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RESEARCH REPORT |
Department of Prosthodontics, University Dental Clinic, University of Erlangen-Nürnberg, Glueckstrasse 11, D 91054 Erlangen, Germany;
* corresponding author, peter.proeschel{at}rzmail.uni-erlangen.de
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS & METHODSRESULTSDISCUSSIONREFERENCES |
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KEY WORDS: activity • bite-force relation • mastication • clenching • estimated chewing force
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS & METHODSRESULTSDISCUSSIONREFERENCES |
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MATERIALS & METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS & METHODSRESULTSDISCUSSIONREFERENCES |
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The bridges covered the chewing center of one quadrant, starting from the first or second premolar. The restorations had been worn for 6 mos without complaints. None of the patients showed signs or symptoms of craniomandibular disorders.
Experimental Procedures
In each person, the original implant abutments were replaced by 2 abutments equipped with strain-gauges (Type FAE-02W-35-S6; BLH, Heilbronn, Germany). Experimental details are described in a previous paper (Morneburg and Proeschel, 2002). Briefly, 4 vertical rectangular walls were cut into each abutment at right angles. Each wall carried a strain gauge with the sensitive direction aligned along the abutment axis. The 2 strain gauges on opposing buccal and oral walls of each abutment were wired in line and constituted the resistance of a Wheatstone quarter-bridge connected to a carrier frequency amplifier (TF19, Hellige, Freiburg, Germany). With this wiring, forces acting along the abutment axis were amplified, while horizontal force components could be widely suppressed. The mesiodistal pairs of strain gauges were not used for the present study. Duplicates of the patients' bridges were set onto the instrumented abutments. Electromyograms of right and left masseter and anterior temporalis muscles were recorded with the use of bipolar Ag/AgCl surface electrodes (Hellige) with 2-cm distance between electrodes. Prior to electrode attachment, the skin was cleaned with alcohol and rubbed with grinding paper for the reduction of impedance. The raw EMGs were filtered (from 10 to 5 KHz), full-wave-rectified, and root-mean-square-integrated with a 40-msec time constant (Digital EMG system 1500®, Disa, Denmark).
Measurement of Bite Force and EMG
In the mastication tasks, the patients chewed winegum unilaterally (Goldbären, Haribo, Bonn, Germany) on the side of the bridge. The EMG and force channels were scanned for 20 sec at a rate of 100 Hz by means of an A/D converter (6944A Multiprogrammer, Hewlett Packard, Palo Alto, CA, USA) controlled by a desktop computer (HP9826, Hewlett Packard, Palo Alto, CA, USA). The force signals arising from the 2 abutments were summed to give the total vertical chewing force acting on the bridge. This allowed us to measure the chewing force without leakage regardless of the site of force impact on the surface of the bridge (Morneburg and Proeschel, 2002).
For isometric biting, an electronic bite-fork (Proeschel and Raum, 2001) was inserted in place of the test food between the bridge and the antagonistic teeth. The patients clenched intermittently in a chewing-like rhythm, with peak loads alternating from low levels up to maximum bite force. For isometric contractions to be ensured, the teeth had to maintain steady contact with the bite-fork, which induced a jaw separation of about 6 mm. The electromyograms and the force signals of the bite-fork and the instrumented abutments were sampled and processed in the same way as in the chewing tasks. The force readings from the bite-fork and the instrumented bridges differed by less than ± 4%. For evaluation, only the bridge recordings were used. The bite-fork mainly served to simulate the experimental conditions of the previous studies in which muscle activities had been calibrated by subjects' clenching on the bite-fork only (Proeschel et al., 1994; Proeschel and Raum, 2001).
Evaluation of Data and Statistics
The peaks of the unilateral bite force recorded in each chewing or clenching task were related to the peak activities of each muscle by linear regression. The strengths of the relationships were characterized by Pearson correlation coefficients (r-values). As an alternative measure for the relationship, we determined an activity/bite-force ratio by dividing the mean peak activity of each muscle by the mean peak force obtained in each chewing or clenching task. To test the validity of EMG-based force estimation, we substituted the mean peak chewing activity for A in the regression equation F = mA + b, determined from isometric bite-fork clenching. Since this was done for each muscle, 4 muscle-related estimates of the chewing force F were obtained in each subject. The estimates were compared with the mean peak chewing force measured by the instrumented bridges. Results are given as mean ± standard deviation in the text and as mean ± standard error in the graphs. Student's t test for paired data was applied for the examination of mean value differences for significance on the 1% level.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS & METHODSRESULTSDISCUSSIONREFERENCES |
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. The mean activity/bite-force ratios of the working-side masseter, the working-side temporalis, and the balancing-side temporalis were between 1.5 and 2 times higher in chewing than in clenching (p < 0.01) (Fig. 2). In contrast, the activity/bite-force ratio of the balancing-side masseter was not significantly different in the 2 biting tasks. The chewing activities exceeded the clenching activities in all muscles (Fig. 2). The working-side masseter activity in chewing was 2 times higher (p < 0.01) than in clenching, while the balancing-side activity was insignificantly enhanced by a factor of 1.2. The temporalis activities were enhanced by factors of 1.4 or 1.9, respectively (p < 0.01).
