Delayed‐onset muscle soreness in human masticatory muscles increases inhibitory jaw reflex responses

The effects of masticatory muscles’ overloading on jaw‐motor control are not yet fully clarified. Therefore, it was tested whether eccentric and concentric exercises of the human masticatory muscles would influence inhibitory jaw reflex responses. Eleven participants (6 males, 5 females) performed 6, 5‐minutes bouts of eccentric‐concentric contractions. Before, immediately after, 24 hours, 48 hours and 1 week afterwards, visual analogue scale (VAS) scores for jaw muscle fatigue and pain, maximum voluntary bite force (MVBF) and inhibitory jaw reflexes were recorded. Reflex data were analysed with the cumulative sum control chart error box method. Immediate and delayed masticatory muscle fatigue and pain were provoked. Further, 24 hours after the exercises, MVBF tended to decrease (P = .056), suggesting that delayed‐onset muscle soreness (DOMS) was provoked in the masticatory muscles. In addition, the inhibitory jaw reflex showed a delayed increase in size 24 hours after the exercise (P < .05). In conclusion, DOMS provoked in the masticatory muscles alters jaw motor control by inducing a delayed increase in the size of the inhibitory jaw reflex.     

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.     

Effect of experimental jaw muscle pain on occlusal contacts

The aim of the study was measuring the effect of experimental jaw muscle pain on number and position of posterior occlusal contacts. Eleven adult voluntary subjects were enrolled. A lower impression was taken for each subject and two dental casts were obtained from each impression. The study was carried out in a randomised cross-over fashion. Each subject participated in two experimental sessions (30-day interval) in which he/she received an injection in the central part of the right masseter muscle consisting of 0·5 mL of either hypertonic or isotonic saline. Each subject was asked to rate pain intensity on a visual analogue scale. Three occlusal bite checks (polyvinylsiloxane) at the maximal intercuspal position were obtained during the experimental session: the first before the injection, the second between 60 and 90 s after the injection and the third 15 min after the injection. Evaluation of contacts was performed on dental casts with the use of different colours (black for baseline, red for intermediate and green for final contacts). Repeated-measures analysis of variance was used to compare the overall number of contacts among groups and the number of contacts of different colours. No significant difference was found between the overall number of occlusal contacts (P>0·05), but significant differences were found between contacts according to different colours: confirmed (P=0·006), disappeared (P=0·007) and new (P<0·001). Assuming different colours as change in contact position, the overall number of contacts did not change, but the position did. Experimentally induced jaw muscle pain affected the pattern of posterior occlusal contacts.     

Myoelectric manifestations of jaw elevator muscle fatigue and recovery in healthy and TMD subjects

The effects of muscle pain and fatigue on the control of jaw elevator muscles are not well known. Furthermore, the myoelectric manifestations of fatigue and recovery from fatigue in the masticatory muscles are not reported in literature. The main aims of this study were (i) to evaluate the possible use of surface electromyography (sEMG) as an objective measure of fatigue of the jaw elevator muscles, (ii) to compare the myoelectric manifestations of fatigue in the temporalis anterior and masseter muscles bilaterally, (iii) to assess recovery of the investigated muscles after an endurance test and (iv) to compare fatigue and recovery of the jaw elevator muscles in healthy subjects and patients with muscle-related temporomandibular disorders (TMD). The study was performed on twenty healthy volunteers and eighteen patients with muscle-related TMD. An intra-oral compressive-force sensor was used to measure the voluntary contraction forces close to the intercuspal position and to provide visual feedback of submaximal forces to the subject. Surface EMG signals were recorded with linear electrode arrays during isometric contractions at 20%, 40%, 60% and 80% of the maximum voluntary contraction force, during an endurance test and during the recovery phase. The results showed that (i) the slope of the mean power spectral frequency (MNF) and the initial average rectified value (ARV) could be used to monitor fatigue of the jaw elevators, (ii) the temporalis anterior and masseter muscle show the same myoelectric manifestations of fatigue and recovery and (iii) the initial values of MNF and ARV were lower in patients with muscle-related TMD. The assessment of myoelectric manifestations of fatigue in the masticatory muscles may assist in the clinical assessment of TMDs.     

