Frameworks for Complex Dental Prostheses

This study presents a digital solution to design and produce implant-supported biometal frameworks for complex dental prostheses (Fig. 7.2) (Ortrop et al., 2000). The intended framework is the metal base structure of the prosthesis. It is supported by oral implants placed in the jawbone of the edentulous patient. Such framework is fixed to the jaw by screws retaining it on the implants. The framework supports the artificial teeth that are attached on top of the framework. The 'upper' shape of the framework (i.e. teeth side) depends on the type of tooth aesthetics. When standard polymer teeth are used, a bar shape is sufficient.

Fig. 7.2 (a) Scheme of implant-supported prosthesis; (b) Framework fitting on upper jaw model


Fig. 7.2 (a) Scheme of implant-supported prosthesis; (b) Framework fitting on upper jaw model

Oral implants

Framework u N

Oral implants

When a veneering porcelain layer is applied to create the individual tooth surfaces, the tooth shapes should be included in the design of the framework.

The framework can be made from ceramics, mainly alumina- or zirconia-based, from precious metal alloys, mainly gold- and platinum-based or from non-precious metal alloys, mainly cobalt-chromium or titanium alloys (Strie-tzel, 2004). This study will focus on the non-precious metal alloys. Cobalt-chromium and titanium alloys combine good mechanical and biocompatible properties and can be processed successfully by SLM. According to the state-of-the-art, laser-based RM processes are yet not able to process ceramics to accurate parts with good mechanical properties.

A framework is patient specific and has to meet strict requirements of accuracy to minimize the risk of mechanical or biological failures of the prosthetic system. To avoid high stresses in the jawbone causing the oral implants to loose and to diminish the risk for colonization of bacteria resulting in infection and eventually bone loss, severe fit criteria below 40 mm are necessary at the framework-implant junctions (Jemt et al., 1998; Riedy et al., 1997).

Digital procedures have been developed for designing and manufacturing of dental frameworks. They replace the traditional methods based on a manual design by 'clay' modeling and a production by lost wax casting. These conventional processes are still widely used, although they are time consuming and inefficient. The lost wax method is a lengthy and labor-intensive process and comprises many manual steps: creating, embedding and burning out the wax pattern, metal casting and post-processing. Moreover, its accuracy is generally lower than for digital procedures.

Figure 7.3 compares the traditional with the digital method for fabricating a dental framework. After installing the oral implants into the jawbone of the patient, a plaster work model with implant replicas is made representing the position of the implants. Upon this work model a tooth set-up is shaped from which the patient validates the aesthetics. This tooth set-up looks the same as the final prosthesis but the internal framework is still absent.

The digital procedure consists of three main steps: the digital geometry capture of implant positions and tooth set-up, the digital design of the framework and the computer-driven production of the framework by NC milling or by SLM. Fixing the artificial teeth (porcelain or polymer) on top of the framework finishes the prosthesis and the final prosthesis can be installed in the mouth of the patient.

In recent years many CAD/CAM systems based on NC milling have been developed for different dental applications. Dedicated dental milling units are

Fig. 7.3 Traditional and digital method to manufacture a dental framework

Fig. 7.3 Traditional and digital method to manufacture a dental framework

commercially available for producing ceramic or metal crowns, small bridges and tooth-supported frames, e.g. Cercon (Degudent), CEREC (Sirona), President DCS (DCS), Lava (3M ESPE), etc. By these milling stations the dental laboratories can fabricate the dental restorations in house. The other possibility is that the dental laboratories handle only the CAD issues, while central production centers deal with the CAM issues and NC milling, e.g. implant-supported frameworks by Procera (Nobel Biocare).

The key advantages of NC milling are the high achievable accuracy and the possibility to process ceramics (mostly in green stage). The subtractive fabrication can create complete frameworks effectively, but at the expense of wasted material. Complex tool path calculations and spatial restrictions can limit the production of complex frameworks by NC milling.

The rest of this chapter will focus on SLM for the manufacturing of frameworks. SLM allows an efficient and customized production of the complex framework for different metal alloys. Remaining unprocessed powder can be reused. Two commercial systems use already SLM as central production process for crowns, small bridges and small tooth-supported frames from cobalt-chromium and precious alloys, Medifacturing (BEGO) and infiniDent (Sirona). K.U.Leuven developed a procedure to manufacture implant-supported frameworks by SLM from titanium and cobalt-chromium alloys (Kruth, Vandenbroucke et al., 2005).

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