2] [p = 0.014]). Similar results for Q523R missense and intronic variants. Conclusions: ACTN3 genotypes influence self‐reported bruxism in patients with dentofacial deformity through specific masseter muscle fiber characteristics. [ABSTRACT FROM AUTHOR]

Background: Previous evidence indicates significant association between genetic polymorphisms and phenotypes related to pain sensitivity in patients with temporomandibular disorders (TMD). Despite the important advances in cataloguing diverse factors such as sleep disorders, anxiety and depression, the interrelated mechanisms of painful TMD aetiopathogenesis still need investigation. Objectives: This case–control study aimed to evaluate the influence of genetic polymorphisms (rs6296, rs6295, rs1799971, rs4680, rs4633, rs4818) and psychosocial factors on the mechanical pain sensitivity and endogenous pain modulation in women with painful TMD and asymptomatic controls. Methods: We evaluated six independent variables: anxiety levels, depression, stress, sleep quality, pain catastrophising and genetic polymorphisms, and four dependent variables: mechanical pain threshold (MPT), pressure pain threshold (PPT), wind‐up ratio (WUR) and conditioned pain modulation (CPM) collected at masseter (trigeminal) and hand (spinal) areas in a sample of 95 painful TMD patients and 85 controls. A regression model was used to test the possible effect of the independent variables on dependent variables. Results: The regression model was significant for MPT (F11,168 = 9.772; R2 =.390). Painful TMD diagnoses and sleep quality were associated with trigeminal MPT (B coefficient = −.499; and B coefficient = −.211, respectively). WUR was associated with rs6295 and rs6746030, respectively, for the spinal and the trigeminal area. Conclusion: Genetic polymorphisms had a slight contribution to endogenous pain modulation as indicated by the significant association with WUR but did not contribute to mechanical pain sensitivity. On the other hand, the presence of painful TMD and the sleep quality contributed significantly to mechanical pain sensitivity. [ABSTRACT FROM AUTHOR]

Background: Movement of the lower jaw, a common behavior observed among vertebrates, is required for eating and processing food. This movement is controlled by signals sent from the trigeminal motor nerve through neuromuscular junctions (NMJs) to the masticatory muscles. Dysfunctional jaw movements contribute to craniomandibular disorders, yet the pathophysiology of these disorders is not well understood, as limited studies have been conducted on the molecular mechanisms of jaw movement. Results: Using erc1b/kimm533 genetic loss of function mutant, we evaluated lower jaw muscle organization and innervation by the cranial motor nerves in developing zebrafish. Using time‐lapse confocal imaging of the erc1b mutant in a transgenic fluorescent reporter line, we found delayed trigeminal nerve growth and disrupted nerve branching architecture during muscle innervation. By automated 3D image analysis of NMJ distribution, we identified an increased number of small, disorganized NMJ clusters in erc1b mutant larvae compared to WT siblings. Using genetic replacement experiments, we determined the Rab GTPase binding domain of Erc1b is required for cranial motor nerve branching, but not NMJ organization or muscle attachment. Conclusions: We identified Erc1b/ERC1 as a novel component of a genetic pathway contributing to muscle organization, trigeminal nerve outgrowth, and NMJ spatial distribution during development that is required for jaw movement. Key Findings: A craniofacial genetic zebrafish mutant, kimble (kimm533), that carries a loss of function variant in the erc1b gene has jaw movement defects.Erc1b function is required for organization of the lower jaw muscles, including tendon attachments.Erc1b is required for trigemeinal nerve outgrowth, and nerve branching architecture during innervation of the jaw muscles.Erc1b/ERC1 is a novel component of a genetic pathway that is involved in the organization of craniofacial muscle attachments and cranial motor nerve innervation. [ABSTRACT FROM AUTHOR]

Introduction: Infrared thermography has the potential to help assess human head and neck muscles, as thermal variation measurements due to muscle imbalance can contribute to the evaluation and therapeutic follow‐up of various clinical conditions in the field of health. Objective: This study investigated, with a scoping review, the use of infrared thermography in complementary assessments of human head and neck muscles to determine the extent of research on the topic, what methodologies are used in thermal assessment and thermographic analysis, what regions are assessed, and what results are expected in thermography. Methods: LILACS, MEDLINE, SciELO, and Web of Science were the databases searched to identify articles published on the topic, with no restriction of language or time of publication. Descriptive, analytical, and experimental observational studies on the assessment of the human head and neck muscle surface temperature with infrared thermography were included. Case studies, case series, methodological accuracy, literature review, animal studies, studies that assessed patients with head and neck cancer, and studies that did not assess head and neck muscle temperature with thermography were excluded. The articles were analysed with a protocol developed by the authors, with data on author, year, country, type of study, sample characterisation, muscles assessed, outcomes investigated, thermal assessment methodology, thermographic analysis methodology and thermography measurements. Results: This review identified and analysed 27 articles. The studies assessed thermal distribution in normal individuals and those with pathologies related to hyper‐ and hypofunctional head and neck muscle conditions for diagnosis or therapeutic follow‐up. The masseter, temporal, digastric, anterior cervical region, orbicularis oris, frontalis, buccinator, suprahyoid, trapezius, sternocleidomastoid and levator scapulae muscles were assessed. Quantitative analyses with area selection tool predominated, considering absolute temperatures and temperature differences. The studies investigated temperature and its relationship with myogenic pain, quantitative assessment of muscle parameters and blood flow velocity. The mean temperature ranged from 32.97°C (±2.21) to 34.90°C, and hyper‐radiant and/or asymmetric regions were observed in hyperfunctional conditions and normal subjects after muscle activation. Conclusion: Thermography is used in complementary assessments of head and neck muscles, identifying hyper‐radiant regions and thermal asymmetry related to muscle tension and activation state. Papers are limited to specific clinical conditions and few muscle groups, besides having great methodological variability. [ABSTRACT FROM AUTHOR]

