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Clenching occurring during the day is influenced by psychological factors
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Journal of Prosthodontic Research 55 (2011) 159–164 www.elsevier.com/locate/jpor
Original article
Clenching occurring during the day is influenced by psychological factors Hiroshi Endo DDS*, Kiyotaka Kanemura DDS, PhD, Norimasa Tanabe DDS, PhD, Jun Takebe DDS, PhD Division of Fixed Prosthodontics, School of Dentistry, Iwate Medical University, 1-3-27 Chuodori, Morioka, Iwate 020-8505, Japan Received 10 September 2010; received in revised form 19 November 2010; accepted 23 November 2010 Available online 5 February 2011
Abstract Purpose: We investigated the relationship between clenching occurring during the day under natural conditions and psychological attributes. Method: Subjects were 37 dentistry students and staff (14 women, 23 men; mean age 27.4 4.7 years). A portable EMG recording device was used to record muscle activity in the temporal muscles for a continuous 5-h period under subjects’ natural conditions, including having lunch. Clenching was identified by electromyography, and subjects were categorized into clenching and non-clenching groups. Psychological testing was performed during the same period using the Modified Taylor Manifest Anxiety Scale (MAS), the Self-rating Depression Scale (SDS), and the Cornell Medical Index (CMI), and the psychological attributes of clenching and non-clenching groups were compared. Results: Of the 37 subjects, 23 were assigned to the clenching group and 14 to the non-clenching group. Thirteen of the 23 subjects of the clenching group and none of the 14 subjects of the non-clenching group were determined to have psychological problems according to the MAS, with a significant difference observed between the clenching and non-clenching groups in terms of anxious tendency ( p < 0.001; Fisher’s exact test). Total muscle activity in the clenching group, excluding eating and talking, was 3.5 times greater compared with the non-clenching group, and a significant difference between the groups was observed ( p < 0.05; Mann–Whitney U-test). Conclusions: Daytime clenching was shown to be associated with anxious tendency. Total muscle activity in the clenching group was 3.5 times greater compared with the non-clenching group. # 2010 Japan Prosthodontic Society. Published by Elsevier Ireland. All rights reserved. Keywords: Bruxism; Daytime clenching; Parafunctional movements; Psychological factors; Portable EMG recording device
1. Introduction Bruxism refers to grinding and clenching of the teeth and psychological factors are believed to contribute to its development. As bruxism can be a major risk factor for problems such as dental attrition, temporomandibular disorder (TMD), periodontal disease, tooth fracture, and breakage of prosthetic devices, its causes require elucidation. A range of studies of nocturnal bruxism have been reported [1–5]. With respect to daytime clenching, however, although some people are aware of their own bruxism, in other cases it is suspected from oral findings [6]. Although there have been some laboratory studies of daytime clenching using electromyography (EMG) for objective evaluation, it is difficult to define in daily life because natural conditions are affected by psychological factors.
* Corresponding author. Tel.: +81 19 651 5111x4127; fax: +81 19 654 3281. E-mail address: [email protected] (H. Endo).
In the present study, we investigated the relationship between clenching occurring during the day and psychological characteristics. To evaluate these parameters we used temporal muscle EMG measurements on the habitual chewing side in daily living, obtained using a portable EMG recording device, and three different psychological tests. 2. Method of research 2.1. Subjects Subjects were 37 students and staff (14 women, 23 men; mean age 27.4 4.7 years) at Iwate Medical University School of Dentistry. Eligibility criteria were (1) not having a denture fitted and (2) individual normal occlusion. Exclusion criteria were (1) orofacial pain; (2) current treatment for acute symptoms of TMD; (3) current orthodontic treatment (including the retention period); (4) severe periodontitis; and (5) unstable occlusion. Informed written consent was obtained from each subject after they had received a detailed explanation
1883-1958/$ – see front matter # 2010 Japan Prosthodontic Society. Published by Elsevier Ireland. All rights reserved. doi:10.1016/j.jpor.2010.11.003
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H. Endo et al. / Journal of Prosthodontic Research 55 (2011) 159–164
of the research protocol. This study was approved by the Human Research Ethics Committee of Iwate Medical University School of Dentistry (No. 01090).
