A Three Dimensional Finite Element Study on the Normal Mandible of the Rabbit
E.P. Freitas1; S.C. Rahal1; L.C. Vulcano1; P.Y. Noritomi2; J.V.L. Silva2
The finite element analysis (FEA) is a computationally intensive engineering technique that estimates how objects of complex design resist loads (Wang et al. 2006). FEA is one of the most promising tools in the study of functional morphology of the craniofacial skeleton in human and from other species (Richmond et al. 2005) which one applied on a finite 3D elements mesh. CT imaging is a useful method to gather three-dimensional physiological data such as geometry and density of bone in vivo (Keyak et al. 1990). Several studies of stress distribution on normal mandibles in human have been performed. These studies have provided detailed knowledge regarding biomechanical properties of normal mandibles (Maurer et al. 2002). The results of von Mises stress distribution found in a human model of normal mandible showed that the model was reliable (Vollmer et al. 2000). Despite advances in commercially available meshing programs for creating subject-specific models from image data, the process is expensive yet, in terms of time, and usually requires high level of user intervention (Wilcox 2007). The software InVesalius can reconstruct 3D virtual models from CT imaging modality based on the DICOM (Digital Imaging and Communication in Medicine) file format (CTI 2009). In this paper, three-dimensional (3D) geometrical and finite element analysis (FEA) of the mandible of a rabbit were built. This study aimed to evaluate the distribution of stresses on mandible of the rabbit applying a masticatory force of 10 Newtons (N).
Materials and Methods
A computerized tomography (CT) examination was performed on the head of the Norfolk rabbit, female, 2 months of age. In order to perform CT studies the animal was submitted to general anesthesia. Sequential transverse images of the head were acquired on a helical scanner (Shimadzu SCT-7800CT) with the rabbit placed in a dorsal recumbency. The scanning parameters were 120kVp, 170 mA, 1.0 mm slice thickness, 1.0mm interval, pitch of 2.0, and 1 s/rotation. A virtual 3D model of the head was generated based on CT image data, in DICOM, by software InVesalius. The 3D model was saved in STL format. The STL model was imported into the software Rhinoceros® in order to create the BioCAD representation of the mandible. The BioCAD was used as a reference to create a more complex 3D model, demanded by the finite element analysis. The complete CAD models were exported to the NEiNastran®, finite elements analysis program, using IGES file format. The finite elements analysis was performed by using a tetrahedral elements mesh. Isotropic, linear and homogeneous material properties were assumed for the bone. The study of stress distribution on mandible and opposite action of the masseter muscle had to be made by NEiNastran® . The masticatory forces of the 10 N were applied on mandible in order to simulate a typical bite of rabbit.
The FEA has show von Mises stress concentration for mandible of the rabbit. There was a major von Mises stress concentration on the cranial region of the mandible, with compressive stress propagation in the ventral side of the mandibular canal, with a tractive stress in the dorsal side of the same mandibular canal, in the middle third of the mandibular body. Both, the compressive and tractive stress fields follow through the mandibular body, drawing a stress triangle with the vertical ramus of the mandible on retro-molar region.
The CT images can be converted to 3D virtual objects with the aid of specially developed medical imaging reconstruction software programs. These programs are commercially available, but can also be distributed free-of-charge as the software InVesalius used in the present experiment (CTI 2009). In this study the results showed that CT image data is necessary to create the virtual 3D model and to perform the finite element analysis. These findings agreed with the literature that suggests that image data can be used in manual segmentation of the bone to generate idealized surfaces for creating finite element models (Keyak et al. 1990). Furthermore, a rapidly produced image-based computational model of bone could provide great physiological detail for presurgical planning (Pfeiler et al. 2007). Previous studies have reported that the distribution of von Mises stresses on the human normal mandible it occurred on the posterior coronoid process, mandibular angle and distal alveolar ridge in the molar area (Tie et al. 2006). In another hand, in this study with rabbit, the von Mises stress concentration occurred on the cranial region of the mandible. Both, the compressive stress and tractive stress distribution occurred through the mandibular body of the rabbit, drawing a stress triangle with the vertical ramus of the mandible on retro-molar region. Analyzing von Mises stress concentration for normal mandible, it was important because the stress distribution has a great effect on biomechanical properties of reconstructed mandibles. If stress distribution is too concentrated, the bone will not regenerate but will be absorbed (Song & Xue 1999).
The finite element analysis in the normal mandible of the rabbit presents the maximum von Mises stress on the cranial region of the mandible.
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