Tutorial Week 4 Biomedical Modelling in Ansys Workbench (The Complete Guide with Anatomy and Implant)

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Ariel Wright
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Tutorial Week 4 Biomedical Modelling in Ansys Workbench (The Complete Guide with Anatomy and Implant) Step 1: Create the Anatomical Model in ScanIP Import the DICOM files for the Proximal Femur

$The files can be found in the ProximalFemur folder within: C:\Program Files\Simpleware\Data\ Perform the image processing by following the instructions provided in the tutorial guide from Week 2$

1 Tutorial Week 4 Biomedical Modelling in Ansys Workbench (The Complete Guide with Anatomy and Implant) Step 1: Create the Anatomical Model in ScanIP Import the DICOM files for the Proximal Femur dataset provided by Simpleware. The files can be found in the ProximalFemur folder within: C:\Program Files\Simpleware\Data\ Perform the image processing by following the instructions provided in the tutorial guide from Week 2 (Segmentation) from Section to Section (pages 59 to 66). Save your work. Remove the head of the femur by following the instructions provided in the tutorial guide from Week 3 (CAD Import in ScanIP) in Section (pages 83 to 86). Ignore the fact that you do not have an implant in your model. Your bone should look similar to the image to the right. Save your work. Step 2: Export the ScanIP Model We will not be exporting a finite element mesh from ScanIP. Instead, we will be exporting surfaces for meshing in Ansys Workbench. Right click on Models in the Dataset Browser and Create a new surface model. Add the femur mask to the newly created Model 1 surface by dragging it in. For this model, we will be ignoring the Femur_Cavity mask because it already has been merged with the Femur mask in a previous step.

In the Surface model tab, click on Setup model. Change the export type from STL (RP) to IGES (triangles). Leave all the values in this window as default. Close the window.

2 In the Surface model tab, click on Setup model. Change the export type from STL (RP) to IGES (triangles). Leave all the values in this window as default. Close the window. Click on Full model to generate a surface. Once complete, export the model. Step 3: Import into Ansys Workbench Launch Workbench from the Start menu and drag the Geometry module from the Toolbox into the Project Schematic. Then double-click on Geometry to open the Design Modeler window. To import your femur model, click on File > Import External Geometry File. Select your femur IGES file. Click on Generate to complete the import. This step may take a while because of the large number of triangles in your model. You should see the figure to the right. When working with a biomedical implant, the normal steps would be to create the geometry in SolidWorks, then export the part as an IGES file. For this tutorial, a femoral implant is provided on the website. Download this file to your directory. Import the FemoralImplant.IGS model using the same steps as before. Click on Generate to complete the import.

At this point, you will notice that the two geometries are not aligned (see image). We need to apply a transformation to correctly position the implant within the femur.

Select the implant in the graphic window, then click on Apply to select it (on the left side). Change the Direction Definition from Selection to Coordinates.

The values you use will ultimately depend on how you cut the head of the femur in the previous section and how deep you want to position it.

3 At this point, you will notice that the two geometries are not aligned (see image). We need to apply a transformation to correctly position the implant within the femur. Luckily, the orientation is correct, so the only transformation required is a translation. In the menu bar, click on Create > Body Transformation > Translate. Select the implant in the graphic window, then click on Apply to select it (on the left side). Change the Direction Definition from Selection to Coordinates. Play around with the numbers for FD3, X Offset, FD4, Y Offset, and FD5, Z Offset to position the implant within the femur. The values you use will ultimately depend on how you cut the head of the femur in the previous section and how deep you want to position it. Therefore, you will need to find the correct values yourself. To do this, enter values (in metres) for each of the three offsets, and then click on Generate. The implant should move to its new position. Keep changing the values until you achieve the result to the right. Ballpark numbers are: FD3, X Offset: to FD4, Y Offset: to FD5, Z Offset: to At this point, you will have two intersecting bodies. To correctly create a cavity in the bone, a Boolean Subtraction will be performed. In the menu bar, click on Create > Boolean. Change the Operation type from Unite to Subtract. Select the bone as the Target Body and the implant as the Tool Body. Change the Preserve Tool Bodies option to Yes to ensure that the implant does not disappear after the Boolean is applied. Then click Generate. To view the hollowed out bone, expand the tree for 2 Parts, 2 Bodies and hide the Solid (the implant) by right-clicking it. Once done, re-show the body.

4 You have successfully imported an anatomical model and correctly positioned a biomedical implant. At this point, you can continue the tutorial by closing the Design Modeler window and performing a Static Structural analysis. Do this by dragging the Static Structural module into the Project Schematic and linking the Geometry node to the new Geometry node in this module. Step 4: Model Setup The imported Femur IGES file is made up many small triangles. In Workbench, each of these triangles is treated as a face. As a result, you will always end up with the same looking mesh unless you make the element edge length really small. The fix for this is to create a Patch Independent mesh. What this means is that Workbench will ignore all the edges of the small triangles when creating the mesh. It will, however, try to match the general shape of the femur. To create a patch independent mesh for the femur, right click on Mesh in the tree and insert a Method. Select the bone for the geometry. Change the Method from Automatic to Tetrahedrons to create a tetrahedral mesh. Change the Algorithm from Patch Conforming to Patch Independent. The Max Element Size option will define how big the elements will be in your mesh of the femur. Change this value to conduct a convergence study. The Min Size Limit will define the edge length of the smallest element in your mesh. By setting this value, you can control how much variation there is in the size of your elements. Update the mesh. Once complete, you will need to click on Mesh to see the final result. After setting these parameters, define the sizing of the implant by clicking on Mesh in the Outline tree. The element size you set here will define the mesh size of the entire geometry. Since we inserted a patch independent mesh for the femur, these settings will be overwritten just for the femur, but will remain for the implant. It is a good idea to play around with these parameters to see what they influence. Complete the other steps for setting up the model: Assign materials to each of the parts

5 Define the contact between the parts Insert the boundary conditions Define the results that you wish to obtain. Step 5: Finding the Result at a Particular Location When you conduct a mesh convergence study, you will need to obtain some kind of result at the same location for each simulation. If not, you will not obtain values that are comparable. To find the result at a particular location, right click on Coordinate Systems and insert a new coordinate system. Make sure you rename this coordinate system so that it will be easily identifiable in later steps. Pick any spot you like on your geometry, ensuring that you are selecting a point. Alternatively, change the Origin Definition from Geometry Selection to Global Coordinates. Now you can define exactly where you want the location to be using x, y, and z coordinates. After defining the coordinate system, insert a Probe (stress, strain, deformation, etc.) into your Solution. Change the Location Method to Coordinate System. Then change the Location to the new Coordinate System you just defined. If you renamed your system, it will be easy to identify here. After solving, you should see the results presented in the bottom left panel for that point.

ANSYS Workbench Guide

ANSYS Workbench Guide Introduction This document serves as a step-by-step guide for conducting a Finite Element Analysis (FEA) using ANSYS Workbench. It will cover the use of the simulation package through