difficult to see from the outside, enabling measurements to be taken without interfering with daily life (Fig. 1a and b). 2.3. Study schedule
2.2. Measurement device The portable EMG recording device was a onechannel EMG recording system comprising a remote control (100 mm 60 mm 18 mm), a main unit (64 mm 21 mm 12.5 mm), and an electrode unit. It is capable of 9 h of continuous recording at a sampling rate of 4 Hz. EMG derivation was performed by attaching a 5-mm diameter Ag–AgCl surface electrode unit to the electrode attachment site, i.e., the skin overlying the temporal muscle (anterior muscle bundle) on the habitual chewing side, with a fixed distance of 20 mm between the centers of the electrodes, and the bipolar induction method was used. Indifferent electrodes were attached to the back of the earlobe. The electrode attachment sites were carefully cleaned with alcoholsoaked cotton before electrodes were applied. This device reduces the effect of motion artifacts generated by shaking of the wires by including a small amplifier in the main unit that performs impedance conversion [7]; it also reduces noise by incorporating a hum filter and a 5.3–450-Hz band path filter. The device is worn behind the ear and is
To identify clenching occurring under natural conditions, a portable EMG recording device developed in our field was used to make continuous 5-h EMG recordings, including having lunch. When measurement was started, muscle activity during the hardest possible bite was measured as maximum voluntary contraction (100% MVC) for calibration purposes (Fig. 2). Subjects were instructed to behave normally, with the exception of washing their face, touching the electrode, or making any other movements that might create undesirable noise during the EMG signals. They were also told to record a keyword indicating their behavior and the time on a record sheet every time they changed their behavior. After the conclusion of measurements, the device was removed from subjects and the EMG data recorded were forwarded to a personal computer via specialized data transmission software for analysis. Three types of psychological test were performed: the Modified Taylor Manifest Anxiety Scale (MAS) of the Minnesota Multiphasic Personality Inventory (MMPI) to measure the degree of anxiety, the Self-rating Depression
Fig. 1. (a) Portable EMG recording device. (b) Left panel: completed set-up of the apparatus. Right panel: the apparatus is covered by the subject’s hair.
H. Endo et al. / Journal of Prosthodontic Research 55 (2011) 159–164
161
Fig. 2. Sample diurnal 5-h EMG recording (subject no. 4).
Scale (SDS) to investigate depressiveness, and the Cornell Medical Index (CMI) to investigate subjective physical and mental symptoms. We used Japanese versions of all these tests. All of these were distributed before the start of EMG recording, and collected several days later.
2.4. Analytical method Functional movements (such as eating and talking) consist of powerful intermittent muscle activity. In contrast, parafunctional movements (clenching) are characterized by weak, continuous muscle activity. In this study, we used this difference to distinguish functional from parafunctional movements in the diurnal 5-h EMG recordings. Subjects were categorized into a clenching group and a non-clenching group (Fig. 3), with clenching defined from previous studies as muscle activity of 20% MVC and 3-s duration observed during parafunctional movements [8–10]. To ensure objectivity, EMG analysis was performed using analysis software. For the three psychological tests, the presence of psychological problems was determined by scores of I and II in the MAS, 48 in the SDS, and III or IV in the CMI. We investigated the relationship between the results of these tests and the occurrence of clenching, and compared them using Fisher’s exact test. Statistical significance was set at 5%. The results of the psychological tests used in this study were scored
and evaluated in accordance with the guides for each test [11– 13] (Table 1). We also calculated the total muscle activity, excluding during eating and talking, in the clenching and non-clenching groups, and compared them using the Mann–Whitney U-test. Statistical significance was set at 5%. The SPSS statistical package (15.0 J for Windows, SPSS Japan, Tokyo, Japan) was used for statistical analysis. The total muscle activity was evaluated using the integral values. 3. Results 1. Of the 37 subjects, 23 (6 women, 17 men) were assigned to the clenching group and 14 (8 women, 6 men) to the nonclenching group based on the EMG measurement results (Fig. 4). 2. Thirteen (3 women, 10 men) of the 23 subjects of the clenching group and none of the 14 subjects of the nonclenching group were determined to have psychological problems according to the MAS, with a significant difference observed between the clenching and non-clenching groups in terms of anxious tendency ( p < 0.001; Fisher’s exact test). In addition, the mean MAS score of the 37 subjects was 16.19. No significant difference in the number of subjects
Table 1 Determination criteria for psychological tests.
