Effect of cortical bone quality and quantity on two orthodontic distalization modalities : a finite element analysis study

Abstract

Introduction: Orthodontic mini-implants have been used to correct Class II malocclusions by distalizing the maxillary dentition via direct anchorage (direct pull from mini-implant to the teeth) and indirect anchorage (teeth pulled against other teeth anchored by the mini-implants). Aims: 1. Develop a scheme for Finite Element Analysis (FEA) that would reflect true human individual variation. 2. Evaluate the effect of cortical bone stiffness and thickness on the rate of tooth movement in the two distalization modalities. Our hypothesis was that cortical bone quality and quantity influence the rate of tooth movement in both conditions. Methods: A 3D model of the maxilla containing the different components (teeth, PDL, trabecular and cortical bones) was generated from a CT scan and material properties were assigned to each component. Cortical bone, the study variable, was divided into several masks, utilizing the data generated by Peterson et al. (2006), who measured stiffness and thickness of the maxilla in human cadavers. The data derived from 13 cadavers were incorporated into the 3D models to simulate individual variation of cortical bone at the different maxillary locations. Subsequently, a finite element analysis was used to simulate the direct and indirect distalization modalities. Outcome measures included stress distribution and displacement of the following permanent teeth: canine, first and second premolars, first and second molars. Statistical methods included t-tests and analyses of variance for group comparisons, and correlation tests for associations among variables. Results: In the direct anchorage modality, 55percent of the total Von mises stress distribution was located at the canine and first premolar, while 68percent of stress amounts were found at the molars in the indirect modality. Moreover, stress amounts decreased from the cervical to the apical parts of the PDL. High correlations were observed between PDL stress and crown displacement of the corresponding tooth in both modalities (0.75 r 0.99)