Abstract
Many reports have been made by researchers in the past on the study of the mandibular movement, which is a matter of primary concern in prosthodontics. It has been made clear by these reports that the movements of mandibular condyle can be divided into two kinetic elements of translation and rotation. However, almost no report has been made so far concerning in what temporal process these two elements occur at the time of physiologic condyle movement, especially when phonation is made.
Therefore, the author particulary took up the phonation of the [a:] sound, which is least influenced by occlusion and the vertical and horizontal overlap of the anterior. The author then paid attention to the movement of mandibular condyle when the [a:] sound was pronounced. And in order to serve for the diagnosis of dysfunction of the stomatognathic system, the subjects used for the present experiment were limited to those who had no abnormality in their stomatognathic system. By photographic analysis using a multiple stroboscope apparatus, and considering the time element, the author recorded and analyzed the translation and rotation of mandibular condyle, and reached the following conclusion.
1) All the seven subjects tended to show very little difference in the amount of translation and the degree of rotation of their mandibular condyle, as well as in the time required until it came to the position ready to pronounce the [a:] sound.
2) The amount of translation of mandibular condyle from the position of maximal intercuspal occlusion to that when the [a:] sound was ready to be pronounced was between 2.31 mm and 6.15 mm ; the degree of rotation was between 5.03°and 7.73°; and the time required was between 0.115 and 0.174 seconds.
3) The amount of translation from the position of the maximal intercuspal occlusion to that when the mandibular movement stopped with the pronunciation of the [a:]sound was between 3.02 mm and 13.37 mm; the degree of rotation was between 6.42° and 12.81°; and the time required was between 0.194 and 0.326 seconds.
4) When compared before and after the [a:] sound was pronounced, the rotation of mandibular condyle tended to occur earlier than translation.
5) When comparison was made by dividing the entire movement of mandibular condyle into three periods, the speed of change, both in translation and rotation, tended to be quickest in the second period; as for rotation, the degree of rotation tended to be smaller in the third period compared to that in the first.
6) From the result of the present research, it is presu med that the mandibular condyle first moves, featuring rotation, to a comparatively fixed position, and later moves, featuring translation, to the position most appropriate for the pronunciation of the [a:] sound.
