RESUMO
To assess the gonial angle changes using BSSRO tecnique for mandibular set back. Lateral cephalograms of 30 patients, 11 male and19 female ranging from 18 to 38 years old who under went BSSRO to treat mandibular prognathism were traced. They were compared before operation, during intermaxillary fixation [IMF] and three months after operation by T - test. The fixation technique was wire osteosynthesis [superior border] - in 24 cases and position screw rigid fixation [2 screws] in 6 cases. Mandibular gonial angle decreased in all patients with a mean of 7 [SD=4.2]. This reduction was 10.3 [SD=5.3] in rigid fixation group and 5.2 [SD=3.5] in nonrigid group. In severe set backs [more than 10 mm] the mean of reduction of gonial angle was 5.4° [SD=3.8]. In set back osteotomies using BSSRO technique there is a decrease in gonial angle but no significant relation exists between the amount of set back and gonial angle changes