In mandibular reconstructive surgery, straight surgical plates are predominantly used for osteotomy. In this process, a straight plate is bent to fit to the shape of the patient's defect. This potentially compromises the stability of the plate because of changes in mechanical properties resulting from plastic deformation; however, the effect of plastic deformation on stability has yet been investigated. In this study, we assess the stability of three types of commonly used bent plates for two types of mandible defect cases, using three-dimensional finite element analysis (FEA). We compare the stability of plastically deformed plates with customized plates having the same shape, but not the residual stresses from plastic deformation. The results indicate that while the bearing strength of the bent plate is sufficient, there is much higher stress in the plate due to the residual stresses from the bending process that may significantly decrease the safety of the reconstruction surgery.