Contribution of Micro-Architecture in Elastic Modulus of Trabecular Bone

It is widely believed the detailed microstructure of trabecular bone such as the bone volume fraction (the ratio of volume of bone tissue to the overall bone volume), trabecular orientation and connectivity (the extent of trabecular connections) is important in governing the mechanical properties and failure of trabecular bone. However, it is not clear the relative contribution of trabecular bone micro-architecture such as trabecular connectivity, trabecular bone type (rod vs. plate), to trabecular bone strength, in addition to bone volume fraction. A new topology preserving skeletonization and classification technique combined with micro computed tomography (µCT) image based finite element (FE) model enables the analysis of the relationship between the trabecular micro-architecture and mechanical properties of trabecular bone. The unique feature of this technique is the preservation of topological properties (rod-like and plate-like structures, connectivities and cavities) while without maintaining bone volume fraction. In addition, rod-like or plate-like trabeculae can be independently recovered from the skeleton structure and their contribution to mechanical properties of trabecular bone can be quantitatively examined. Therefore, the focus of this study is to characterize quantitatively the contribution of micro-architecture to mechanical properties of trabecular bone using image based finite element analysis technique.

3D Morphological Analysis of Human Trabecular Bone Based on Individual Trabeculae Segmentation

It is clear that the microstructural type of trabeculae (plate vs. rod) is critical in determining the strength of trabecular bone while a dramatic change of trabeculae from plate-like to rod-like occurs with aging and osteoporosis. However, the quantitative contribution from the plate-like and rod-like trabeculae to mechanical properties of trabecular bone has not been fully identified. This is due to the lack of explicit classification of trabecular types with existing morphological analysis techniques. A novel morphological analysis technique based on the µCT image was developed. This technique has the capacity to segment trabecular bone image into different individual trabeculae by following their topological features. The morphological parameters such as the plate fraction (pBV/BV), rod fraction (rBV/BV), trabecular plate thickness (pTb.Th) , trabeculae rod thickness (rTb.Th), trabecular plate number density (pTb.N), trabecular rod number density (rTb.N), trabecular orientation and junction density can be further determined .

Top Figure: Segmentation of trabecular rod;

Left Figure: Segmentation of continuous curved trabecular plate;

Related Publications

1. Liu XS, Saha PK, Wehrli FW, Sajda P, Guo XE, A 3D Morphological Analysis based on Individual Trabeculae Segmentation for Human Trabecular Bone, Baltimore, MD, September 28-October 1, 2005
2. Liu XS, Saha PK, Wehrli FW, Sajda P, Guo XE, A 3D Morphological Analysis of Trabecular Bone Based on Individual Trabeculae Segmentation, Chicago, Illinois, March 19-22, 2006

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With aging, the strength of vertebral bones is impaired by the trabecular bone loss and trabecular network disruption. It has been observed that the replacement of plate-like trabeculae with the rod-like trabeculae leads to increased bone fragility. Moreover, the reduced horizontal struts have been suggested to further reduce the buckling strength of vertical trabeculae. However, there are no available quantitative data on the contribution of plate-like trabeculae vs. rod-like trabeculae. In addition, the loss of horizontal trabeculae in aging vertebral trabecular bone has led to the controversy regarding the relative contribution of horizontal and longitudinal trabeculae in the strength of vertebral trabecular bone. To quantitatively characterize the micromechanics of trabecular bone failure at the individual trabeculae level will improve our understanding of the etiology of age-related vertebral fractures and identify the important microstructural features such as trabecular plates or horizontal trabecular rods in determining mechanical integrity of vertebral trabecular bone, in addition to bone volume fraction.
We have developed a new segmentation technique of individual trabecular plates/rods, in conjunction with materially and geometrically nonlinear micro-computed tomography (µCT) based finite element analysis, which allows explicit determination of micromechanics of individual trabecula failure. The aim of the current study is to determine the role of (1) trabecular types (plates vs. rods) and (2) trabecular orientation (horizontal vs. longitudinal/oblique) in the initiation and progression of plastic yielding in vertebral trabecular bone. This represents a first quantitative study of micromechanics of human vertebral trabecular bone at the individual trabecula level.

Related Publications

1. Liu XS, Gupta A, Bevill G, Keaveny KM, Guo XE, Micromechanical Analysis of Individual Trabeculae in a µCT Based Nonlinear Finite Element Model of Human Vertebral Trabecular Bone, Chicago, Illinois, March 19-22, 2006
2. Liu XS, Gupta A, Bevill G, Keaveny KM, Guo XE, Micromechanical Analysis of Human Vertebral Trabecular Bone At Individual Trabecula Level, ASME Summer Bioengineering Conference , Amelia island, FL, June 21-25, 2006

Simulating 3D Architectural and Mechanical Changes in Human Trabecular Bone During Menopause