Normal
≤47
48−52
≥53
Normal
Neurosis
Depression
Cornell Medical Index
I, II
IV
CMI
Normal
III Neurotic tendency
MAS Self-rating Depression Scale
SDS
Fig. 3. Method of distinguishing clenching events.
I Severe anxiety
II Moderate anxiety
Manifest Anxiety Scale
III, IV, V
Psychologically normal
Neurosis
Psychological problems present
162
H. Endo et al. / Journal of Prosthodontic Research 55 (2011) 159–164 Table 3 Psychological test results for each subject. Non-clenching group 14 subjects (37.8%) 8 women, 6 men
Clenching group 23 subjects (62.2%) 6 women, 17 men
Clenching group (n = 23) Subject No.
Subjects n = 37 Fig. 4. Categorization of subjects according to presence or absence of clenching events.
with psychological problems was observed between the clenching and non-clenching groups according to the SDS (6 (3 women, 3 men)/23 vs. 1 (a woman)/14) or the CMI (5 (all men)/23 vs. 1 (a man)/14). Furthermore, no significant gender difference was observed between the two groups (Tables 2 and 3). 3. Total muscle activity, excluding eating and talking, was 3.5 times greater in the clenching group than in the non-clenching group, a significant difference was observed between the two groups ( p < 0.05; Mann–Whitney U-test) (Fig. 5).
4. Discussion In this study, significantly more subjects in the clenching group than the non-clenching group were determined to have psychological problems according to the MAS, indicating an association between daytime clenching and severe anxious tendency. In addition, although the differences were not significant, the fact that some subjects who had psychological problems according to the MAS also had problems according to the SDS and CMI indicates that the involvement of psychological factors may be considerable. These findings suggest that when dealing with patients with suspected clenching, it is important to take psychological factors into account. This study is not capable of determining whether psychological problems occur as a result of clenching or vice versa. On this point, Asano et al. [14] have reported the involvement of severe anxious tendency as a factor contributing to TMD onset. Carlson et al. [15] and Curran et al. [16] reported that muscle activity at a time of rest was increased by experimental stress. Hurrell et al. [17] reported that job stressors and their resulting stress reactions, such as anxiety, led to health problems. Taken together with the fact that bruxism is noted for its relation to TMD onset, this suggests that anxiety Table 2 Numbers of subjects with psychological problems. Subjects, n = 37
MAS*
SDS
CM I
Clenching group, n = 23 Non-clenching group, n = 14
13 0
6 1
5 1
*
p < 0.001: Fisher’s exact test.
18 5 13 16 35 36 8 23 26 2 3 14 20 1 4 7 9 10 11 12 15 17 19
MAS
SDS
CMI
Non-clenching group (n = 14) Subject No.
MAS
SDS
CMI
32 30 6 21 22 24 25 27 28 29 31 33 34 37
Psychological problems present
caused by some sort of stress may induce clenching, leading to TMD. The mean MAS score among healthy Japanese individuals is 16.90 [13], and since the mean of subjects in the present study was almost the same as this value, they can be regarded as comprising a general population. No gender difference was observed in this study. However, because it has been reported that women are more sensitive to stress than men [18,19], further studies with an increased number of participants may be required. There was a significant difference in total muscle activity between the clenching and non-clenching groups, which were categorized according to a cut-off value in this study. This may be considered an unsurprising result if clenching events are regarded as being reflected in the data as considerable amounts of muscle activity. The characteristic of the clenching group in this study, however, was the presence of considerable amounts of muscle activity of <20% MVC. Fig. 6 shows one example. Clenching that occurs during the day, even if relatively weak, has been reported to have an adverse effect if continued over a long period, and instructing patients not to allow their upper and lower teeth to touch has been reported to be effective in counteracting this [6,20]. The results of this study may be regarded as an EMG record of tooth contacting habits (TCH). In this study, diurnal EMG recordings were performed only once. Concerning this point, in their report of four-day recordings of muscle activity in the temporal muscles, Watanabe et al. [10] state that there was no variation in the amount of muscle activity in the clenching group on different days. Because changes in the long-term occurrence of clenching events in habitual clenchers have not yet been confirmed, however, this is a question for further study. Criteria used to determine nocturnal bruxism include number of masticatory muscle bursts and strength of muscle
H. Endo et al. / Journal of Prosthodontic Research 55 (2011) 159–164
163
(μV/s)
* Clenching group Non-clenching group
*p < 0.05: Mann-Whitney U-test
Clenching group:non-clenching group = 3.5:1 Fig. 5. Total muscle activity over a 5-h diurnal period.
