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

Abstract

Bone is a vital, dynamic connective tissue that gives form to the body, supporting its weight, protecting vital organs, and facilitating locomotion by providing attachments for muscles to act as levers. It also acts as a reservoir for ions, especially for calcium and phosphate, the homeostasis of which is essential to life. These functions place serious requirements on the mechanical properties of bone, which should be stiff enough to support the body’s weight and tough enough to prevent easy fracturing, as well as it should be able to be resorbed and/or formed depending on the mechanical and biological requirements of the body. Under normal physiological conditions, the structure/function relationships observed in bone, coupled with its role in maintaining mineral homeostasis, strongly suggest that it is an organ of optimum structural design. To fulfill these structure/function relationships adequately, bone is constantly being broken down and rebuilt in a process called remodeling. Bone has the potential to adapt its architecture, shape, and mechanical properties via a continuous process termed adaptation in response to altered loading conditions (Burr et al., 2002; Forwood & Turner, 1995; Hsieh & Turner, 2001). Under normal states of bone homeostasis, the remodeling activities in bone serve to remove bone mass where the mechanical demands of the skeleton are low, and form bone at those sites where mechanical loads are transmitted sufficiently and repeatedly. An early hypothesis about the dependence of the structure and form of bones, and the mechanical loads they carry, was proposed by Galileo in 1638 (Ascenzi, 1993), and was first described in a semiquantitative manner by Wolff (Wolff, 1892). The adaptive response of bone has been a subject of research for more than a century and many researchers have attempted to develop mathematical models for functional adaptation of bone. In this chapter, a brief explanation about the bone structure and mechanics will be provided first. Then, the bone remodeling process and its relation with osteoporosis will be discussed. The important issue of bone quality makes another section of this chapter.