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Coordination of Tongue Pressure and Jaw Movement in MasticationDivision of Oromaxillofacial Regeneration, Osaka University Graduate School of Dentistry, 1-8 Yamadaoka, Suita City, Osaka, 565-0871, Japan Correspondence: * corresponding author, ono{at}dent.osaka-u.ac.jp
The tongue plays an important role in mastication and swallowing by its contact with the hard palate. Using an experimental palatal plate with 7 pressure sensors, and recording jaw movement using mandibular kinesiography, we assessed, in healthy subjects, the coordination of tongue and jaw movements during the entire masticatory sequence of solids, by measuring tongue pressure against the hard palate. Tongue pressure appeared during the occlusal phase, reached a peak near the start of opening, and disappeared during opening. Specific patterns in order, duration, and magnitude of tongue pressure were seen at the 7 pressure sensors in each chewing stroke. Magnitude and duration were significantly larger in the late stage of chewing (8 strokes before initial swallowing) than in the early stage (until 8 strokes after starting mastication). The normal pattern of tongue contact against the hard palate, control of tongue activity, and coordination with jaw movement during mastication is described.
Key Words: tongue • palate • mastication • jaw movement
The reduced ability to comminute food or to process the bolus in patients with movement disorders (Schneider et al., 1986; Leopold and Kagel, 1996), cerebrovascular disease (Umapathi et al., 2000), tongue cancer (Davis et al., 1987; Logemann et al., 1989), and complete-denture wearers (Koshino et al., 1997) might be due to lack of coordination between tongue and jaw movements, as well as to limited movement and/or volume of the tongue. However, coordination of tongue and jaw movements during the masticatory sequence of solids has not been well-documented, and clarification of the normal process could benefit the rehabilitation of these patients. The sequence model of handling solid food, which included tongue and jaw movements, was proposed from videofluorographic studies (Palmer et al., 1992, 1997; Hiiemae et al., 1996; Thexton and Hiiemae, 1997; Hiiemae and Palmer, 1999). The role of the tongue in the formation and transport of the bolus, however, has yet to be fully elucidated, since contact of the tongue against the hard palate cannot be evaluated precisely by x-ray motion pictures or ultrasound images. Tongue activity during mastication of solids should be analyzed, because it is involved in comminution of food and transportation of the bolus to the mesopharynx (Palmer, 1997). Although muscle activity in mastication and swallowing can be evaluated directly by electromyography (Palmer et al., 1992; Takada et al., 1996; Shiozawa et al., 1999), few studies have investigated coordination of tongue and jaw movements in humans during natural mastication, due to the difficulty of measuring tongue activity with surface electrodes. Measurement of tongue pressure is useful for evaluating the biomechanical performance of the tongue during normal functional movements, as well as the effectiveness of transpalatal orthodontic appliances (Chiba et al., 2003). Although the Iowa Oral Performance Instrument (Pouderoux and Kahrilas, 1995; Robbins et al., 1995; Crow and Ship, 1996) and the handy probe (Hayashi et al., 2002) were useful in the simple assessment of tongue pressure, those devices could not be used for the evaluation of natural mastication and swallowing, since they must be used without bringing the teeth into occlusion. Electric pressure sensors, installed in an artificial palate (Shaker et al., 1988; Ono et al., 2004) or upper denture, can provide information on the state of contact between the tongue and hard palate. Previously, we clarified the normal functional characteristics of tongue movement and pressure against the hard palate during swallowing of water by analyzing the order, duration, and magnitude of tongue pressure. For this purpose, we used a specially designed experimental palatal plate having 7 pressure sensors (Ono et al., 2004). Those parameters of tongue pressure appeared to be useful in the study of the biomechanical performance of the tongue during mastication. The aim of the present study was to describe the coordination of tongue and jaw movements in chewing strokes of solid food, by recording tongue pressure and jaw movement trajectory, and to elucidate control of tongue activity in the early and late stages of the masticatory sequence.
