Design, Modeling, Additive Manufacturing, and Polishing of Stiffness-Modulated Porous Nitinol Bone Fixation Plates Followed by Thermomechanical and Composition Analysis

Abstract

The use of titanium bone fixation plates is considered the standard of care for skeletal reconstructive surgery. Highly stiff titanium bone fixation plates provide immobilization immediately after the surgery. However, after the bone healing stage, they may cause stress shielding and lead to bone resorption and failure of the surgery. Stiffness-modulated or stiffness-matched Nitinol bone fixation plates that are fabricated via additive manufacturing (AM) have been recently introduced by our group as a long-lasting solution for minimizing the stress shielding and the follow-on bone resorption. Up to this point, we have modeled the performance of Nitinol bone fixation plates in mandibular reconstruction surgery and investigated the possibility of fabricating these implants. In this study, for the first time the realistic design of stiffness-modulated Nitinol bone fixation plates is presented. Plates with different levels of stiffness were fabricated, mechanically tested, and used for verifying the design approach. Followed by the design verification, to achieve superelastic bone fixation plates we proposed the use of Ni-rich Nitinol powder for the AM process and updated the models based on that. Superelastic Nitinol bone fixation plates with the extreme level of porosity were fabricated, and a chemical polishing procedure used to remove the un-melted powder was developed using SEM analysis. Thermomechanical evaluation of the polished bone fixation plates verified the desired superelasticity based on finite element (FE) simulations, and the chemical analysis showed good agreement with the ASTM standard.

Keywords

• nitinol
• additive manufacturing
• patient specific implant
• bone fixation plate
• porous structure
• stiffness-modulation