Effects of denture wearing on coordinated features of jaw and neck muscle activities during chewing in partially edentulous elderly patients of Prosthodontic Research

Purpose: This study was performed to examine how denture wearing improves jaw and neck muscle coordination during chewing in partially edentulous elderly patients. Methods: Sixteen patients classified as Eichner’s index B2 or B3 and 16 young dentate subjects were enrolled. Jaw and neck muscle activities during chewing were recorded using electromyography with and without denture wearing, then analyzed using intermuscular Electromyography (EMG)-EMG transfer and EMG-EMG coherence function analyses to clarify quantitative, temporal, and functional coordination of jaw and neck muscle activities while chewing. Occlusal force and masticatory scores were also determined. Results: Denture wearing increased the power values for jaw closing muscle activities, and improved occlusal area and force, and masticatory score. Gain values for jaw closing and opening muscle activities were decreased in those wearing dentures compared to those not wearing dentures. Denture wearing resulted in equivalent gain values for jaw closing and opening muscle activities as compared to the young subjects. Coherence values for chewing and non-chewing side neck muscle activities were increased as compared to not denture wearing. Conclusions: The suitability of denture wearing can be evaluated from the viewpoint of gain as a quantitative parameter showing coordination between jaw closing and opening muscle activities. Such evaluation can be performed from the viewpoint of coherence as a parameter of functional coordination between jaw and neck muscle activities during chewing in partially edentulous elderly patients. The gain parameter in regard to jaw muscle activities may be compensated to a state equivalent to that seen in young subjects by wearing an appropriate denture.

Recently, a quantitative evaluation of coordinated jaw and neck muscle activities during chewing performance was conducted with healthy subjects with full dentition using intermuscular Electromyography (EMG)-EMG transfer and EMG-EMG coherence function analyses [18]. The principal power spectrum frequency revealed in those findings was the same as that seen in a previous examination of rhythmic gait performance conducted by Nielsen et al. [19].
Intermuscular EMG-EMG transfer function analysis is considered useful for evaluations of interrelationships among input and output signals, and may clarify gain and phase characteristics of jaw and neck muscle activities during chewing performance. Gain parameter has been speculated to reflect the relative quantitative coordination of the variability of output signaling in response to input signaling, while the phase parameter may show temporal coordination [18,20,21]. In addition, intermuscular EMG-EMG coherence function analysis is also considered useful for evaluations of functional coordination revealed in synchronous input and output variabilities [18,22,23].
Increased activation of the chewing side masseter muscle caused by denture wearing promptly regulates chewing activities with a low variation coefficient, which may be advantageous for obtaining occlusal stability in partially edentulous patients [24]. Accordingly, intermuscular EMG-EMG transfer and EMG-EMG coherence function analyses of input signals from chewing side masseter muscle activities are considered suitable for examining the effects of denture wearing from the viewpoint of chewing-related jaw and neck muscle coordination in partially edentulous elderly individuals.
Examinations of the effects of denture wearing from the viewpoint of prefrontal cortex activity while chewing in young dentate subjects and partially edentulous elderly patients with or without denture wearing showed that prefrontal activation caused by denture wearing was not much different from that in the young dentate subjects [25]. Based on comparison of these subjects, the present study was conducted to investigate the effects of denture wearing so as to properly interpret spectrum parameters, because intermuscular EMG-EMG transfer and EMG-EMG coherence function analyses are quite new approaches for understanding jaw and neck muscle coordination. As compared with partially edentulous patients with and without a denture, our findings with the young dentate subjects may suggest excessive deficits without a denture and compensative effects with denture wearing as a preferable interpretation for the spectrum parameters of gain, phase, and coherence in regard to jaw and neck muscle coordination. The findings obtained in this study may provide new insight for evaluation of the suitability of denture wearing based on the coordinated features of jaw and neck muscle activities during chewing.