The reduced level of estrogen during menopause leads to an increase in bone remodeling activation and a subsequent decrease in bone mass, despite normal bone formation activities. However, kinetic simulations of bone remodeling have indicated that both a maintained increase in bone remodeling activation and a transient imbalance in local bone remodeling (i.e., a small, local deficit between bone formation and bone resorption) are required to predict the clinical course of bone mineral density loss during post-menopausal osteoporosis. The later assumption has not been supported by the available bone histomorphometric data that the resorption depth in older women is the same as in young women. Interestingly, a three-dimensional (3D) simulation of age-related bone remodeling using micro-CT images of trabecular bone also suggests that a bone formation deficit is a dominate cause of post-menopausal bone loss. Therefore, the mechanism of post-menopausal bone loss is still uncertain. In this study, a rigorous 3D image topological analysis technique has been incorporated in the 3D simulation of trabecular bone remodeling with a specific consideration of three types of microscopic bone loss: trabecular perforation, breakage, and isolation. The available literature in bone biology suggests that perforated holes in trabecular plates, broken trabeculae, or small isolated bone volumes broken off from the main architecture are not refilled or re-connected during the remodeling process. In this study, we simulate the bone remodeling process during and after menopause by a detailed 3D human trabecular bone model and quantify the amount of the bone loss due to each mechanism.

Related Publications

1. Liu XS, Huang AH, Sajda P, Guo XE, Simulating 3D Architectural and Mechanical Changes in Human Trabecular Bone During Menopause, Chicago, Illinois, March 19-22, 2006
2. Liu XS, Huang AH, Sajda P, Guo XE, Realistic Simulation of 3D Architectural and Mechanical Alterations in Human Trabecular Bone During Menopause, ASME Summer Bioengineering Conference , Amelia island, FL, June 21-25, 2006

3D Image Analysis and Pattern Recognition for Modeling Trabecular Bone Microstructures

Trabecular bone is composed of a complex 3D microstructure which can be acquired in great detail by high resolution imaging techniques such micro computed tomography (µCT) or micro magnetic resonance imaging (µMRI). In this project, a distinctly new approach to handle microstructural modeling of trabecular bone is proposed. We are developing image analysis procedures on detailed µCT images of trabecular bone for bone tissue segmentation, bone surface compression, bone volume skeletonization and feature analysis for rods/plates such that a reduced finite element microstructural model can be produced using beam/plate structural elements alone. In this approach, bone volume fraction, trabecular orientation and connectivity will be maintained while we expect that a 200-1000 fold reduction of the model size such that the nonlinear behavior of a large bone sample (even whole bone organ) can be analyzed. In the current study, the accuracy of the developed algorithms was validated in idealized trabecular rod and rod-plate models.

Schematics of the Image Processing Approach and its Application to one of the Idealized Structures

Original µCT image of Trabecular Bone	Voxel Based Model	Reduced Model

Related Publications

1. Wei, L., Sajda, P., Laine, A. F. and Guo, X. E. A Novel Approach to Model Trabecular Bone Using Topological Image Analysis, 49th Orthopaedic Research Society Annual Meeting, 2003. Download

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Finite Element Analysis of In Vivo Virtual Bone Biopsy from Hypogonadal Male Patients (Collaborative Project with Laboratory for Structural NMR Imaging, University of Pennsylvania Medical Center)

The change of bone biomechanical properties is important for the evaluation of treatment efficacy in various metabolic diseases such as male hypogonadism that affects primarily trabecular bone and results in impaired mechanical competence secondary to gonadal steroid depletion. Analysis of trabecular bone images from the in vivo micro magnetic resonance imaging (µMRI), virtual bone biopsy (VBB), has been shown to provide detailed insight into the three-dimensional trabecular network topology and scale at the distal tibia. Large-scale finite element (FE) models of trabecular bone have been developed in mapping each image voxel into an element. These voxel-based, specimen-specific FE models of trabecular bone have improved tremendously the fundamental understanding of trabecular bone mechanics. As shown in our collaborative project with the University of Pennsylvania Medical Center, in vivo µ-MRI data from trabecular bone can be used as input into large-scale FE models from which the implications of the disease on the bone's mechanical competence can be assessed.

Related Publications

1. ML Chan, XS Liu, B Vascillic, FW Wehrli, M Benito, PJ Snyder, X Ed Guo. Lower Stiffness Detected in Finite Element Analysis of In Vivo Virtual Bone Biopsy from Hypogonadal Male Patients, 2005 ASME Summer Bioengineering Conference.
2. ML Chan, XS Liu, B Vascillic, FW Wehrli, M Benito, PJ Snyder, X Ed Guo, Mechanical and Three-Dimensional Morphological Changes in Tibial Trabecular Bone of Hypogonadal Patients, Chicago, Illinois, March 19-22, 2006

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Research Grants

1. BES-9875633 CAREER: An Efficient 3D Representation for Modeling Microstructure of Trabecular Bone and Development of An Integrated Program in Computational Biomechanics, The National Science Foundation, 7/1/99-6/30/2003.

2. Micromechanical Modeling of Trabecular Bone, NIH RO1 AR051376, 5/2006~5/2011.

3. Structural MRI of Trabecular Bone for Therapy Response Monitoring (PI: Felix Wehrli), NIH RO1 AR052020, 2005~2010