10% MVC (times)
20% MVC
≥30% MVC
(times)
(s) Clenching group (subject No.7)
(s) Non-clenching group (subject No.32)
Fig. 6. Breakdown of muscle activity in each group.
potentials [1,3,21]. We have been unable to find any reports, however, that clearly indicate criteria for the determination of daytime clenching. One possible reason may be that previously there were no devices capable of recording masticatory muscle activity during daily activities. The portable EMG recording device used in this study differentiates between functional and parafunctional movements in terms of a combination of amount of muscle activity and sustained duration of muscle activity. It is capable of distinguishing between functional movements such as eating and parafunctional movements such as clenching with an accuracy rate of 75% [8,9]. It also possesses a sound biofeedback function to provide feedback to subjects when clenching occurs. A combination of muscle activity of continuous 20–30% MVC for a duration of 3 s was used when setting biofeedback training thresholds for clenching occurring during the day [10]. Taking this as standard, in this study we determined that clenching was present if 20% MVC was maintained for a duration of 3 s. The fact that total muscle activity in the clenching group was significantly greater than
that in the non-clenching group is another result that supported the appropriateness of the cut-off value used in this study. Even if strong clenching did not occur in the clenching group, the frequent occurrence of weak clenching may result in the buildup of muscle fatigue, leading to TMD. Nevertheless, it is still necessary to validate the cut-off value set in this study for determining the presence or absence of clenching, and we intend to make this a subject for further study. 5. Conclusions From EMG recordings of temporal muscle activity under natural conditions and psychological test results, (1) daytime clenching occurring under natural conditions was shown to be associated with severe anxious tendency; and (2) total muscle activity in the clenching group, excluding during eating and talking, was 3.5 times greater compared with the non-clenching group.
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Acknowledgments The authors wish to thank Professor Kanji Ishibashi, Division of Fixed Prosthodontics, School of Dentistry, Iwate Medical University, for his helpful advice. This research was supported in part by a Grant-in-Aid for Scientific Research (No. 21791914) from the Japan Society for the Promotion of Science. References [1] Lavigne GJ, Rompreˆ PH, Montplaisir JY. Sleep bruxism: validity of clinical research diagnostic criteria in a controlled polysomnographic study. J Dent Res 1996;75:546–52. [2] Camparis CM, Formigoni G, Teixeira MJ, Bittencourt LRA, Tufik S, Siqueira JTT. Sleep bruxism and temporomandibular disorder: clinical and polysomnographic evaluation. Arch Oral Biol 2006;51:721–8. [3] Haketa T, Baba K, Akishige S, Fueki K, Kino K, Ohyama T. Utility and validity of a new EMG-based bruxism detection system. Int J Prosthodont 2006;16:422–8. [4] Yamaguchi T, Mikami S, Okada K. Validity of a newly developed ultraminiature cordless EMG measurement system. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 2007;104:22–7. [5] Rossetti LMN, Araujo CDRP, Rossetti PHO, Conti PCR. Association between rhythmic masticatory muscle activity during sleep and masticatory myofascial pain: a polysomnographic study. J Orofac Pain 2008;22:190–200. [6] Kino K, Sugisaki M, Haketa T, Takaoka M, Ota T, Shibuya T, et al. Exploratory survey for contributing factors observed in generations in the investigation with multi-dimensional questionnaire for patients with painful TMD. J Jpn Soc TMJ 2007;19:210–7 [in Japanese]. [7] Fujisawa M, Uchida K, Yamada Y, Ishibashi K. Surface electromyographic electrode pair with built-in buffer-amplifiers. J Prosthet Dent 1990;63:350–2. [8] Gohdo Y, Fujisawa M. Determination of electromyogram biofeedback threshold for patients with clenching behavior. Prosthdont Res Pract 2004;3:46–54.
