6th International DAAAM Baltic Conference INDUSTRIAL ENGINEERING April 2008, Tallinn, Estonia. Radu, C. & Roşca, I.C.

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1 6th International DAAAM Baltic Conference INDUSTRIAL ENGINEERING April 2008, Tallinn, Estonia ON THE DESIGN OF A MEDICAL IMPLANT USED FOR OSTEOSYNTHESIS OF THE TRANSSINDESMOTIC FIBULAR FRACTURE PART I: ANKLE JOINT AND FIBULAR BONE 3D MODEL BY TOMOGRAPHIC SLICES Radu, C. & Roşca, I.C. Abstract: The studies and the results which will be discuss in this scientific paper follow the researches concerning the determination of some dynamic parameters using an analytic model of the ankle foot system and the determination of dynamic and kinematic parameters using a mechatronic system and the main objective of these biomechanical researches is to conceive a 3D model of the ankle joint respectively fibular bone based on tomographic slices. The proposed solution is based on a method, witch combines the image processing techniques and 3D computer graphics. For these techniques it has been used a medical imagistic software, which is a software suite that performs the segmentation of the anatomy through sophisticated three-dimensional selection and editing tools. Key words: implant, fracture, model, fibula, computer tomographic 1. COMPUTATIONAL MODELING In medical application, attention of researchers has now turned toward using combined 3D reconstruction and virtual environment technologies to train clinicians and to help surgeons plan patient-specific, complex procedures like plastic surgery, surgery for trauma from accidents and reconstruction surgery. The 3D models are very useful in simulation of bone fractures and internal fixations with implants. These models are also important to understand how human musculoskeletal structures adapt to external forces disturbances. Because of the complexity of these structures and because not all the biological and anatomical data about them are known, there are many possibilities for how the system might work. Computational modelling techniques applied to the human body and skeleton provides a possibility to analyse without interference in the human body [ 1 ][ 3 ]. 2. ANKLE JOINT AND FIBULA BONE 3D MODELING The proposed solution is based on a method that combines the image processing techniques and 3D computer graphics. Our modelling methodology of the ankle joint system includes three major stages, such as: importing and processing of the impute data; segmentation of the tomographic slices and automatic identification of the objects from imported images; three-dimensional reconstruction of the anatomical segment [ 1 ][ 2 ][ 3 ]. 2.1 Importing and processing of impute data Importing and processing of the images obtained by tomographic computer is an essential step to obtain the virtual model of the ankle foot anatomical system. For these technique it has been used medical imagistic software (MIMICS), which is a software suite that performs the segmentation of the anatomy through

2 sophisticated three-dimensional selection and editing tools [ 2 ]. Impute data of the modelling process are represented by tomographic slices, in.jpg format, which are the results of a specific analyse. The 73 tomographic slices, which are represented in figure 1, belong to a 25 years person (male) with a body weight of 84 kg. Fig. 1. The succession of the input data represented by the 73 tomographic slices As variable input data are provided the tomographic slice number (73), the dimension of the each file (512 x 512 pixels), the distance between them (1 mm) and their type (.jpg format). After importing process, all the images are displayed in three views, as is represented in figure 2. In the first view are represented the imported images in xoz plane (frontal plane), the second work plane reveals the images in yoz plane (sagittal plane) and the third view shows the images in the xoy plane (transversal plane) [ 1 ][ 3 ]. 2.2 Segmentation of the impute data The second stage of our research includes thresholding method, which means that the segmentation object (visualized by a collared mask) will contain only those pixels of the image with a value higher than or equal to the threshold value [ 3 ]. The detection of bone tissue can be obtained by using the optimal grey value, established between minimum value of 1628 and maximum value of 3056 Hounsfield units (HU) (see figure 3) [ 4 ]. The 1628 HU value represents the lower value in which case the soft tissue appears. The right choice of these values is important for the accuracy of the final model. If the threshold value is too higher, then the bone tissue it will be neglected and if it is too lower, then the final model will have the unwanted soft tissue areas. Fig. 2. Work planes of the medical imagistic software (MIMICS): frontal plane, sagittal plane and transversal plane Fig. 3. The diagram segmentation s process of the homogenous regions