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). The group mean of the clenching force amounted to 236 N ± 92 N. In eight patients, the clenching force was bigger than or equal to the chewing force. Only one rather cautious subject (Fig. 1
, patient B; and Fig. 3, number 3) exerted a smaller force in clenching than in mastication. The EMG-based force estimates (Fig. 3) were, in most cases, considerably higher than the measured forces and depended on the muscle. A deviation of less than 15% between estimated and measured chewing forces was obtained from the activity of 5 muscles in three persons (numbers 2, 3, and 8). In one subject (Fig. 1, patient B; Fig. 3, number 3), the force estimates from 3 muscles coincided approximately with the measured chewing force. From working-side masseter activities, a group mean force of 475 N ± 248 N was estimated. A likewise high value of 442 N ± 245 N was obtained from the balancing-side temporalis activity. A more moderate prediction of 329 N ± 125 N resulted from the working-side temporalis. The balancing-side masseter provided 273 N ± 123 N, which came closest to the group mean of the actual chewing force of 220 N. Except for the balancing-side masseter, all differences between EMG-based force estimates and measured chewing forces were significant.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS & METHODSRESULTSDISCUSSIONREFERENCES |
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, patient B). The weak activity/bite-force relations in chewing do not allow conclusions to be drawn concerning the quality of relations between activity and muscle forces (Hylander and Johnson, 1989). In contrast to the subjects of Ahlgren and Öwall, a few of our patients revealed quite good correlations, with r-values up to 0.85 (Fig. 1). Such stronger relationships were not common to all muscles but could appear together with weak correlations in the same person (Fig. 1, patient A).
With respect to the validity of EMG-based force estimates, the activity/bite-force ratios (Fig. 2) were more useful than the regression analysis. These ratios clearly showed that muscle activity associated with a certain bite force was considerably higher in chewing than in isometric biting. A similar observation was reported for sums of elevator muscle activities (Schindler et al., 1998). Our study, in addition, revealed that the enhancement of activity per unit bite-force in chewing depended on the muscle and on its function as a working- or balancing-side elevator. The obvious task-dependence of activity/bite-force ratios violates the validity of masticatory-force-estimation from EMG, in particular when only 1 muscle is used in this procedure (Hagberg, 1987; Slagter et al., 1993; Proeschel et al., 1994; Tate et al., 1994). If the respective muscle generates higher activity per unit bite-force in mastication than in isometric biting, the masticatory force will be overestimated when the chewing EMG is used in the isometric activity/bite-force regression. Just as illustrated in patient A (Fig. 1), this caused the high number of excessive force estimates (Fig. 3). The different activity/bite-force ratios in the 2 motor tasks could reflect different activity/muscle-force relations as well as different combinations of muscle and reaction forces facilitated by the redundancy of the craniomandibular force system (Van Eijden et al., 1990). In the latter case, a valid estimation of chewing force from EMG would require a three-dimensional force-model in which all involved muscle and reaction forces could be considered together (Hatcher et al., 1986; Trainor et al., 1995). Data provided by one of these models (Hatcher et al., 1986) allow for a rough estimate of occlusal and temporomandibular joint loads based on the activities found in our patients: If muscle forces in chewing would be enhanced (with respect to clenching) by the same factors (chewing/clenching activity ratios in Fig. 2) as muscle activities, the chewing force derived from the cited model would exceed the clenching force by 30 to 40%. This would contradict our experimental finding of equal mean forces in both biting tasks. A hypothetical depressor counteraction could, in principle, reduce the masticatory force. However, compensation of bite-force by antagonistic co-activation was found only in isometric biting (Pruim et al., 1978; Miles and Madigan, 1983) but not in chewing. These considerations indicate that increased muscle activities in chewing could hardly be associated with proportionately increased muscle forces. Rather, they imply task-dependent relations between muscle activities and muscle forces and thus between muscle activities and bite-forces. Higher muscle activities per unit bite-force were observed in biting with smaller jaw gapes than with bigger ones (Manns and Spreng, 1977; Lindauer et al., 1993). Likewise, isotonic contractions of limb muscles are known to produce more activity than isometric contractions (Komi, 1973). Additional muscle activity was also evoked when the neuromuscular system could anticipate a counteracting force during jaw closing (Ottenhoff et al., 1992). Such effects could be relevant, since opposing teeth approach a distance of about 0.5 mm in chewing (Proeschel and Raum, 2001) but remain separated by 6 mm in clenching on the bite-fork. Further, the contraction in chewing is partly non-isometric, and the neuromuscular system may anticipate the resistance of food during the closing movement. These features could possibly explain a general increase of muscle activity per unit bite-force in chewing. However, since no specific side preference could be assigned to the quoted effects, the asymmetric enhancement of the masseter EMGs remains puzzling. Side-related differences in activity/bite-force relations could be provoked by oblique bite-force directions (Van Eijden et al., 1990; Mao and Osborn, 1994). The asymmetric increase of activity/bite-force ratios in our patients would roughly comply with activity/bite-force relations obtained for a medially directed bite-force in a previous study (Van Eijden et al., 1990). Analysis of the present data, however, provides no information concerning the direction of peak force during chewing.
In summary, the prediction of chewing force from dynamic EMGs and isometric activity/bite-force relations usually resulted in considerable overestimation. The reason for this was that a certain bite-force in chewing was associated with higher muscle activity than the same bite-force in clenching. The surplus masticatory activity differed between muscles and was smallest for the balancing-side masseter. Biomechanical considerations suggest that the higher muscle activity in chewing was not associated with likewise higher muscle forces. This should be considered in all attempts to model occlusal or temporomandibular joint loads using EMG data obtained from chewing.
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ACKNOWLEDGMENTS |
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Received November 13, 2001; Last revision May 1, 2002; Accepted May 15, 2002
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REFERENCES |
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  P.A. Proeschel and J. Raum Task-dependence of Jaw Elevator and Depressor Co-activation J. Dent. Res., August 1, 2003; 82(8): 617 - 620. [Abstract] [Full Text]
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