The purposes of this study are efficiency evaluation of thick wafer in comparison with standard wafer on assessment of maxillary sagittal position during orthognathic surgery. This is a single arm Quasi-experimental trial study, which will be performed on 9 patients with skeletal discrepancy of the jaws, class 2 and 3, who would be referred to oral and maxillofacial surgery clinic of Shariati hospital of Tehran. The inclusion criteria is skeletal discrepancy of the jaws, class 2 or 3, without previous history of orthognathic surgery. The exclusion criteria is the patients who are unwilling to sign the written consent for. In each case, mandibular and maxillary casts would be provided and fixed to the arthiculator. Then model surgery of the patient will begin and maxillary cast separates from the arthiculator base and will be fixed to the arthiculator in 2 mm more anterior than the primary position (0 position). Then maxillary cast will be separated from arthiculator, and fixed to it in 1 mm more anterior than 0 position (a-1 position). Now, a standard wafer and a thick wafer with 5 mm thickness, would be provided. This procedure would be replicated in 2 mm and 3 mm more anterior than 0 position (a-2 position and a-3 position). During the operation, horizontal osteotomy line will be marked on maxillary bone with a bur and a vertical line would be drown on the bone at molar area. Now, lefort 1 osteotomy and maxillary down fracture will be performed. At this time maxilla would be repositioned with the standard wafer for a-1 position, as two segments of the maxilla interconnect at the osteotomy line. Then amount of displacement of vertical line on down fractured maxillary segment from vertical line on the base of the maxilla would be measured. Now, thick wafer for a-1 position will be placed and the same measurement will be performed . This procedure would be repeated with provided wafers in a-2 and a-3 positions, and finally data from this study would be analyzed.