Biomedical Engineering Reference
In-Depth Information
1 Introduction
Aging takes its toll on the tissues through multiple facets such as regular wear and
tear, cumulative exposure to chemical and physical factors, diet, exercise and other
environmental factors [
100
]. There is also a hereditary aspect to the rate at which
aging occurs. The net effect of aging is gradual impairment in the function of the
tissue. Even though bone is one of the few tissue systems with self-regeneration
capacity, it is also not immune to aging. The fabric-level changes in mineral and
collagen framework results in organ, and thereby, organism level deterioration of
function, which are discerned as alterations in gait, posture and even macro
anatomy such as shortening of spine due to accumulation of creep deformation in
vertebral bodies. This chapter surveys the existing literature on age-related
changes in bone's mineral and microstructural aspects. It is important to underline
that the present review does not refer aging as pathology, rather, a natural process
during which fractures do not occur. Studies emerging from such sample sets are
categorized as within the aging context. On the other hand, the body of literature
which screened their sample set for osteoporosis, be it by the way of bone mineral
density measurements or by the way of sample harvested from patients with
osteoporotic fractures, is presented separately in
Sect. 4
. Another important detail
that affects data interpretation is the distinction between growth and maturation
(taking place between birth and adulthood) versus aging (taking place from
adulthood to end of life). Finally, the review is limited to humans and primates, as
the longevity of life and physiology of bone in humans is not sufficiently reflected
by rodent models.
2 Age-Related Changes in Bone's Mineral Phase
2.1 Bone Matrix Mineral Content
Bone is mainly composed of mineral, collagen and water, where the mineral
composition resembles carbonated apatite, Ca
10
(PO
4
)
6-x
(OH)
2-y
(CO
3
)
x+y
(Fig.
1
). The mineral phase of bone has often been erroneously referred to as
hydroxyapatite. While the presence of Ca and PO
4
3-
ions are common in both, the
presence of OH
-
ion has been contested, particularly in the recent literature
[
73
,
121
]. The structure of mineral lattice, due to a wide array of non-stoichi-
ometric substitutions, makes it even more divergent from hydroxyapatite.
Therefore, carbonated apatite or bone apatite would be more accurate in referring
to bone's mineral phase.
Generally, changes in bone mineral are discussed with regard to content, size
and shape of the mineral which are measured using variety of methods. Mineral
content at the organ level is often characterized by bone mineral content (BMC),
or bone mineral density (BMD). Specifically, dual-energy X-ray absorptiometry
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