A METHODOLOGY FOR HUMAN PROSTHESIS MODELLING IN THE CAD/CAM SYSTEM

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1 A METHODOLOGY FOR HUMAN PROSTHESIS MODELLING IN THE CAD/CAM SYSTEM Osiris Canciglieri Junior Pontifícia Universidade Católica do Paraná (PUCPR) Rua Imaculada Conceição, Prado Velho - CEP: Curitiba - Paraná. osiris.canciglieri@pucpr.br Alan Eduardo Lam Pontifícia Universidade Católica do Paraná (PUCPR) Rua Imaculada Conceição, Prado Velho - CEP: Curitiba - Paraná. alan.lam@hotmail.com Jefferson de Oliveira Gomes ITA Instituto Tecnológico de Aeronáutica - São José dos Campos São Paulo. gomes@ita.br Abstract: The union of discoveries in various different areas results in great technological jumps, as for example, the union of the Radiology with the Computer Science that is the base of the technology of the diagnosis by image. With the incorporation of the computerized tomography scan with the three dimensional processing (3D) of the image, it was possible to have a virtual visualization of the body. Considering the great technological advances in medicine and engineering, this work aimed to present a Methodology for translating information from computerized tomography to CAD/CAM system, in order to design and to manufacture the model of human prosthesis. The integration of different areas such as Engineering assisting Medicine, Computer Science in Engineering and Physics in Engineering is other very important side of this work. Keywords: CAD/CAM, Prosthesis Modelling, Conversion of Images, Deformities, Geometric Modelling, Tomography Scan, Bio models. 1 Introduction In the last two centuries there was a quick technological development and consequently a lots of important discoveries appeared in several areas of knowledge. One of them occurred in 1985, when the German physicist Wilhem Konrad Rontgen

2 had found out the X Ray, which allowed the indirect access to the human body, resulting in a fundamental technology to the diagnostics by image. Another was the incorporation of the computerized tomography in the 70 s, and followed by the three dimensional processing of image (3D). These developments had created the possibilities for the human body visualization, in perspective, in a graphic monitor. In this way, the analysis of the radiologist and the surgeon was facilitated, being unnecessary the mental and subjective reconstruction of the various two dimensional tomography sections. There are patient that have congenital or acquired deformities, whose treatment is a constant challenge to the surgeon team, including, in the most of the cases, multiples and complexes surgeries, that can have high costs and with the results, in some cases, not satisfactory. Therefore, the Health Care Organizations and the Health professionals started to search and to develop studies about new ways of to make the diagnosis and to treat the deformities. In Brazil, it is estimate that 12% of the people are physical disable. It is about 19 million of people. There are several reasons for their deformities but the main reasons are: car accident, accident at the work, radical sports, bad formation with genetics origin, and the pathological and degenerative illness (D URSO, 1998). In the product development area, the CAD systems (Computer Aided Design) was consolidated as powerful and indispensable tool to the technological improvement in the conception and development of the products, in the industries in a general way. Other important aspect was the manufacturing processes evolution and in the radical change in the geometry form of the products. This was possible only through the integration of the Computer Aided Design (CAD) that offers support to the Computer Aided Manufacturing (CANCIGLIERI AND YOUNG, 2003). This scenery showed the opportunity to unite the knowledge of the Medicine, Engineering and Information Technology to create new conception ways for prosthesis and bio models to increase and make easier for the medicine in the surgeries planning. The current way of prosthesis production, in the most of the cases, are handmade and this has a direct influence in the quality and in the operational costs, since that normally the prosthesis are geometrically irregular obligating the surgeon to make adjusts during the surgery process. These adjustments increase substantially the surgery time and costs and, also, can cause, in certain cases, traumas to the patient that can be psychological or terrible sequelae.

