(B-47) Evaluation of Trabecular Bone Mechanics using Additive Manufacturing and Finite Element Analysis

Introduction: Trabecular bone is a honeycomb-like network of spongy and porous material present in vertebrae and long bones 1. Trabecular bone microstructure is an indicator for bone health. Osteoporosis is a metabolic disease characterized by low bone mineral density that reduces bone strength and increases fracture risk 2. Therefore, assessing bone strength and fracture risk is important for staging and monitoring of osteoporosis. Micro-finite element analysis (µFEA) of micro-computed tomography (µCT) images of trabecular bone has been used to evaluate stiffness and strength 3,4. Additive manufacturing allows for the reproduction of bone microarchitecture in 3D printed phantoms 5. Recently, additive manufactured trabecular bone phantoms are used to gauge the mechanical properties of trabecular bone6,7. Both µFEA and mechanical testing of additive manufactured phantoms are useful approaches for estimating mechanical properties of trabecular bone. However, it is unclear if measurements from the two approaches are consistent and requires systematic analysis. In this work, we investigate two approaches for evaluating mechanical properties of trabecular bone: mechanically testing additive manufactured phantoms and µFEA modeling. We assess the agreement between stiffness results derived from µFEA simulations and mechanical testing of trabecular bone phantoms.
Materials and Methods: A set of 20 lumbar vertebrae body regions-of-interests (ROIs) from different patients were segmented, meshed, and additive manufactured using a selective laser sintering printer to produce trabecular bone phantoms. Additional phantoms evaluated include control phantoms, shrew trabecular bone phantoms, and emu trabecular bone phantoms. Select phantoms were printed multiple times to assess the repeatability of mechanical testing and additive manufacturing. The phantoms were scanned with µCT. The phantoms were tested in uniaxial compression. Linear elastic µFEA was also performed using the same boundary conditions. The control phantom was tested to evaluate the elastic modulus of 3D printing resin to use for µFEA simulations. Bland-Altman analysis was then performed to evaluate agreement between the mechanical testing and µFEA simulation values and an image registration assessment was performed to evaluate the fidelity of the 3D printed phantoms using the Dice score to assess consistency between phantoms and in silico models.

Results and Discussion: We observed good correlation between the mechanical stiffness and µFEA stiffness (R^2=0.84), Kendall rank correlation coefficient of 0.86, and with normalized root mean square deviation (NRMSD)=8.1%. There is high repeatability in mechanical testing and 3D printing with coefficient of variance less than 5% for all tested phantoms. In terms of the 3D printing fidelity, there is strong agreement (Dice score=0.97 (95% CI, [0.96,0.98]) between the macrostructure of the trabecular bone ROI and the µCT image of the phantom. The control phantoms printed with different orientations show there is anisotropy in the material, as their stiffness results have a difference of 9.64%. The discrepancy between the mechanical and µFEA stiffness results can be attributed to mechanical testing errors, artifacts from 3D printing, and material properties of the 3D printing resin, such as anisotropy.

Conclusion:
This work compares µFEA modeling and additive manufacturing as methods for evaluating trabecular bone mechanics. The results indicate that µFEA can effectively model and evaluate the stiffness of trabecular bone with similar results to additive manufacturing.
Acknowledgements: This work was supported by the FDA Office of Women’s Health. This project was supported in part by an appointment to the ORISE Research Participation Program at the Center for Devices and Radiological Health, U.S. Food and Drug Administration, administered by the Oak Ridge Institute for Science and Education through an interagency agreement between the U.S. Department of Energy and FDA/ Center for Devices and Radiological Health.
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