Model-based analysis of jaw-movement kinematics using jerk-optimal criterion: simulation of human chewing cycles

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Abstract

Large intra- and/or inter-individual variability in human masticatory motor behaviors may reduce objectivity in assessing normality/abnormality for jaw movements. Analytical simulation by kinematic modeling based on a universally accepted objective criterion is a potentially useful clinical tool for credible diagnostic quantification of jaw motion. This study aimed to test whether a minimum-jerk (maximum-smoothness) model could accurately simulate the kinematics in each separate gum-chewing cycle of 10 healthy adults, and if so, to examine whether simulation errors reflected an extent of abnormality for chewing movements for 10 patients caused by temporomandibular joint disorders (TMD). The model predicted the lower incisor-point movements of the control group with reasonable accuracy. Overall, the prediction errors for the patients group were greater than the control group, demonstrating that prediction errors for jaw opening velocity were sensitive to the criteria for joint disorders. It was concluded that the present simulation emerged as being capable of scaling the not so smooth masticatory jaw movements performed in the presence of TMD.

Introduction

A skilled sensory-motor system produces an economy of motion that is dependent on a specific task, e.g., saving energy or reducing degrees of variability of the motion [3], [21]. The regularity of human arm reaching in terms of its optimal smoothness has been identified [8], [9]. The movement smoothness relates to regularity in the movement jerk, where the jerk is the rate of change in movement acceleration/deceleration. Jaw movement during chewing is one of the most skillful human motor functions, and the minimum-jerk (maximum smoothness) kinematic model is capable of representing the features underlying the three-dimensional jaw trajectories during the breakage of various foodstuffs, i.e., the “power phase” of the chewing cycle [29].

The opening and closing phases of the chewing cycle have been subjectively classified into various types according to the shapes [19], [20] and direction of the trajectory [20], [24], suggesting that there exist relationships between representative types of jaw movement paths and malocclusions [1], [16]. However, these types of evaluations present many difficulties that relate to the influences of the large intra- and/or inter-individual variabilities of human masticatory cycles [12]. The jaw motor system alters the movement on a cycle-by-cycle basis according to intra-oral conditions then present.

Clinically valid quantification of masticatory movements is crucial for dental practitioners to in practice account for necessity of occlusal adjustments and to demonstrate effect of therapeutic interventions upon a patient’s jaw motor function. Here, we assume that an analytical simulation by kinematic modeling based on a universally accepted objective criterion is a potentially useful clinical tool for credible diagnostic quantification of jaw motion, because it allows the various patterns to be characterized in a logical and quantitative manner. The high accuracy of the model’s predictions implies that there exists a common intrinsic feature underlying the variety of curvilinear trajectories of the chewing cycles. As well, if the proposed model can be shown to be capable of accounting for the inter-individual variations of the chewing cycles performed in the absence of any clinical signs of discomfort or dysfunction, the ranges of the prediction accuracy of the proposed model show the specific optimality of masticatory jaw movements under normal conditions. Further, the ranges of normality may allow determination of extent of abnormality for masticatory motor function. Previous methods have not allowed reliable estimations of the extent to which temporomandibular joint disorders (TMD) interfere with masticatory jaw movements.

The present study was aimed to examine whether jerk-cost optimization (smoothness maximization) can be generalized to encompass the variety of masticatory cycles under various chewing conditions in adults without any discomfort and dysfunction of the jaw. In addition, the aim of this study included investigation of whether the errors in the model predictions reflect an extent of abnormality of the chewing jaw movements with presence of the TMD. The present simulation may provide us a novel insight into normality/abnormality of chewing jaw movements.

Section snippets

Subjects and patients

Ten healthy volunteer adults (five males and five females, mean age, 26y 6m; SD, 2m) with good occlusions served as a control group. None of the subjects had discernible clinical signs of jaw dysfunction or discomfort. All gave informed consent to participate after receiving a full explanation of the aims and design of the study. The Ethics Committee of the dental school approved the experiment. In addition, 10 patients (five males and five females, mean age, 19y 10m; SD, 6y9m), who showed

Movement paths

The representative findings for the comparisons between the measured and the predicted trajectories in the control subjects are shown in Fig. 3. Overall, there were reasonable matches between the predicted and the actual trajectories. The comparisons of the average paths and areas of ±1 standard deviations around the average paths for the two control subjects were shown in Fig. 4. Within each control subject, the areas of positional variation of the data agreed with those predicted by the

Discussion

The kinematic features predicted by the mathematical models quantitatively matched the masticatory jaw movements recorded from the healthy adults under varied conditions. The experimental conditions induced a wide variety of jaw motions during manipulation of soft coherent bolus and those during reducing the size of hard crushable food-particles. The favorable performance of the proposed model under the varied experimental conditions indicates that the observations of the present study were not

Acknowledgments

This work was supported by Grant-in-Aid for Scientific Research (B-14370695 and B-14370696) sponsored by the Japanese Ministry of Education, Science and Culture.

Dr. Kohtaro Yashiro received his D.D.S. degree in 1987, and received his Doctor of Philosophy in Dental Science Degree (PhD) in 1996 from Osaka University, Osaka Japan. He is a research assistant in the Department of Orthodontics and Dentofacial Orthopedics, Graduate School of Dentistry, Osaka University. His research interests involve mathematical modeling, analytical simulation and measurement of human jaw motor performance. His research concern includes human jaw biomechanics. Currently, his

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  • Dr. Kohtaro Yashiro received his D.D.S. degree in 1987, and received his Doctor of Philosophy in Dental Science Degree (PhD) in 1996 from Osaka University, Osaka Japan. He is a research assistant in the Department of Orthodontics and Dentofacial Orthopedics, Graduate School of Dentistry, Osaka University. His research interests involve mathematical modeling, analytical simulation and measurement of human jaw motor performance. His research concern includes human jaw biomechanics. Currently, his scientific activities are focused on an individual modeling of human temporomandibular joint (TMJ) for clinical diagnosis. Dr. Yashiro is a member of Japanese Orthodontic Society, International and Japanese Associations of Dental Research. From 2000 to 2001, he was a Visiting Scientist at Department of Oral Health Sciences, Faculty of Dentistry, University of British Columbia, Canada.

    Dr. Kenji Takada is a professor and chair in the Department of Orthodontics and Dentofacial Orthopedics, Graduate School of Dentistry, Osaka University. Dr. Takada is a member of council in Japanese Orthodontic Society. He received the Doctor of Philosophy in Dental Science Degree (PhD) from Osaka University, Faculty of Dentistry, Osaka, Japan. From 1990 to 1996, he was in office as Associate Professor and Chair in the Department of Orthodontics and Dentofacial Orthopedics at Graduate School of Dentistry of Osaka University. His research concern includes mathematical formulation of decision-making process in orthodontic diagnosis and treatment planning, with particular foci on jaw motion control and automatic recognition of Dentofacial form. He has given lectures in Belgium, Denmark, France and UK as well as in North America.

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