Fundamental Study On Applications Of Key CAD Technologies To Occlusal Surface Adjustment Of Dental Restoration

The applications of CAD/CAM technologies to dental restorations'design and fabrication evoked the third technological revolution of prosthodontics. It is changing the traditional method of false tooth fabrication, and is the only way to realize the false tooth fabrication with high efficiency and precision. Adjustment of occlusal surface is a crucial problem and a technical difficult in the whole CAD system of dental restorations. At present, the domestic study of dental restorations'CAD/CAM is deficient. Furthermore, although many practical systems have been developed abroad, there are still some issues about the adjustment of occlusal surface not solved well so far. In this thesis, the key CAD technologies for adjusting the occlusal surface are studied deeply and systemically. The main creative achievements are as follows:(1) The data structure for reconstructing the topology of dental restorations'triangular meshes is designed, and the algorithm flow for fast topology reconstruction is presented. When searching the vertexes of the triangular meshes model, a media file is introduced to record non-repeated vertexes and the topology of triangles and vertexes. It increases the searching speed of the direct method. Furthermore, AVL trees and Hash table are applied to the process of vertex searching, both of which effectively wiped off the massive repeated vertexes with high speed as well. The comparison of the three vertex searching methods demonstrates that the Hash table method is most applicable to the topology reconstruction of dental restorations, which have massive irregular vertexes.(2) A method able to acquire the opposite occlusal restriction of prepared tooth is proposed. It makes use of the virtual orientation of the lower and upper jaws model on the position of centric occlusion, during which the simple articulator is used as a medium. Firstly, an improved method for feature identification of the cylinder is used to compute the rotation axis equation of the simple articulator. Then, by constructing OBB trees for the lower and upper jaws model, updating the bounding box trees during the rotation of the lower jaw around the axis of the articulator, and traversing these trees, real-time collision detection between lower and upper jaws at sample positions is realized. As a result, the occlusal relationship between the lower and upper jaws on the position of centric occlusion is built. It provides accurate static opposite occlusal restriction for the occlusal surface adjustment of the prepared tooth's standard crown.(3) The concepts of two distance maps defined along the occlusal surface's direction and the normal direction respectively, along with the calculation methods of the distance maps are proposed for the lower and upper jaws model. The calculation of the distance map along the occlusal surface's direction is based on the collision detection algorithm in the image space, in which the distances are obtained from the depth buffer and stencil buffer by rending the lower and upper jaws model in camera along the occlusal surface's direction. The calculation of the distance map along the normal direction utilizes the selection mode of OpenGL, in which the distances from the vertexes on the model to the opposite model are achieved from the selection buffer by rending the opposite occlusal model along the normal direction of vertexes on the tooth model. Because the two calculation methods for the distance map fully utilize the good computation performance of the graphics hardware, they have high efficiency and accuracy. And they provide distance changes between occlusal surfaces for conducting the adjustment of the standard crown's occlusal surface.(4) A method for adjusting the conflict areas is proposed. According to the distance map of the lower and upper jaws model, the method realizes the adjustment by interpolating the conflict distance between occlusal surfaces using a volume spline function. Firstly, control points on or nearby the standard crown model are selected, and the volume spline interpolation function is calculated by variational methods. Then the conflict distances of the vertexes on the standard crown model are computed using the interpolation function. Finally, the conflict areas on the occlusal surface of the standard crown are adjusted according to the results. The adjusted occlusal surface can accord with the opposite tooth. The shape inherent to the occlusal surface is well maintained, and the requirement of occlusal surface's smoothness is satisfied.(5) A surface fitting algorithm suitable to the dental restorations is proposed, which fits surface by using patches of smooth B-spline surfaces. For the triangular meshes model of the restoration, its Voronoi partition, delaunay domain partition, quadrilateral domain combination and resampling, and the surface fitting using patches of smooth B-spline surfaces are discussed in succession detailedly. The fitting results of triangular meshes models of the first molar, canine and central incisor in left maxillary fully demonstrate the effectiveness of the proposed algorithm. They show that the algorithm has a strong adaptability of topology and a capability of producing high quality spline surface. Therefore it satisfies the clinical applications. At last, a base of porcelain fused to metal crown is designed and processed based on the reconstructed surface, which further verifies the practicability of the algorithm.