Background: It has not yet been clarified how the type of the chewing task affects related muscle activity and how the suprahyoid muscles contribute to masticatory function in humans. Objectives: This study aimed to investigate the difference in the suprahyoid muscle activity between the freely and unilaterally chewing tasks and between the working and non‐working sides during chewing. Materials and Methods: Twenty healthy volunteers were instructed to chew peanuts and two different types of rice crackers in two ways: freely and unilaterally while surface electromyograms of the masseter and suprahyoid muscles were recorded. The chewing duration, number of chewing cycles and chewing rate were compared between the tasks. Furthermore, the masseter and suprahyoid muscle activities per chewing cycle were compared between the sides. Results: The chewing duration was significantly longer, and the chewing rate was significantly higher during unilaterally chewing than freely chewing. The chewing duration differed significantly among the different foods; the harder the food, the longer the duration. Chewing rate and suprahyoid activity were significantly higher during soft rice cracker chewing. Masseter activity was higher on the chewing side than on the non‐chewing side while there was no difference in suprahyoid activity between the sides. Conclusion: The current results demonstrate a difference in the masticatory efficacy between the chewing tasks and a functional role of the suprahyoid muscles during chewing, which does not differ between the chewing and non‐chewing sides. [ABSTRACT FROM AUTHOR]

Objective: To establish a new rat model of craniofacial myalgia, and to clarify which central nervous system pathways are activated in the model. Background: Craniofacial myalgia, represented by myogenous temporomandibular disorder and tension‐type headache with pericranial tenderness, is more common in female patients. The pain is thought to be a type of multifactorial disorder with several coexisting causes. To our knowledge, there are no models of craniofacial muscle hyperalgesia caused by multiple types of stimuli. Methods: We injected nerve growth factor into the trapezius muscle of female and male rats and repeatedly stimulated the masseter muscle (MM) electrically for 10 days. We determined the mechanical head‐withdrawal threshold of MM and extent of phosphorylated extracellular signal‐related kinase 1/2 (pERK) immunoreactivity in various regions of the lower brainstem. We conducted retrograde tract‐tracing to determine the projection of mechanosensitive MM‐innervating secondary neurons to the lateral parabrachial nucleus. Finally, we administered morphine in rats to determine whether increases of pERK immunoreactivity were dependent on noxious inputs. Results: In female rats, but not male rats, the mechanical head‐withdrawal threshold was decreased significantly from days 9 to 12. The number of pERK‐immunoreactive neurons in the brainstem was increased significantly in female rats in the group with both stimuli compared to rats in other groups with a single stimulus. Mechanosensitive MM‐innervating neurons in the brainstem projected to the parabrachial nucleus. Morphine administration blocked the increase in the number of pERK‐immunoreactive neurons in both the brainstem and parabrachial nucleus. Conclusions: We established a model of craniofacial myalgia by combining trapezius and MM stimuli in female rats. We found mechanical hyperalgesia of the MM and activation of the pain pathway from the brainstem to parabrachial nucleus. The model reflects the characteristics of patients with craniofacial myalgia and might be helpful to clarify the pathogenic mechanisms underlying these disorders. [ABSTRACT FROM AUTHOR]

Background: Botulinum toxin type A (BoNT‐A) injection is a popular cosmetic procedure in East Asian countries, and there are multiple types of toxins on the market. Whether different toxin types induce different changes in the masseter has not been studied. Objective: We aimed to investigate and compare the changes induced by injection of different types of BoNT‐A in the masseter. Methods: Upon retrospective review of clinical records and routine ultrasound examination results of patients who underwent BoNT‐A injection, four groups were set for comparison: unapproved toxin, OnabotulinumtoxinA, LanbotulinumtoxinA, and non‐injected. Clinical records and ultrasound images were collected. Ultrasound images were taken in both horizontal and coronal planes. Elasticity of the masseter muscle was also investigated by measuring the surface wave speed. Results: A total of 24 patients were included (six patients per group). All patients were young females. For the non‐injected group, the deep inferior tendon was clear and the muscle bundles were arranged, with little fibrous content. Patients who received unapproved toxin injection showed highly disarranged muscle and fiber bundles, with much fiber content present. The changes in OnabotulinumtoxinA and LanbotulinumtoxinA groups were in between those in the non‐injected and unapproved toxin groups. Muscle rigidity was the highest in the unapproved toxin group, and generally higher in injected groups compared with the non‐injected group. Conclusion: Different types of BoNT‐A may induce different changes in the masseter. [ABSTRACT FROM AUTHOR]