Use of CAD/CAM technology to fabricate a removable partial denture framework
R. J. Williams, BA, PhD,aRichard Bibb, BSc, PhD,bDominic Eggbeer, BSc,cand John Collis, BDSd Centre for Dental Technology and the National Centre for Product Design and Development Research, University of Wales Institute, Cardiff, UK; Prince Charles Hospital, Merthyr Tydfil, UK
This article reports on the first patient-fitted chromium cobalt removable partial denture framework pro- duced by computer-assisted design, computer-assisted manufacture and rapid prototype technologies.
Once the dental cast was scanned, virtual surveying and design of the framework on a 3-dimensional com- puter model was accomplished. A rapid prototype machine was used for direct fabrication of the alloy framework. Traditional finishing techniques were applied, the framework was assessed by a clinician in a conventional manner, fitted to the patient, and judged to be satisfactory by both the patient and clin- ician. (J Prosthet Dent 2006;96:96-9.)
T
he introduction of computer-aided design and computer-assisted manufacturing (CAD/CAM) into crown, inlay, and fixed partial denture fabrication is well advanced.1,2Furthermore, CAD/CAM and rapid prototyping (RP) have been used in areas of maxillofa- cial technology.3,4Recent work has shown that, in prin- ciple, CAD/CAM/RP technologies can be successfully applied to the fabrication of removable partial denture (RPD) alloy frameworks.5,6Within the latter field, elec- tronic surveying and the production of sacrificial pat- terns through digital technologies have been achieved, but there have been no reports of direct RP alloy builds or of the application of these achievements to the treat- ment of patients. This article describes the further de- velopment of CAD/CAM/RP technologies in RPD framework fabrication whereby the casting stage is omit- ted. This article also describes how digital processes are verified by successfully fitting the RPD framework to a patient. In essence, an RPD alloy framework was pro- duced without the use of wax and without casting through the application of new technologies.CLINICAL REPORT
A 75-year-old woman, requiring a bilateral distal extension prosthesis, presented to the Royal Gwent Hospital, Newport, UK, for treatment. The design requirements necessitated a lingual plate, infrabulge clasps, appropriate support, and indirect retention.Fig- ure 1 shows the unrestored condition of the patient.
Dental casts were produced according to accepted
principles.7 CAD/CAM/RP procedures were then undertaken as follows.
A 3-dimensional scan of the patient’s partially den- tate cast was obtained using a structured white light dig- itizer (Comet 250; Steinbichler Optotechnik GmbH, Neubeuern, Germany). Multiple overlapping scans were used to collect point cloud data that were aligned using software (PolyWorks; InnovMetric Software Inc, Que- bec, Canada). Other software (Spider; Alias-Wavefront Inc, Ontario, Canada) was used to produce a polygon surface in the standard triangulation language (STL) file format.
The CAD package used in this study (FreeForm;
SensAble Technologies, Inc, Woburn, Mass) was se- lected for its capability in the design of complex, well- defined shapes that are required when designing custom prosthesis devices that must fit human anatomy. In addition to this capability, several precise functions within the software allowed electronic surveying to be completed.6
The CAD package has tools analogous to those used in physical sculpting and enables a manner of working that mimics physical surveying and wax pattern fabri- cation. The software uses a haptic interface (Phantom Desktop haptic interface; SensAble Technologies Inc) that incorporates positioning in 3-dimensional (3-D) space and allows rotation and translation in all axes,
Fig. 1. Unrestored condition of patient.
aSchool Director, School of Health and Social Sciences, University of Wales Institute Cardiff.
bHead of Medical Applications, National Centre for Product Design and Development Research, University of Wales Institute Cardiff.
cResearch Assistant in Medical Applications, National Centre for Product Design and Development Research, University of Wales Institute Cardiff.
dConsultant, Department of Oral and Maxillofacial Surgery, Prince Charles Hospital.
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transferring hand movements into the virtual environ- ment. It also allows the operator to feel the object being manipulated in the software. The combination of tools and force feedback sensations mimics working on a physical object and allows shapes to be designed and modified, sometimes by using the phantom arm in a freehand manner and at other times by precisely defin- ing sizes, shapes, and positions. The software also allows the import of scan data to create reference objects or
‘‘bucks’’ onto which fitting objects may be designed.
The RPD metal framework in this report used this CAD software, and the RPD framework design was built on a 3-D scan of the patient’s cast (Fig. 2). The computer- aided design of RPD frameworks using the CAD software has been previously described.5,6
In a previous study,6the application of RP methods was investigated for the production of sacrificial patterns that were used to cast RPD frameworks in cobalt- chrome alloy. In the present study, direct manufacture was attempted with the aim of eliminating the time and material-consuming investment-casting process.
