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After completing this chapter, you will
be able to:
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- 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?
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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.
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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.
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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 (62). Collagen is a protein that provides
bone with flexibility and contributes to its tensile
strength.
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The water content of bone makes up approximately 25–30% of
total bone weight. The water present in bone tissue is an important
contributor to bone strength. For this reason, scientists and engineers
studying the material properties of different types of bone tissue
must ensure that the bone specimens they are testing do not become
dehydrated. The flow of water through bones also carries nutrients
to and waste products away from the living bone cells within the
mineralized matrix. In addition, water transports mineral ions to
and from bone for storage and subsequent use by the body tissues
when needed (18).
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Structural Organization
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The relative percentage ...