Subjects
Sixteen partially edentulous patients [8 males, 8 females; mean age ± standard deviation (SD) 70.8 ± 5.7 years, range 62-78 years] with removable partial dentures who were being treated at the Division of Prosthodontics of Nihon University Hospital at Matsudo were included in this study. The mean numbers of remaining and prosthetic teeth in the patients were 19.8 ± 3.0 and 7.8 ± 3.0, respectively ( Fig. 1), and all were considered to be bilateral maxillary and/or mandibular freeended edentulous cases [26,27]. Sixteen young full dentate subjects (8 males, 8 females; mean age 24.1 ± 1.9 years, range 20-26 years) also participated as the young denate subjects (Control group) and were recruited from among staff members and dental students of Nihon University School of Dentistry at Matsudo. All subjects in each group were free from pain and dysfunction in the oromandibular, maxillofacial, head, neck, and shoulder regions. A screening questionnaire created by the Nihon University of Dentistry at Matsudo was used for recruitment and during patient examination. Prior to the start of the study, each patient signed an informed written consent form in accordance with the World Medical Association's Declaration of Helsinki. The protocol was approved by the committee on ethics of Nihon University School of Dentistry at Matsudo (EC-12-008).

Experimental procedures
Edentulous patients were treated with a sufficient conventional removable partial denture prosthesis during the 3 months prior to the start of the study. None of the patients had any complaints about the prosthesis, such as discomfort, pain, or chewing difficulties. They were requested to fill out a food intake questionnaire to evaluate chewing ability [28,29] while not wearing (Unwearing) and wearing (Wearing) a removable partial denture. The maximal occlusal force and occlusal contact areas were also measured using pressure-sensitive sheets [30] under the Unwearing and Wearing conditions. The Control group subjects were also measured while in an occlusal state for area and force. Furthermore, EMG recordings were obtained to evaluate the activities of the jaw and neck muscles during gum chewing in the patients under both conditions as well as in the Control group.

Maximal occlusal force and area
Maximal occlusal force and occlusal area were measured using pressure-sensitive sheets (Dental Prescale 50H Rtype, Fuji Film Co., Tokyo, Japan) [30] in both the patient and Control groups. Each subject was asked to perform maximal clenching in an intercuspal position with a sheet placed on the dental arch. Occlusal force and occlusal contact area values were calculated using special analytical equipment (Occluzer FPD703, Fuji Film Co., Tokyo, Japan).

Masticatory score
The masticatory scores of all patients were determined using a questionnaire designed by Hirai et al. [28] and according to a method presented in our previous study [25]. A mastication score greater than 80 was considered to indicate favorable chewing activity [28,29].

Surface EMG recording
EMG signals from the jaw and neck muscles of the patients under the Unwearing and Wearing conditions, and in the Control group subjects were recorded using surface EMG electrodes (Fig. 2a, b). Details regarding the recording protocol have been described [18]. The electrodes were placed bilaterally on the masseter (Mm: jawclosing muscle), anterior temporal (Ta: jaw-closing muscle), anterior digastric (AD: jaw-opening muscle), and sternocleidomastoid (SCM: neck extensor/protrusion/rotator muscle) muscles. For this study, SCM activity was used as neck muscle activity because that has been reported to coordinate with jaw muscle activities and head movement while chewing [10][11][12][13][14][15][16][17][18]. A piece of chewing gum (1.0 g, Freezone, Lotte, Tokyo, Japan) was used as the test food. The patients performed two chewing sessions for 80 s, one session each on the right and left side in random order under both the Unwearing and Wearing conditions. The Control group also performed two chewing sessions in a similar manner. EMG signals were recorded at a sampling rate of 1 kHz and the high-frequency cutoff was filtered with a time constant of 0.03 s (POLYGRAPH BIOELECTRIC AMPL 1253A, San-ei MED, Tokyo, Japan).

