Geometric Stiffness Control Techniques for Metal-based Implants
Prevailer Mba Mentor: Jaejong Park Mechanical Engineering Introduction: Bone defects can develop from various situations such as trauma, infections, tumor, surgery, etc. In some cases where the defect is large, making bone not being able to regenerate itself, the defect site needs to be replaced for functional requirements and aesthetic reasons. Bone grafting is one of such methods available for such purposes. This method includes repurposing another bone in the body (or using a substitute) by modifying it to fit into the affected area. Metal-based bone implants are another option. Titanium and its alloys are widely used in reconstruction surgeries. Since the design and fabrication are done using virtually Ex vivo, metal-based bone implants have geometric freedom compared to bone grafts. However, one of the main shortcomings of metal-based bone implants is their inherent strong mechanical properties. A substantial mismatch in mechanical properties between the metal implant and the bone may lead to a failure in the long run. The property gap between the metal implant and neighboring bone needs to be closed to avoid stress shielding. This study aims to focus on various techniques for stiffness control by changing the geometry of the overall structure confined in a design domain. We are studying how to make the structures ‘porous’ to control the stiffness of the structures. While making structures porous intuitively reduces the stiffness of the structure, the study explicitly pursues algorithms where the stiffness can be systematically controlled. Various internal structuring techniques, such as lattice structures with different geometric configurations, architectural materials, and auxetics, are being investigated. Methods and Materials: In our review of published manuscripts, different methods were identified for stiffness control. This summer, we decided to focus our efforts on understanding two of these methods. They include creating pores on implants and creating lattice structures. To be able to effectively control the creation of the pores, software was needed that supported the creation of designs from an algorithm. This ensured creativity and flexibility in design. We utilized the combination of Rhino and Grasshopper to learn the basics of creating new designs from an algorithm. For the lattice structures, Crystallon was considered an excellent choice for the work. It was to be used as a plug-in to Rhino for the creation of the lattice structures. Results and Discussion: As mentioned in the last progress report, the ability to edit imported implant designs is crucial to this work. However, we have been having some issues in this regard. We have not been able to edit imported designs such as .stl files of previously optimized bone-implant geometries in Rhino. It is crucial that we are able to do this as not all implant models will be created in Rhino. It is essential that imported models can be edited. Some solutions that we found include converting the files to a surface and working with them from there. Another option is to use the local code import plug-in to import the .obj files directly into a grasshopper. We started testing out these possible solutions; however, we haven’t been able to completely solve the problem. In the case of the file conversion, the conversion was successful; however, the file was converted into an indefinite list of surfaces, which makes it difficult to work with. The image in figure 2 above shows this obstacle. The Rhino program has to be able to recognize the model as a single surface for effective use. This issue is currently being mitigated. The local code import was successfully implemented to a grasshopper. The plug-in is still being explored, and as such, it has not yet been used on implant models.
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