3 From this, it is the fundamental importance to develop a Methodology that can be able to conceive and manufacture prosthesis which can be used as support base to the CAD/CAM design systems. Therefore, the aim of this research is to presents a methodology to the conception of virtual models of human prosthesis through the CAD/CAM UNIGRAPHICS system. It shows, also, a case study applied to this methodology through de modelling of the top of a human skull. 2 Methodology to Design/Conception eption and Manufacturing of Prosthesis The methodology to the conception of prosthesis through de CAD/CAM systems must start with the conversion of the computerized tomography sections in two dimensional poly lines figure that will generate the geometrical three dimension models in the CAD system. With the 3D models it is possible to realize the virtual machining through the CAM module. The figure 1 shows the whole methodology divided in two main function: i) the acquiring and conversion of the tomography image and the creation of the geometrical model in CAD; and ii) the virtual machining of the model. Capture the tomography Image in DICOM format Tomography Image Convert the digital image to JPG format Create profiles in DXF format Extract clouds of points from DXF format Export the clouds of points to The UNIGRAPHICS NX3 Acquiring and Conversion of the tomography image in the 3D model at the CAD Define raw material Define part geometry Create 3D model Virtual machining of 3D model (CAM) Define the references Define the cutting tools Define the strategies and the cutting operations Virtual 3D model machined Machining simulation Generate de CNC program Figure 1 Conception and manufacturing of prosthesis models.

4 2.1 Acquiring and Conversion of Tomography image a 3D Model in the CAD System The acquiring of the tomography image starts at the tomography sections generated by a computerized tomography, preferably sections made in the parallel planes to one of the reference axis of the object to be modelled, that in this case is a human skull, in the DICOM (Digital Imaging and Communications in Medicine DICOM, 2004) format. The geometrical modelling of the three dimensional model implicates in the conversion process from the DICOM format to a public format, that in this research specifically is the JPG format. This conversion is realized through the Dicom Works software. With the JPG format images is realized the conversion to the DXF (Data Exchange Format) format. This operation is made using the Vextractor 2.95 software. With the DXF format profiles, it is necessary the appliance of an algorithm developed in the AutoLisp language and implemented in the AutoLisp 2004 software to capture the clouds of points, that is, it has been made the Cartesian mapping of the borders based in a three dimensional system where the Z axis is define to the high of the tomography transversal section. All the borders (tomography sections) should have the same reference. To the acquiring of the data, the routine divides the borders in Internal borders and External borders generating a DAT (data files in ASCII code) file that holds the co-ordinates of every vertices of each type of borders: internal or external. With the clouds of points of all sections in text format (DAT), it is, now, necessary the replacement of the parallel planes derived from the computerized tomography in the Computer Aided Design (CAD) environment, which in this research is the Unigraphics NX3. As all profiles are parallel lined up the function through Curves is used and so that the model is automatically rebuilt in the graphic environment of the UNIGRAPHICS NX3. The great restriction of the three dimensional reconstruction based in the surfaces is the solution for the bifurcation and open borders problems, since until now there is no CAD software that has a resource capable to solve these problems automatically. It occurs because of the focus definition for these products that are geometrical complexes. It is hard to find mechanic components with such complexity. In the several CAD systems with solid and surface modellers that exist in the market, normally the functions loft, sweept, rule, through curve, among others, are utilized to generate the surface of the borders. The union of these surfaces, since the geometry stays totally

5 closed, can be transformed in a solid geometry. In short, the essence of this environment is to create surfaces or solids from the parallels borders. The surfaces are obtained by the interpolation of a built curves sequence, most of them, in parallel plane. Among the restrictions of this technique we can outstanding: Limitation of only one border by plane; The border/curve must be closed; In some systems may not have the superposition of the borders/curves. The limitation as for the numbers of the borders should be solved by the creation of auxiliary planes in the same position for the exceeded borders. In this way, various curves are obtained, and where the interpolations must be defined by the costumer for the surface formation. In the other hand, the open borders can not be interpolated in the CAD. To minimize these problems an option must be made for a simplification in the selection of the region to be built. In this research the following criteria are adopted: The interesting region must be between the transversal sections that comprehend in the maximum two closed borders; Must not have the superposition of the borders in the same surface in the choose direction to the cutting. Besides these simplifications, it must observe that there is the necessity of the conversion of the images before the introduction of the borders into the CAD systems to modelling the surfaces. The necessity of work in polygonal format (vector) and not in the matrix format (raster) is because of the CAD/CAM software work only in this way. 2.2 Model Virtual Machining (CAM) Since the solid geometry or the surface were generate, the geometry and the blank dimensions can be define, that is, the raw material for the CAM process (machining). To obtain the whole virtual machining process, the model was divided basically in two parts: external surface, for the representation of the external part of the bones; and internal surface for the representation of the internal part of the bones. Then, for each part was defined in the software the geometry of the model and the raw material, as well as the definition of the point of reference (zero part). With the defined geometry, to generate the trajectory of the machining, it is necessary to select the suitable tools, to obtain the more suitable surface finishing quality and the

