The selection of an appropriate fixation system is critical for maintaining postoperative stability after sagittal split ramus osteotomy (SSRO), especially in cases involving large mandibular advancements and counterclockwise rotation, where mechanical stresses may compromise treatment outcomes. This in vitro study evaluated the biomechanical stability of five fixation systems following sagittal split ramus osteotomy (SSRO) under two mandibular advancement conditions. Fifty polyurethane hemimandibles were allocated into two experimental groups: Group 1, submitted to 10-mm linear advancement, and Group 2, submitted to 10-mm advancement associated with 20° counterclockwise rotation. Each group was further divided into five subgroups according to the fixation design employed: (A) conventional straight plate, (B) angled plate, (C) sagittal plate, (D) 10-hole miniplate, and (E) two 4-hole miniplates. Biomechanical performance was assessed by compression testing using a universal testing machine. Complementary analyses were performed using strain gauges and digital image correlation. Group 1 in fixation B, C and D demonstrated higher biomechanical resistance than Group 2, however without significance difference. Among the evaluated configurations, subgroup E exhibited the highest resistance values in both experimental conditions. Statistically significant differences were observed among fixation systems within each group (p < 0.05). The fixation systems using two miniplates demonstrated superior biomechanical stability, particularly in mandibular advancements associated with counterclockwise rotational movements. These findings suggest that this fixation strategy may contribute to enhanced postoperative stability in orthognathic surgery involving large mandibular advancements and complex rotational movements.