抄録
The position of the hyoid bone as related to the cranial base and to the mandible was investigated. The materials used in this study were twenty Japanese adults of both sexes with normal occlusion, and twenty orthodontic cases with upper protrusion (Angle Class II division 1), and twenty lower protrusion cases (Angle Class III). The Hellman’s dental age analysis of these clinical cases was between III-C and IV-A. The position of the body of the hyoid as related to the cranial base showed a significant difference among· these three groups. Compared with the normal occlusion group, the body of the hyoid was located slightly backward in the upper protrusion group and slightly forward in the lower protrusion. However, in relation to the mandible, position of the body of the hyoid was constant in these three groups. In order to confirm the stability of the relative position of the hyoid to the mandible, following two cases were investigated; an ankylosis of the temporomandibular joint and a case of lower protrusion with severe distortion of the cervical vertebra caused by tuberculosis of spines. In the former case, the mandible was located far backward in relation to the cranium, while that of the latter was located far forward. However, the relative position of the hyoid to the mandible seemed to be stable in these two cases. The hyoid as related to the cranial base varied when the mandible took anteroposterior malpositions to the cranium. Considering the fact that the hyoid position as related to the mandible was stable, the muscles which connect them seemed to play an important role in the displacement of the mandible.
