In Vivo Evaluation of Novel Implant Topologies Designed for Bone Fixation under Multi-Directional Loading

Introduction While contemporary prosthetic devices provide some restoration of function to individuals who have lost a limb, there are ongoing efforts to develop bio-integrated prostheses that would enhance functionality by providing motor control and sensory feedback. A critical step in the development of a bio-integrated prosthesis will be establishing longterm, secure fixation to the remnant bone. This would allow the transfer of multi-axial and multi-directional loads generated during normal daily activity, and establish a secure interface for the acquisition and transmission of neural or muscular data. As part of a large program to study technologies for bio-integrated prosthetic limb development, we investigated mechanisms for establishing long-term, robust fixation in bone under complex loading environments. Specifically, the purpose of this study was to test the potential of using a topologic optimization strategy to design implant interface conditions that would promote secure fixation under multidirectional loading in a unique in vivo model. Methods Topology optimization coupled with a finite element (FE) model was used to find optimal implant structures under a specified loading condition by distributing limited implant material in a specified domain [1]. For initial evaluation, our loading condition consisted of uni-axial forces with equivalent tension and compression. The 3-D FE model had a cylindrical implant design domain surrounded by trabecular bone. The optimization objective was to minimize the total compliance of the bone-implant system (analogous to minimizing interface motion) subject to the implant filling no more than 50% of the design domain. The resulting implant designs were axisymmetric due to model symmetries. Two optimized implants made of Ti-alloy were selected for in vivo analysis. Both were based upon the same optimized design; one was derived directly from the output of the topology optimization and had a solid structure, while the other had the additional step of converting the design domain into a hierarchical scaffold resulting in a porous structure [2] (Fig 1A). The addition of the scaffold served to facilitate bone infiltration and improve implant fixation. A third design, a solid cylinder with a porous Ti-bead coating [3], was fabricated for comparative analysis of a non-optimized control.