Mandibular ramus anatomy as it relates to the medial osteotomy of the sagittal split ramus osteotomy.

The sagittal split ramus osteotomy is probably the most frequently used procedure for correction of mandibular skeletal dentofacial deformities. Despite numerous improvements in the technique in the 30 years since the procedure was introduced, a number of troublesome complications still occur. These include unfavorable fracture during surgery, paresthesia, and relapse. The purpose of this study was to determine where fusion of the buccal and lingual cortical plates occurs in the upper mandibular ramus, as it is thought that placement of the horizontal medial osteotomy above the point of fusion (without any intervening medullary bone) may lead to unfavorable fracture during splitting. Forty-nine human mandibles were sectioned vertically at three locations perpendicular to the surface of the ramus and the occlusal plane. Measurements were made to locate vertically the point of fusion of the buccal and lingual cortical plates relative to the lingula and to the depth of the sigmoid notch. The point of fusion occurred between 7.5 and 13.3 mm above the lingula. Only 2% of mandibles had fusion at or below the level of the lingula in the anterior ramus, whereas in the posterior ramus, 6.1% of mandibles were fused at that level. At a level halfway from the lingula to the sigmoid notch, 20% of mandibles were fused in the anterior ramus, whereas in the mid- to posterior ramus, the incidence was as high as 39%. The location of the medial horizontal osteotomy should be at or just above the tip of the lingula. A higher level of cut may be associated with an increased difficulty in splitting or incidence of unfavorable fracture.