1. OBJECTIVE: By establishing a three-dimensional finite element model of the nasal maxillary complex, we explored the method of establishing a three-dimensional finite element model to provide a basis for studying the characteristics of the biomechanics of the nasal maxillary complex.
2. METHODS: The spiral CT thin-layer scan was obtained in DICOM format, imported into the 3D reconstruction software Mimics, and the finite element software Ansys was applied to establish the 3D finite element model of the nasal maxillary complex.
3. RESULTS: The 3D finite element model of the nasal maxillary complex established by this method has good similarity with the solid, and the operational error is small and the modeling resolution is high.
4. CONCLUSION: The application of DICOM data computer-aided modeling method is a fast, accurate and efficient finite element modeling method, and the 3D finite element model of the nasal maxillary complex built by this experiment has high accuracy and good reproducibility of operation, which can provide a basis for clinical otolaryngology to study the mechanism of the biomechanics of the nasal maxillary complex fracture.
The nasomaxillary complex is located in the middle of the face and protrudes from the face, with a complex geometric structure. It is a bony complex composed of the maxilla, zygoma, nasal bone, lacrimal bone, upper dentition, and part of the cranial bone that is closely related to the maxilla anatomically, so the chance of fracture due to trauma is relatively high. The influence of the structure itself and the structural state on the biomechanical properties is the first thing to be understood. In recent years, with the continuous progress of computer computing speed and calculation methods, finite element method has gradually become the most important analysis method in the study of mechanics, especially in the study of oral biomechanics, and the rapid and accurate establishment of three-dimensional finite element model is the primary problem of biomechanical finite element research, and also the basis of three-dimensional finite element analysis. At present, a variety of modeling methods have been used at home and abroad to establish the 3D finite element model of the mandible, while less finite element models have been established for the nasal maxillary complex due to its complex structure. In this paper, we applied CT scanning technology, used DICOM data to import Mimics software, and finally applied Ansys finite element software to establish the 3D finite element model of the nasal maxillary complex. While exploring how to establish a new method and model of 3D finite element model of nasal maxillary complex, it provides a reliable basis for clinical otolaryngology and craniomaxillofacial surgery to further study the mechanism of biomechanics of nasal maxillary complex fracture.
1. Materials and methods
(1) Modeling materials A healthy adult male volunteer with no history of head and facial trauma or surgery was selected.
(2) Experimental equipment
①CT machine: The CT machine used for the study was a 16-row spiral CT (LIGHTSPEED 16) made by GE, USA.
②Working platform: PC with Windows XP sp3 operating system, CPU Pentium D 2.8GHz, 4G memory
③ Software environment: Medical 3D reconstruction software Mimics10.01 (Materialise’s Interactive Medical Image Control System) and finite element analysis software Ansys10.0 (Analysis System).
(3) CT scan
Sixteen-row spiral CT from GE General was used to scan the subject’s skull in the supine position with 120 KV, 100 mAs, 1.3 mm layer thickness, 1000 window width, 200 window positions, and more than 200 CT tomographic images were obtained. 80 of the CT tomographic images were selected for the reconstruction of the nasal maxillary complex, and the 2D tomographic images were processed by the CT workstation into DICOM. The 2D tomographic images were processed into DICOM format data files by CT workstation and burned to disk.
(4) Image processing and 3D reconstruction
The DICOM format data files obtained from the CT scans were imported into the medical 3D reconstruction software Mimics 10.01 software to determine the range of images to be reconstructed in 3D, set the Thresholding range to show only the skeletal tissues, and use Mask to edit the tissues to be imaged on the CT images to reconstruct the 3D images of the nasal maxillary complex. Because the obtained 3D model unit shape is irregular and the number of units is too much, which will affect the accuracy of mechanical analysis and have a great impact on the analysis speed, so the surface triangle quantity and quality of the 3D model are optimized by using the remesh module. Then Mimics10. 01 outputs the surface (Area) file of the 3D model to Ansys10.0, and finally finite element modeling is completed in Ansys software.
(5) Finite element working method
The elastic object under study is discretized into a finite number of cells. Select the unit displacement function, establish the unit stiffness matrix and the overall stiffness matrix, introduce the boundary conditions, and solve the equations. All the nodal displacement components are obtained, and the stresses in each unit are derived from the nodal displacements. The whole solving process and related data processing are done automatically by computer. In the study, the cortical bone and cancellous bone of the maxillary complex were assumed to be isotropic homogeneous continuous elastic material.