Data analysis
Data for activities of the jaw and neck muscles during chewing were analyzed using the software package Multi Scope EMG (version 1.8.4, Medical Try System, Tokyo, Japan). Frequency analysis of the EMG signals was conducted with full-wave rectification of the signals (Fig.  2c, d). Fast Fourier transform analysis [31] was performed to calculate the power spectrum (Fig. 2e, f). Details of the frequency analysis procedure were described in a previous study [18].
Hereafter, the letter "C" is used as a prefix to indicate chewing-side and "NC" for non-chewing-side muscles. For transfer and coherence function analyses, C-Mm was used as the input signal, because it plays an agonist role and may also receive a profound effect from the central pattern generation in the brainstem, whereas synergist (NC-Mm, C/ NC-Ta), antagonist (C/NC-AD), and neck (C/NC-SCM) muscles were used as the output signals. The gain value shown in the transfer function analysis was used to indicate the variability in output signals in response to input signal changes [32][33][34] (Fig. 2g, h), and the phase Fig. 1. Dental state of 16 partially edentulous subjects (A to P). Using Eichner's intermaxillary tooth contact classification, subjects A-I were classified as B2 and subjects J-P as B3. Remaining teeth are indicated in black and prosthetic teeth in red. The mean numbers of remaining, lost, and prosthetic teeth in these subjects were 19.8 ± 3.0, 8.2 ± 2.9, and 7.8 ± 3.0, respectively.
value was considered to represent the temporal relationship between the input and output signals [32,34,35] (Fig. 2i, j). Coherence refers to the synchrony between input and output signals using values from 0 to 1 [32] (Fig. 2k, l). Several peaks were presented in the power spectrum, with the frequencies expressed as integer multiples of f, 2f, 3f, 4f, etc., and the first harmonic termed the fundamental frequency [18]. According to Nielsen [19], the first peak of the power spectrum fits with gait rhythm during walking and Ishii et al. [18] also showed chewing rhythm in the first peak of the power spectrum in masticatory muscle activities. Because the first peak is evident in the power spectrum, chewing rhythm, gain, phase, and coherence were analyzed in the first peak frequency (Fig. 2e, f). Details of the transfer and coherence function analyses are described in our previous study [18].

Analysis items
Masticatory scores under the Unwearing and Wearing conditions, as well as occlusal force and area under those conditions were evaluated, and compared to findings in the Control group. Spectrum analyses examined first peak frequency, gain, phase, and coherence of jaw and neck muscle activities during chewing in the patients under both Unwearing and Wearing conditions, as well as in the Control group.

Statistical analyses
In order to examine the effects of denture wearing, a paired t-test and Wilcoxon's signed rank test were used to compare the occlusal contact area, occlusal force, masticatory scores, and first peak frequency between the Unwearing and Wearing conditions. Two-way repeatedmeasures analysis of variance (ANOVA) and Bonferroni t-test were used to compare the power, gain, phase, and coherence in jaw and neck muscle activities during chewing between the Unwearing and Wearing conditions. In order to examine compensation from denture wearing for excessive modulation seen in the Unwearing condition, one-way ANOVA and Dunnett's test, Kruskal-Wallis one-way ANOVA on ranks and Dunnett's test were used to compare occlusal force and occlusal area, and also first peak frequency under the Unwearing and Wearing conditions, which were compared to Control group findings. Twoway ANOVA and Dunnett's test were also used to compare the power, gain, phase, and coherence during chewing under the Unwearing and Wearing conditions, as well as with findings of the Control group. Statistical analyses were performed using SigmaStat, v. 3.11 (Systat Software, Inc., San Jose, CA, USA) and all results were considered to be significant at p < 0.05.

Occlusal contact area and force
There were significant increases in occlusal contact area (Wilcoxon's signed rank test, p < 0.001) (Fig. 3) and occlusal force (paired t-test, p < 0.001) (Fig. 4) under Wearing as compared to the Unwearing condition.

