NDLI: Evaluating a NiTi Implant Under Realistic Loads: A Simulation Study

Content Provider	The American Society of Mechanical Engineers (ASME) Digital Collection
Author	Amerinatanzi, Amirhesam Moghaddam, Narges Shayesteh Ibrahim, Hamdy Elahinia, Mohammad
Copyright Year	2016
Abstract	Additive manufacturing (i.e. 3D printing) has only recently be shown as a well-established technology to create complex shapes and porous structures from different biocompatible metal powder such as titanium, nitinol, and stainless steel alloys. This allows for manufacturing bone fixation hardware with patient-specific geometry and properties (e.g. density and mechanical properties) directly from CAD files. Superelastic NiTi is one of the most biocompatible alloys with high shock absorption and biomimetic hysteresis behavior. More importantly, NiTi has the lowest stiffness (36–68 GPa) among all biocompatible alloys [1]. The stiffness of NiTi can further be reduced, to the level of the cortical bone (10–31.2 GPa), by introducing engineered porosity using additive manufacturing [2–4]. The low level of fixation stiffness allows for bone to receive a stress profile close to that of healthy bone during the healing period. This enhances the bone remodeling process (Wolf’s Law) which primarily driven by the pattern of stress. Also, this match in the stiffness of bone and fixation mitigates the problem of stress shielding and detrimental stress concentrations. Stress shielding is a known problem for the currently in-use Ti-6Al-4V fixation hardware. The high stiffness of Ti-6Al-4V (112 GPa) compared to bone results in the absence of mechanical loading on the adjacent bone that causes loss of bone mass and density and subsequently bone/implant failure. We have proposed additively manufactured porous NiTi fixation hardware with a patient-specific stiffness to be used for the mandibular reconstructive surgery (MRS). In MRS, the use of metallic fixation hardware and double barrel fibula graft is the standard methodology to restore the mandible functionality and aesthetic. A validated finite element model was developed from a dried cadaveric mandible using CT scan data. The model simulated a patient’s mandible after mandibular reconstructive surgery to compare the performance of the conventional Ti-6Al-4V fixation hardware with the proposed one (porous superelastic NiTi fixation plates). An optimized level of porosity was determined to match the NiTi equivalent stiffness to that of a resected bone, then it was imposed to the simulated fixation plates. Moreover, the material property of superelastic NiTi was simulated by using a validated customized code. The code was calibrated by using DSC analysis and mechanical tests on several prepared bulk samples of Ni-rich NiTi. The model was run under common activities such as chewing by considering different levels of the applied fastening torques on screws. The results show a higher level of stress distribution on mandible cortical bone in the case of using NiTi fixation plates. Based on wolf’s law it can lead to a lower level of stress shielding on the grafted bone and over time bone can remodel itself. Moreover, the results suggest an optimum fastening torque for fastening the screws for the superelastic fixations causes more normal distribution of stress on the bone similar to that for the healthy mandible. Finally, we successfully fabricated the stiffness-matched porous NiTi fixation plates using selective laser melting technique, and they were mounted on the dried cadaveric mandible used to create the finite element model.
Sponsorship	Aerospace Division
File Format	PDF
ISBN	9780791850497
DOI	10.1115/SMASIS2016-9287
Volume Number	Volume 2: Modeling, Simulation and Control; Bio-Inspired Smart Materials and Systems; Energy Harvesting
Conference Proceedings	ASME 2016 Conference on Smart Materials, Adaptive Structures and Intelligent Systems
Language	English
Publisher Date	2016-09-28
Publisher Place	Stowe, Vermont, USA
Access Restriction	Subscribed
Subject Keyword	Torque Gaussian distribution Additive manufacturing Density Finite element model Absorption Biomimetics Lasers Nickel titanium alloys Surgery Materials properties Titanium Biocompatibility Stiffness Hardware Mechanical testing Manufacturing Shapes Failure Stress concentration Melting Alloys Mechanical properties Computer-aided design Stress Computerized tomography Geometry Simulation Screws Stainless steel Plates (structures) Metal powders Bone Porosity Shock (mechanics)
Content Type	Text
Resource Type	Article

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