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Head Movements during Chewing: Relation to Size and Texture of Bolus
1 Department of Odontology, Clinical Oral Physiology, Umeå University, S-901 87 Umeå, Sweden; and Correspondence: * corresponding author, per-olof.eriksson{at}odont.umu.se
Coordinated mandibular and head-neck movements during jaw opening-closing activities suggest a close functional linkage between the jaw and the neck regions. The present study investigated whether size and texture of bolus can influence head-neck behavior during chewing. Using an optoelectronic 3-D recording technique, we analyzed concomitant mandibular and head-neck movements in 12 healthy adults chewing small (3 g) and large (9 g) boluses of chewing gum and Optosil®. The main finding was a head extension during chewing, the amount of which was related mainly to bolus size. Furthermore, each chewing cycle was accompanied not only by mandibular movements, but also by head extension-flexion movements. Larger head movement amplitudes were correlated with larger size and, to some extent, also with harder texture of the bolus. The results suggest that head-neck behavior during chewing is modulated in response to changes in jaw sensory-motor input.
Key Words: bolus • chewing • head • neck • jaw
Jaw opening during chewing varies with differences in the size and weight of the bolus. Thus, larger size and weight will result in larger jaw opening (Thexton et al., 1980; Lucas et al., 1986; van der Bilt et al., 1991; Daet et al., 1995). Larger jaw opening has also been reported in response to harder texture of the bolus (Horio and Kawamura, 1989; Peyron et al., 1997, 2002). However, other studies have reported no relation (Plesh et al., 1986; Bishop et al., 1990) or even decreased jaw opening with harder texture (Pröschel and Hofmann, 1988; Karlsson and Carlsson, 1989). We have previously reported that ‘functional jaw movements’ are the result of coordinated activation of jaw as well as neck muscles, leading to simultaneous movements in the temporomandibular, atlanto-occipital, and cervical spine joints (Eriksson et al., 2000). This suggests that jaw behavior such as eating (e.g., mouth opening, biting, chewing, swallowing), yawning, and speech relies on linked motor control of the jaw and neck motor systems. Thus, it can be assumed that varying the size and texture of the bolus might alter the control of movements of not only the mandible, but also of the head-neck during chewing. However, this question has as yet not been addressed. Previous studies on the effects of size and texture of the bolus during chewing have taken only mandibular movements into account. The general aim of this study was to gain further knowledge about the functional coupling between the jaw and the head-neck motor systems in man during natural jaw behavior. Specifically, using a wireless 3-D movement recording technique, we evaluated the influence of size and texture of the bolus on head-neck movements during chewing (Häggman-Henrikson et al., 1998).
Subjects and General Procedure Six males and six females (ages, 22–37 yrs; median age, 25 yrs) participated in the study. All subjects were free from pain and dysfunction in the craniomandibular and neck regions and were unaware of the underlying aim of the investigation. They had given their informed consent according to the World Medical Association’s Declaration of Helsinki. The investigation was approved by the Ethics committee, Umeå University. The subjects were seated comfortably in an upright position with back support up to the mid-scapular level, but without a headrest. Movements of the mandible and the head were simultaneously monitored in 3 dimensions (3-D), by means of a wireless optoelectronic recording system with a sampling rate of 50 Hz (MacReflex®, Sävedalen, Sweden) (Josefsson et al., 1996). Spherical low-weight retro-reflective markers (5 mm in diameter) were attached to the mandible (tip of the chin) and to the head (at the bridge of the nose) by trimmed double-sided adhesive tape applied at the midline of the face. The reliability of skin-attached markers in recordings of mandibular and head movements during chewing has been evaluated in a previous study (Häggman-Henrikson et al., 1998). In the present set-up, a recording volume of 45*55*50 cm was used, which provided a spatial resolution of 0.02 mm. Verification of marker identity and marker trace continuity, as well as computation of the markers’ 3-D position co-ordinates, was assessed with dedicated software. Off-line analyses were performed with standard software. Further descriptions of the experimental set-up and procedures for off-line conditioning have been presented previously (Häggman-Henrikson et al., 1998; Eriksson et al., 2000; Zafar et al., 2000).
Test Procedure
Definitions The amount of head extension was defined as the position of the head in space at the start of a chewing cycle in relation to the head position at the start of the recording. Head position was calculated at the start of chewing cycles nos. 1, 2, 5, 10, 15, and 20 for each recording. In addition to the head position at the start of each cycle, the additional head extension-flexion movement amplitudes were calculated for each jaw-opening/-closing cycle. These mandibular and head movement amplitude estimates were calculated from the first 10 consecutive cycles from each test.
Analysis
where s and p indicate start and peak positions. This enabled the mandibular and head movement amplitudes to be calculated as the shortest 3-D distance between the different positions. After graphed display of the movement traces for visual inspection and identification of the defined key events, the parameters under study were quantified from the recorded signals.
