Effects of Twin-block therapy on protrusive muscle functions

Protrusive mandibular function, including maximum protrusive force and fati gue time, was investigated in 66 children displaying Class II Division 1 ma locclusion. Thirty-two children were treated with the Clark Twin-block appl iance and the other 34 children served as untreated controls. The observati on period was 6 months. Cross-sectional data based on pretreatment records showed that maximum protrusive force ranged from 18.5 N to 160 N, with a me an of 80.3 +/- 30.7 N, Maximum protrusive force was significantly higher in males than in females (P < .001). The correlation between maximum protrusi ve force and chronologic age was low (r = 0.20) and did not reach significa nce. Maximum protrusive force in the group of children with disk displaceme nt was not significantly different from that of the group without disk disp lacement. Comparison of pretreatment and 6-month records in the untreated c ontrol group revealed a significant increase in maximum protrusive force (P < .01) as a result of normal growth, while the measured change in the Twin -block-treated children did not reach significance. Fatiguing the protrusiv e muscles did not alter mandibular position in the Twin-block group after 6 months of treatment. The present study does not support the lateral pteryg oid hypothesis, as there was no evidence of an increase in mandibular protr usive function after treatment with the Twin-block functional appliance.