Hierarchical Structure of Porosity in Cortical and Trabecular Bones
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Hierarchical Structure of Porosity in Cortical and Trabecular Bones Ekaterina Novitskaya1, Elham Hamed2, Jun Li2, Zherrina Manilay3, Iwona Jasiuk2, and Joanna McKittrick1, 3 1 University of California, San Diego, Materials Science and Engineering Program, 9500 Gilman Dr., La Jolla, CA 92093, USA 2 University of Illinois at Urbana-Champaign, Department of Mechanical Science and Engineering, 1206 West Green Street, Urbana, IL 61801, USA 3 University of California, San Diego, Department of Mechanical and Aerospace Engineering, 9500 Gilman Dr., La Jolla, CA 92093, USA ABSTRACT In this paper the amount and morphology of cortical and trabecular bone porosities were estimated using optical microscopy and micro-computed tomography technique. The hierarchical structure of porosity at different structural scales spanning from a single lacuna (sub-microscale) to trabecular or cortical bone levels (mesoscale) was characterized and described. This study was conducted by using samples of untreated, deproteinized and demineralized bones, to obtain better insight into the bone structure and porosities. The motivation of this work is that the porosity in bone has a major effect on its mechanical response, yet it is often neglected in bone models. Investigations of the mechanical properties of bone and its main components (collagen and mineral phases), complemented by modeling, are of importance in orthopedics. INTRODUCTION Bone is a hierarchically structured composite material. It is mainly composed of a biopolymer (type-I collagen), a mineral phase (carbonated hydroxyapatite), and water. In mammalian skeletal bone, there is 33-43 vol.% mineral, 32-44 vol.% organic, and 15-25 vol.% water [1]. Bone is a multifunctional material which in addition to supporting the body and aiding in locomotion, protects internal organs and produces marrow for mineral storage. There are two main types of bone: cortical and trabecular. The cortical bone forms a dense outer sheath, while highly porous trabecular bone is sandwiched between the cortical layers and mostly provides energy absorbent properties (for example, in ribs, skull, and femoral heads). As a structural material, bone is strong, stiff, tough, and light weight (due to porosity) and moreover possesses the ability to heal and regenerate. Excellent mechanical properties of bone are the result of its multilayered hierarchical structure spanning from nano- to macro- levels. The bone porosity is also hierarchically arranged. Bone porosity has a significant effect on its mechanical properties. Therefore, its characterization is needed to better assess bone quality and evaluate bone strength and risk of fracture in elderly and osteoporotic patients. The relation between cortical bone porosity and its mechanical properties was obtained experimentally [2-4]. Recently, micro-computed tomography (ยต-CT) and nano-computed tomography (nano-CT) techniques were used to characterize the structure and porosity of cortical [5-7] and trabecular bones [8-10] at different length scales. However, a systematic hi
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