RESEARCH PAPER
The Correlation Between Surface Electromyography and Bite Force of Mastication Muscles in Asian Young Adults Cheng-I Yen, MD,* Shih-Hsuan Mao, MD,* Chih-Hao Chen, MD,* Chien-Tzung Chen, MD,† and Ming-Yih Lee, PhD‡ Background: Mastication function is related to mandible movement, muscle strength, and bite force. No standard device for measuring bite force has been developed. A linear relationship between electromyographic activity and bite force has been reported by several investigators, but data on the reliability of this relationship remain limited in Asian young adults. Aim and Objectives: The purpose of this study was to develop a clinically applicable, reliable, quantitative, and noninvasive system to measure the kinetic mastication function and observe the correlation between surface electromyography (sEMG) and bite force. Materials and Methods: The study group consisted of 41 young healthy adults (24 men and 17 women). Surface electromyography was used to evaluate bilateral temporalis and masseter muscle activities, and an occlusal bite force system was used concurrently to measure the bite force during maximal voluntary biting. Bilateral symmetry was compared, and the correlation between EMG and bite force was calculated. Results: The sEMG signals were 107.7 ± 55.0 μVand 106.0 ± 56.0 μV (P = 0.699) on right and left temporalis muscles and 183.7 ± 86.2 μV and 194.8 ± 94.3 μV (P = 0.121) on right and left masseter muscles, respectively. The bite force was 5.0 ± 3.2 kg on the right side and 5.7 ± 4.0 kg on the left side (P = 0.974). A positive correlation between sEMG and bite force was observed. The correlation coefficient between the temporalis muscle and bite force was 0.512, and that between the masseter muscle and bite force was 0.360. Conclusion: No significant difference between the bilateral electromyographic activities of the temporalis and masseter muscles and bilateral bite force was observed in young healthy adults in Taiwan. A positive correlation between sEMG signals and bite force was noted. By combining sEMG and bite force, we developed a clinically applicable, quantitative, reliable, and noninvasive system for evaluating mastication function by using characteristics of biofeedback. Key Words: mastication, mandible, electromyography (EMG), bite force (Ann Plast Surg 2015;74: S168–S172)
M
astication is a highly coordinated neuromuscular function involving mandible movements and continual modulations of force.1 When efficiently performed, mastication facilitates swallowing and nutrient absorption.2 The strength of masticatory muscles is a wellrecognized parameter, and the capacity to exert sufficient bite force is an indicator of normal masticatory function.3 In addition, one study Received September 19, 2014, and accepted for publication, after revision, December 17, 2014. From the *Department of Plastic and Reconstructive Surgery, Chang Gung Memorial Hospital, Chang Gung University College of Medicine, Taoyuan, Taiwan, ROC; †Department of Plastic and Reconstructive Surgery, Chang Gung Memorial Hospital at Keelung, Keelung, Taiwan, ROC; and ‡Graduate Institute of Medical Mechatronics, Chang Gung University, Taoyuan, Taiwan, ROC. Conflicts of interest and sources of funding: none declared. Reprints: Chih-Hao Chen, MD, Department of Plastic and Reconstructive Surgery, Chang Gung Memorial Hospital, Chang Gung University College of Medicine, 5 Fu-Hsin St, Kwei-Shan, Taoyuan 333, Taiwan, ROC. E-mail: [email protected]. Ming-Yih Lee, PhD, Graduate Institute of Medical Mechatronics, College of Engineering, Chang Gung University, 259 Wen-Hwa First Road, Kuei-Shan, Taoyuan 333, Taiwan, ROC. E-mail: [email protected]; [email protected]. Copyright © 2015 Wolters Kluwer Health, Inc. All rights reserved. ISSN: 0148-7043/15/7402–S168 DOI: 10.1097/SAP.0000000000000468
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demonstrated a significant relationship between occlusal force and masticatory performance.3 Occlusal force is related to dental conditions, the type of prosthetic reconstruction, the presence of temporomandibular disorders, and the fracture of the bones involved in the masticatory apparatus.4–9 Occlusal force can be measured directly between a pair of teeth by using a suitable transducer.3,7,8,10–12 However, no standard device for measuring occlusal force has been developed, and thus, dental occlusion has largely been a matter of guesswork for dentists. Articulation paper, waxes, and pressure indicator paste are the main tools dentists use to assess and balance the forces of occlusion. Most of these methods are not sufficiently sensitive for detecting simultaneous contact, and none of them can be used to measure both masticatory activity and bite force simultaneously. An ideal measurement system is noninvasive, providing protection from artifactural loading by preventing an increase in the vertical dimension of the occlusion and interference with dentition. In this study, we used surface electromyography (sEMG) to detect the activities of masticatory muscles and a thin custom-made splint with a built-in force transducer to measure bite force. We calculated the variations between men and women and tested for the presence of side-to-side facial asymmetry. In addition, we combined the 2 devices to determine the degree of correlation between sEMG signals and bite force. Because no previous related data have been collected in a Taiwanese population, before applying the system to patients, we sought to establish a standard model in a young healthy population.