The RP development of selective laser melting (SLM) technology showed potential application to dental
technology due to its ability to produce complex-shaped objects in hard wearing and corrosion resistant metals and alloys directly from CAD data.
To build the RPD framework on an SLM machine (SLM Realizer 2; MCP-HEK, SLM Tech Center, Borchen, Germany), adequate supports had to be cre- ated using software (Magics Version 9.5; Materialise NV, Leuven, Belgium) (Fig. 3). The purpose of the sup- ports is to provide a firm base for the part to be built onto while separating the part from the substrate plate.
In addition, the supports conduct heat away from the material as it melts and solidifies during the build pro- cess. Inadequate support results in incomplete parts or heat-induced curl, which results in build failure. Because the supports need to be removed with tools, the part was oriented such that the supports avoided the fitting surface of the RPD, and thus, the resultant framework would not be affected or damaged by the supports or their removal. The chromium cobalt (Sandvik Osprey Fig. 2. Screen captures of RPD being created using FreeForm
software.A,Use of tool in FreeForm.B,Completed virtual build.
Fig. 3. Screen capture of virtual RPD and supports prepared for building.
Fig. 4. Framework emerging from RP machine.
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Ltd, Neath, UK) RPD framework is shown inFigure 4 in the built stage emerging from the RP machine.
The framework was finished and polished using tradi- tional dental laboratory procedures,7then evaluated on the original physical cast (Fig. 5). A prosthodontic con- sultant assessed the quality of fit according to recom- mended practice,8and the framework was successfully fitted to the patient (Fig. 6).
DISCUSSION
Although the framework exhibited slight surface po- rosity in some small areas, this was not considered likely to prohibit normal function, and the RPD framework was positioned on the mandibular teeth and other den- ture bearing areas. The fit was subjectively judged to be excellent. The RPD framework produced using the SLM process in conjunction with cobalt-chrome alloy resulted in an RPD framework that is comparable in terms of accuracy, quality of fit, and function to those produced by existing methods commonly used in dentistry.
The current costs of using the CAD/CAM approach are high. For example, the cost of FreeForm software and a Phantom arm is approximately $30,000. A suit- able scanner (Roland DGA Corp, Irvine, Calif) costs ap- proximately $25,000, including the software to produce a surface. However, the majority of the cost is in this initial investment, and such costs are likely to decrease with further technical development. To produce a scanned model with a surface takes approximately 5 to 15 min- utes of operator time and 1 hour of machine time.
Time taken to virtually survey the 3-D cast is comparable to physical surveying or perhaps slightly faster. Building the framework on screen may take about 40 minutes, but it is likely that with practice, this time could be greatly reduced. In future, this process could be con- densed dramatically if software were developed to allow
components to be ‘‘dragged and dropped’’ from icons on the screen to position on the virtual cast. However, SLM machines are expensive, and it is likely that labora- tories would send data to a center to obtain alloy frame- works. SLM production costs are currently high. For example, one firm (MCP Tooling Technologies, Stone, UK) charges approximately $269 per framework for a batch of 6 different RPD frameworks, plus the cost of the alloy. However, methods and equipment in the field are developing rapidly, and it is expected, for example, that positioning supports will be automated, reducing operator time. Once the alloy build has been completed, finishing times and methods are the same as finishing a framework in the ‘‘as cast’’ condition.
The method described offers potential advantages.
For instance, in the future, it may be possible to virtually build a framework on a scanned cast using a ‘‘drag and drop’’ facility by means of RPD components displayed as icons on screen and dragged onto the electronically surveyed digital model. In such a situation, quality as- surance could be built into the software. This would reduce interoperator variability, and increase speed and economy over traditional handcrafting and investment casting techniques. However, there is need for refine- ment and development of the software and equipment for dental requirements and for further studies in this area.
SUMMARY
The successful application of CAD/CAM/RP tech- nologies for the fabrication of RPD alloy frameworks has been confirmed by this report. Electronic processes are in existence that can be applied to RPD fabrication.
The framework showed an accuracy of fit judged to be at least comparable to the results obtained by traditional casting methods.
CONTRIBUTING AUTHOR
Lyndon Camillier,Chief Dental Technologist, Royal Gwent Hospital.
Fig. 5. SLM Co/Cr RPD fitted to cast.
Fig. 6. Finished framework fitted to patient.
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doi:10.1016/j.prosdent.2006.05.029
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