Statistics
During chewing, mandibular movements were accompanied by head movements. Both mandibular and head movements varied with size and texture of the bolus (Fig. 1  ). There was a marked individual variability in the patterns and amplitudes for both mandibular and head movements. Since no significant differences were found between the two repeated tests for any parameter, the data from the two tests were pooled, and mean values were calculated for each parameter and subject.
Head Position With reference to the baseline at the start of the recording, a small preparatory shift in head position was generally seen before the start of chewing (Fig. 2 ). This shift was related to size, but not texture, of the bolus (Table). A general finding in all sessions was also a change in head position, i.e., head extension at the start of chewing. Larger bolus size was correlated with larger head extension throughout the chewing sequence (i.e., cycles 1–20). Harder bolus texture was correlated with larger head extension during the first part of the chewing sequence (cycles 1–5) (Table). For all boluses, the largest increase in head extension was seen during the first part of the chewing sequence (Fig. 2). There were no significant differences between males and females, although the mean values were lower in the female group.
Mandibular and Head Movements In addition to the preparatory head extension before the start of chewing and the head extension during the complete chewing sequence, each chewing cycle was also accompanied by head extension-flexion movements. Larger amplitudes of both mandibular and head movements were found for the larger (9 g) boluses compared with the 3-g boluses. For the small boluses, harder texture was related to larger amplitude of both mandibular and head movements (Fig. 3 ).
The present study investigated whether size and texture of bolus can influence head-neck behavior during chewing. The main findings were that bolus intake resulted in a change in head posture, and that larger size and harder texture of the bolus were correlated with larger head extension and larger amplitudes of both mandibular and head movements. Our present results suggest that head-neck behavior is modulated by jaw sensory-motor input in a task-dependent way during chewing. The small shift in head position seen before the start of the first chewing cycle, ‘preparatory head extension’, implies a feed-forward, anticipatory, activation of neck motoneurons to reposition the gape in a favorable position for expected jaw activities. This could correspond to the forward anticipatory activation that has been demonstrated for jaw elevators during chewing (Ottenhoff et al., 1992, 1993). Taken together, the present and previous findings suggest that head-neck behavior during chewing relies on both ‘feed-forward’ anticipatory and ‘feed-back’ reactional activation of neck motoneurons to modulate head-neck position in a favorable way during chewing behavior. Size and texture of the bolus affected the head position to different extents. Compared with the soft bolus (chewing gum), we found a more extended head position for harder texture for the first chewing cycles. That texture of bolus affects the first part of a chewing sequence is also suggested by findings that assessment of characteristics of the bolus—like hardness, crispness, and firmness—is made mostly during the first bite (Peyron et al., 1997). The finding that the larger size of the bolus resulted in a more extended head position compared with that of the small bolus throughout the chewing sequence supports and extends our previous observations of an adjustment of head extension during rhythmic jaw activities (Eriksson et al., 2000). An extended head position will probably gain biomechanical advantages, by positioning the gape for optimal direction and force production. Such an interpretation is corroborated by the finding, in man, of increased maximum bite force following extension of the head (Hellsing and Hagberg, 1990), and the finding that head extension can increase the stability of mandibular closing movements (Yamada et al., 1999). The present results showed a significant increase in head extension in the first part of the chewing sequence. For the second part of the chewing sequence, an increase was found for the large gum and small silicone boluses. However, it seems reasonable to assume that, for a longer chewing sequence, as food softens and breaks down, head extension will reach a steady level or possibly decrease. Thus, it has been shown that, during a masticatory sequence, the mandibular movement amplitudes will decline gradually, due to softening of the food (Gibbs et al., 1981) and the breakdown of the foodstuff (Jemt et al., 1979; Thexton et al., 1980; Wickwire et al., 1981). In conclusion, the present finding—that head position and head-neck movements were related to size and texture of the bolus—suggests that jaw sensory-motor input can influence head-neck motor behavior during chewing. The results reinforce the hypothesis that the jaw and neck motor systems are tightly linked in jaw function (Eriksson et al., 1998, 2000). Our present results are in line with previous suggestions (Häggman-Henrikson et al., 2002) that impaired function in the atlanto-occipital and cervical spine joints and neck muscles can hamper natural jaw activities.
The skillful technical assistance of Mr. Jan Öberg and the programming assistance of Mr. Mattias Backén are gratefully acknowledged. This research is supported by the Faculty of Medicine, Umeå University, the Swedish Dental Society, and The Public Dental Health Service, Västerbotten, Sweden. Received for publication December 19, 2003. Revision received July 19, 2004. Accepted for publication August 23, 2004.
Journal of Dental Research, Vol. 83, No. 11,
864-868 (2004)
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