Biomedical Engineering Reference
In-Depth Information
Bone fractures are a major health care concern for postmenopausal women and the
elderly of both genders. Postmenopausal and age-related osteoporosis/osteopenia is
a major contributor to the risk of such fractures. Since cortical bone is the major load
bearing tissue, age-related and pathological changes in cortical bone tissues have
been of interest to bone researchers. Although there are multiple factors, cortical
bone fragility is ultimately determined by two biomechanical factors: its micro-
structure and intrinsic tissue quality. The microstructural integrity is mainly related
to porosity and some other microstructural features (e.g., osteon/interstitial, cement
lines), whereas the intrinsic tissue quality is primarily affected by the composition
and ultrastructural features of bone constituents (i.e., mineral, collagen, and water).
This chapter intends to provide a fundamental review regarding the composition and
mechanical behavior of cortical bone and how these are affected by aging and gender.
2 Hierarchy of Human Cortical Bone
Human cortical bone has a complex, hierarchical structure. At its most simple
form it can be considered a two-phase material (*95% solid, *5% porous).
A typical section of a human long bone reveals the cortical microstructure of
osteons and interstitial tissue between periosteal and endosteal lamellae (Fig.
1
).
Lamellae are layered sheet-like structures and serve as basic building units of
human cortical bone. Osteons (also called Haversian systems) are tube-like
structures of multiple concentric lamellae with a canal in the center that accom-
modates blood vessels for transportation of nutrients and removal of wastes by
bone cells (e.g., osteocytes). The interstitial tissue is also lamellar by nature and is
actually the remnants of either primary bone or previously formed osteons that are
partially removed by the bone remodeling process.
2.1 Composition of Cortical Bone
Solid bone tissue can be characterized as a natural composite of mineral (apatite
crystals), organic matrix (mostly type I collagen fibrils), and water [
2
-
5
]. The
mineral phase comprises *60% by weight (*40% by volume), the organic matrix
*30% by weight (*40% by volume), and water *10% by weight (*25% by
volume) [
6
,
7
].
2.1.1 Mineral Phase
The mineral phase mainly consists of crystals that have a composition of calcium
(Ca
2+
) and phosphate (PO
4
3-
) with a small fraction of carbonates (CO
3
2-
) and
other ''impurities'' (sodium, magnesium, potassium, citrate, fluoride, HPO
3-
)[
8
].
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