Approximately 1.1 million individuals in US have severe maxillofacial skeletal deformity of developmental origin and need surgical correction. Current surgical planning systems can provide prediction on the anatomical restoration but not functional improvement. The present study intended to integrate a patient-specific finite element analysis into the surgical planning stage and provide predictions on the functional improvement based upon the biomechanical response of the stomatognathic system. The specific aim was to verify the hypothesis that the mandible advancement reduces the contact pressure along the TMJ. A comparison study were conducted to evaluate the alteration of the contact pressure on the TMJ using three-dimensional finite element models of a pediatric patient specific stomatognathic system that include mandible, articular disk and glenoid fossa. Analytical models and optimization technique were used to determine the masticatory muscle forces based upon the patient-specific CT scan. The simulation results revealed that the mandible advancement leads to a considerable reduction of the contact pressure on the TMJ.