English Abstract CHANGES IN MASTICATORY MUSCLES FOLLOWING ORTHOGNATHIC SURGERY AFTER MANDIBULAR PROGNATHISM CORRECTION ABSTRACT Aim: To evaluate the postoperative electromyographic changes of temporalis and masseter muscles activity, in patients with orthognathic surgery BSSO,for correction of mandibular prognathism. Material and Methods: Eight adult patients, their ages ranged from 22-45 years, five females and three males, suffering from mandibular prognathism. underwent BSSO, for performing mandibular setback. EMG examinations(peak to peak amplitude, for detection of the strength of contraction of both temporalis and masseter muscles) were done presurgical and twelve months postoperatively. Results: Significant increase in peak-to-peak amplitude values of both temporalis muscles was founded. Moreover, an increase in peak-to-peak amplitude values of both masseter muscles was found,.No significant differences were found in the mean frequency score (as one of the variables of the EMG measurement) of the masseter muscles before bilateral sagittal split osteotomy and that undertaken 12 months after bilateral sagittal split osteotomy (P = 0.06). An increase in the mean peak to peak amplitude score for the right and left temporalis muscle was found, with significant differences between the measurement found 12 months after bilateral sagittal split osteotomy, and that measured before bilateral sagittal split osteotomy fig (6) (P = 0.002,0.0001 respectively) Conclusion: No change in the strength of contraction of both temporalis and masseter muscle had detected after performing BSSO for correction of mandibular prognathism ,as detected by EMG examination.