Biomechanics of Osteoporosis: The Importance of Bone Resorption and Remodeling Processes

Osteoporosis 60Two mixture models of bone resorption, a bi-and a tri-phasic model of bone resorption will be reviewed, followed by a 2D model investigating the effects of osteocytes number and mechanosensitivity on bone loss.Discussion and conclusions make the last section of this chapter. Bone structure and mechanicsBone is the main constituent of the skeletal system and differs from the connective tissues in rigidity and hardness.The rigidity and hardness of bone enable the skeleton to maintain the shape of the body; to protect the vital organs; to supply the framework for the bone marrow; and also to transmit the force of muscular contraction from one part to another during movement.It is made basically of the fibrous protein collagen, impregnated with a mineral closely resembling calcium phosphate (Currey, 2002).The mineral content of bone acts as a reservoir for ions, particularly calcium (almost 99% of the calcium of our body is stored in bone), and it also contributes to the regulation of extracellular fluid composition.It also contains water, which is very important mechanically, some not well understood proteins and polysaccharides, living cells and blood vessels.The organic matrix of bone consists of 90% collagen, the most abundant protein in the body, and about 10% of various noncollagenous proteins (Behari, 1991).The protein part, mainly collagen type I, forms a model for the subsequent deposition of hydroxyapatite, the mineral phase of bone which p r o v i d e s r i g i d i t y t o t h e s t r u c t u r e .F r o m m e c h a n i c a l p o i n t o f v i e w , b o n e i s a nonhomogeneous and anisotropic material.Spongy and cortical bones can be considered as orthotropic and transversely isotropic materials, respectively.In the physiological range of loading, bone can be assumed as a linear elastic material, with negligible viscoelastic effects (Rouhi, 2006a).Bone is stronger in compression than in tension, and much greater young's moduli of elasticity than shear modulus (Bartel et al., 2006).Outstanding mechanical properties of bone can be achieved by a very complex hierarchical structure of bone tissue, which has been explained in a number of reviews (Weiner and Wagner, 1998;Fratzl et al., 2004;Fratzl and Weinkamer, 2007).The mechanical performance of bone tissue depends on all levels of hierarchy.The term composite is usually employed for those materials in which two or more distinct phases are separated on a scale larger than the atomic, and in which their material properties such as stiffness and strength are altered compared with those of a homogeneous material.On the basis of the definition of a composite and also by considering bone structure, it is clear that bone is a composite material.Bone, as a biocomposite, shows hierarchical structures at different scales (Lakes, 1993).For example, in cortical bone, on the microstructural level, there are osteons or Haversian systems, which are large hollow fibers (200 to 250 μm outer diameter) composed of concentric lamellae and of pores.The lamellae are made up of fibers, and the fibers contain fibrils.At the molecular level, the sophisticated structural interaction between the organic and inorganic phases is one of the fundamental determinants of the astonishing mechanical properties of bone.The underlying assumption is that a strong bonding between mineral and collagen allows the former to stiffen the collagen matrix through shear stress transfer.There are some important questions related to the composite nature of bone, which need to be addressed in order to make one able to understand the mechanics of bone as a composite at different hierarchical levels, such as: What are the properties of organic and mineral www.intechopen.com

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