3 In figure 4 is represented the histogram of the threshold values ( HU) to detect de bone tissue. The method for this processing was segmentation s histogram method with two-threshold value, minimum and maximum, which correspond to the bone density interval. On the Ox axis are represented the threshold values and on the Oy axis are represented the pixels displayed on the segmentation interval. Thus, applying the segmentation process we obtained the bone tissue of the anklefoot anatomical system that is represented by the yellow mask (see figure 2). The detection of the interest regions is achieved for all images of the stack. For each individual 3D model of the main bones of the ankle foot anatomical system, we have used segmentation process for each component and implicit three coloured masks: the red mask for fibula, blue for tibia and yellow for talus, calcaneus, tarsian and metatarsian bones (see figure 5). The major problem that we meet in this case was the right delimitation of the tissues. The artefacts affected this delimitation, which was captured by the computer tomograph during scanning procedure. To eliminate these artefacts we have used the editing tools of the medical imaging software, for each imaging which demand to be repaired. 2.3 Three dimensional reconstruction of the ankle foot anatomical system The next step we followed was the three dimensional reconstruction of the interest anatomical parts. Using the right selection of the threshold values, all the pixels bordered in this interval will be attributed to one coloured mask. The mask was the role to create individual 3D model for each interest region. During three dimensional reconstruction processes, each pixel of the formed mask is converted into voxel. The value of each voxel depends of the scanning distance between images. Fig. 4. The histogram of the threshold to detect the bone tissue Fig. 5. Masks attribution to the main bones of the ankle foot anatomical system: sagittal view (left), frontal view (right) Fig. 6. The 3D model of the ankle foot anatomical model that presents scale effect

4 The main problem, which can be meet, is the interpolation between 2D tomographic sections. The virtual model obtained by the 2D slices, without the interpolation between them, presents a roughness surface, with precipitous gaps on al three directions Ox, Oy, Oz (it is present so called scale effect, the size of gap is equal to the distance between two consecutive sections on the Oz direction and equal to the size of the pixel on the Ox and Oy directions. As it can see in figure 6, the 3D model is poor in details and there is a possibility that it may provide wrong virtual and tactile information. The main advantage of MIMICS software is the fact that it s possible to perform the interpolation between sections using a cube algorithm (see figure 7) [ 2 ][ 3 ]. Practically, this medical imagistic software hides a complex mathematically process, and the effect of the imaging processing can be visualised by displaying the 3D model on the calculator screen. contained in the interest anatomical system (figure 10). Fig. 8. The values of the parameters used to generate the ankle foot 3D model [ 1 ] In figure 9 is represented the threedimensional gross model of the ankle foot anatomical system before editing process. In the first step, we generate the 3D gross model of the interest anatomical system by using the parameters from figure 8. In this case, the segmentation process was applied to each section, in whole image stack. In figure 10 is represented the threedimensional model of the ankle foot anatomical system after editing process. In this case, the segmentation process was applied to each section, for each component (bone) of the anatomical system. Fig. 7. The cube algorithm used by MIMICS software [ 3 ] Using the tomographic slices, from the computer tomograph, and the resolution and smoothing parameters (figure 8), we have obtained the gross model of the ankle foot anatomical system (figure 9) and the individual model of each bone that is Fig. 9. 3D dimensional representation of the bone tissue of the ankle foot anatomical system before editing process

