Ten children with normal occlusion and another ten children with mandibular.prognathosis were-selected. They range in age from 6 to 12 years. The electromyographic study was limited .: . to the anterior inferior part and posterior superior part of masseter muscle and the posterior.;:: . and anterior part of the temporal muscle. The electromyogram was taken on a 4 channel Grass ,. ~ Polygraph Model VII electroencephalograph modified for electromyograph. Four pairs of
surface. electrodes were used. exclusively, with the ear lobe as reference point. The contractionpatterns of the masticatory muscles was observed in rest position, centric occlusion, mandibular,
protrusive and retrusive positions. In rest position electrical silences or little electrical potentials were found in four muscles in children with normal occlusion. In centric occlusion all four, muscles showed almost the same electrical activity and displayed synchoronous activity. When the mandible comes forward into protrusive occlusion then the activity of the temporal muscles dropped considerably while the activity of the masseter muscles was greater than that of the temporal muscle. Occlusion in forced retruded position showed high activity of temporal muscle but the greatest changes in activity were seen in the masseter muscle. The activity of this muscle dropped. In mandibular prognathous children it was impossible to obtain a balanced rest position recording and electrical silence of all four muscles. The uneven contraction and electrical potential of four muscles showed in centric occlusion. An irregular action potential of all four muscles during mandibular protrusive retrusive movement was found in mandibular prognathous children. Unfortunately in mandibular prognathous patients, although clinically normal in appearence, normal electromyography has not been achieved yet. The primary purpose of the research reported in this paper is to analyse the muscles contraction patterns of Korean normal occlusion children, and mandibular proganthous children, so that these comparative electromyographic studies may help in diagnosis and treatment planning for mandibular prognathosis.
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