Study On Modeling Of Bionic Scaffold Oriented To Bone Tissue Engineering

The large defect of bone is one of the key problems of clinical medicine treatment, due to disease, accident, and so on. The technology of bone tissue engineering is developing quickly in recent years. A new treatment pattern of bone defect repairing is that the defective bone is repaired with tissue-engineered bone as transplant, and the tissue-engineered bone is generated by co-combining with cultured osteogenic cells and bionic scaffold in vitro. This is a hotspot in the research field of bone tissue engineering too. The bionic scaffold provides suitable growth space and enough mechanical support for regenerating bony tissue, mediates the signal and the interaction between osteocytes, and induces the regeneration of osteocytes in the tissue-engineered bone regeneration. Therefore, the fabrication of bionic scaffold is the key technology of bone tissue engineering and most important research.The fabrication of bionic scaffold includes the fabrication of anatomical shape and the generation of inner porous structure. The traditional method focuses much more on the generation of inner porous structure, and anatomical shape is fabricated by die or handcraft. The precision of scaffold is fairly low. Moreover, the performance of inner structure of scaffold can not be evaluated before being fabricated, and some parameters, such as porosity and connectivity, are difficult to be controlled. With the continuous development of applying the technology of Rapid Prototyping (RP) to the field of bone tissue engineering, a better method to fabricate bionic scaffold using RP is set up. Not only precise anatomical shape and complicated inner porous structure of scaffold can be synchronously generated by using RP, but also the performance of inner structure of scaffold can be evaluated on the basis of three-dimensional model information before being fabricated. Therefore, the modeling is a key technology of fabricating bionic scaffold by using RP.According to the research background and application requirement above, bionic scaffold is chosen as research object in this paper, and a series of research oriented to bone tissue engineering are developed, including modeling of anatomical shape and inner structure of scaffold, performance evaluation of inner structure, and so on. The main research contents and contributions are described as following.Firstly, based on the theory of Delaunay triangulation, three-dimensional reconstruction method of surface model of defective bone is set up. The pixel cube model is constructed based on data of CT images. The data of space point cloud is acquired by comparing the gray threshold of image of bone tissue. The algorithm of k-nearest neighbors and 2D-Delaunay neighbors are applied to simplify Delaunay triangulation of space point set and accelerate the surface reconstruction. It is better to adopt the algorithm of local deformable spherical map to solve the problem of topological structure reconstruction of holes, and then the precise surface of defective bone is generated. Moreover, the reconstructed surface provides a more precise digital model for modeling of anatomical shape and inner porous structure of bionic scaffold. Secondly, on the basis of analyzing clinicopathological characteristics and type of bone defect, the modeling method of shape model of bionic scaffold is set up. The boundary edge of hole is automatically searched to generate 3D polygons by adopting the dihedral angle criteria algorithm. Delaunay triangulation of 3D polygon is finished based on the minimum area principle, and the mesh is refined based on the shortest edge principle. The weighted umbrella-operator is applied to control the curvature transformation of the patching mesh to smooth it, and the fairing surface patch is merged into the surface around it. Then, the shape model of bionic scaffold is generated by Boolean operation between every two key models. Thus, the complexity of modeling is greatly reduced.Thirdly, the modeling method of inner porous structure of bionic scaffold is set up by combining the structure of multi-constrained knapsack problem model with hybrid genetic algorithm. Based on the structure of multi-constrained knapsack problem model, and using ellipsoid as the basic unit, the inverse model of porous structure is generated, and hybrid genetic algorithm is used to solve this model. In the design of genetic operators, the hybrid selection operator of combining proportional selection with optimal Elitist Model is adopted to improve the operating efficiency and the convergence. The uniform crossover operator is adopted to avoid the diversity deterioration of population, and then to improve the bio-diversity of porous structure. The uniform mutation operator is adopted to enhance the local searching ability of hybrid genetic algorithm, and then to promote the bio-diversity evolution of population. The perturbation operator is adopted to locally adjust the solution space, and then to enhance the overall searching ability of hybrid genetic algorithm. Then, the model of bionic scaffold containing porous structure is generated by Boolean operation between the shape model and the inverse model.Fourthly, according to factors influencing the performance of inner structure of bionic scaffold, the evaluation index system of porous structure performance of scaffold is constructed, including porosity, connectivity, uniformity, twist degree and specific surface area. The calculation method of every evaluation index is set up based on the inverse model of porous structure of scaffold. Every evaluation index influencing bioactivation, biomechanical strength and degradation rate is analyzed, and the weight value of every evaluation index is calculated based on the theory of AHP. The integrated evaluation model of inner structure performance of bionic scaffold is constructed based on the theory of gray relation grade, and the gray relation value of every evaluation index is calculated to synthetically evaluate inner structure performance of bionic scaffold.Lastly, according to the theory and method above, the modeling prototype system of bionic scaffold is designed by adopting object-oriented technology and visualization technology. This system can perform three functions, including three-dimensional reconstruction of surface model of defective bone, prosthesis modeling, inner structure modeling of bionic scaffold and evaluation of its performance. The scientificity, rationality and validity of the described modeling theory and method are validated.