[9] Ogawa Y, Fujisawa M. Electromyogram biofeedback threshold determination for clenching behavior. Dent Jpn 2005;41: 54–6. [10] Watanabe A, Kanemura K, Tanabe N, Fujisawa M. Effect of electromyogram biofeedback on daytime clenching behavior in subjects with masticatory muscle pain. J Prosthodont Res; in press, doi:10.1016/ j.jpor.2010.09.003. [11] Fukuda K, Kobayashi S. The manual of SDS (Japanese version). Kyoto: Sankyobo; 1983. [12] Kanehisa T, Fukamachi K, Nozoe S. The manual of CMI (Japanese version). Kyoto: Sankyobo; 2001. [13] Abe M, Takahashi N. The manual of MAS (Japanese version). Kyoto: Sankyobo; 2007. [14] Asano A, Tanabe N, Fujisawa M. A 2.5-year prospective cohort study of the psychological properties associated with the incidence of TMD in young adults. J Jpn Soc TMJ 2008;20:166–73 [in Japanese]. [15] Carlson CR, Okeson JP, Falace DA, Nitz AJ, Curran SL, Anderson D. Comparison of psychologic and physiologic functioning between patients with masticatory muscle pain and matched controls. J Orofac Pain 1993;7:15–22. [16] Curran SL, Carlson CR, Okeson JP. Emotional and physiologic responses to laboratory challenges: patients with temporomandibular disorders versus matched control subjects. J Orofac Pain 1996;10: 141–50. [17] Hurrell Jr JJ, McLaney MA. Exposure to job stress—a new psychometric instrument. Scand J Work Environ Health 1988;14: 27–8. [18] Dao TTT, LeResche L. Gender differences in pain. J Orofac Pain 2000;14:169–85. [19] Wang J, Korczykowski M, Rao H, Fan Y, Pluta J, Gur RC, et al. Gender difference in neural response to psychological stress. Soc Cogn Affect Neurosci 2007;2:227–39. [20] Glaros AG, Tabacchi KN, Glass EG. Effect of parafunctional clenching on TMD pain. J Orofac Pain 1998;12:145–52. [21] Ikeda T, Nishigawa K, Kondo K, Takeuchi H, Clark GT. Criteria for the detection of sleep-associated bruxism in humans. J Orofac Pain 1996;10:270–81.
Journal of Prosthodontic Research 55 (2011) 159–164 www.elsevier.com/locate/jpor
Original article
Clenching occurring during the day is influenced by psychological factors Hiroshi Endo DDS*, Kiyotaka Kanemura DDS, PhD, Norimasa Tanabe DDS, PhD, Jun Takebe DDS, PhD Division of Fixed Prosthodontics, School of Dentistry, Iwate Medical University, 1-3-27 Chuodori, Morioka, Iwate 020-8505, Japan Received 10 September 2010; received in revised form 19 November 2010; accepted 23 November 2010 Available online 5 February 2011
Abstract Purpose: We investigated the relationship between clenching occurring during the day under natural conditions and psychological attributes. Method: Subjects were 37 dentistry students and staff (14 women, 23 men; mean age 27.4 4.7 years). A portable EMG recording device was used to record muscle activity in the temporal muscles for a continuous 5-h period under subjects’ natural conditions, including having lunch. Clenching was identified by electromyography, and subjects were categorized into clenching and non-clenching groups. Psychological testing was performed during the same period using the Modified Taylor Manifest Anxiety Scale (MAS), the Self-rating Depression Scale (SDS), and the Cornell Medical Index (CMI), and the psychological attributes of clenching and non-clenching groups were compared. Results: Of the 37 subjects, 23 were assigned to the clenching group and 14 to the non-clenching group. Thirteen of the 23 subjects of the clenching group and none of the 14 subjects of the non-clenching group were determined to have psychological problems according to the MAS, with a significant difference observed between the clenching and non-clenching groups in terms of anxious tendency ( p < 0.001; Fisher’s exact test). Total muscle activity in the clenching group, excluding eating and talking, was 3.5 times greater compared with the non-clenching group, and a significant difference between the groups was observed ( p < 0.05; Mann–Whitney U-test). Conclusions: Daytime clenching was shown to be associated with anxious tendency. Total muscle activity in the clenching group was 3.5 times greater compared with the non-clenching group. # 2010 Japan Prosthodontic Society. Published by Elsevier Ireland. All rights reserved. Keywords: Bruxism; Daytime clenching; Parafunctional movements; Psychological factors; Portable EMG recording device
1. Introduction Bruxism refers to grinding and clenching of the teeth and psychological factors are believed to contribute to its development. As bruxism can be a major risk factor for problems such as dental attrition, temporomandibular disorder (TMD), periodontal disease, tooth fracture, and breakage of prosthetic devices, its causes require elucidation. A range of studies of nocturnal bruxism have been reported [1–5]. With respect to daytime clenching, however, although some people are aware of their own bruxism, in other cases it is suspected from oral findings [6]. Although there have been some laboratory studies of daytime clenching using electromyography (EMG) for objective evaluation, it is difficult to define in daily life because natural conditions are affected by psychological factors.