MATERIALS AND METHODS The study group consisted of 41 healthy people, 24 men and 17 women, with class I occlusion, intact dentition, and no craniofacial anomalies, previous facial trauma or facial surgeries, malocclusion, orthodontic treatment, or symptoms or clinical signs of temporomandibular disorders. The participants ranged in age from 23 to 32 years, and the mean age was 28.4 years. Each participant was administered a history survey and clinical examination to assess occlusion and jaw movement. Two well-trained research assistants performed all measurements concurrently. Each movement was repeated 3 times to determine the average and ensure that readings were accurate. Informed consent was obtained from all participants, and the protocol was reviewed and approved by an institutional review board.
Surface Electromyography The sEMG machine (Zebris EMG 4, Zebris GmbH, Isny im Allgau, Germany) is a noninvasive tool used to analyze electrical signals that emanate during muscular contraction in the stomatognathic system (Fig. 1). We used this machine to test the bilateral temporalis and masseter muscles. The participants sat upright in a comfortable chair and a relaxed environment. The skin was prepared and draped using 75% alcohol cotton balls.13 Pregelled disposable electrodes were applied to the bilateral temporalis and masseter muscles, which were detected and located by performing palpation while asking the patient to clench.13 Another reference electrode was placed on the left shoulder. Annals of Plastic Surgery • Volume 74, Supplement 2, May 2015
Copyright © 2015 Wolters Kluwer Health, Inc. All rights reserved.
Annals of Plastic Surgery • Volume 74, Supplement 2, May 2015
The Correlation Between sEMG and Bite Force
Examination Protocol After the values of the sEMG signal had stabilized, the resting tonus was recorded for 6 to 8 seconds as a relaxation test. To determine the maximal voluntary contraction (MVC), a bite test was conducted in habitual intercuspation.13 Each participant bit as forcefully as possible for 2 seconds13 and then relaxed his or her mouth. The investigator then placed the bite force device in the mouth gently and instructed the participant to bite for 2 seconds again to determine the correlation between sEMG activity and bite force (Fig. 3). To determine the firing pattern, the participants were asked to press their teeth together with gradually increasing force for 10 seconds.13 In a fatigue test, the participants bit their teeth by using their full strength for 10 seconds. Each test was conducted 3 times, and the average value was calculated.
Statistical Analysis The data were statistically analyzed using the SPSS package version 20.0 for Windows. Means and standard deviations were computed for each independent variable. Differences in the mean values were assessed using the Mann-Whitney U test and Wilcoxon signed rank test. The level of significance was set at P < 0.05.
RESULTS FIGURE 1. Positions of the surface electrodes on temporalis and masseter muscles.
Bite Force An occlusal bite force system was designed specifically for measuring bite force (Fig. 2). It is a complete system comprising sensors, a sensor-connecting device, and software. The 3 sensors on the occlusal pad were positioned on the incisor and bilateral first molar region. Participants whose first molars were missing were measured at the second molars. The software converted pressure values into forces. In all experimental sessions, measurement devices (an occlusal pad with a sensor attached to a laptop computer system) were inserted intraorally to measure the maximal bite force in kilograms.
FIGURE 2. The occlusal bite force system comprising sensors, sensor-connecting device, and software. © 2015 Wolters Kluwer Health, Inc. All rights reserved.
