After completing this chapter, you will be able to:
Explain how the material constituents and structural organization of bone affect its ability to withstand mechanical loads.
Describe the processes involved in the normal growth and maturation of bone.
Describe the effects of exercise and of weightlessness on bone mineralization.
Explain the significance of osteoporosis and discuss current theories on its prevention.
Explain the relationship between different forms of mechanical loading and common bone injuries.
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What determines when a bone stops growing? How are stress fractures caused? Why does space travel cause reduced bone mineral density in astronauts? What is osteoporosis and how can it be prevented?
The word bone typically conjures up a mental image of a dead bone—a dry, brittle chunk of mineral that a dog would enjoy chewing. Given this picture, it is difficult to realize that living bone is an extremely dynamic tissue that is continually modeled and remodeled by the forces acting on it. Bone fulfills two important mechanical functions for human beings: (a) it provides a rigid skeletal framework that supports and protects other body tissues and (b) it forms a system of rigid levers that can be moved by forces from the attaching muscles (see Chapter 12). This chapter discusses the biomechanical aspects of bone composition and structure; bone growth and development; bone response to stress, osteoporosis, and common bone injuries.
lever a relatively rigid object that may be made to rotate about an axis by the application of force
COMPOSITION AND STRUCTURE OF BONE TISSUE
The material constituents and structural organization of bone influence the ways in which bone responds to mechanical loading. The composition and structure of bone yield a material that is strong for its relatively light weight.
The major building blocks of bone are calcium carbonate, calcium phosphate, collagen, and water. The relative percentages of these materials vary with the age and health of the bone. Calcium carbonate and calcium phosphate generally constitute approximately 60–70% of dry bone weight. These minerals give bone its stiffness and are the primary determiners of its compressive strength. Other minerals, including magnesium, sodium, and fluoride, also have vital structural and metabolic roles in bone growth and development. Collagen is a protein that provides bone with flexibility and contributes to its tensile strength.
stiffness ratio of stress to strain in a loaded material—that is, the stress divided by the relative amount of change in the structure's shape