Effects of directions of maxillary protraction forces on biomechanical changes in craniofacial complex

The purpose of this study was to investigate the effect of directions of extraoral maxillary protraction forces on biomechanical changes in the craniofacial complex, using the three-dimensional finite element method (FEM). A three-dimensional FEM model was developed on the basis of a young, human dry skull. The model consisted of 2918 nodes and 1776 solid elements. An anteriorly directed 1.0 Kg force was applied to the buccal surface of the maxillary first molar in directions varying from -90 to 90 degrees to the occlusal plane. The displacement pattern of the entire craniofacial complex was evaluated. Further, the stress distributions were determined in three transverse planes associated with parallel, and 30 degrees upward and downward forces. As the force direction was more upward, repositioning of the craniofacial complex became larger in both the horizontal and vertical directions. Displacements were most translatory in loading with the forces applied in the directions ranging from -45 to -30 degrees to the occlusal plane. High stress levels were observed in the nasomaxillary complex and its surrounding structures. However, the patterns of stress distribution within the complex were different for three loading conditions. A downward protraction force produced the most uniform stress distribution. It is shown that the force direction plays an important role in determining the repositioning and the stress distributions in the craniofacial complex.