Orthognathic surgery is important for correcting craniofacial deformities and restoring occlusion. However, in edentulous patients with severe maxillary atrophy, traditional fixation methods, such as mini plates, may not provide sufficient stability and support for prosthetic rehabilitation. Advances in additive manufacturing have enabled the development of patient-specific subperiosteal implants, offering improved biomechanical performance and more favourable load distribution. This study utilized finite element analysis to compare the biomechanical performance of traditional mini plates and customized subperiosteal implants in maxillary orthognathic surgery. Computed tomography data were used to construct patient-specific models, and a Le Fort I osteotomy with a 9-mm maxillary advancement was simulated. Displacement and stress distribution were analysed under vertical and oblique loading conditions, focusing on critical regions such as osteotomy sites and screw interfaces. Subperiosteal implants exhibited superior biomechanical performance, with significantly lower displacement (0.58 mm) compared to mini plates (4.50 mm). Stress levels in mini plates frequently exceeded the yield strength of grade IV titanium, whereas subperiosteal implants remained within the elastic limit of Ti6Al4V. Additionally, screw stresses were reduced by 38-42% in the subperiosteal implant group, thereby reducing the risk of mechanical failure. Customized subperiosteal implants provided enhanced stability, reduced stress concentrations, and improved load distribution compared to traditional mini plates. These findings highlight their potential as a transformative solution in orthognathic surgery, particularly for edentulous patients with severe maxillary atrophy. Future research should focus on long-term clinical outcomes and cost-effectiveness to further establish their role in maxillofacial reconstruction.