In vitro comparison of parameters affecting the fixation strength of sagittal split osteotomies

Abstract

Purpose: The goal of this study was to determine how different parameters affect the bending strength of human cadaver mandibles that have undergone a sagittal split osteotomy. Materials and Methods: The effects of screw material (titanium [Ti] vs polylactic acid/polyglycolic acid [PLA/PGA]), screw configuration (linear vs inverted L-shape), screw diameter (2.0 mm vs 2.7 mm), material into which screws were inserted (human mandible, bovine rib, synthetic polymer), and loading rate (1.0 mm/min vs 10.0 mm/min) were quantified. Also, biomechanical principles were used to model shear stress and displacement. Variable lever arms, screw material, screw diameter, screw configuration, distance between screws, and bone properties were all evaluated in this model. Results: Accounting for variable mandible geometries and differentiating between deflections (and shear stresses) due to bending and due to torsion, in vitro mechanical testing revealed that there was a statistically significant difference in total shear stress at 3 mm of deflection depending on screw material (Ti > PLA PGA ), screw diameter, and material into which screws are inserted (mandibles > ribs = synthetic polymer). There was no significant difference in total shear stress depending on screw configuration or strain rate. Conclusion: Total shear stress and deflections are important and more viable parameters than load to assess parameters of clinical importance in osteotomy or fracture fixation.