6 machining strategies. After these steps, a simulation is realized and if all elements are visually correct, the code CNC is sent to the machining tool via DNC Machining Strategy for Prosthesis using 5 axes The generation of tool paths for the simultaneous 5-axes milling must contain all necessary technical resources to avoid tool gouging and machine collisions, as respectively the information about the tip tool trajectory, with the definition of the cutter contact and the cutter location, and also the way how constant the tool axis is inclined toward the normal vector of surface (ELBER, 1995; ALTMÜLLER, 2001; GRAY, 2001). The traditional methods (isoparametric and isoscallop) for the setting of tool axis inclination are based on adjust of the trajectory from the edges of the surface. Thus the adjacent trajectory always depends on the last definite path and so on (figure 2) (BALASUBRAMANIAM, 2000; GRAY, 2001; CHIOU, 2002). Second tool path based on the first one First tool path based on the board surface a e Figure 2: Isoparametric method for the generation of tool path th. For the isoparametric method the tool path generation depends on the parameters u and v of the parametric surface. The main advantage of this method is its low mathematic complexity. However, as the interval between adjacent steps are constant in the parametric space, in the three-dimensional space this can diminish or increase, what leads frequently to the generation of redundant trajectories. Another inconvenience of this method is the high dependency on the mode of surface

7 construction and its discreetization grade (LEE, 1998; BALASUBRAMANIAM, 2002; GOMES, 2005; PARK, 2005). For the simultaneous five-axes milling the tool axis has to have a constant inclination with the local normal vector of surface. In this case, two angles have to be defined, respectively longitudinal (β) and transversal (α) to the feed direction. The both are defined by the programmer (Figure 3). This is a method of attempt and error, where different angles are tested, until it finds the lesser possible inclination where interferences will not occur. This becomes this process very dependent of the interaction between the man and the trustworthiness of the graphical simulation systems. It is necessary to check eventual interferences and/or collisions and, if identified some interference, the program must be remade. These steps become the generation of programs very delayed and the generated tool paths are generally subexcellent (LEE, 1998; BALASUBRAMANIAM, 2002; GOMES, 2005; PARK, 2005). Nomal to Surface Tool axis Nomal to Surface Feed direction Feed direction Feed direction Figure 3 Programmed inclinations in the simultaneous s five-axes milling. In the process CAD/CAM a solid object is frequently represented by a combination with two or more generic geometric entities. Thus, predefined parametric surfaces are usually subjected to Boolean operations of trimming and extending. This type of manipulation involves, many times, in intersection curves that do not coincide with the original isoparametric curves (YANG, 2003), that results in inadequate trajectories. In this case, it is necessary a reparametrization of surface. Therefore, the reparametrization of trimmed surfaces is fundamentally important for the 5 axes milling CAD/CAM chain. The Method of Algebric Interpolations or Coons Method and the Laplace Method are the two main existing methods, the first one for its simplicity and second for deciding some problems of the first one. In both the methods,

8 the parametric domain 2D of the trimmed surface is reparametrized in a new unitary parametric domain 2D. Thus, the tool trajectories are generated in accordance with the new set of parametrization. However, many times the only solution is the complete remodelling of surface (LEE, 1998; GOMES, 2005; PARK, 2005). The Cartesian methods calculate the trajectory to be covered by the cutting tool from projections of geometric entities on the surface to be machined. The methods isoscallop are a variation of this group where the calculation considers the curvatures of the surface to define the distance between adjacent steps, to always generate scallops with the same height (ALTMULLER, 2001). 3 Methodology Application A Case Study In order to test the methodology, the authors have limited the demographics cuts from cut number 110 until number 149, that is, the initial height in the Z axis correspond 110 millimetres. The methodology steps application are: i) skull preparation; ii) tomography images in the DICOM format acquisition; iii) Converting from DICOM images format to JPG format; iv) generating profiles in the DXF format; v) generating of cloud of points; vi) export the cloud of points to a CAD system; vii) modelling 3D model in the CAD system; viii) a virtual machining simulation of 3D model (CAM). 3.1 Skull Preparation The skull was fixed to acrylic support, guaranteeing in this way, that the skull always use the same references between tomography cuts. The support is transparent to avoid influence in the tomography image. The skull and support were depicted in figure 4a. 3.2 Tomography images in the DICOM format acquisition The obtainment of DICOM images were made through of DicomWorks freeware software (DICOMWORKS, 2004), as it is illustrated in figure 4b. 3.3 Converting from DICOM images format to JPG format This conversion was made through the same freeware software DicomWorks and an example of this cut is showed in figure 4c.

