A Bone Density Based Finite Element Study of the Efficacy of Maxillary Protraction Protocols With and Without Mini-Implants

Objectives: The purpose of this study was to develop a geometrically accurate and adaptable finite element head model for orthodontic and orthopedic simulations. This finite element head model will use a grayscale conversion and image-based meshing method to apply anatomically accurate bone densities of the human skull and craniofacial sutures. The model can then be used to simulate different clinical maxillary protraction protocols using a conventional facemask appliance and a mico-implant assisted rapid palatal expander (MARPE). Our goal was to improve upon the existing finite element model and to analyze how a bone and suture density based human skull model can improve the analysis of the skeletal effects of these two clinical treatment protocols. Methods: A 3-dimensional cranial mesh model with associated circummaxillary sutures was developed from CT images and Simpleware modeling software. Using a novel image-based meshing approach rather than the conventional computer-aided design (CAD) approach, the different regions of interests such as bone and sutures are separated into different masks. These masked are defined based on the image grayscale within the region of interest. Mesh generation was completed using a multi-part Extended Volumetric Marching Cubes (EVOMAC) approach where voxels are converted directly into finite elements. The material properties of the craniofacial bone and sutures were extracted from the underlying grayscale intensity Hounsfield (HU) of the CT image. Utilizing ANSYS simulation software, two different maxillary protraction protocols including conventional facemask therapy and micro-implant assisted rapid palatal expander (MARPE) were simulated. The stress distribution and displacement of these two different protraction protocols were analyzed. Superimpositions and video animations were generated to visualize the skeletal effects and to compare the results from the newly generated image-based finite element mesh model with the previous CAD-based mesh model generated from the Mimics software.Results: Both the newly designed image-based finite element mesh model and the previous CAD-based mesh model produced similar results in regards to direction of maxillary protraction: Conventional facemask results in counter-clockwise rotation of the maxillary complex. In contrast, facemask with the use of micro-implant assisted rapid palatal expander (MARPE) produces translation of the maxilla in a downward and forward direction. However, the newly designed image-based finite element mesh model produced less magnitudes of movement and less mesh deformation. Conclusion: The newly generated image-based finite element mesh model produced more clinically accurate results compared to the previous CAD-based mesh model. While the direction of the maxillary movement for the two different protocols is the same for both models, the geometrically accurate bone and suture density based model produced less mesh deformation and more accurately represents actual clinical outcomes. Further maxillary protraction protocols can be tested with this newly developed finite element mesh model