Influence of removable prostheses on mastication in elderly subjects with rheumatoid arthritis

Rheumatoid arthritis (RA) is a prevalent disease in the elderly population, and it may affect the temporomandibular joint (TMJ) and the stomatognathic system. This study evaluated masticatory function in elderly patients with RA before and after oral rehabilitation with removable prostheses. Forty‐five elders with partial or total edentulism who were using unsatisfactory removable prostheses were selected and assigned to 3 groups: RA with TMJ involvement, RA without TMJ involvement and healthy controls. Masticatory function was assessed in terms of masticatory performance (MP) and maximum bite force (MBF). The former was determined by the sieving method, and the latter was measured by pressure sensors placed in the bilateral molar regions. The variables were first evaluated in elderly subjects wearing unsatisfactory prostheses and re‐assessed after participants had received new removable prostheses. Comparisons between groups and among time points were performed with analysis of variance for repeated measures and the Tukey‐Kramer test (P < .05). Comparison among groups showed decreased MP in elders with RA before new prosthesis insertion (P < .05). Irrespective of TMJ involvement, MP improved after treatment in subjects with RA. Rheumatoid arthritis groups also showed decreased MBF (P < .05), which improved after new prosthesis insertion. Rheumatoid arthritis might impair masticatory function, and well‐fitted removable prosthesis insertion might be very beneficial in elders with RA.     

Masseter thickness,endurance and exercise-induced pain in subjects with different vertical craniofacial morphology

The aim of the study was to compare neuromuscular features of the masseter muscle in subjects with different vertical craniofacial morphology. Fifteen short-faced (mandibular plane-Frankfurt plane angle < 15 degrees) and 15 normal- to long-faced (mandibular plane-Frankfurt plane angle > or = 23 degrees) male students participated. The thickness of the masseter was assessed by ultrasonography. Onset and endurance of exercise pain were recorded during sustained biting at a level of 15% of maximum voluntary contraction and 30 micro V electromyographic activity. Pain and fatigue was measured on visual analog scales before and after the biting, as well as before and after 10 min chewing. Statistical comparison showed that the masseter muscle was significantly thicker (+15%) in the short-faced than the normal- to long-faced subjects. The pain onset time and endurance time were also consistently shorter in short-faced subjects, whereas the intensity of pain and fatigue did not differ significantly between the two groups. Multiple stepwise regression showed positive influence from the mandibular plane inclination and the masseter thickness on the pain onset time and endurance time. The present findings support the concept that subjects with different craniofacial morphology show neuromuscular differences.     

Effect of a brief episode of experimental muscle pain on jaw movement and jaw‐muscle activity during chewing

The aims of this study were to determine whether: (i) the jaw motor system develops a new pattern of jaw movement and/or jaw‐muscle activity after resolution of an acute episode of jaw‐muscle pain; and (ii) if jaw‐muscle activity and jaw‐movement features change progressively with repetition of a chewing sequence. Jaw movement and jaw muscle (masseter, anterior temporalis, and digastric) activity were recorded during free and rate‐standardized chewing in eight asymptomatic participants (pain infusion group), before and at three time blocks up to 45 min after a single 0.2‐ml bolus infusion of 5% hypertonic saline into the right masseter muscle. The same procedure, without infusion, was performed in another eight participants (control group). There were no significant main effects of group on jaw movement and muscle activity, suggesting that there were no persistent post‐pain effects on chewing. Across groups, repetitions of free and unstandardized chewing movements were associated with progressive increases in velocity and amplitude of jaw movement and masseter and temporalis electromyographic (EMG) activity. These findings suggest that factors unrelated to pain, such as practice effects, may be playing a role in the changes in jaw movement and jaw‐muscle activity observed after resolution of an acute episode of jaw‐muscle pain.     

Comparison of ambulatory and polysomnographic recording of jaw muscle activity during sleep in normal subjects

