ABSTRACT
Bone tissue consists of different cell types, a mineralized extracellular matrix (ECM) and a highly organized neurovascular supply. Chemically, it contains about 45% inorganic minerals (predominantly calcium phosphate in the form of hydroxyapatite), 30% organic material (mostly type I collagen) and 25% water. The inorganic components are deposited by a process called mineralization, which is initiated by osteoblasts (see below). Because of its mineralization, bone is able to carry high compressive loads (compressive strengths of the tissue). The organic matrix and, especially, the collagen fibres and their distribution are able to withstand tensile forces (tensile strength of the tissue). Together, the inorganic and organic components enable bone tissue to carry bending and shearing
stresses (bending strength). The compressive strength is higher than the tensile and, especially, the bending strength, which appears to be the weakest. Since all components are arranged in specific axes, bone tissue is an anisotropic material; therefore, the mechanical properties are different in different directions. The water content of bone tissue has also been shown to be important, since bone is to some extent a viscoelastic tissue that shows stiffening with increasing loading rates, a behaviour which is usually related to increasing friction during interstitial fluid flow. Drying of the tissue, therefore, increases the moduli of elasticity, hardness and even the tensile strengths. It is suggested that bone tissue-related biomechanics studies should always be carried out under normal moisture content conditions.