* Corresponding author. Tel.: +81 19 651 5111x4127; fax: +81 19 654 3281. E-mail address: [email protected] (H. Endo).
In the present study, we investigated the relationship between clenching occurring during the day and psychological characteristics. To evaluate these parameters we used temporal muscle EMG measurements on the habitual chewing side in daily living, obtained using a portable EMG recording device, and three different psychological tests. 2. Method of research 2.1. Subjects Subjects were 37 students and staff (14 women, 23 men; mean age 27.4 4.7 years) at Iwate Medical University School of Dentistry. Eligibility criteria were (1) not having a denture fitted and (2) individual normal occlusion. Exclusion criteria were (1) orofacial pain; (2) current treatment for acute symptoms of TMD; (3) current orthodontic treatment (including the retention period); (4) severe periodontitis; and (5) unstable occlusion. Informed written consent was obtained from each subject after they had received a detailed explanation
1883-1958/$ – see front matter # 2010 Japan Prosthodontic Society. Published by Elsevier Ireland. All rights reserved. doi:10.1016/j.jpor.2010.11.003
160
H. Endo et al. / Journal of Prosthodontic Research 55 (2011) 159–164
of the research protocol. This study was approved by the Human Research Ethics Committee of Iwate Medical University School of Dentistry (No. 01090).
difficult to see from the outside, enabling measurements to be taken without interfering with daily life (Fig. 1a and b). 2.3. Study schedule
2.2. Measurement device The portable EMG recording device was a onechannel EMG recording system comprising a remote control (100 mm 60 mm 18 mm), a main unit (64 mm 21 mm 12.5 mm), and an electrode unit. It is capable of 9 h of continuous recording at a sampling rate of 4 Hz. EMG derivation was performed by attaching a 5-mm diameter Ag–AgCl surface electrode unit to the electrode attachment site, i.e., the skin overlying the temporal muscle (anterior muscle bundle) on the habitual chewing side, with a fixed distance of 20 mm between the centers of the electrodes, and the bipolar induction method was used. Indifferent electrodes were attached to the back of the earlobe. The electrode attachment sites were carefully cleaned with alcoholsoaked cotton before electrodes were applied. This device reduces the effect of motion artifacts generated by shaking of the wires by including a small amplifier in the main unit that performs impedance conversion [7]; it also reduces noise by incorporating a hum filter and a 5.3–450-Hz band path filter. The device is worn behind the ear and is
To identify clenching occurring under natural conditions, a portable EMG recording device developed in our field was used to make continuous 5-h EMG recordings, including having lunch. When measurement was started, muscle activity during the hardest possible bite was measured as maximum voluntary contraction (100% MVC) for calibration purposes (Fig. 2). Subjects were instructed to behave normally, with the exception of washing their face, touching the electrode, or making any other movements that might create undesirable noise during the EMG signals. They were also told to record a keyword indicating their behavior and the time on a record sheet every time they changed their behavior. After the conclusion of measurements, the device was removed from subjects and the EMG data recorded were forwarded to a personal computer via specialized data transmission software for analysis. Three types of psychological test were performed: the Modified Taylor Manifest Anxiety Scale (MAS) of the Minnesota Multiphasic Personality Inventory (MMPI) to measure the degree of anxiety, the Self-rating Depression
Fig. 1. (a) Portable EMG recording device. (b) Left panel: completed set-up of the apparatus. Right panel: the apparatus is covered by the subject’s hair.
