Primary Research On Three-dimensional Finite Element Analysis Of A Cracked Maxillary First Molar

This study is to investigate the stress distribution and crack propagation of a cracked maxillary first molar in early stage when it is enduring the masticatory force based on constructing a three-dimensional finite element model of a cracked maxillary first molar according to fracture mechanics theory, and to simulate changing the cusp inclination to observed the changes before and after the therapy so as to make a preliminary assessment on the effect of adjustment of occlusion.Objectives1. To reconstruct a meticulous three-dimensional finite element model of maxillary first molar2. To reconstruct a three-dimensional finite element model of a cracked maxillary first molar and to investigate the stress distribution3. To make a fatigue crack growth analysis of the cracked maxillary first molar, and to make a preliminary assessment on the effect of adjustment of occlusion Methods1. An intact maxillary first molar was scanned with a micro-CT. After pictures of every cross-section were adjusted, surface contours of enamel and dentin were fitted following tooth segmentation based on different gray scale using an interactive medical image control system (Mimics, Materialise, Belgium). Enamel and dentin were then separately converted into STL files which were consisted of the nodal points and surfaces information. A STL handling software (Magics RP, Materialise, Belgium) was used in order to reestablish the congruence of the interfacial mesh between enamel and dentin. The remesh operations were then applied to the STL model to improve the quality of the triangles. Then, these high quality STL files were imported into finite element analysis software and a three-dimensional finite element model was reconstructed2. The crack part was meshed with particular method based on fracture mechanics theory by using singular element,and than the three-dimensional finite element model of a cracked maxillary first molar by the solid crack modeling method. The force perpendicular to the occlusal surface contact area was loaded and the stress distribution was analyzed.3. Three finite element models of cracked maxillary first molar with three different crack depths (0.5mm, 1.5mm, and 2.5mm) were reconstructed. Fatigue crack growth analysis of each model was made before and after the adjustment of occlusion by using a finite element post-processing software (MSC.Fatigue, MSC.software, USA) to observed the changes before and after the therapy.Results1. A meticulous three-dimensional finite element model of maxillary first molar which can reflect the physical details of the tooth specimens was reconstructed. The enamel and dentine of this model are close connected with each other and the meshes of the enamel and dentine in one place is coincident. This model can ensure that the finite element analysis in next step is reliable.Cloud of horizontal stress tensor is acquired in witch it can be seen that stress distribution near the crack tip is in accordance with singularity. Maximum of horizontal stress tensor is 103MPa.3. Maximum of horizontal stress tensor of each one in three finite element models with different crack depths was significantly smaller after adjustment of occlusion, and each one's safety fatigue life is also significantly increased. The model with 2.5mm deep crack, However, still cracks after undergoing 1.73×106 times'loads.Conclusions1. The three-dimensional finite element model of tooth tissue reconstructed by Micro-CT data is meticulous. STL files contain triangular surface meshes with high quality which can be used to generate three-dimensional meshes in finite element analysis software after the appropriate treatment2. The cracked tooth is more likely to fracture than the intact one as a result of the singularity near the crack tip. The results indicate that an unstable fracture may occur when a cracked tooth is enduring the masticatory force.3. The effect of adjustment of occlusion is obvious in improving the safety fatigue life of the cracked tooth with shallow crack. But it is insufficient to achieve the goal of therapy in treating a cracked tooth with deep crack although it can improve the tooth's safety fatigue life too.