[Mandibular movement and electromyogram investigation of normal occlusion and reversed occlusion before and after correction of over jet].

The purpose of this investigation was to understand how the mandibular movement and myofunction in reversed occlusion, approach to normal occlusion after the correction of over jet. The experiment was carried out in 30 normal and 26 reversed occlusion school children patients who were classified into 3 groups by morphological analysis. A K6 diagnostic system was used. It recorded muscle activity of maximum clenching, free way space, path of closure, and the maximum velocity of opening and closing mandibular. And recorded a condylar test at the same time. Reversed occlusion classified 3 groups: upper and lower incisor had improper inclinations (D group, 19%), construction bite possible but recognized back and forth discrepancy between maxilla and mandibular (FS group, 50%), discrepancy larger than FS group and construction bite not possible (S group, 31%). Toward upper forth group indicated 77% and toward upper back group indicated 23% in normal occlusion. As for reversed occlusion the former indicated 92% and the latter indicated 8%. Differences in connection with orthodontic appliance and period of treatment were not so large. Back and forth mandibular movement, as measured by a condylar test, was greater for reversed occlusion than for normal occlusion, but this value tended to decrease after orthodontic treatment. In reversed occlusion, maximum opening, free way space, maximum velocity of opening and closing and muscle activity of rest position didn't show significant difference (p less than 0.05) among each group before and after the correction of over jet. Temporalis and masseter activity of maximum clenching, except temporalis among the D group, approached the values for normal occlusion. In the S group, muscle activity of maximum clenching with a cotton-roll, was lower before treatment, but approached to data in normal occlusion. These results prove that improvement of over jet in reversed occlusion, by ordinary orthodontal diagnosis and treatment plan, quasi-normalizes mandibular movement and muscle activity in reversed occlusion.