H. Endo et al. / Journal of Prosthodontic Research 55 (2011) 159–164
161
Fig. 2. Sample diurnal 5-h EMG recording (subject no. 4).
Scale (SDS) to investigate depressiveness, and the Cornell Medical Index (CMI) to investigate subjective physical and mental symptoms. We used Japanese versions of all these tests. All of these were distributed before the start of EMG recording, and collected several days later.
2.4. Analytical method Functional movements (such as eating and talking) consist of powerful intermittent muscle activity. In contrast, parafunctional movements (clenching) are characterized by weak, continuous muscle activity. In this study, we used this difference to distinguish functional from parafunctional movements in the diurnal 5-h EMG recordings. Subjects were categorized into a clenching group and a non-clenching group (Fig. 3), with clenching defined from previous studies as muscle activity of 20% MVC and 3-s duration observed during parafunctional movements [8–10]. To ensure objectivity, EMG analysis was performed using analysis software. For the three psychological tests, the presence of psychological problems was determined by scores of I and II in the MAS, 48 in the SDS, and III or IV in the CMI. We investigated the relationship between the results of these tests and the occurrence of clenching, and compared them using Fisher’s exact test. Statistical significance was set at 5%. The results of the psychological tests used in this study were scored
and evaluated in accordance with the guides for each test [11– 13] (Table 1). We also calculated the total muscle activity, excluding during eating and talking, in the clenching and non-clenching groups, and compared them using the Mann–Whitney U-test. Statistical significance was set at 5%. The SPSS statistical package (15.0 J for Windows, SPSS Japan, Tokyo, Japan) was used for statistical analysis. The total muscle activity was evaluated using the integral values. 3. Results 1. Of the 37 subjects, 23 (6 women, 17 men) were assigned to the clenching group and 14 (8 women, 6 men) to the nonclenching group based on the EMG measurement results (Fig. 4). 2. Thirteen (3 women, 10 men) of the 23 subjects of the clenching group and none of the 14 subjects of the nonclenching group were determined to have psychological problems according to the MAS, with a significant difference observed between the clenching and non-clenching groups in terms of anxious tendency ( p < 0.001; Fisher’s exact test). In addition, the mean MAS score of the 37 subjects was 16.19. No significant difference in the number of subjects
Table 1 Determination criteria for psychological tests.
Normal
≤47
48−52
≥53
Normal
Neurosis
Depression
Cornell Medical Index
I, II
IV
CMI
Normal
III Neurotic tendency
MAS Self-rating Depression Scale
SDS
Fig. 3. Method of distinguishing clenching events.
I Severe anxiety
II Moderate anxiety
Manifest Anxiety Scale
III, IV, V
Psychologically normal
Neurosis
Psychological problems present
162
H. Endo et al. / Journal of Prosthodontic Research 55 (2011) 159–164 Table 3 Psychological test results for each subject. Non-clenching group 14 subjects (37.8%) 8 women, 6 men
Clenching group 23 subjects (62.2%) 6 women, 17 men
Clenching group (n = 23) Subject No.
Subjects n = 37 Fig. 4. Categorization of subjects according to presence or absence of clenching events.
with psychological problems was observed between the clenching and non-clenching groups according to the SDS (6 (3 women, 3 men)/23 vs. 1 (a woman)/14) or the CMI (5 (all men)/23 vs. 1 (a man)/14). Furthermore, no significant gender difference was observed between the two groups (Tables 2 and 3). 3. Total muscle activity, excluding eating and talking, was 3.5 times greater in the clenching group than in the non-clenching group, a significant difference was observed between the two groups ( p < 0.05; Mann–Whitney U-test) (Fig. 5).
4. Discussion In this study, significantly more subjects in the clenching group than the non-clenching group were determined to have psychological problems according to the MAS, indicating an association between daytime clenching and severe anxious tendency. In addition, although the differences were not significant, the fact that some subjects who had psychological problems according to the MAS also had problems according to the SDS and CMI indicates that the involvement of psychological factors may be considerable. These findings suggest that when dealing with patients with suspected clenching, it is important to take psychological factors into account. This study is not capable of determining whether psychological problems occur as a result of clenching or vice versa. On this point, Asano et al. [14] have reported the involvement of severe anxious tendency as a factor contributing to TMD onset. Carlson et al. [15] and Curran et al. [16] reported that muscle activity at a time of rest was increased by experimental stress. Hurrell et al. [17] reported that job stressors and their resulting stress reactions, such as anxiety, led to health problems. Taken together with the fact that bruxism is noted for its relation to TMD onset, this suggests that anxiety Table 2 Numbers of subjects with psychological problems. Subjects, n = 37
MAS*
SDS
CM I
Clenching group, n = 23 Non-clenching group, n = 14
13 0
6 1
5 1
*
p < 0.001: Fisher’s exact test.
