In vitro biomechanical evaluation of the effect of an additional L-shaped plate on straight or box plate fixation in sagittal split ramus osteotomy using a bioabsorbable plate system

Purpose: Relapse caused by clockwise (opening) rotation of the distal segment (dentate segment) in the sagittal plane is one of the postoperative complications sometimes seen after sagittal split ramus osteotomy. The force involved in this movement is primarily exerted by the masticatory and suprahyoid muscles. For postoperative stability, we have used two plates on each side: a straight plate placed over the vertical osteotomy line at the buccal surface of the first molar and an L-shaped plate placed at the distal or lateral part of the last molar over the osteotomy line, just crossing over the upper-lateral edge of the buccal shelf, to prevent the tail of the distal segment from moving upward. Although postoperative stability was clinically fine, experimental evaluations have not previously been performed. To clarify the effects of the L-shaped plate, we performed an experimental study using a bioabsorbable plate system. Methods: A custom-fabricated jig was made to simulate rotational movement of the segments. Two segments made from polyoxymethylene resin were fixed with a four-hole straight or four-hole box poly-L-lactate ioabsorbable plate. An L-shaped plated was then added for rigidity, and mechanical testing was performed. Result: The yield load exerted by the four-hole straight plate alone was 152.4±11.0 N. This increased significantly to 273.8±43.7 N with addition of an L-shaped plate (p