A method was designed to determine the mandibular positions during speech in which three phenomena, namely, -(1) the path of the mandibular jnovements, (2) the spectrogram of the voice, and (3) the voice sound-were simultaneously recorded on the same frame of 16mm motion picture films.
The method was applied to predict acceptable occlusal vertical dimention (O. V. D.) for a patient with mandibular prognathisrn before surgical correction.
The O. V. D. was gradually increased by setting resin blocks on the occlusal surface of the mandible.
At various O. V. D., the mandibular positions during pronunciation of/s, sh, m/were observed. As a result, it was found that the mandibular positions of each consonants were almost constant regardless of the difference in O. V. D. to some extent.
Within this range, the O. V. D. was increased and the mandibule was retruded to the esthetically ideal position by surgery.
Just before the operation, provisional bridges with guide pins and holes were cemented to keep predetermined intermaxillary relation during surgical retrusion of the mandible.
Two and six months after the operation, the mandibular movements during speech showed normal pattern and the precise intercuspal position was established.