Clinicians and investigators need a simple and reliable recording device to diagnose or monitor sleep bruxism (SB). The aim of this study was to compare recordings made with an ambulatory electromyographic telemetry recorder (TEL-EMG) with those made with standard sleep laboratory polysomnography with synchronised audio-visual recording (PSG-AV). Eight volunteer subjects without current history of tooth grinding spent one night in a sleep laboratory. Simultaneous bilateral masseter EMG recordings were made with a TEL-EMG and standard PSG. All types of oromotor activity and rhythmic masseter muscle activity (RMMA), typical of SB, were independently scored by two individuals. Correlation and intra-class coefficient (ICC) were estimated for scores on each system. The TEL-EMG was highly sensitive to detect RMMA (0·988), but with low positive predictive value (0·231) because of a high rate of oromotor activity detection (e.g. swallowing and scratching). Almost 72% of false-positive oromotor activity scored with the TEL-EMG occurred during the transient wake period of sleep. A non-significant correlation between recording systems was found (r = 0·49). Because of the high frequency of wake periods during sleep, ICC was low (0·47), and the removal of the influence of wake periods improved the detection reliability of the TEL-EMG (ICC = 0·88). The TEL-EMG is sensitive to detect RMMA in normal subjects. However, it obtained a high rate of false-positive detections because of the presence of frequent oromotor activities and transient wake periods of sleep. New algorithms are needed to improve the validity of TEL-EMG recordings.     

Influence of psychological symptoms on home-recorded sleep-time masticatory muscle activity in healthy subjects

The present investigation attempts to describe the correlation between sleep-time masticatory muscle activity (MMA) and psychological symptoms by the use of a four-channel electromyography (EMG) home-recording device in a group of 15 healthy volunteers completing a battery of psychometric questionnaires for the assessment of anxiety, depression and anger. The integrated EMG signal was adopted to quantify the work (μV × s) produced by each of the four muscles (bilateral masseter and temporal) during the 5-h recording span and per each 1-h increment. The duration of MMA events and the muscle work during the first hour of sleep was related to trait anxiety scores for both masseter (P = 0·007) and temporalis muscles (P = 0·022). Trait anxiety was also significantly correlated to the total amount of MMA duration (in seconds) of the temporalis muscles (r = 0·558; P = 0·031). The present investigation provides support to the hypothesis that the duration of sleep-time masticatory muscle activity, especially during the early phases of a night's sleep, may be related to anxiety trait and not to anxiety state, depression or anger. These findings may support the view that features related to the individual management of anxiety, viz. trait, are likely to be more important than acute episodes of anxiety, viz. state, in the aetiology of sleep-time masticatory muscle activity. The role of other psychological symptoms is likely to be less important.     

Effect of experimental anterior temporalis muscle pain on jaw movements

To test the hypotheses that experimental noxious stimulation of the anterior temporalis muscle results in significant decreases in jaw movement amplitude and velocity, and there are significant correlations between scores of mood or pain‐related cognitions and amplitude and velocity. The jaw movements of 14 asymptomatic participants were recorded during standardised open/close jaw movements and free and standardised chewing tasks. Tonic infusion of hypertonic saline into the right anterior temporalis muscle maintained pain intensity between 40 and 60 mm on a 100‐mm visual analogue scale. Tasks were performed in a single session in the following sequence: baseline condition, test 1 condition (during hypertonic or isotonic saline infusion), test 2 condition (during saline infusion) (10‐min rest between conditions). Participants completed the Depression, Anxiety and Stress Scale (DASS‐21) and the Pain Catastrophizing Scale (PCS). Amplitude and velocity of opening and closing were compared between conditions with a repeated‐measures analysis of variance (anova ), and Spearman's rank correlation coefficient explored correlations; statistical significance: P < 0·05. For any of the three tasks, there were no significant differences in kinematic variables between any condition and no significant correlations between DASS‐21 or PCS scores and kinematic variables during hypertonic saline infusion. The absence of a significant reduction in velocity or amplitude of open/close or chewing jaw movements during experimental temporalis muscle pain is not consistent with the Pain Adaptation Model proposing decreases in kinematic measures in pain. The lack of significant correlations between psychological variables and measures of jaw movement may reflect the low scores reported by our study sample.     

Pathophysiology of TMD pain – basic mechanisms and their implications for pharmacotherapy