The mean age of the study group was 28.40 ± 3.0 years. When MVC was applied in habitual intercuspation, the mean EMG signals were 107.7 ± 55.0 μV and 106.0 ± 56.0 μV (P = 0.699) on the right and left temporalis muscles, and 183.7 ± 86.2 μV and 194.8 ± 94.3 μV (P = 0.121) on the right and left masseter muscles, respectively (Fig. 4A). In the male group, the EMG activity was 119.5 ± 64.5 μVand 114.9 ± 43.3 (P = 0.191) on the right and left temporalis muscles, and 170.8 ± 93.1 μV and 187.1 ± 103.2 μV (P = 0.191) on the right and left masseter muscles. In the female group, the EMG activity was 95.3 ± 45.6 μV and 100.9 ± 63.4 (P = 0.872) on the right and left temporalis muscles, and 144.4 ± 64.1 μV and 138.3 ± 72.5 μV (P = 0.387) on the right and left masseter muscles. No significant differences in the right and left temporalis muscle (P = 0.339 and P = 0.286) or the right
FIGURE 3. Surface EMG with bite force assembly ready to connect to measuring apparatus after intraoral positioning. www.annalsplasticsurgery.com
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and left masseter muscle were noted between men and women (P = 0.369 and P = 0.177) (Fig. 4B). The bite force was 5.0 ± 3.2 kg on the right side and 5.7 ± 4.0 kg on the left side (P = 0.974) (Fig. 5). In the male group, the bite force was 6.3 ± 3.2 kg on the right side and 7.0 ± 3.7 kg on the left side (P = 0.859). In the female group, the bite force was 4.1 ± 3.0 kg on the right side and 4.8 ± 4.1 kg on the left side (P = 0.972). The value of bite force was significantly higher in the male group than in the female group. Regarding the correlation between sEMG signals and bite force, the Pearson correlation coefficient was 0.512 between the temporalis muscle and bite force (P = 0.000) and 0.360 between the
FIGURE 4. A, Graphic comparison of bilateral sEMG activities on temporalis and masseter muscles. B, Graphic comparison of temporalis and masseter sEMG activities between female and male. RTA, right temporalis muscles; LTA, left temporalis muscles; RMAS, right masseter muscles; LMAS, left masseter muscles. *P < 0.05.
FIGURE 5. Graphic comparison of bite force between female and male. *P < 0.05. S170
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FIGURE 6. A, Correlation between bite force and sEMG activity of temporalis muscle. B, Correlation between bite force and sEMG activity of masseter muscle. © 2015 Wolters Kluwer Health, Inc. All rights reserved.
Copyright © 2015 Wolters Kluwer Health, Inc. All rights reserved.
Annals of Plastic Surgery • Volume 74, Supplement 2, May 2015
The Correlation Between sEMG and Bite Force
DISCUSSION
FIGURE 7. A, Improvement of sEMG activities of temporalis and masseter muscles at postoperative 1, 3, and 6 months. B, Improvement of bite force at postoperative 1, 3, and 6 months. R, right side; L, left side.
masseter muscle and bite force (P = 0.014); both correlations were positive (Fig. 6). No significant differences in MVC and maximal bite force were observed between the bilateral temporalis and masseter muscles in a normal population.
CLINICAL APPLICATION A 21-year-old male patient experienced a right zygoma closed fracture in a motorcycle crash. Open reduction and internal fixation were performed 5 days after the injury. After the operation, the Zebris sEMG system was used to evaluate the contraction strength of his temporalis and masseter muscles at 1, 3, and 6 months after operation. The temporalis muscle signal improved from 49.5 μV to 109 μV to 124.9 μV on the right side and from 65.5 μV to 86.1 μV to 157.3 μV on the left side. The masseter muscle improved from 45.8 μV to 119.3 μV to 163 μV on the right side and from 32.1 μV to 185.6 μV to 255.9 μVon the left side (Fig. 7). Although the right-side mastication muscles did not fully recover to the reference range compared with the left-side muscles, we observed a significant improvement in muscle contraction strength after early rehabilitation and physical therapy. © 2015 Wolters Kluwer Health, Inc. All rights reserved.
