Poster AA8 - Quantifying the Relation Between Osteoporosis-related Trabecular Bone Microstructural Changes and Mechanical Properties of Human Vertebrae with Digital Volume Correlation Approach

Introduction: The overall prevalence of spine fracture is approximately 30.5%. Due to the complicated structure of the spine, its intricate biomechanical function remains inadequately understood. This study focuses on studying the biomechanics of healthy and osteoporotic human spines using a combination of micro-computed tomography and image correlation method, to characterize the 3D (three-dimensional) mechanical deformation inside the vertebrae under axial compressive loads. Investigating the biomechanics of the spine is fundamental to understanding its health and function and provides significant implications for the pathological fragility of the spine affected by various diseases such as osteoporosis. Mechanical testing coupled with micro X-ray computed tomography (micro-CT) and digital volume correlation (DVC) is a non-contact method that can measure both the structures and 3D full-field internal mechanical deformation vector field in bone under external forces at sub-millimeter resolution. In this study, ex vivo mechanical tests were performed on healthy and osteoporotic human cadaveric vertebrae to investigate the relation between microarchitecture and mechanical properties of trabecular bone.

Materials and

Methods: Healthy and osteoporotic vertebrae were placed inside a loading device (CT5000TEC, Deben, Suffolk, UK), and the loading device was placed inside a micro-CT chamber (NSI Micro-CT System, North Star Imaging Inc., Rogers, MN). A multi-step axial mechanical loading procedure was performed multiple CT scans were collected: no-load (Scan#1)---1000 N---hold---(Scan#2)---2000 N (or fracture)---hold---(Scan#3)---Fracture---hold---(Scan#4). Prior to each scan, the sample will be held in position for 50-minutes to relax bone from viscoelastic effect. A 30-minutes scan with 160 kV, 0.175 mA and aluminum filter will give images with isometric resolution of 25.4 µm. Image segmentation was performed on the micro-CT images using the watershed tool in Avizo 3D analysis software (Thermal Fisher Scientific, Burlington, MA) to digitally remove background and extract only the spine volume in the images. The 3D full-field mechanical deformation in spine was calculated by correlating the corresponding loaded images with images at no-load state (Scan#1) in DaVis software (LaVision Inc., Germany). Mechanical strain was calculated by taking partial derivative of the 3D deformation vector field.

Results, Conclusions, and Discussions: The principal strains in healthy and osteoporosis vertebrae under different levels of compressive forces were experimentally measured. The results indicate that under axial compressive loads, strain concentration appeared close to the endplate in both healthy and osteoporosis spine. Both healthy and osteoporosis vertebrae failed in endplate region. Regression analysis was performed to quantitatively correlate strain level inside vertebrae with the local microarchitecture and mineral density. The complicated mechanical strain distribution in spine observed in this study is due to the nonhomogeneous distribution of the trabeculae structure inside the vertebra.
As a proof-of-concept, this study investigated the 3D full-field biomechanics in healthy and osteoporotic human spinal vertebrae using mechanical test coupled with micro-CT and DVC method. Specifically, the variation of tensile and compressive strain concentration contours under increasing levels of forces in healthy and osteoporotic vertebral bone were experimentally measured. The results are helpful for determining the role of bone microarchitecture and mineral density in bone mechanical properties from CT images.

Acknowledgements (Optional): This research is supported by the J. Mike Walker’66 Department of Mechanical Engineering and the Texas A&M Engineering Experiment Station (TEES). We would also like to thank Dr. Jessie Maisano, Dr. Matthew Colbert, and Mr. David Edey at the Department of Geological Sciences, University of Texas at Austin for their help with micro-CT imaging, and the support from the NSF Division of Earth Science Instrumentation and Facilities Program (NSF EAR-1762458) and NASA (80NSSC23K0199).