Abstract
Electromyograms of masticatory muscles on tonic neck reflex on dorsal flexion in 18 children were recorded in order to evaluate the correlation between the neuromuscular activities of the masticatory muscles, and the dental and cephalometric analysis.
The following findings were obtaine:
(1) In 9 children out of 18, the activities elicited were different between the right and left side muscles and these children were refered to as Group II, and the ot her children who showed no differences were refered to as Group I. Temporal mu scles showed more sensitivity than the other two muscles, e. g. m. masseter, m. diga s tricus.
(2) There was a high correlation between the asymmetric muscle activities and occlusal factor, such as over bite, over jet, canine relationship, the degree of attritio n and the ratio of upper to lower jaw length. The occlusion of Group II in wh ich muscular activities were shown to be asymmetric, presented a tendency towa rd a deeper dite, and an increased difference of the length between lower jaw against upper jaw.
(3) Cephalometric analysis disclosed a tendency for antero-posterior differences, such as angle of convexity, A-B plane angle, A-B difference, and for the length of mandible, such as Gn-Cd, Cd-Go. The length of the mandibular body of Group II i s shorter than that of Group I.
(4) Based on the above findings with respect to the optimal occlusion ratio, the following was recognized. The over bite was 0.87±0.76mm, the over jet was 1.73±0.48mm, and the degree of the labial surface of the mandibular canine which covered that of the maxilla is less than half. The occlusion of the bilateral canines was symmetrical. Occlusal wear was within the normal range. The above factors were co nsidered to correspond to the optimal occlusion.