18 5 13 16 35 36 8 23 26 2 3 14 20 1 4 7 9 10 11 12 15 17 19
MAS
SDS
CMI
Non-clenching group (n = 14) Subject No.
MAS
SDS
CMI
32 30 6 21 22 24 25 27 28 29 31 33 34 37
Psychological problems present
caused by some sort of stress may induce clenching, leading to TMD. The mean MAS score among healthy Japanese individuals is 16.90 [13], and since the mean of subjects in the present study was almost the same as this value, they can be regarded as comprising a general population. No gender difference was observed in this study. However, because it has been reported that women are more sensitive to stress than men [18,19], further studies with an increased number of participants may be required. There was a significant difference in total muscle activity between the clenching and non-clenching groups, which were categorized according to a cut-off value in this study. This may be considered an unsurprising result if clenching events are regarded as being reflected in the data as considerable amounts of muscle activity. The characteristic of the clenching group in this study, however, was the presence of considerable amounts of muscle activity of <20% MVC. Fig. 6 shows one example. Clenching that occurs during the day, even if relatively weak, has been reported to have an adverse effect if continued over a long period, and instructing patients not to allow their upper and lower teeth to touch has been reported to be effective in counteracting this [6,20]. The results of this study may be regarded as an EMG record of tooth contacting habits (TCH). In this study, diurnal EMG recordings were performed only once. Concerning this point, in their report of four-day recordings of muscle activity in the temporal muscles, Watanabe et al. [10] state that there was no variation in the amount of muscle activity in the clenching group on different days. Because changes in the long-term occurrence of clenching events in habitual clenchers have not yet been confirmed, however, this is a question for further study. Criteria used to determine nocturnal bruxism include number of masticatory muscle bursts and strength of muscle
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163
(μV/s)
* Clenching group Non-clenching group
*p < 0.05: Mann-Whitney U-test
Clenching group:non-clenching group = 3.5:1 Fig. 5. Total muscle activity over a 5-h diurnal period.
10% MVC (times)
20% MVC
≥30% MVC
(times)
(s) Clenching group (subject No.7)
(s) Non-clenching group (subject No.32)
Fig. 6. Breakdown of muscle activity in each group.
potentials [1,3,21]. We have been unable to find any reports, however, that clearly indicate criteria for the determination of daytime clenching. One possible reason may be that previously there were no devices capable of recording masticatory muscle activity during daily activities. The portable EMG recording device used in this study differentiates between functional and parafunctional movements in terms of a combination of amount of muscle activity and sustained duration of muscle activity. It is capable of distinguishing between functional movements such as eating and parafunctional movements such as clenching with an accuracy rate of 75% [8,9]. It also possesses a sound biofeedback function to provide feedback to subjects when clenching occurs. A combination of muscle activity of continuous 20–30% MVC for a duration of 3 s was used when setting biofeedback training thresholds for clenching occurring during the day [10]. Taking this as standard, in this study we determined that clenching was present if 20% MVC was maintained for a duration of 3 s. The fact that total muscle activity in the clenching group was significantly greater than
that in the non-clenching group is another result that supported the appropriateness of the cut-off value used in this study. Even if strong clenching did not occur in the clenching group, the frequent occurrence of weak clenching may result in the buildup of muscle fatigue, leading to TMD. Nevertheless, it is still necessary to validate the cut-off value set in this study for determining the presence or absence of clenching, and we intend to make this a subject for further study. 5. Conclusions From EMG recordings of temporal muscle activity under natural conditions and psychological test results, (1) daytime clenching occurring under natural conditions was shown to be associated with severe anxious tendency; and (2) total muscle activity in the clenching group, excluding during eating and talking, was 3.5 times greater compared with the non-clenching group.
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