9 (a) skull and acrylic support. (b) DiconWorks tomographics cut. (c) converting DICOM to JPG format. (d) converting JPG to DXF format. (e) DAT file (x, y and z coordinates) (f) 3D CAD model (g) machining simulation (CAM) Figure 4 Methodology Application

10 3.4 Generating profiles in the DXF format Now, through the Vextractor 2.95 software the profiles created in JPG format were converted to the DXF format. In this step an analysis is made to solve and take out the imperfections (small open areas), privileging only the borders, as in the figure 4d Generating of cloud of points A computer routine in Visual Lisp was created in order to obtain the co-ordinates (starting and final) of the profiles for the preparation of the image and the cloud of points acquisition. The algorithm developed for the co-ordinates acquisition divides the DXF image into two borders: external and internal. To the data transference A DATA file was used, as illustrated in figure 4e. This file has the whole cloud of points (Cartesian Mapping) of the borders based in the reference axes Exporting the cloud of points to a CAD system The acquisition routine of the cloud organizes the DATA file according to a standard format, required by the Unigraphics NX3 system, where the three dimensional model will be automatically build. The file formatting requires the division of the cloud of points into three distinct columns. The first correspond to the Cartesian axis x, the second to the y axis and the last one to the z axis Modelling 3D model in the CAD system From this files of points, parallel planes can be constructed in the Uigraphics NX3, inserting curves by poles. This application is showed in the figure 4f. To transform the present curves in parallel planes it was used the Through Curves command, but firstly some adjusts had to be made in the curves, in order to correct the conversion mistakes Virtual machining simulation of 3D model (CAM) From the 3D model created in the CAD, a virtual machining of the model was made using the CAM module of the Unigraphics NX3. The graphic visualization of the

11 machining can be seen in the figure 4g. 4 Discussion and Conclusion The methodology developed in this research can be discussed focusing two aspects: the conversion from the tomography to the three dimensional model and the virtual machining. In the conversion from the tomography to the three dimensional model was presented an exploratory study that needs new experimentations in order to quantify the dimensional deviation found in each border to evaluate the several techniques used in the image conversion, moreover the necessity to increase the translation of the borders in the way to avoid the resulting intersections that make more difficult the three dimensional reconstruction process in the CAD. Regarding to the virtual machining, the complexity of the human bones geometry to be used can present some difficulties, therefore, new approaches as the milling process in 5 axes that should be refined in the acquirement of the whole prosthesis model machining process. The research showed that through the proposed methodology it is possible convert the tomography images of the DICOM pattern in to a geometry in the CAD. It was developed using the graphic environment of the UNIGRAPHICS NX3 software. The generation of the splines or poly lines from the conversion need to be adjusted to avoid mistakes in the three dimensional modelling. Through the function through curves it was possible generate the solids automatically. These solids and the Bolean operations allowed the final geometry in the CAD. Inside the Unigraphics environment NX3 is possible realize virtually the machining since it has the integrated CAD and CAM systems. Through this methodology new researches are recommended: i) a more accurate study about the conversion from images to the CAD profiles (splines); ii) a study to optimize the time and method of the model machining; and iii) in order to expand the proposed methodology, and its validation, a model with relative complexity should be used. References 14 GRAY, P. et al. Comparison of 5-axis and 3-axis finish machining of hydroforming die inserts. The International Journal of Advanced Manufacturing Technology, [S.l]: Springer-Verlag, v. 17, p , ALTMÜLLER, S. Fünf-achs-fräsen von freiformflächen aus titan. Aachen, f. Dissertation, Rheinisch-Westfälischen Hochschule Aachen, 2001.

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