Biomedical Engineering Reference
In-Depth Information
1.6 Bone Metabolism
Material metabolism is a chemical process by which cells produce the
substances and energy needed to sustain life. As part of metabolism,
organic compounds are broken down to provide heat and energy in the
process called catabolism. Simpler molecules are also used to build more
complex compounds like proteins for growth and repair of tissues, as part
of anabolism (the constructive phase of metabolism characterized by the
conversion of simple substances into the more complex compounds of
living matter).
Many metabolic processes are brought about by the action of enzymes.
The overall speed at which an organism carries out its metabolic processes
is termed its metabolic rate (or, when the organism is at rest, its basal meta-
bolic rate). Birds, for example, have a high metabolic rate, since they are
warm-blooded, and their usual method of locomotion, flight, requires
large amounts of energy. Accordingly, birds usually need large amounts of
high-quality, energy-rich foods such as seeds or meat, which they must eat
frequently. As an example of metabolism, bone metabolism is a lifelong pro-
cess in which mature bone tissue is removed from the skeleton (a process
called bone resorption) and a new bone tissue is formed (a process called
ossification or new bone formation). These processes also control the
reshaping or replacement of bones following injuries like fracture as well as
following microdamage that occurs during normal activities. Metabolism
also responds to the functional demands of mechanical loading. In the first
year of life, almost 100% of the skeleton is replaced. In adults, remodeling
proceeds at about 10% per year.
The cells responsible for bone metabolism are known as osteoblasts, which
secrete new bone, and osteoclasts, which break down bone. The structure
of bones as well as an adequate supply of calcium requires close coopera-
tion between these two types of cells. Complex signaling pathways are
used to achieve proper rates of growth and differentiation. These signaling
pathways include the action of several hormones, such as PTH, vitamin D,
growth hormone, steroids, and calcitonin, as well as several cytokines. In
this way the body is able to maintain adequate levels of calcium required for
physiological processes.
After appropriate signaling, osteoclasts move to resorb the surface of bone,
followed by deposition of bone by osteoblasts. Together, the cells that are
responsible for bone remodeling are known as the basic multicellular unit
(BMU), and the temporal duration (i.e., life span) of the BMU is referred to as
the bone remodeling period [10].
As was mentioned and stated in Kalfas [11], bone metabolism is under con-
stant regulation by a host of hormonal and local factors. Three of the calci-
tropic hormones that most affect bone metabolism are parathyroid hormone,
vitamin D, and calcitonin. Parathyroid hormone increases the flow of calcium