In our study, EMG signals were measured at the temporalis and masseter muscles, which have also been used in previous studies. These muscles provided easy access for localization and measurement and exhibit the most definite contraction during maximal occlusion.14 The bite force was measured while the participants clenched the first molar area.14 According to Ramfjord and Ash, the first molar area exhibits the largest bite force, and this force is similar to the bite force at centric occlusion.14 Several investigators have reported a linear relationship between electromyographic activity and bite force, but data on the reliability of this relationship are limited.15 Gonzalez et al15 reported that the slope of the sEMG activity versus bite force for a given biting situation was reliable for temporalis and masseter muscles. In our study, we observed a positive correlation between bite force and sEMG activity on both the temporalis and masseter muscles. The contribution of this study was that we used 2 devices concurrently, increasing the accuracy and efficiency of detection and thus facilitating progress toward a goal of developing a comprehensive device. Moreover, no data on the bite force and EMG activity have been collected in a Taiwanese population, and no previous studies have compared differences among participants. The standard deviation and variation in sEMG signals and bite force between each participant were substantial, indicating that various factors, such as the mastication pattern, occlusal contact point, mastication muscle strength, psychological factors, and measurement errors, may affect the results.14 In addition, sEMG can be used to detect the activity of the masticatory muscles on the skin but not the activity inside the muscle. For some participants, we anticipated that the signal would be small when conducting palpation to determine the location of the masseter muscle before examination because the bulging of the muscle while the participant clenched was weak. Interpretation of the sEMG data was affected by technical specifications and physiologic limitations.13 To improve the repeatability of the study, we controlled the experimental protocol, standardized the electrode positioning,13 and used the average of 3 measurements for analysis. The graphs recorded showed high uniformity and consistency between the repeated examinations, indicating that the intermeasurement variability was low and that the study is repeatable and reliable. Although some people may use one side of a muscle more often than the other side, no statistically significant difference between left and right temporalis and masseter muscle contractions and bite force was observed in a normal population. Thus, the system developed in this study can be used to determine the muscle strength and mastication condition in people with unilateral craniofacial trauma or deformity. This system may also be useful in assessing the degree of improvement after treatment and comparing the outcomes of various surgical and nonsurgical techniques. The advantages of the system are that it is noninvasive, efficient (examination requires only 20 minutes), and easy to understand and use; moreover, real-time results can be displayed on a computer screen, which is educational and has characteristics of biofeedback. The disadvantage of the device is that applying it to noncooperative people, such as elderly people and children, as well as unconscious patients, is difficult. We hope that the system can be applied clinically to patients with temporomandibular disorders, facial bone fracture, and head and neck cancer with trismus. It can be useful in preoperative evaluation, postoperative follow-up, and outcome evaluation, and can facilitate the design of rehabilitation programs and physical therapy. In addition, the system can be combined with a jaw motion analyzer or sensory test to increase the comprehensiveness of functional analysis.
CONCLUSION We developed a clinically applicable, quantitative, reliable, and noninvasive system for evaluating mastication function with characteristics www.annalsplasticsurgery.com
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of biofeedback. No difference between electromyographic activity on the bilateral temporalis and masseter muscle and bilateral bite force was observed in young healthy adults in Taiwan. A positive correlation between sEMG signals and bite force was noted. An sEMG machine and an occlusal bite force system can be combined to reduce the time and increase the comprehensiveness of examinations. We hope that this simple system can be combined with a jaw motion analyzer with an allin-one setting to facilitate application in facial trauma management in the future.
REFERENCES 1. Gibbs CH, Messerman T, Reswick JB, et al. Functional movements of the mandible. J Prosthet Dent. 1971;26:604–620. 2. Hannam AG, Scott JD, De Cou RE. A computer based system for the simultaneous measurement of muscle activity and jaw movement during mastication in man. Arch Oral Biol. 1977;22:17–23. 3. Messerman T, Reswick JB, Gibbs C. Investigation of functional mandibular movements. Dent Clin North Am. 1969;13:629–642. 4. Svensson P, Graven-Nielsen T. Craniofacial muscle pain: review of mechanisms and clinical manifestations. J Orofac Pain. 2001;15:117–145.
