Biomedical Engineering Reference
In-Depth Information
or cellular link of this finely organized network is chronically disrupted,
such as in osteoporosis, hyperparathyroidism, osteomalacia, and corti-
costeroid-induced osteopenia. Remodeling also permits the restoration
of microdamage caused by fatigue and shock. This constant care of the
bone matrix prevents its premature deterioration and maintains its overall
strength.
As a major reservoir of body calcium, bone is under the hormonal control
of PTH [8]. PTH is the most important hormone regulating calcium homeo-
stasis and bone remodeling. Moreover, PTH is currently involved in numer-
ous clinical trials as an anabolic agent for the treatment of low bone mass
in osteoporosis [20]. Interestingly, the overall effect of PTH on bone mass
depends primarily on its mode of administration; whereas a continuous
increase in PTH levels decreases bone mass, intermittent PTH adminis-
tration increases bone mass [21-25]. Despite many attempts to identify the
source of these differential dosing effects on bone turnover, the precise
mechanism remains elusive. Prevention and reversal of bone loss require
a thorough understanding of the remodeling process in bones and of the
mechanism of bone formation and resorption, including the actions of hor-
mones such as PTH.
Long-term physical activity on a regular basis plays a particularly impor-
tant role in maintaining healthy bones. Exercise can maintain and increase
bone strength by increasing bone mass or by changing bone structures at
micro and macro levels. Two main types of exercise are beneficial to bone
health: weight-bearing exercise and resistance exercise (lifting weights with
arms or legs).
Weight-bearing exercise involves any exercise that is performed while a
person is standing so that gravity is exerting a force. Examples of weight-
bearing activities are jogging, walking, tennis, dancing, golf, and netball.
Activities that are high impact, such as aerobics, running, and jumping, have
a greater effect on bone strength than low-impact activities, such as walking
and cycling.
Resistance exercises, also called strength training, can also have a positive
effect on the health of bones. In moving a heavy weight, the strong mus-
cle contractions place stress or strain on the bone to which the muscles are
attached. When a bone is repeatedly strained (as also happens in regular
exercise training), it responds by increasing bone mass to become stronger.
Generally speaking, physical activities place mechanical loading on the
skeleton and the loading has profound influences on bone remodeling.
Disused or reduced loading due to long-term bed rest, cast immobiliza-
tion, or microgravity conditions (such as that experienced by astronauts in
a space station or shuttle) induces obvious bone loss and mineral changes.
Conversely, overuse or increased loading has the opposite effect. In tennis
players, the bones of their racquet arms display significantly higher bone
mineral density and cortical bone content than the bones of their nonplay-
ing arms. Although a great deal of general knowledge exists about the role
