ITS and FEA of skeletons

In the area of image based microstructural and finite element analyses of human skeletons, we have innovated and advanced an individual trabeculae segmentation (ITS) technique and demonstrated its unique power in basic micromechanics of human trabecular bone. More importantly, we have successfully translated this ITS technology into clinical studies of metabolic bone diseases. The most exciting development is the recent breakthrough in identifying dramatic and striking microstructural differences in bones between Chinese-Americans and Caucasians. It will have significant clinical, basic science, and anthropological implications as we begin to explore the genetic and environmental causes of these remarkable differences. Projects in this area are listed below.

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.

(Collaborative Project with Berkeley Orthopaedic Biomechanics Laboratory, UC Berkeley)

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.

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.

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.

(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.

The property of cement line has been hypothesized to play an important role in strength of cortical bone, and may also be a crucial factor for understanding lamellar structures in both cortical and trabecular bone tissues. The lamellar properties of bone tissue are crucial in determining mechanical properties at sub-microstructural level

Application of fiber-matrix composite fracture mechanics methods to predict strength, fracture process in osteonal cortical bone. It has been long hypothesized that cortical bone behaves like a fiber-matrix composite material without any verification. The purpose of this study is to verify applicability of current fracture mechanics techniques for fiber-matrix composites to cortical bone, to quantify contributions of various microstructural components to fracture properties of cortical bone.

In collaboration with the Orthopaedic Research Laboratory at Columbia University, we are developing a mixed finite element formulation for triphasic mechano-electrochemical theory for charged, hydrated biological soft tissues, such as cartilage and cells. The finite element formulation is developed using the standard Galerkin weighted residual method. The finite element formulation has been used to investigate a triphasic stress relaxation problem in the confined compression configuration and a triphasic free swelling problem.

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