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5. Cooper BC. The role of bioelectronic instrumentation in the documentation and management of temporomandibular disorders. Oral Surg Oral Med Oral Pathol Oral Radiol Endod. 1997;83:91–100. 6. Nielsen IL, Marcel T, Chun D, et al. Patterns of mandibular movements in subjects with craniomandibular disorders. J Prosthet Dent. 1990;63:202–217. 7. Gysi A. The problem of articulation. Dent Cosmos. 1910;52:1–19. 8. Posselt U. Range of movement of the mandible. J Am Dent Assoc. 1958;56:10–13. 9. Weinberg LA. A cinematic study of centric and eccentric occlusions. J Prosthet Dent. 1964;14:290–293. 10. Mongini F, Tempia-Valenta G, Conserva E. Habitual mastication in dysfunction: a computer-based analysis. J Prosthet Dent. 1989;61:484–494. 11. Howell PG, Johnson CW, Ellis S, et al. The recording and analysis of EMG and jaw tracking. I. The recording procedure. J Oral Rehabil. 1992;19:595–605. 12. Howell PG, Ellis S, Johnson CW, et al. The recording and analysis of EMG and jaw tracking. II. Reproducibility of jaw tracking. J Oral Rehabil. 1993; 20:33–43. 13. Ko EW, Huang CS, Lo LJ, et al. Alteration of masticatory electromyographic activity and stability of orthognathic surgery in patients with skeletal class III malocclusion. J Oral Maxillofac Surg. 2013;71:1249–1260. 14. Park MK, Cho SM, Yun KI. Change in bite force and electromyographic activity of masticatory muscle in accordance with change of occlusal plane. J Oral Maxillofac Surg. 2012;70:1960–1967. 15. Gonzalez Y, Iwasaki LR, McCall WD Jr, et al. Reliability of EMG activity versus bite-force from human masticatory muscles. Eur J Oral Sci. 2011; 119:219–224.
© 2015 Wolters Kluwer Health, Inc. All rights reserved.
Copyright © 2015 Wolters Kluwer Health, Inc. All rights reserved.
The Correlation Between Surface Electromyography and Bite Force of Mastication Muscles in Asian Young Adults Cheng-I Yen, MD,* Shih-Hsuan Mao, MD,* Chih-Hao Chen, MD,* Chien-Tzung Chen, MD,† and Ming-Yih Lee, PhD‡ Background: Mastication function is related to mandible movement, muscle strength, and bite force. No standard device for measuring bite force has been developed. A linear relationship between electromyographic activity and bite force has been reported by several investigators, but data on the reliability of this relationship remain limited in Asian young adults. Aim and Objectives: The purpose of this study was to develop a clinically applicable, reliable, quantitative, and noninvasive system to measure the kinetic mastication function and observe the correlation between surface electromyography (sEMG) and bite force. Materials and Methods: The study group consisted of 41 young healthy adults (24 men and 17 women). Surface electromyography was used to evaluate bilateral temporalis and masseter muscle activities, and an occlusal bite force system was used concurrently to measure the bite force during maximal voluntary biting. Bilateral symmetry was compared, and the correlation between EMG and bite force was calculated. Results: The sEMG signals were 107.7 ± 55.0 μVand 106.0 ± 56.0 μV (P = 0.699) on right and left temporalis muscles and 183.7 ± 86.2 μV and 194.8 ± 94.3 μV (P = 0.121) on right and left masseter muscles, respectively. The bite force was 5.0 ± 3.2 kg on the right side and 5.7 ± 4.0 kg on the left side (P = 0.974). A positive correlation between sEMG and bite force was observed. The correlation coefficient between the temporalis muscle and bite force was 0.512, and that between the masseter muscle and bite force was 0.360. Conclusion: No significant difference between the bilateral electromyographic activities of the temporalis and masseter muscles and bilateral bite force was observed in young healthy adults in Taiwan. A positive correlation between sEMG signals and bite force was noted. By combining sEMG and bite force, we developed a clinically applicable, quantitative, reliable, and noninvasive system for evaluating mastication function by using characteristics of biofeedback. Key Words: mastication, mandible, electromyography (EMG), bite force (Ann Plast Surg 2015;74: S168–S172)
M
astication is a highly coordinated neuromuscular function involving mandible movements and continual modulations of force.1 When efficiently performed, mastication facilitates swallowing and nutrient absorption.2 The strength of masticatory muscles is a wellrecognized parameter, and the capacity to exert sufficient bite force is an indicator of normal masticatory function.3 In addition, one study Received September 19, 2014, and accepted for publication, after revision, December 17, 2014. From the *Department of Plastic and Reconstructive Surgery, Chang Gung Memorial Hospital, Chang Gung University College of Medicine, Taoyuan, Taiwan, ROC; †Department of Plastic and Reconstructive Surgery, Chang Gung Memorial Hospital at Keelung, Keelung, Taiwan, ROC; and ‡Graduate Institute of Medical Mechatronics, Chang Gung University, Taoyuan, Taiwan, ROC. Conflicts of interest and sources of funding: none declared. Reprints: Chih-Hao Chen, MD, Department of Plastic and Reconstructive Surgery, Chang Gung Memorial Hospital, Chang Gung University College of Medicine, 5 Fu-Hsin St, Kwei-Shan, Taoyuan 333, Taiwan, ROC. E-mail: [email protected]. Ming-Yih Lee, PhD, Graduate Institute of Medical Mechatronics, College of Engineering, Chang Gung University, 259 Wen-Hwa First Road, Kuei-Shan, Taoyuan 333, Taiwan, ROC. E-mail: [email protected]; [email protected]. Copyright © 2015 Wolters Kluwer Health, Inc. All rights reserved. ISSN: 0148-7043/15/7402–S168 DOI: 10.1097/SAP.0000000000000468