Subjects Subjects were ten healthy volunteers (eight men and two women; age range, 24–30 yrs; mean age, 27.7 yrs) without (i) disturbance of mastication and swallowing, (ii) abnormality in the number or position of teeth except for third molars, (iii) history of orthodontic treatment and temporomandibular disorder, or (iv) abnormality of occlusion. Written informed consent was obtained from each subject after explanation of the aim and methodology of the study. This study received approval by the ethical committee in the Osaka University Graduate School of Dentistry.
Fabrication of Experimental Palatal Plate
Seven disk-shaped pressure sensors (PS-2KA; diameter, 6 mm; thickness, 0.6 mm; rated capacity, 200 kPa; safety overload range, 150%; non-linearity, ± 1% RO; hysteresis, ± 1% RO; thermal effect on output, ± 1%/°C; compensated temperature range, 0–50°C; Kyowa Electric Instruments, Tokyo, Japan) were installed in the experimental palatal plate. The location of pressure sensors was chosen with reference to the dental arch and the anatomical landmarks of the incisive papilla and Hamuller’s notch, since we considered that our evaluation method could also, in future, be applied to edentulous patients (Fig. 1
Measuring System and Condition Tongue pressure was measured by pressure sensors installed in the experimental palatal plate and recorded on a computer through an interface board (PCD 100A, Kyowa Electric Instruments, Tokyo, Japan). Jaw movement data was measured by mandibular kinesiography (MKG, K6-I, Myotronics Co., Taren Point, Australia) and recorded on digital audiotape. The two datasets were synchronized by the computer so that the coordination of tongue and jaw movements could be analyzed. Before the measurement, subjects wore a MKG sensor array on their heads vertical to the Frankfort plane, and a magnet was attached at the center of the labial surface of the anterior mandibular teeth. An experimental palatal plate was then positioned on the maxilla. The subject was instructed to chew a gummy jelly (No. 3, 5.5 g, Ezaki Glico Co., Osaka, Japan), a test food previously developed for the measurement of masticatory efficiency (Okiyama et al., 2003) on the habitually masticatory side. Recordings were made while the subject was chewing the gummy jelly and stopped when the subject performed the initial voluntary swallow. The subject was told to chew only on the habitual masticatory side. Recording was performed 8 times for each subject, so that intra-subject variability could be taken into account.
Analysis of the Data
Statistical Analysis To evaluate patterns of tongue pressure with the jaw movement, we sought the differences in the order of tongue pressure production, and duration and magnitude of tongue pressure among the sensors, in the early and late stages of masticatory sequences. In those analyses, uniformity of variance was determined by a Bartlett test. When uniform variance was found, significant differences were determined by a repeated-measures analysis of variance (ANOVA) and comparison testing, as performed with Scheffé’s post hoc test. We performed Student’s t test to determine the differences in duration and magnitude of tongue pressure between the early stage and the late stage of mastication. Statistical significance was set at P < 0.05.
Coordination of Tongue and Jaw Movements Tongue pressure occurred during occlusion, reached a peak near the start of the opening phase of mastication, then ceased during the opening phase at each sensor in all subjects (Fig. 2B  ). Time of pressure onset at Ch.1 was earlier than at any other sensor, and that at Ch.3 was later than at Chs.6 and 7 (non-habitual masticatory side). Time of pressure offset at Ch.5 (habitual masticatory side) was later than Chs.2 and 3 on the median line, and later than Chs.6 and 7 (non-habitual masticatory side) (Fig. 3 ).
Duration and Maximum Magnitude of Tongue Pressure Durations of tongue pressure at Ch.1 (early stage, 0.12 sec; late stage, 0.25 sec) and Ch.5 (early stage, 0.13 sec; late stage, 0.25 sec) were significantly longer than that at other sensors (P < 0.05). In the early stage, duration of tongue pressure at Chs.2 and 3 was significantly shorter than that at other sensors (P < 0.05). In every subject, duration of tongue pressure in the late stage was 2.0 to 7.8 times longer than that in the early stage (P < 0.05). These findings suggest that the tongue tended to be in contact with the hard palate for a longer time (i) at the anterior part than at the middle or posterior part on the median line, (ii) on the habitual masticatory side rather than on the non-habitual masticatory side in the postero-circumferential part, and (iii) in the late stage of mastication rather than in the early stage (Fig. 4A ).
