Masseter muscle thickness as a predictive variable in treatment outcome of the twin-block appliance and masseteric thickness changes during treatment.

OBJECTIVES To estimate the association of initial masseter muscle thickness with treatment outcomes using functional appliances and the effect of treatment on masseter muscle thickness. MATERIAL AND METHODS Twenty-two children, aged 8-12, with skeletal and dental class II relationships and increased overjet were treated with twin-block appliances for 9-17 months, until a class I molar relationship and decreased overjet was achieved. Dental casts, lateral cephalograms, and ultrasonographic measurements of the masseter muscle were performed before and after treatment. Twenty-two children, aged 8-12, without immediate need for orthodontic treatment, served as controls. They were observed for 11-17 months, and ultrasonographic masseter muscle measurements were taken before and after the observation period. RESULTS Masseter muscles in treated children were thinner at the end of treatment, while untreated controls showed an increase in thickness. Treated children with thinner pre-treatment muscles showed greater mandibular incisor proclination, distalisation of maxillary molars, and posterior displacement of the cephalometric A point during treatment. CONCLUSION Treatment of a dental class II relationship with functional appliances leads to mild atrophy of the masticatory muscles, possibly because of their decreased functional activity. The initial condition of the muscles may be associated with mandibular incisor proclination, and the position of maxillary first molars and A point.