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demonstrated a significant relationship between occlusal force and masticatory performance.3 Occlusal force is related to dental conditions, the type of prosthetic reconstruction, the presence of temporomandibular disorders, and the fracture of the bones involved in the masticatory apparatus.4–9 Occlusal force can be measured directly between a pair of teeth by using a suitable transducer.3,7,8,10–12 However, no standard device for measuring occlusal force has been developed, and thus, dental occlusion has largely been a matter of guesswork for dentists. Articulation paper, waxes, and pressure indicator paste are the main tools dentists use to assess and balance the forces of occlusion. Most of these methods are not sufficiently sensitive for detecting simultaneous contact, and none of them can be used to measure both masticatory activity and bite force simultaneously. An ideal measurement system is noninvasive, providing protection from artifactural loading by preventing an increase in the vertical dimension of the occlusion and interference with dentition. In this study, we used surface electromyography (sEMG) to detect the activities of masticatory muscles and a thin custom-made splint with a built-in force transducer to measure bite force. We calculated the variations between men and women and tested for the presence of side-to-side facial asymmetry. In addition, we combined the 2 devices to determine the degree of correlation between sEMG signals and bite force. Because no previous related data have been collected in a Taiwanese population, before applying the system to patients, we sought to establish a standard model in a young healthy population.
MATERIALS AND METHODS The study group consisted of 41 healthy people, 24 men and 17 women, with class I occlusion, intact dentition, and no craniofacial anomalies, previous facial trauma or facial surgeries, malocclusion, orthodontic treatment, or symptoms or clinical signs of temporomandibular disorders. The participants ranged in age from 23 to 32 years, and the mean age was 28.4 years. Each participant was administered a history survey and clinical examination to assess occlusion and jaw movement. Two well-trained research assistants performed all measurements concurrently. Each movement was repeated 3 times to determine the average and ensure that readings were accurate. Informed consent was obtained from all participants, and the protocol was reviewed and approved by an institutional review board.
Surface Electromyography The sEMG machine (Zebris EMG 4, Zebris GmbH, Isny im Allgau, Germany) is a noninvasive tool used to analyze electrical signals that emanate during muscular contraction in the stomatognathic system (Fig. 1). We used this machine to test the bilateral temporalis and masseter muscles. The participants sat upright in a comfortable chair and a relaxed environment. The skin was prepared and draped using 75% alcohol cotton balls.13 Pregelled disposable electrodes were applied to the bilateral temporalis and masseter muscles, which were detected and located by performing palpation while asking the patient to clench.13 Another reference electrode was placed on the left shoulder. Annals of Plastic Surgery • Volume 74, Supplement 2, May 2015
Copyright © 2015 Wolters Kluwer Health, Inc. All rights reserved.
Annals of Plastic Surgery • Volume 74, Supplement 2, May 2015
The Correlation Between sEMG and Bite Force
Examination Protocol After the values of the sEMG signal had stabilized, the resting tonus was recorded for 6 to 8 seconds as a relaxation test. To determine the maximal voluntary contraction (MVC), a bite test was conducted in habitual intercuspation.13 Each participant bit as forcefully as possible for 2 seconds13 and then relaxed his or her mouth. The investigator then placed the bite force device in the mouth gently and instructed the participant to bite for 2 seconds again to determine the correlation between sEMG activity and bite force (Fig. 3). To determine the firing pattern, the participants were asked to press their teeth together with gradually increasing force for 10 seconds.13 In a fatigue test, the participants bit their teeth by using their full strength for 10 seconds. Each test was conducted 3 times, and the average value was calculated.
