Twelve cases of anterior cross-bite in the deciduous dentition which has been treated with chin cap appliance, were observed until the eruptions of anterior permanent teeth _??__??__??_. These were then divided into two groups;
Group I revealed normal occlusion (consist of 7 cases),
Group II revealed anterior cross-bite (consist of 5 cases).
This investigation, researching into the differences in the function of masticatory muscles between Group I and Group II, was made in order to serve as the prognosis after treatment of reversed occlusion.
Electromyographs (EMG) were recorded bilaterally on the anterior temporal muscle (TA), the posterior temporal muscle (TP) and the superficial portion of the masseter (M), duringfree chewing of gum and maximum biting. Maximum amplitudes were measured on bursts of 20 strokes in chewing and of 5 strokes in maximum biting. Thereupon, TP/TA and M/TrAa tio and TA %, TP%, M% were calculated from the maximum amplitudes. These EMG data of Group I and Group II were compared with those of the normal occlusion group. The differences of the EMG data between Group I and II were then evaluated.
The results obtained are summarized as follows:
1. The aspects of masticatory muscle activity in Group I were similar to those of the normal occlusion group as compared with Group II.
2. The prognosis after treatment in anterior cross-bile with high activity of the masseter muscle is considered to be favorable.
3. There developed a tendency that TP/TA ratio or TP% in Group I after treatment increased compared with before treatment and decreased on the periodical check up (3 to B months after treatment). On the other hand, in Group II there were not any certain changes of the TP/TA ratio or the TP% before and after treatment, and from the period after treatment to the periodical check up, these increased.
These results serve as an aid to the prognosis after treatment of anterior cross-bite in deciduous dentition.