| With the development of computer technology,information technology and graphics technology,digital restorative dentistry and prosthodontics which is efficient,accurate and intelligent,is gradually replacing traditional dental prosthesis mode,and has led to a revolution in dental repair field.Currently,foreign dental CAD/CAM systems monopolize the domestic market and domestic digital restorative dentistry and prosthodontics are still interaction-intensive,inefficient and inaccurate.Under these circumstances,starting from the combination of computer graphics and prosthetics,two key techniques of tooth segmentation and standard crown positioning are studied deeply in this dissertation.Main contents and contributions include:1.Many published tooth segmentation approaches directly separate teeth from dental mesh without dealing with the region fusion,which lead to inaccurate results and incomplete segmented teeth with side shape lacked.Meanwhile,existing tooth shape modeling schemes are interaction-intensive and inefficient.Therefore,a novel tooth segmentation approach based on segmentation of feature line is proposed.Feature region is selected according to mean curvature,and morphologic algorithm is used to extract dentition line.The fusion region is automatically recognized by the feature line segmenting and branch points matching algorithm as well as morphologic dilation.The restoration result is automatically obtained by repairing holes with matched branch points.After the gingival margin lines betwwen adjacent teeth are extracted,the teeth are segmented by all the gingival margin lines.Experimental results demonstrate that the poposed approach is accurate and the segmented teeth have complete side feature.What's more,the approach avoids user interactions in the stage of tooth shape modeling,thus improving the whole efficience by 60%~90%.2.In order to facilitate the design operations in dental CAD system,such as scaling,positioning and matching,a simple and reliable digital denture measurement method is proposed.Firstly,three planes including mesial-distal symmetry plane,buccolingual/labiolingual symmetry plane and the horizontal plane of cusp are determined as datums for tooth positioning constraints.And from a practical point of view,mesiodistal diameter and buccolingual diameter are determined as two shape parameters of digital tooth.Then,the identification method of the high profiles of tooth is improved by setting the dynamic threshold,which can ensure the reliability of the identification results.Finally,The shape and position parameters are calculated with the five identified high profiles.3.During the design process of dental prostheses such as standard crown,crown bridge,and inlay,standard crowns imported from the system database are used to simulate the position and orientation of the missing tooth and serve as the outer surface(or maxillofacial).A simple and efficient solution is proposed to solve the problem of complex operation,low efficiency and poor effect in existing standard crown positioning technique.After importing the reference objects,the abutment coordinate system is adjusted firstly,and then the shape and position parameters of standard crown and reference objects are calculated.After that,the size and position of the standard crown are adjusted by performing homogeneous coordinate transformation.Further,according to the constraint information of the adjacent teeth and the abutment,standard crown positioning is classified into four cases: based on the left and right adjacent teeth,based on a single abutment,based on an abutment and an adjacent teeth,and based on an inlay preparation.These four cases cover almost all scenes of restoration design,and all of them are realized in detail in the thesis.The experimental results show that the size of the positioned standard crown is very coordinated with the abutment,the mesiodistal surface is naturally fit with the adjacent tooth,and the position and orientation are reasonable. |
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