First peak frequency
Each of the patients exclusively showed the first peak frequency of the power spectrum throughout all jaw-closing and jaw-opening, and neck muscle activities during rhythmical chewing. There were no significant (Wilcoxon's signed rank test, left: p = 0.813, right: p = 0.250) differences seen between the Unwearing and Wearing conditions during both left and right side chewing (Fig. 5).

Fig. 2.
Raw and rectified EMG signals, first peak frequency, representative gain, phase, and coherence in jaw and neck muscles under Unwearing and Wearing conditions in representative patient. Raw and rectified EMG signals from jaw and neck muscles under the Unwearing (a, c) and Wearing (b, d) conditions while chewing gum for 80 s. The area between vertical lines indicates analyzing phase (61.5 s). Power spectrum of jaw and neck muscle EMG signals (e, f). Power spectrum of rectified EMG signals for all muscles are characterized from the first peak frequency. Gain, phase, and coherence of chewing side muscles under Unwearing (g, i, k) and Wearing (h, j, l) conditions. Arrows indicate first peak frequency. Horizontal lines in k and l denote 95% confidence level (0.19) for coherence spectrum.

Power
The Wearing condition showed a significantly (two-way repeated measures ANOVA, p < 0.001 and Bonferroni t-test, p < 0.05) increased power in first peak frequency of the jaw-closing (C-/NC-Mm, C-/NC-Ta) muscles as compared to the Unwearing condition during both left and right side chewing (Table 1). In contrast, there were no significant (two-way repeated measures ANOVA and Bonferroni t-test) differences between the conditions for power of jaw-opening (C-/NC-AD) and neck (C-/NC-SCM) muscles during both left and right side chewing (Table 1).

Gain
Gain in the first peak frequency of the jaw-closing (NC-Mm, C-Ta) and jaw-opening (C-/NC-AD) muscles with left and right side chewing under the Wearing condition was significantly (two-way repeated measures ANOVA, p < 0.05 and Bonferroni t-test, p < 0.05) decreased as compared to Unwearing (Fig. 6). In contrast, there were no significant (two-way repeated measures ANOVA and Bonferroni t-test) differences in the values for gain in the jaw-closing (NC-Ta) and neck (C-/NC-SCM) muscles during both left and right side chewing between the conditions (Fig. 6).

Phase
There were no significant (two-way repeated measures ANOVA, left: p = 0.850, right: p = 0.202) differences between the Wearing and Unwearing conditions for phase in the first peak frequency in the jawclosing (NC-Mm, C-/NC-Ta), jaw-opening (C-/NC-AD), or neck (C-/ NC-SCM) muscles during both left and right side chewing (Fig. 7).

Coherence
There were no significant (two-way repeated measures ANOVA and Bonferroni t-test) differences between the conditions in regard to coherence in the first peak frequency of the jaw-closing (NC-Mm, C-/NC-Ta) and jaw-opening (C-/NC-AD) muscles during left and right side chewing (Fig. 8). In contrast, under the Wearing condition, a significant (two-way repeated measures ANOVA, p < 0.05 and Bonferroni t-test, p < 0.05) increases in the coherence of neck (C-/NC-SCM) muscles were noted during both left and right side chewing (Fig.  8).

Occlusal contact area and occlusal force
In comparison with the Control group, there was no significant (Kruskal-Wallis one way ANOVA on ranks, p = 0.053) difference for occlusal contact area noted for the Unwearing or Wearing condition (Fig. 3). In contrast, occlusal force in the Control group was significantly (Kruskal-Wallis one way ANOVA on ranks, p < 0.001 and Dunnett's test, p < 0.05) greater as compared to Unwearing, but not significantly (Kruskal-Wallis one way ANOVA on ranks and Dunnett's test) different as compared to Wearing (Fig. 4).

First peak frequency
In comparisons with the Control group, there were no significant (Kruskal-Wallis one way ANOVA on ranks, left: p = 0.218, right: p = 0.421) differences for first peak frequency of the power spectrum of jaw and neck muscle activities noted for the Unwearing and Wearing conditions during both left and right side chewing (Fig. 5).

