This study aims to evaluate the clinical efficacy and positioning accuracy of a custom-made proximal segment positioning appliance designed for use in sagittal split ramus osteotomy (SSRO). The appliance is produced using STL-based digital planning and 3D printing technologies. In each patient, the appliance is applied to one side of the mandible while the contralateral side is positioned using the conventional manual method. This within-subject design enables direct comparison by eliminating inter-individual anatomical variability. The primary outcome is the accuracy of segment positioning, evaluated by 3D superimposition and deviation analysis of pre- and postoperative STL models. Secondary outcomes include surgical time, ease of use as rated by the surgeon, and postoperative temporomandibular joint symptoms. The study will enroll 30 adult patients undergoing SSRO or double-jaw surgery due to dentofacial deformities. The results are expected to provide high-level clinical evidence for the reliability of patient-specific appliances in orthognathic surgery.