2. Results
The CT scan tomographic images were imported into Mimics medical 3D reconstruction software in DICOM format, and the 3D surface model was reconstructed from the 2D tomographic images, and the Remesh module optimized the surface of the 3D model, and the 3D reconstruction of the solid model of the normal human nasal maxillary complex was completed in Ansys software. A 3D finite element model of the nasal maxillary complex with 236120 cells and 117894 nodes was generated. The important anatomical features were accurately reproduced in the whole modeling process, and the connections with the skull base, zygomatic bone and other surrounding bone tissue structures were completely preserved, the model image was of high resolution with 3D solid information, and the data format of the model could be smoothly imported into various finite element analysis software.
3.Discussion
The finite element method (FEM) is a method of mechanical analysis that divides a continuous elastomer into a finite number of units, replaces the original elastomer with its union, and studies the properties of each unit one by one to obtain the whole elastomer. It was first successfully applied by Turner in 1956 in the aerospace industry. Since then, with the progress of computer technology, FEA has gradually developed into a widely used method in engineering. Later, Friedenberg applied it to the medical field. Finite element method is one of the important tools in biomechanics research, it can model complex geometry objects, find out the overall and local stress and displacement values and their distribution laws, and can change the mechanical parameters such as loading and boundary conditions according to the need, and can easily compare and analyze the changes of stress magnitude and distribution while keeping the original model geometry unchanged. It is gradually being applied to the field of orthopaedics, and bone stress analysis is one of the main uses of FEM.
FEM is one of the main uses of bone stress analysis. While the stress distribution cannot be described comprehensively and precisely by traditional experimental methods, FEM can well represent the overall trend of stress distribution in bone tissue, and can perform a variety of mechanical analyses without the need to establish an in vitro solid model, and its analysis method is efficient, accurate, and highly reliable.
The current modeling methods that can be used for the finite element of nasal maxillary complex are: (1) Grinding and cutting method The method has been eliminated because of the need to cut and destroy the model, and in the case of thin sections, it is also difficult to obtain a consistent section thickness, which is prone to errors in the selection of materials, image processing, edge extraction and other aspects. (2) Three-dimensional measurement method Three-dimensional measurement data acquisition cost is high, data processing time is long, can only get the surface data can not reflect the inherent material properties of the tissue, and is not commonly used. (3) CT image processing method CT image processing method whose main process is: (a) CT scan to obtain the original data; (b) the CT film through scanning, camera and other methods into the computer to obtain two-dimensional images; (c) in the image analysis software to form contour line bitmap, to obtain image boundary data; (d) the data obtained into the three-dimensional finite element analysis software processing, and finally obtain the finite element model. This method requires manual map format and manual accurate alignment, and a lot of information is easily lost in the process of data transfer through film scanning, and the inaccuracy of alignment also directly affects the accuracy of the established model.
In this study, we applied DICOM data modeling method to simplify the procedure of CT modeling, avoid the distortion or loss of data caused by repeated operations, and realize the automated modeling. Mimics is a set of inverse software between medical and mechanical fields, which can display, segment and reconstruct CT and MRI scans in 3D, and convert them into data formats (such as STL format) that can be processed by CAD/CAM, Ansys and other finite element analysis software. In this study, we used Mimics to directly read the Dicom data to directly build a 3D model with a 3D surface model and a non-3D solid model, and then converted to a format recognizable by Ansys 3D finite element software, and completed finite element meshing in Ansys to build a 3D finite element solid model. In this modeling process, the 3D images of the nasal maxillary complex are directly derived from the thin-layer CT scan data, and there is no form of image conversion in the preprocessing, which greatly reduces the workload and removes the errors caused by human factors. The finite element solid model of the nasal maxillary complex constructed by this method (1) has good similarity with the CT 3D reconstructed medical biomodel. (2) The model has powerful assembly function and can be re-modeled based on it. (3) The model can be observed and studied from any angle in the three-dimensional space, and the distribution of its internal stress and displacement can be visualized through the stress distribution map after loading. (4) The model establishes a complete morphology of the nasomaxillary complex, including the posterior part of the maxilla and the cranial bone connected to the nasomaxillary complex, thus making the calculation of stress distribution more accurate and providing a model basis for further study of the biomechanics of nasomaxillary complex fractures in clinical otolaryngology.
Due to the complex geometry of the nasomaxillary complex, it is not only time-consuming and labor-intensive to achieve full conformity between the model and the anatomical parts of the entity, but also difficult to model, and sometimes, for the purpose of finite element analysis, some simplifications are made during the pre-processing of the finite element model of the nasomaxillary complex (e.g., the material properties are set to continuous, homogeneous, isotropic linear elastic material, which is less deformed by the forces). The inaccuracy of some information may have an impact on the mechanical similarity of the model.