Abstract
Purpose : The purpose of this study was to investigate how the head and trunk behave during basic mandibular movement, and to clarify whether there is relationship between the range of motion of mouth opening and the range of motion of head or trunk movement.
Methods : We used a six-degree-freedom system (TRIMET II) for recording mandibular and head movement, and a motion-capture system (ProReflex) for recording the body movement. The subjects were six men (age 24~29 years, mean age 25.8 years) without stomatognathic problems. The subjects were instructed to open their mouths as widely as they possible without discomfort and to repeat the open-close movements at rate of 3Hz for 20 seconds. The reference points were upper and lower incisor points and the sternal bone point. We analyzed the vertical components of the mandibular and head movements, and the forward-backward horizontal component of the trunk movements.
Results : Head moved backward during mouth-opening and moved forward during mouth-closing, as the subjects were standing. The body moved forward as mouth opened and backward while the mouth closed. The range of head movement was 0.6~10.7mm, and the ratio of head movement to width of mouth opening was 5.2~40.5%. The range of trunk movement was 0.2~1.8mm, and the ratio of trunk movement to width of mouth opening was the 1.3~6.8%. According to the Spearman's rank correlation coefficient, the width of mouth opening significantly correlated with the range of head and trunk movement.
Conclusion : During open-close mandibule movements, co-operative head and trunk movements were observed. Conclusion for instance, when the mouth opened, the head tilted backward, resulting in a shift of balance with the weight of the head over the cervical segments. To compensate for the backward head motion, the body tended to move forward, and vice versa. Also, a wider opening of the mouth correlated to a larger the range of head and trunk motion was observed. These findings suggest that head and trunk movements facilitate the smoothness of jaw movements.