Statistical Analysis The data were statistically analyzed using the SPSS package version 20.0 for Windows. Means and standard deviations were computed for each independent variable. Differences in the mean values were assessed using the Mann-Whitney U test and Wilcoxon signed rank test. The level of significance was set at P < 0.05.
RESULTS FIGURE 1. Positions of the surface electrodes on temporalis and masseter muscles.
Bite Force An occlusal bite force system was designed specifically for measuring bite force (Fig. 2). It is a complete system comprising sensors, a sensor-connecting device, and software. The 3 sensors on the occlusal pad were positioned on the incisor and bilateral first molar region. Participants whose first molars were missing were measured at the second molars. The software converted pressure values into forces. In all experimental sessions, measurement devices (an occlusal pad with a sensor attached to a laptop computer system) were inserted intraorally to measure the maximal bite force in kilograms.
FIGURE 2. The occlusal bite force system comprising sensors, sensor-connecting device, and software. © 2015 Wolters Kluwer Health, Inc. All rights reserved.
The mean age of the study group was 28.40 ± 3.0 years. When MVC was applied in habitual intercuspation, the mean EMG signals were 107.7 ± 55.0 μV and 106.0 ± 56.0 μV (P = 0.699) on the right and left temporalis muscles, and 183.7 ± 86.2 μV and 194.8 ± 94.3 μV (P = 0.121) on the right and left masseter muscles, respectively (Fig. 4A). In the male group, the EMG activity was 119.5 ± 64.5 μVand 114.9 ± 43.3 (P = 0.191) on the right and left temporalis muscles, and 170.8 ± 93.1 μV and 187.1 ± 103.2 μV (P = 0.191) on the right and left masseter muscles. In the female group, the EMG activity was 95.3 ± 45.6 μV and 100.9 ± 63.4 (P = 0.872) on the right and left temporalis muscles, and 144.4 ± 64.1 μV and 138.3 ± 72.5 μV (P = 0.387) on the right and left masseter muscles. No significant differences in the right and left temporalis muscle (P = 0.339 and P = 0.286) or the right
FIGURE 3. Surface EMG with bite force assembly ready to connect to measuring apparatus after intraoral positioning. www.annalsplasticsurgery.com
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and left masseter muscle were noted between men and women (P = 0.369 and P = 0.177) (Fig. 4B). The bite force was 5.0 ± 3.2 kg on the right side and 5.7 ± 4.0 kg on the left side (P = 0.974) (Fig. 5). In the male group, the bite force was 6.3 ± 3.2 kg on the right side and 7.0 ± 3.7 kg on the left side (P = 0.859). In the female group, the bite force was 4.1 ± 3.0 kg on the right side and 4.8 ± 4.1 kg on the left side (P = 0.972). The value of bite force was significantly higher in the male group than in the female group. Regarding the correlation between sEMG signals and bite force, the Pearson correlation coefficient was 0.512 between the temporalis muscle and bite force (P = 0.000) and 0.360 between the
FIGURE 4. A, Graphic comparison of bilateral sEMG activities on temporalis and masseter muscles. B, Graphic comparison of temporalis and masseter sEMG activities between female and male. RTA, right temporalis muscles; LTA, left temporalis muscles; RMAS, right masseter muscles; LMAS, left masseter muscles. *P < 0.05.
FIGURE 5. Graphic comparison of bite force between female and male. *P < 0.05. S170
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FIGURE 6. A, Correlation between bite force and sEMG activity of temporalis muscle. B, Correlation between bite force and sEMG activity of masseter muscle. © 2015 Wolters Kluwer Health, Inc. All rights reserved.
Copyright © 2015 Wolters Kluwer Health, Inc. All rights reserved.
Annals of Plastic Surgery • Volume 74, Supplement 2, May 2015
The Correlation Between sEMG and Bite Force
DISCUSSION
FIGURE 7. A, Improvement of sEMG activities of temporalis and masseter muscles at postoperative 1, 3, and 6 months. B, Improvement of bite force at postoperative 1, 3, and 6 months. R, right side; L, left side.
masseter muscle and bite force (P = 0.014); both correlations were positive (Fig. 6). No significant differences in MVC and maximal bite force were observed between the bilateral temporalis and masseter muscles in a normal population.