The magnitude of tongue pressure at Ch.1 (early stage, 5.2 kPa; late stage, 14.7 kPa) and Ch.5 (early stage, 5.5 kPa; late stage, 17.1 kPa) was significantly larger than that at other sensors (P < 0.05). The magnitude of tongue pressure at Ch.2 (early stage, 1.2 kPa; late stage, 7.2 kPa) and Ch.3 (early stage, 0.8 kPa; late stage, 5.6 kPa) was significantly smaller than that at other sensors (P < 0.05). In every subject, the magnitude of tongue pressure in the late stage was 2.8 to 6.9 times larger than that in the early stage (P < 0.05), suggesting that the tongue tended to come into contact with the hard palate more strongly (i) at the anterior part than at the middle or posterior part on the median line, (ii) on the habitual masticatory side rather than on the non-habitual masticatory side in the postero-circumferential part, and (iii) in the late stage rather than in the early stage of mastication (Fig. 4B ).
We investigated the biomechanical coordination of tongue and jaw movements through parameters representing the state of tongue pressure production and the control of tongue activity in the early and late stages of the masticatory sequence. Simultaneous recording of tongue pressure and jaw movement by MKG, with our newly designed pressure sensors installed in a palatal plate, enabled us to analyze quantitatively the coordination between tongue and jaw movements during mastication. We found that onset of tongue pressure against the hard palate occurred during the occlusal phase of mastication, reached a peak close to the start of the opening phase, and was offset within the opening phase. These results suggest that the tongue ’scooped up’ comminuted pieces of gummy jelly spilled from the occlusal table of the posterior teeth and gathered them around the teeth by moving upward in the occlusal phase, then located them on the occlusal table again by close contact with the hard palate during the opening phase. Furthermore, the tongue usually came into contact with, and always parted from, the hard palate on the non-masticatory side earlier than on the masticatory side. Long duration and large magnitude of tongue pressure in the postero-circumferential part of the hard palate on the masticatory side may suggest that the tongue performed rotational as well as forward and backward movement (Hiiemae and Palmer, 1999) to manage the comminuted food pieces.
Videofluorographic images of mastication and swallowing of gummy jelly have shown that Processing (comminuting of gummy jelly) and Stage II Transport (transport of the bolus produced from comminuted pieces of gummy jelly from the oral cavity to the mesopharynx) in the Process Model (Palmer, 1997) occur during the late stage of mastication (Ono et al., 2003). The tongue starts to move in the occlusal phase and then forward and upward for gathering comminuted food pieces on the dorsum during the start of Stage II Transport (Hiiemae and Palmer, 1999). The order of tongue pressure production observed in the present study is considered to be the result of this forward and upward movement of the tongue, which coordinates jaw movement. It was shown that contact between the tongue and the hard palate in each chewing cycle was short in duration and low in magnitude during the early stage of mastication. Statistically significant changes in duration and magnitude of tongue pressure found during the late stage of mastication suggest that Stage II Transport required long and close contact between the tongue and the hard palate. It should be noted that irregular jaw movements in the first cycle of the early stage and the last cycle of the late stage of mastication (Fig. 2A Our findings here clearly show that the tongue plays a series of important roles in the total sequence of mastication of gummy jelly, by moving in coordination with jaw movement as well as controlling pressure against the hard palate. We believe that these findings will be useful for developing diagnostic criteria of tongue movement in the intake of solid food. They might also be applied to objective evaluation in the rehabilitation of patients with oropharyngeal dysphagia.
The Ministry of Education, Science and Culture of Japan, Grant 14370631, and a Grant-in-Aid for the 21st Century COE program from the Japan Society for the Promotion of Science supported this study. This work is based on a thesis submitted to Osaka University Graduate School of Dentistry, in partial fulfillment of the requirements for the PhD degree. Received for publication August 3, 2004. Revision received September 11, 2005. Accepted for publication October 3, 2005.
Journal of Dental Research, Vol. 85, No. 2,
187-191 (2006)
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ABSTRACT
INTRODUCTION
Duration of tongue pressure in the late stage is longer than that in the early stage.