| Sign In to gain access to subscriptions and/or personal tools. |
Human Masticatory Muscle Forces during Static Biting
1 University of Nebraska Medical Center College of Dentistry, Departments of Growth and Development and Correspondence: 3 corresponding author, liwasaki{at}unmc.edu
Muscle forces determine joint loads, but the objectives governing the mix of muscle forces involved are unknown. This study tested the hypothesis that masticatory muscle forces exerted during static biting are consistent with objectives of minimization of joint loads (MJL) or muscle effort (MME). To do this, we compared numerical model predictions with data measured from six subjects. Biting tasks which produced moments on molar and incisor teeth were modeled based on MJL or MME. The slope of predicted vs. electromyographic (EMG) data for an individual was compared with a perfect match slope of 1.00. Predictions based on MME matched best with EMG activity for molar biting (slopes, 0.89-1.16). Predictions from either or both models matched EMG results for incisor biting (best-match slopes, 0.95-1.07). Muscle forces during isometric biting appear to be consistent with objectives of MJL or MME, depending on the individual, biting location, and moment.
Key Words: numerical modeling • TMJ • loading • electromyography
In humans, muscle recruitment during biting and chewing depends in part on bite force direction (van Eijden et al., 1990, 1993). Directional information is provided by periodontal ligament mechanoreceptors (Trulsson and Johansson, 1994, 1996). However, it is unknown whether there is a governing neuromuscular objective for the mix of masticatory muscle forces produced for a given biting situation. Data from living subjects are sparse. The mix of muscle forces determines the magnitudes and directions of temporomandibular joint (TMJ) loads (Throckmorton et al., 1990; Trainor et al., 1995), and thus, is clinically important, especially since the likelihood of degenerative joint disease increases with increased TMJ loads (Iwasaki et al., 1997). Recent analysis of the mix of muscle outputs measured in post-orthognathic surgery patients suggested that the neuromuscular objectives of minimization of joint load (MJL) or minimization of muscle effort (MME) determined the magnitudes of joint loads (Nickel et al., 2002). The aim of this project was to elucidate neuromuscular control of craniomandibular mechanics by testing the hypothesis that masticatory muscle forces, in response to static biting which produces moments on molars and incisors, can be predicted on the basis of MJL or MME, or both. To test this hypothesis, we compared data from living subjects with numerical model predictions of muscle forces calculated from the three-dimensional geometry of the individual and an objective function of MJL or MME.
One female and five male subjects, aged 23 to 32 yrs, participated. Informed consent was obtained, and the project was approved by the appropriate Institutional Review Board.
Anatomical Geometry
Modeled Eminence Shape The effective sagittal TMJ eminence morphology was defined as the shape of hard- and soft-tissue structures articulating with the mandibular condyle in sagittal view. We used a custom program with optimization based on unconstrained MJL and the subject’s geometry to predict the effective sagittal eminence morphology (Trainor et al., 1995). In brief, it operated as follows. Joint load directions were calculated for a series of bilateral vertical bite forces applied in 20 steps from first molars to central incisors. The mandibular position, consistent with bite force position for each calculation, ranged from relatively retruded (molar biting) to protruded (incisor biting). For equilibrium, the joint load must be directed perpendicular to the effective eminence. Therefore, the effective eminence shape for an individual was delineated by a series of 20 lines, each perpendicular to the predicted joint load direction for a given step, to which a cubic polynomial was fitted.
Modeled Muscle Forces
Modeled conditions produced moments on the molar and incisor teeth that were similar to those tested in vivo (see below). Bite forces of 100 units, at angles from vertical (
Experimental Eminence Shape A set of 10 protrusion/retrusion movements, with upper and lower teeth separated by 1 to 3 mm, was recorded at each of 2 sessions, at least 3 days apart. Recordings were viewed frame-by-frame, and the condylar marker position was traced on acetate mounted on the video screen. Points along the path of the marker were quantified in two dimensions with occlusal plane and a perpendicular line as axes. The occlusal plane was chosen as a reference axis, given its importance in the mechanics of biting. A custom computer program was used to correct for scale, to record x- and y-coordinates for each point, and to calculate a best-fit third-order polynomial which delineated the effective eminence shape. Intra- and inter-session measurement errors in condylar position were, on average, + 0.3 mm or + 10% of full scale.
Experimental Biting Tasks
Bipolar surface electromyographic (EMG) data were measured as surrogates for forces during biting tasks. The muscle bulk center was located by palpation for masseter, anterior temporalis, and anterior digastric muscles. The skin over each muscle was debrided, and paired electrodes (E5SH Standard Silver®, 7 mm, Grass Instrument Company, Quincy, MA, USA) with conducting paste (Liqui-Cor®, Burdock Corp., Milton, WI, USA) were held in place with adhesive pads. Surface impedances were less than 30 k Muscle output data were replayed and analyzed with commercial software (Microsoft Excel®, Microsoft Corp., Seattle, WA, USA). Muscle activities over a two-second period were sampled at 1.7 kilo-samples/second/channel and expressed as root-mean-square (RMS) values (mV).
Eminence Shape Comparisons
Muscle Output Comparisons
where I and C were the ipsilateral and contralateral muscle output for lateral molar biting, and R and L indicated right and left sides. In vivo intra- and inter-sessional ratio variabilities were, on average, + 13%. Predicted vs. measured I/C muscle output ratios for biting moments were plotted for each subject. Linear regressions were calculated and slopes were tested for significant differences from a perfect-match slope of 1.00. Incisor biting tasks were centered on the mandible, so RMS values for right and left muscles were averaged for 5 bites at each of 3 bite positions. These data for each subject and recording session were normalized relative to the peak RMS value for a given muscle. Similarly, predicted muscle forces for incisor biting were normalized relative to the peak predicted muscle force for the subject and muscle. Intra- and inter-sessional normalized RMS value variabilities were, on average, + 14%. Predicted vs. measured normalized muscle output values for biting moments were plotted for each subject. Linear regressions were calculated and slopes were tested for significant differences from a perfect-match slope of 1.00.