CLINICAL APPLICATION A 21-year-old male patient experienced a right zygoma closed fracture in a motorcycle crash. Open reduction and internal fixation were performed 5 days after the injury. After the operation, the Zebris sEMG system was used to evaluate the contraction strength of his temporalis and masseter muscles at 1, 3, and 6 months after operation. The temporalis muscle signal improved from 49.5 μV to 109 μV to 124.9 μV on the right side and from 65.5 μV to 86.1 μV to 157.3 μV on the left side. The masseter muscle improved from 45.8 μV to 119.3 μV to 163 μV on the right side and from 32.1 μV to 185.6 μV to 255.9 μVon the left side (Fig. 7). Although the right-side mastication muscles did not fully recover to the reference range compared with the left-side muscles, we observed a significant improvement in muscle contraction strength after early rehabilitation and physical therapy. © 2015 Wolters Kluwer Health, Inc. All rights reserved.
In our study, EMG signals were measured at the temporalis and masseter muscles, which have also been used in previous studies. These muscles provided easy access for localization and measurement and exhibit the most definite contraction during maximal occlusion.14 The bite force was measured while the participants clenched the first molar area.14 According to Ramfjord and Ash, the first molar area exhibits the largest bite force, and this force is similar to the bite force at centric occlusion.14 Several investigators have reported a linear relationship between electromyographic activity and bite force, but data on the reliability of this relationship are limited.15 Gonzalez et al15 reported that the slope of the sEMG activity versus bite force for a given biting situation was reliable for temporalis and masseter muscles. In our study, we observed a positive correlation between bite force and sEMG activity on both the temporalis and masseter muscles. The contribution of this study was that we used 2 devices concurrently, increasing the accuracy and efficiency of detection and thus facilitating progress toward a goal of developing a comprehensive device. Moreover, no data on the bite force and EMG activity have been collected in a Taiwanese population, and no previous studies have compared differences among participants. The standard deviation and variation in sEMG signals and bite force between each participant were substantial, indicating that various factors, such as the mastication pattern, occlusal contact point, mastication muscle strength, psychological factors, and measurement errors, may affect the results.14 In addition, sEMG can be used to detect the activity of the masticatory muscles on the skin but not the activity inside the muscle. For some participants, we anticipated that the signal would be small when conducting palpation to determine the location of the masseter muscle before examination because the bulging of the muscle while the participant clenched was weak. Interpretation of the sEMG data was affected by technical specifications and physiologic limitations.13 To improve the repeatability of the study, we controlled the experimental protocol, standardized the electrode positioning,13 and used the average of 3 measurements for analysis. The graphs recorded showed high uniformity and consistency between the repeated examinations, indicating that the intermeasurement variability was low and that the study is repeatable and reliable. Although some people may use one side of a muscle more often than the other side, no statistically significant difference between left and right temporalis and masseter muscle contractions and bite force was observed in a normal population. Thus, the system developed in this study can be used to determine the muscle strength and mastication condition in people with unilateral craniofacial trauma or deformity. This system may also be useful in assessing the degree of improvement after treatment and comparing the outcomes of various surgical and nonsurgical techniques. The advantages of the system are that it is noninvasive, efficient (examination requires only 20 minutes), and easy to understand and use; moreover, real-time results can be displayed on a computer screen, which is educational and has characteristics of biofeedback. The disadvantage of the device is that applying it to noncooperative people, such as elderly people and children, as well as unconscious patients, is difficult. We hope that the system can be applied clinically to patients with temporomandibular disorders, facial bone fracture, and head and neck cancer with trismus. It can be useful in preoperative evaluation, postoperative follow-up, and outcome evaluation, and can facilitate the design of rehabilitation programs and physical therapy. In addition, the system can be combined with a jaw motion analyzer or sensory test to increase the comprehensiveness of functional analysis.
CONCLUSION We developed a clinically applicable, quantitative, reliable, and noninvasive system for evaluating mastication function with characteristics www.annalsplasticsurgery.com
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of biofeedback. No difference between electromyographic activity on the bilateral temporalis and masseter muscle and bilateral bite force was observed in young healthy adults in Taiwan. A positive correlation between sEMG signals and bite force was noted. An sEMG machine and an occlusal bite force system can be combined to reduce the time and increase the comprehensiveness of examinations. We hope that this simple system can be combined with a jaw motion analyzer with an allin-one setting to facilitate application in facial trauma management in the future.
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© 2015 Wolters Kluwer Health, Inc. All rights reserved.
Copyright © 2015 Wolters Kluwer Health, Inc. All rights reserved.