Summary This article discusses the pathophysiology of temporomandibular disorders (TMD)‐related pain and its treatment with analgesic drugs. Temporomandibular disorders are comprised of a group of conditions that result in temporomandibular joint pain (arthralgia, arthritis) and/or masticatory muscle pain (myofascial TMD). In at least some patients with TMD, a peripheral mechanism contributes to this pain. However, there is often a poor correlation between the severity of TMD‐related pain complaints and evidence of definitive tissue pathology. This has led to the concept that pain in some patients with TMD may result from altered central nervous system pain processing and further that this altered pain processing may be attributable to specific genes that are heritable. Psychosocial stressors are also thought to contribute to the development of TMD‐related pain, particularly masticatory muscle pain. Finally, substantially more women suffer from TMD than men. Although there are arguably multiple reasons for sex‐related differences in the prevalence of TMD, one candidate for the increased occurrence of this disorder in women has been suggested to be the female sex hormone oestrogen. Analgesic drugs are an integral part of the primary treatment for TMD‐related pain and dysfunction with more that 90% of treatment recommendations involving use of medications. The most commonly used agents include non‐steroidal anti‐inflammatory drugs, corticosteroids, muscle relaxants, anxiolytics, opiates and tricyclic antidepressants, however, evidence in support of the effectiveness of these drugs is lacking. Continued research into the pathophysiology of TMD‐related pain and the effectiveness of analgesic treatments for this pain is required.     

Delayed‐onset muscle soreness does not influence occlusal sensitivity and position sense of the mandible

Masticatory muscle‐pain patients often complain about sensorimotor changes, but the effects of pain on the psychophysical properties remain unclear. This study aimed to investigate the effects of delayed‐onset muscle soreness (DOMS) on the jaw's position sense (PS) and occlusal sensitivity (OS). In all, 12 participants underwent intense concentric–eccentric jaw exercises. Self‐reported muscle fatigue and pain, pain‐free maximum mouth opening (MMO), pain pressure thresholds (PPTs) at right and left masseter and right and left anterior temporalis, maximum voluntary bite force (MVBF), PS and OS were recorded before, immediately after, 24 h, 48 h and 1 week after the exercises. Data were analysed with repeated measures anova . Pain and fatigue increased significantly after the exercises, while fatigue also increased 24 h afterwards. Time and site had a significant effect for PPTs, not for MVBF. MMO decreased significantly 24 h after the exercises. OS and PS did not change significantly. Experimentally induced DOMS does not influence the psychophysical properties of the masticatory system.     

Influence of sustained submaximal clenching fatigue test on electromyographic activity and maximum voluntary bite forces in healthy subjects and patients with temporomandibular disorders

This study aimed to investigate whether the fatigue induced by sustained motor task in the jaw elevator muscles differed between healthy subjects and patients with temporomandibular disorder (TMD). Fifteen patients with TMD and thirteen age‐ and sex‐matched healthy controls performed a fatigue test consisting of sustained clenching contractions at 30% maximal voluntary clenching intensity until test failure (the criterion for terminating the fatigue test was when the biting force decreased by 10% or more from the target force consecutively for >3 s). The pre‐ and post‐maximal bite forces (MBFs) were measured. Surface electromyographic signals were recorded from the superficial masseter muscles and anterior temporal muscles bilaterally, and the median frequency at the beginning, middle and end of the fatigue test was calculated. The duration of the fatigue test was also quantified. Both pre‐ and post‐MBFs were lower in patients with TMD than in controls (P < 0·01). No significant difference was found in the percentage change in MBF between groups. The duration of the fatigue test in TMD patients was significantly shorter than that of the controls (P < 0·05). Our results suggest that, compared to healthy subjects, patients with TMD become more easily fatigued, but the electromyographic activation process during the fatigue test is similar between healthy subjects and patients with TMD. However, the mechanisms involved in this process remain unclear, and further research is warranted.     

慢性咀嚼肌疼痛患者面部和手指电刺激痛阈的初步观察

目的 通过测定和对比慢性咀嚼肌痛(masticatory muscle pain,MMP)患者及健康对照组双侧咬肌区、双侧颞肌区及左手中指的电刺激感觉阈和痛阈,初步探讨慢性MMP患者的三叉神经感觉功能是否出现变化.方法 收集慢性MMP患者12例(慢性MMP患者组),以及年龄、性别与慢性MMP患者组相似的健康对照者12名(健康对照组),分别行咬肌区、颞肌区疼痛的视觉模拟量表(visual analogy scale,VAS)评分;对两组双侧咬肌区、双侧颞肌区和左手中指的电刺激感觉阈和痛阈进行榆查.对两组5个部位的电刺激感觉阈和痛阈进行分析对比.结果 与健康对照组相比,慢性MMP患者组左侧咬肌、右侧咬肌、左侧颞肌、右侧颞肌和左手中指的电刺激痛阈均明显降低[分别为(5.9±1.1)、(5.7±1.0)、(5.3±1.4)、(5.5±1.7)和(5.2±1.0)mA],差异均有统计学意义(F=24.433,P＜0.001);而两组电刺激感觉阈的差异无统计学意义(F=0.341,P=0.959).慢性MMP患者组VAS得分与咀嚼肌电刺激感觉阈和痛阈之间未发现相关关系.结论 相比健康对照组,慢性MMP患者的咬肌、颞肌及手指的电刺激痛阈均显著降低,提示患者三叉神经感觉功能出现变化,其机制可能涉及高级神经中枢.     