Eminence Shapes The maximum vertical difference between predicted and experimental results for the group was 0.4 mm for protrusion up to 5 mm. Plots of predicted vs. measured effective sagittal eminence shapes for each subject showed slopes from 0.87 to 1.13, and a range of correlations, 0.92 < R2 < 0.99 (Fig. 2A  ). The mean slope was 0.94 with a standard deviation (SD) of 0.12. An independent t test showed no significant difference between predicted and experimental results for eminence shape. However, in vivo eminence shapes showed inter-subject variability, where the individual slopes of fitted polynomials differed by up to 3 to 1 (Fig. 2B).
Muscle Outputs Anterior digastric muscle force predictions and RMS values were similar for the biting tasks investigated, and near or at baseline levels. The 2 models predicted quite different I/C masseter and anterior temporalis muscle force ratios for moments resulting from lateral and medial molar biting angles (Fig. 3A ). Plots of predicted vs. measured muscle output during molar biting were compared for the models (Table). All subjects’ molar biting I/C muscle output ratios were predicted better according to the MME model (mean slope, SD: 0.98, 0.10) compared with the MJL model (mean slope, SD: 3.71, 3.74). Similarly, plots of predicted vs. measured normalized masseter and anterior temporalis muscle output values for incisor biting were compared for the two models (Table). The predictions from the MJL and MME models were the same for two subjects (e.g., f1, Fig. 3B) and matched well with the EMG data for these subjects. Predictions from the 2 models were not the same for the other four subjects (e.g., m5, Fig. 3B). The MJL model results matched best with the EMG data from two of these subjects, while the MME model results matched best with the EMG data from the two remaining subjects. Slopes from plots of predicted vs. measured results for molar and incisor biting (Table) were compared with perfect-match slopes of 1.00. Separate independent t tests showed no significant differences between best model-predicted and experimental data for each biting condition (p > 0.05). However, post hoc power analyses indicated that sample sizes were too small for statistically significant differences between predicted and measured values to be excluded.
Computer-generated models based on human anatomy render solutions based on an optimal strategy or objective function, which is of biological importance and thus represents a theory of underlying neuromuscular control (Pruim et al., 1980; Osborn and Baragar, 1985, 1992; Smith et al., 1986; Koolstra et al., 1988; Herzog and Binding, 1994; Koolstra and van Eijden, 1995, 1997, 1999; Osborn, 1995; Trainor et al., 1995; Buchanan and Shreeve, 1996; Itoh et al., 1997; Raikova, 1999). A neuromuscular objective of MJL is of interest because cartilage has limited repair potential, and theoretically, this objective reduces the likelihood of mechanical fatigue. However, MJL tends to increase muscle forces for given biting tasks, theoretically increasing the likelihood of muscle fatigue. In contradistinction, an objective of MME reduces muscle forces for a biting task, but increases joint loading, especially in the TMJ contralateral to a molar bite force. The data presented here are the first, to our knowledge, to test model predictions of masticatory muscle output for changes in biting moments in individual subjects. Variance seen between predicted and experimental data is likely due to factors such as the use of synthetic crowns, a static biting task rather than routine masticatory activity, and the limitations of the numerical model, which necessitated that the in vivo biting tasks be represented by bite force positions that would produce equivalent moments, but not identical force vectors. Experimental evidence comparable with model predictions from the current study has been reported (van Eijden et al., 1993). Human I/C masseter muscle peak EMG ratios calculated from unilateral gum-chewing data ranged from 1.18 to 2.66. In the current study, mean predicted I/C masseter muscle force ratios were 1.5 (SD 0.7) for non-vertical bite forces. In summary, predictions of muscle recruitment were tested with experimental data. The mix of masticatory muscle outputs during isometric unilateral molar biting and centered incisor biting was generally consistent with MJL or MME for the individuals investigated. The specific objective function involved appeared to depend on the individual’s anatomy and the specific location and direction of bite force. These factors are the focus of further inquiry.
A supplemental appendix to this article is published electronically only at http://www.dentalresearch.org. Received for publication December 5, 2001. Revision received October 30, 2002. Accepted for publication November 7, 2002.
Journal of Dental Research, Vol. 82, No. 3,
212-217 (2003)
|
|
||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
 |
|
|
 |
|
Sign In to gain access to subscriptions and/or personal tools.
TOP
ABSTRACT
INTRODUCTION
is the moment arm vector, 
y is the angle away from vertical, and 
, applied input impedance was 20 M
) and subject m4 ("best-case", *). (B) Sagittal morphology of the effective TMJ eminence in each of the six subjects. The vertical axis represents the eminence height (mm) relative to a retruded condylar position and perpendicular to the occlusal plane, and the horizontal axis represents condylar protrusion (mm) from the retruded position. Function beyond 5 mm of protrusion was unlikely, since this placed mandibular incisors anterior to maxillary incisors in all subjects.
) and anterior temporalis (
) muscles and MME model predictions for masseter (
) and anterior temporalis (X) muscles are shown for subject f1. Please note the discontinuity of scale in the vertical axis. (B) Predicted normalized muscle forces for incisor biting tasks at a range of bite force angles that created moments equivalent to in vivo biting at posterior, center, and anterior positions. Masseter muscle force predictions from MJL model (
subject m5), and anterior temporalis muscle force predictions from MJL model (
subject m5) and from MME model (
subject f1, • subject m5).