Preliminary Study On CAD/CAM Of Removable Partial Denture Framework

ObjectiveThe goal of this study is to preliminarily present a complete procedure for computer - aided design and computer - aided manufacture (CAD/CAM ) of removable partial denture ( RPD) framework, from the acquisition of data to the production of framework. Advances in CAD/CAM are providing a new option for dentistry, creating an alternative technique for producing dental restorations.Compared with the conventional method, named the lost - wax method, the CAD/CAM technology can provide prosthesis with higher precision and strength, as well as excellent esthetics. In addition, the CAD/CAM process also delivers an inherent repeatability, which facilitates to eliminate the operator variation and improve the quality control in the dental laboratory.Since the CAD/CAM system was introduced to the dental profession in 1985, CAD/CAM for inlay/onlay, veneer, individual crown and fixed partial denture restoration have long been used in clinical practice of the prosthodontics and have greatly succeeded just in the past few years. So far, however, few literatures on CAD/CAM for RPD have been published in comparison to those of fixed partial denture. A similar situation is that many systems are launched on the market in recent years but no one was reported for the RPD.Prof. Maeda presented an idea of CAD/CAM for RPD inl993 and considered the rapid prototyping (RP) technology is a way to fabricate the framework in 2000, but the design method of RPD framework was not described. In 2004, Prof. Williams performed the design of 2 simple clasps, guide planes and a connector using CAD on the basis of scanning of the cast of the posterior section for a patient's mandibular left arch and implemented the production of sacrificial pattern by RP technique. However, all the above did not give the details of CADprocedure.CAD/CAM technology relies on having an accurate Three dimension (3D) - data acquisition of partially edentulous cast. Dense, accurate and intact point cloud data are essential to creating a precise digital model by CAD and the final production by CAM. In this study, three kinds of method were used to obtain the 3 D point cloud data of a plaster model and to reconstruct CAD model respectively. Each method has its own characteristic. A study on the application of CAD technique to make the design of RPD framework by using two different CAD/CAM software systems respectively was reported in the present work. The data were imported to the RP equipment and the sacrificial patterns in resin for metal RPD framework was. fabricated through a RP technique termed Stereo Lithography (SL).The present work, as a preliminary study, contributes to the development of a specialized software system used in restorative dentistry and the application of CAD/CAM to the RPD framework.Methods1. 3D - data acquisitionThree kinds of method, LSH600 linear laser scanner, low energy industrial computed tomography (ICT) and 3D CaMega optical scanner, were used to capture the 3D data points of a mandibular partially edentulous cast classified as Kennedy Class II Modification 2 arch originating from the standard dental cast. A layer of relief wax approximately 0.5 ~ lmm thick was uniformly placed on the residual ridges. After corresponding processing, the 3D data were transferred to the CAD/CAM software system and the 3D model of edentulous cast was established respectively using triangulation technique.2. CAD of the RPD frameworkTwo different CAD/CAM software systems were used respectively to bulid the 3D model of RPD framework.Based on Geomagic studio 6, which is an universal CAD/CAM softwaresystem, the outline of framework was drawn on the reconstructed triangle mesh model. The inner side data were picked up, processed as the data of tissue surface and shelled for the 3D model of framework.Employing a domestically developed Tanglong'CAD/CAM system, the a-bove point cloud data were reduced, and the digital surveying line and the inserting path were determined. In terms of the principle of clinical design, the tissue surfaces and polishing surfaces of every component of RPD were built, such as clasp, rest, lingual bar and mesh construction, etc. As well as the characteristic structures of the framework, including the half - pear shaped cross section of lingual bar, the internal and external finish lines, the tissue stop at the tissue surface of mesh construction, etc, were fabricated on the base of reduced data model. 3D surface model of the RPD framework was created.3. CAM of the RPD frameworkThe data of 3D model of the framework based on the Geomagic studio 6 were converted to a stereolithography (STL) file format and then transferred to the RP equipment for manufacture. SL method was used to fabricate the framework in resin that can be cast into metal framework in the following stage.ResultsPreliminarily accomplished the whole procedure of the CAD/CAM from data acquisition, 3D computer modeling and fabrication of RPD framework.Dense and complete point cloud data were achieved without scanning blind zone basically by three kinds of scanning method respectively.The data of point cloud were converted to STL file format that could be read by most CAD/CAM software system.In general, among the three kinds of method, 3D CaMega optical scanner was the fastest and had no harm to person and ICT could obtain regular and complete data directly.The anatomic structure of reconstructed 3D model was sharp and clear.Geometric model of the RPD framework was fabricated successfully, whichtook on good fitting, high visibility and easy to modify.Compared with the results of the other reported researches, the CAD model of RPD framework developed here, especially that built on the base of the domestic Tanglong' CAD/CAM system, was more complex and more coincident with the clinical requests.The reconstructing precision in the CAD model of RPD framework by domestic CAD/CAM software system met the needs of clinical practice.Complex shape of RPD framework was fabricated in resin by RP method.Assessed visually, this resin framework appeared to be a little lack of fit to the original plaster model, which might be due to the accumulated error throughout the whole procedure from data acquisition, CAD to rapid prototyping. But it was robust and had an acceptable surface finish.ConclusionEach of the scanning methods presented here provided better spatial resolution , which can satisfies the complex and high precision modeling requests in the following CAD of RPD, finite element analysis and RP.Multiple view combination is an available technique for eliminating scanning blind zone.The present work showed that it is feasible to design geometric models of the RPD framework both with universal CAD/CAM system and domestically developed CAD/CAM software.RP may be a more appropriate CAD/CAM fabrication method than milling for the thin - wall and complex shapes of RPD frameworks.The resin framework here may be used as a sacrificial pattern for casting metal framework. A new RP technique that can build parts directly for metals will be explored in the future study.This method, as an integrated procedure including data acquisition, 3D computer modeling and fabrication by RP, is practical to implement CAD/CAM of RPD framework.