Chemistry Reference
In-Depth Information
a great variety of structures are obtained due to the adaptation of bone
structure to the actual needs. Nevertheless, the basic material is always
a collagen-mineral nanocomposite, which has already quite remarkable
properties well suited for its mechanical function. The composite con-
tains mineral platelets (essentially nanocrystalline carbonated hydroxy-
apatite [HA]), protein (predominantly collagen type I), and water. This
material combines two components with extremely different properties,
namely, the mineral, which is stiff but brittle, and the (wet) protein,
which is tough but much less stiff than the mineral, resulting in outstand-
ing fracture resistance to the bone material despite the inherent brittle-
ness of the mineral [1] .
As a consequence, the overall structure of bone (from the material
level [Figure 9.1d] to shape and architecture [Figure 9.1a]) needs to be
assessed in diseases which affect bone fragility, such as osteoporosis.
Osteoporosis is a disease which affects roughly 75 million people in
Japan, the United States, and Europe [2]. Loss of bone mass, measured
clinically as change in bone mineral density (BMD), is considered an
important risk factor for bone fragility. However, BMD is not the sole
predictor of whether an individual will experience a fracture [3,4] , and
there is considerable overlap in BMD between populations that do and
do not develop fractures [5-7]. It has been demonstrated that for a
given bone mass, an individual's risk to fracture increases with age [8].
Consistent with these fi ndings, numerous investigators have shown that
mechanical variables directly related to fracture risk are either inde-
pendent [9] or not totally accounted for bone mass itself [10 - 14] . Epi-
demiological evidence also shows considerable overlap of bone density
values between fracture and nonfracture groups, suggesting that low
bone quantity alone is an insuffi cient cause of fragility fractures
[15 - 17] .
It is becoming evident then, that in addition to BMD, bone quality
should also be considered when assessing bone strength and fracture
risk. Bone quality is a broad term encompassing a plethora of factors
such as geometry and bone mass distribution, trabecular bone micro-
architecture, microdamage, increased remodeling activity, along with
genetics, body size, environmental factors, and changes in bone mineral
and matrix tissue properties [6,7]. In addition, new fracture risk factors
are emerging and it is still unclear how these are manifested in BMD
changes (if at all). For example, recent clinical/epidemiological data
[18 - 21] show a defi nite correlation between homocysteine in the blood
and fracture risk. Homocysteine is known to interfere with lysyl oxidase
action [22], thus altering collagen posttranslational modifi cations and
thus collagen cross- link profi les. Moreover, BMD measurements may
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