Digital Design

The second step of any digital solution to fabricate dental parts is the virtual design based on the digitized input geometry. Many dedicated CAD software programs are commercially available to design dental restorations like crowns, bridges and small frames (Rudolph et al., 2003). However, no commercial CAD system exists today for complete design of implant-supported frameworks for complex prostheses replacing multiple teeth. K.U.Leuven has developed a design method for these frameworks based on the captured implant positions and digitized tooth set-up.

This digital design replaces the conventional manual design, requiring a lot of experience and good skill of the dental technician who builds up the framework from wax material. Based on the many manual actions of the technician, several design rules were defined and were put in a certain sequence. This design strategy was implemented in a software module by translating the design rules to computer tools. Some user interaction is still required to design the framework since a fully automated CAD system seemed not possible for these complex parts.

An important issue during the design step is the manipulation of the complex dataflow. The different and large data files have to be filtered and combined to one geometrical model in a fast and convenient way. Because the input data files are mainly point clouds without mathematical surface representation and because the file format for SLM is the STL (Standard Triangulation Language) format, it is preferred to use STL-based CAD software instead of conventional CAD software. K.U.Leuven used the STL-based 3Maticsoftware from Materialise (Ref. Materialise website).

The design of the framework depends on the type of tooth aesthetics. For standard polymer teeth, a bar-shaped framework is sufficient, while for veneering porcelain teeth, the tooth shapes should be included in the design of the framework.

The framework design process for porcelain tooth aesthetics is illustrated in Fig. 7.5 (left). Firstly, the measured point cloud data of the tooth set-up is converted into a triangular mesh model (a), representing the desired shape of the final prosthesis (b). Each artificial tooth of the final prosthesis needs a support surface for porcelain veneering on top of the framework. The different teeth can be identified by means of a curvature analysis (c). Each separate tooth surface, obtained by cutting the tooth set-up model, is incomplete because the side surfaces are missing (d). These side surfaces are needed to compute the offset of the total tooth surfaces. Therefore, each incomplete tooth surface is matched with a full model of the corresponding standard tooth surface (e). A digital library of standard tooth surfaces is available. The support surfaces of the framework are then computed by a defined offset of the completed tooth surfaces (f). The measured position and inclination of the implants are transferred to the design environment. Conical fitting structures are designed according to these data (g) and connected to the support surfaces of the teeth (h). Detail features like screw holes and fillets are added which finishes the digital design of the framework (i).

A similar framework design process for polymer tooth aesthetics is developed. Figure 7.5 (right) shows the bar-shaped outcome for a framework with six

Fig. 7.5 (left) Framework design strategy for veneering porcelain teeth; (right) Framework bar-shaped design for standard polymer teeth (Source: K.U.Leuven)

implant-junctions. Small ball-features were added to the surface to increase the mechanical retention and adhesion between teeth and framework. Dr. Leu at the University of Missouri-Rolla has developed a similar design method for dental bars (Leu et al., 2006).

State-of-the-art CAD packages allow designing complex dental parts in an efficient way. Further developments may include simpler GUI (Graphical User Interface) and integration of virtual articulators, which would lead to further automation of dental restoration design (Strub et al., 2006).

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