5 3. CAD MODELING TECHNIQUE OF THE FIBULA BONE Fig D dimensional representation of the bone tissue of the ankle foot anatomical system after editing process Fig D model of fibula bone Also, using the 3D model of the interest anatomical area, after editing, we can extract different models of the anatomical parts, which can used it afterwards for other operations. For example, we have used the 3D model of the fibula to generate the model of the osteosysnthesis implant. CAD modelling technique of the fibula bone is the link between 3D model of the anatomical part and the model of the osteosysnthesis implant s 3D model. The MedCAD module of the MIMICS software does this technique. Using this technique we have obtained the model of the fibula bone in IGES format, which afterwards is used as reference model in SolidWorks software, to design an osteosysnthesis implant for fibula fracture [ 2 ][ 3 ]. Thus, after we have obtained the 3D model of fibula bone, the next step is to generate the IGES model. In this case, the IGES model is described by a surface witch wraps and copy with fidelity all the irregular parts of the natural bone and it is taken as reference model for ulterior design of the fibular implant by CAD software (SolidWorks) [ 1 ][ 3 ]. To obtain an IGES format of the anatomical part takes three stages [ 2 ][ 3 ]: 1. Determination of polylines resulting the outer contour of the bone. 2. Verification and patching of contours determinated by polylines. The analysis can be affected by the influence of artefacts. The artefacts are some erroneous signals received during scanning time and they are produced by the metal implants witch are already in the human body. 3. Obtaining of the IGES model. Because of the complicated geometrical configuration of the 3D model, this cannot be exported directly in CAD software. Thus, the solid model of fibula is transformed in a detailed 3D model, which contains IGES surfaces. The transfer from a solid to a surface is done by the polylines, which determine the exterior bone contour. For each section it has been generated a polyline, so for 73 tomographic slices we have obtained 73 polylines [ 1 ]. Because of the incomplete delimiting contour of the fibula model, because of artefacts, we had to use the edit tool of the software, by adding the pixels on the

6 sections 32, 33, 34 and 37 (see figure 12). If we should not have been precede that, certainly our model would not be precise and as a fallowing the virtual model of the implant would not be precise too [ 1 ]. for Osteosynthesis of the Transsindesmotic Fibular Fracture Part II: The Locking Orthopaedic Plate, as a base model to design an osteosynthesis implant used for transsindesmotic fibula fractures. 5. REFERENCES Fig. 12. The editing process by adding the pixels on the sections 32, 33, 34 and 37 The next step is to generate the 3D model of the IGES surface. The final model is obtained by inserting a surface, which is tangent to all 73 polylines, from up to bottom. This surface copied with fidelity all the 3D model s irregularities that were obtained using tomograph slices (see figure 13). Doing that we have obtained the final model, which now can be exported in IGES format to Solid Works software Fig. 13. Generating 3D model of IGES surface, which is tangent to all 73 polylines, from up to bottom. 4. CONCLUSIONS The IGES model of the fibula bone will be used afterwards, in the next scientific paper On the Design of a Medical Implant Used 1. Radu, C., Rosca, I.C. On the design of a medical implant used for fibular fracture. Bulletin of the Transilvania University of Braşov, 2006, 13, en/92462why+choose+mimics.html, Leondes, C., T. Medical imaging systems technology. Word Scientific Publisher, California, HnsfldUnt.htm ADDITIONAL DATA ABOUT AUTHORS 1) Author names: Radu, C., Lecturer and Rosca, I.C., Professor 2) Title of manuscript: On the Design of a Medical Implant Used for Osteosynthesis of the Transsindesmotic Fibular Fracture Part I: Ankle Joint and Fibular Bone 3D Model by Tomographic Slices 3) Full address: Radu, C., Lecturer of Mechanical Engineering Faculty, Fine Mechanics and Mechatronic Department, Transylvania University from Braşov; Barbu Lautaru, No. 13, Bl. 26, Sc. C. Ap. 15, Braşov, Romania; tel: ; ciprian1_radu@yahoo.com; Rosca, I.C., Professor of Mechanical Engineering Faculty, Fine Mechanics and Mechatronic Department, Transylvania University from Braşov; 29, Eroilor Avenue, Braşov, , Romania; tel: ; e- mail: roscaileana@yahoo.com. 4) Corresponding author: Radu, C., Barbu Lautaru, No. 13, Bl. 26, Sc. C. Ap. 15, Braşov, Romania; tel: ; e- mail: ciprian1_radu@yahoo.com.