Power
Power values for the first peak frequency of jaw-closing (C-/NC-Mm, C-/NC-Ta) and jaw-opening (C-/NC-AD), and neck (C-/NC-SCM) muscles in the Control group were significantly (left: two-way      to the Unwearing and Wearing conditions during both left and right side chewing (Table 1).

Gain
Gain for the first peak frequency of jaw-closing (NC-Mm, C-/ NC-Ta) and jaw-opening (C-AD) muscles in the Control group was significantly (left: two-way ANOVA, p < 0.001 and Dunnett's test, p < 0.05; right: two-way ANOVA, p = 0.018 and Dunnett's test, p < 0.05) lower than that under the Unwearing condition, whereas no significant (two-way ANOVA and Dunnett's test) differences were seen regarding gain for NC-AD and C-/NC-SCM muscles (Fig. 6). Furthermore, gain for C-/NC-Ta in Control was significantly (left: two-way ANOVA, p < 0.001 and Dunnett's test, p < 0.05; right: two-way ANOVA, p = 0.018 and Dunnett's test, p < 0.05) lower as compared to Wearing, whereas no significant (two-way ANOVA and Dunnett's test) differences were presented regarding gain for jaw-closing (NC-Mm) and jaw-opening (C-/NC-AD), or neck (C-/NC-SCM) muscles between the Control group and Wearing condition (Fig. 6).

Phase
Phase for the first peak frequency of the jaw-closing (NC-Mm and C-/NC-Ta) and jaw-opening (C-/NC-AD), and neck (C-/NC-SCM) muscle activities was not significantly (two-way ANOVA, left: p = 0.061; right: p = 0.938) different between the Control group and the Unwearing or Wearing condition with both left and right side chewing (Fig. 7).

Coherence
Coherence of the first peak frequency of jaw-closing (NC-Mm, C-/NC-Ta) and jaw-opening (C-/NC-AD), and neck (C-/NC-SCM) muscle activities was not significantly (two-way ANOVA, left: p = 0.273; right: p = 0.429) different between the Control group and the Unwearing or Wearing condition with both left and right side chewing (Fig. 8).

