This report describes a robotic system, the 6DOF/R0, which simulates human jaw movement with six degrees of freedom using dental cast models. The robot consists of a personal computer, a controller, six actuators and a set of links. A mandibular dental cast model was moved under a parallel mechanism formula using data obtained with a mandibular movement recorder system (JM-1000T, ONO SOKKI) . These data were converted to the coordinates of the movement of robot table with the personal computer. The range of movement of the robot was±14 mm in the X, Y and Z-axes, and the simulation speed was approximately two thirds of the original movement of the subject. To investigate the accuracy of simulation, the movement of the lower dental cast on the robot was recorded with the JM-1000T. Both the original data and the robot data were analyzed to confirm reproducibility. To evaluate the mechanical accuracy of the robot, a 3D-measurement device (UMC850S, Zeiss) was used. The patterns of the simulated movement path of the robot system were similar to the original path of the subject. The maximal error of the movement path of the robot system was about 1 mm. Though the accuracy of the simulation of the jaw movement is not enough to observe details of the tooth-to-tooth contact of the posterior part of the dentition during the final phase of mouth closing, we can show the movement of the mandible under a controlled moving speed. This robot will be useful for dental education and research.

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The clinical accuracy of an optical recording system for mandibular movement was evaluated with the following methods: (1) three-dimensional assessment of the accuracy of measurement using the least-squares method, (2) two dimensional assessment of the reproducibility of the hinge axis movement path by an auto-pendulum device, and (3) an assessment of the effects of illumination and vibration of the Light Emitting Diode (LED) holders on measurement accuracy.
The following results were obtained:
1. The three-dimensional accuracy in terms of distance was 0.12mm.
2. Measurement error values at the center and radius coordinates in terms of hinge axis movement were 0.27mm and 0.02mm, respectively.
3. Measurement accuracy was affected by the lighting conditions.
4. Measurement error at the coordinates during the standing still point of the LED marker was below 0.1mm.
5. No vibration was observed in the LED holders, head-frame or mandibular face bow during jaw movement.
6. A measurement error of 0.55mm was detected when the face of the LED was inclined 30 degrees.
The present results indicate the usefulness of our system for clinical application.

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It is important to consider the relationship between tooth guidance and condylar movements from the point of view of occlusal reconstruction and analysis of the physiological relationship between stomatognathic function and occlusion. We examined the movements of working and balancing side condyles during lateral excursions, when the anterior guidance was altered but without changing the intercuspal position. When the incisal path angle became steeper than the natural tooth guidance angle, the distance of the balancing side condyle paths (center of the condyle paths) decreased significantly, whereas the distance of the working side condyle paths (center of the condyle pathways) remained unchanged. The trajectories of the working side condyle paths were found to be random, whereas those of the balancing side condyle paths remained stable. The analysis of six points around the center of the condyle indicated that the working side condyle paths were affected passively following an alteration in the tooth guidance angle and that the balancing side condyle performed an active role during lateral excursions.

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To investigate the relationship among jaw movements, jaw, tongue, and hyoid muscle activities during chewing and swallowing, and clarify how physical characteristics of foods affect the coordination of these muscle activities, we recorded jaw movement trajectories and muscle activities in the freely behaving rabbit. The phase duration, gape and excursion sizes of jaw movements as well as the duration and peak values of electromyographic burst activities were obtained and compared among three foods (pellet, bread and banana) .
1 . Chewing cycles
During chewing, the opening phase duration was most closely related to the total phase duration, suggesting that the opening phase controlled the masticatory cycle duration. The chewing cycles consisted of three phases (fast closing, slow closing, and opening phases) in pellet and bread chewing, however, only two phases (closing and opening phases) were observed in banana chewing. The muscles recorded can be divided into two groups; one muscle group consists of a jaw-closing muscle [masseter muscle (Mass) ] and a tongue retracting muscle [styloglossus muscle (SG) ], and the other group consisted of a jaw-opening muscle [digastric muscle (Dig) ], a suprahyoid muscle [mylohyoid muscle (MH) ] and a tongue protrusive muscle [genioglossus muscle (GO) ] . The former group appeared to be mainly activated in the jaw-closing phase and the latter in jaw-opening phase, and these patterns were similar among foods. The results suggest that the MH and SG muscles play a critical role in compressing the bolus between the tongue and palate, particularly in banana chewing.
2 . Swallowing cycles
The swallowing cycle was found to have longer cycle duration than that of the chewing cycle as if an extra phase (a pause) occurred in the opening phase. The swallowing cycle consisted of five phases (fast closing, slow closing, opening 1, opening 2, and opening 3 phases) in pellet and bread swallowing, however, only three phases (closing, slow opening and fast opening phases) in banana swallowing. Swallow-related activities that began in the middle of the jaw-closing phase were observed in the GG, MH and SG muscles. The MH and SG muscles were thought to participate in a leading complex activity of tongue movements during swallowing. The MH activity in pellet swallowing had a second peak during the jaw-opening phase, and this suggested that the MH muscle plays a critical role of pulling the laryngeal cartilage and creating a space at the esophagus for food passage.
During swallowing the Dig muscle was active as with the Mass muscle in the jaw-closing phase, this may be due to co-activation of the antagonist (the Mass and Dig) muscles which might stabilize the jaw near the rest position.
We concluded that the mechanism underlying the coordination of the jaw and tongue muscle movements might be maintained not only during chewing but also during swallowing and may be modulated depending on the physical characteristics of foods. Particularly in chewing soft foods like banana, the tongue could play a critical role in compressing and transporting the bolus against the palate during the bucco pharyngeal stage of swallowing and this could lead to modulation of jaw movements as well as muscle activities.

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It is well known that mechanical stimulation of the pharyngeal areas innervated by the glossopharyngeal nerve (GPN) or the laryngeal areas innervated by the superior laryngeal nerve (SLN) readily elicits reflex swallowing. However, electrical stimulation of the SLN elicits reflex swallowing more readily than does stimulation of the GPN. This paradox remains unexplained; hence, the purpose of this study was to solve it.
Mechanical stimulation with light pressure easily elicited reflex swallowing from the pharynx in the rat. The most effective reflexogenic regions were the palatopharyngeal arch, the edge of the pharyngeal surface of the epiglottis, the soft palate (extending region of the palatopharyngeal arch) and the aryepiglottic fold. Sectioning the pharyngeal branch of the GPN (GPN-ph) abolished the reflex swallowing from the palatopharyngeal arch and the soft palate out of these regions. The relationship between the frequency of electrical stimulation and the latency of swallowing for the GPN-ph was quite similar to that for the SLN. Electrical stimulation of the lingual branch of the GPN (GPN-li) was not effective in elicitation of swallowing. When electrical stimulation was applied simultaneously to both the GPN-ph and SLN, the latency of swallowing became shorter as compared with those for stimulation of each nerve. On the contrary, when electrical stimulation of the GPN-li was applied in addition to the stimulation of the SLN, reflex swallowing was delayed or inhibited.
These results indicate that the GPN-ph plays a major role in initiation of reflex swallowing from the pharynx and that the electrophysiological properties of the GPN-ph for initiating swallowing are almost the same as those of the SLN, whereas the GPN-li plays a minor role and contains the inhibitory fibers for initiation of swallowing. Our findings provide a solution for the paradox regarding the initiation of swallowing.

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