Reorganization of motor unit activity at different sites within the human masseter muscle during experimental masseter pain

The aims were to test the hypotheses that experimental masseter muscle pain leads to recruitment and/or derecruitment of motor units at different sites within the masseter and that the patterns of change in motor unit activity differ between sites. Single motor unit (SMU) activity was recorded at two sites within the right masseter [superior/anterior, inferior/posterior (IP)] during isometric biting tasks (ramp, step level) on an intraoral force transducer in 17 participants during three experimental blocks comprising no infusion (baseline), 5% hypertonic saline infusion (pain), or isotonic saline infusion (control). A visual analog scale (VAS) was used to score pain intensity. The VAS scores were statistically significantly greater during infusion of hypertonic saline than during infusion of isotonic saline. No significant differences in force levels and rates of force change were found between experimental blocks. In comparison with isotonic saline infusion, SMUs could be recruited and derecruited at both sites during hypertonic saline infusion. The frequency of recruitment or derecruitment, in comparison with no change, was statistically significantly greater at the IP site than at the superior/anterior site. Experimental noxious masseter stimulation results in a reorganization of motor unit activity throughout the muscle, and the pattern of reorganization may be different in different regions of the muscle.     

Evaluation of occlusal splint therapy in temporomandibular joint disorder patients using real‐time ultrasonography

Aim: The aim of the present study was to evaluate the effect of occlusal splint therapy by determining the cross‐sectional dimension of masseter muscle using ultrasound in patients with temporomandibular joint disc displacement with reduction. Methods: Twenty‐seven patients aged between 20 and 40 years were included in the study. A detailed history was obtained, and a comprehensive clinical examination was carried out. Subjective assessment with structured pro forma was performed pre‐occlusal and post‐occlusal splint therapy. The objective measurement of the bilateral masseter muscle thickness was recorded using real‐time ultrasonography before and after splint therapy. Results: The mean ultrasound thickness of the masseter muscle in the pre‐clenching state before splint therapy was 9.45 mm (SD 1.39), and the post‐clenching state was 13.15 mm (SD 2.23). After splint therapy, the mean thickness in the pre‐clenching state was 9.14 mm (SD 1.31), and the post‐clenching state was 12.78 mm (SD 2.23; P < 0.001). Conclusion: Ultrasonographic measurements of the masseter muscle in patients with temporomandibular joint disc displacement with reduction can be a useful tool to assess the effectiveness of occlusal splint therapy. Such conservative measures can significantly reduce masticatory muscle tenderness, especially of the temporalis and masseter.     

Influence of masticatory muscle pain on electromyographic activities of cervical muscles in patients with myogenous temporomandibular disorders

The present study tested the hypothesis that the resting electromyographic (EMG) activities of sternocleidomastoid and trapezius muscles of myogenous temporomandibular disorder (TMD) patients would be significantly greater than healthy individuals. Eight masticatory muscle pain patients without disc interference disorders (DID) and 30 patients with DID along with 41 matched control subjects were included in the study. The resting EMG activities of sternocleidomastoid and trapezius muscles were recorded with a portable EMG machine. Pain intensities described over the muscles were recorded with a 100 mm visual analogue scale. The two groups of patients had significantly higher resting activities compared with control subjects. Patients (in both groups) who had pain in sternocleidomastoid and trapezius muscles reported significantly higher resting activities in the respective muscles than patients who were free of pain and the controls. The presence of pain over the examined muscles showed significant association with the myogenous TMDs without DID. The observed association of the cervical muscles with myogenous TMDs is expected to be a result of functional link between masticatory and cervical muscles. The present findings suggest that the myogenous TMDs without DID could be a distinct sub-group that can be a part of broader regional or generalized diseased entity.