Discussion
The present findings showed that denture wearing resulted in increased power values for jaw closing muscle activities, and simultaneously decreased gain values for jaw closing and opening muscle activities as compared to the Unwearing condition, as well as improved masticatory score, and occlusal area and force results. In addition, our findings indicate that denture wearing attenuates the excessive modulation of gain in regard to jaw-closing and jaw-opening muscle activities seen when not wearing a denture as compared to the young dentate subjects.
Previous studies have found that denture wearing improves jaw muscle activities during chewing [11,25], with prominent activation obtained in chewing side masseter muscle activity [24,[36][37][38]. This decrease in gain induced by use of a denture also suggests improved quantitative coordination of jaw muscle activities, because increased activity for the chewing side masseter muscle while chewing may be particularly enhanced by occlusal reconstruction while wearing a denture [27,[39][40][41][42]. It has also been reported that a good fitting denture may induce bilateral masseter muscle activities, whereas a denture with a poor fit may cause unilateral masseter muscle activities [43]. The present findings showing a decrease in gain suggest that wearing a suitable denture may provide coordination of bilateral power jaw muscle activities during chewing performance in partially edentulous elderly patients. Furthermore, based on results showing attenuation of excessive modulation of gain by denture wearing in comparison with the young dentate subjects, it is considered that denture use may lead to compensative jaw muscle coordination until there is no significant difference with young dentate individuals. Additionally, those also revealed improved occlusal force by denture wearing.
Our results also indicated that denture wearing improves coherence between chewing side jaw masseter muscle activity and both chewing and non-chewing side neck sternocleidomastoid muscle activities during chewing, as compared to not wearing a denture. Such improved functional coordination between jaw and neck muscle activities has been noted as another novel benefit in elderly patients classified as Eichner's index B2 or B3 when wearing a denture [22,23,44]. Thus, use of an appropriate denture may improve not only quantitative coordination of jaw-closing and jaw-opening muscles, but also functional coordination related to jaw and neck muscle coherence, which may be reflexively induced by facilitation of somatosensory inputs when wearing a denture [22,45,46]. Also, the coherence seen under the present Wearing and Unwearing conditions indicated no significant differences as compared with the young dentate subjects. When considering the large variability of coherence parameters between jaw masseter muscle and neck sternocleidomastoid muscle activities [18], and the complexity of head movements during chewing performance [17] in healthy dentate individuals, the improved functional coordination between jaw and neck muscle activities caused by denture wearing may limit the changes in partially edentulous elderly patients with maxillary and/or mandibular posterior tooth loss.
Chewing rhythm for the first peak frequency and phase parameter for temporal coordination was not significantly different between the Unwearing and Wearing conditions in the present study. We considered that this is because chewing rhythmicity and the temporal pattern of jaw and neck muscle activities centrally generated in the brainstem are maintained without denture wearing, in contrast to above-mentioned quantitative power coordination of gain parameters under a nonwearing condition [47,48]. Also, when considering the neural balance between automaticity and executive control seen with rhythmical chewing [47][48][49], it can be speculated that high-order regulation may contribute to maintain physiological chewing rhythmicity and temporal coordination of jaw muscle activities by changing the chewing region and mandibular movement activities while chewing both with and without denture wearing in elderly patients classified as Eichner's index B2 or B3 [49][50][51][52].
Results of intermuscular EMG-EMG transfer and EMG-EMG coherence function analyses performed for quantitative evaluation of chewing-related jaw and neck muscle coordination suggested the following two important points. First, the parameter of gain for jawclosing and jaw-opening muscle activities is valuable to determine the suitability of denture wearing from the viewpoint of quantitative coordination of those muscle activities. Furthermore, that may also be an important variable for evaluation of the compensative effects of denture wearing to prevent excessive modulation of jaw-closing and jaw-opening muscle coordination as compared with young dentate individuals. Second, the coherence parameter is beneficial for evaluation of the suitability of denture wearing from the viewpoint of functional coordination of chewing-related jaw and neck muscle activities in elderly patients classified as Eichner's index B2 or B3.
This study focused on the suitability of denture wearing from the viewpoints of quantitative and functional jaw and neck muscle coordination during rhythmical chewing in partially edentulous elderly patients. Our findings indicate the need for further investigations of jaw and neck muscle coordination in order to understand therapeutic effects gained during the process of denture treatment in partially edentulous elderly patients as well as the effects of denture wearing on masticatory aging in aged edentulous subjects as compared to young and aged dentate individuals [53].

Conclusion
This study was conducted to clarify the effects of denture wearing on coordinated features of jaw and neck muscle activities during chewing in maxillary and/or mandibular bilateral free-ended edentulous patients, classified as Eichner's index B2 or B3. Our findings show that use of a denture improves jaw closing muscle activities, quantitative coordination of jaw-closing and jaw-opening muscle activities, and functional coordination of jaw and neck muscle activities during chewing. In addition, they indicate that intermuscular EMG-EMG transfer and EMG-EMG coherence function analyses may be applicable for quantitative evaluation of the suitability of denture wearing for determining the coordinated features of jaw and neck muscles activities during chewing in partially edentulous elderly patients.

Conflicts of interest
TU and IK, employed by Dental Support Co. Ltd., and have no conflicts of interest with respect to the research, authorship, and/or publication of this article to declare. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Informed consent
Informed consent was obtained from all individual participants included in this study.

Ethical approval
All procedures performed in this study involving human participants were in accordance with the ethical standards of a relevant institutional and/or national research committee, and with the 1964 Helsinki Declaration and its later amendments, or comparable ethical standards. The study protocol was approved by the Ethics Committee of Nihon University School of Dentistry at Matsudo (No. EC-12-008).