Page 246 - Biomedical Engineering and Design Handbook Volume 1, Fundamentals

BONE MECHANICS 223

FIGURE 9.1 The four levels of bone microstructure, from the level of
mineralized collagen fibrils to cortical and trabecular bone. It is generally
assumed that at the former level, all bone is equal, although there may be
subtle differences in the nature of the lamellar architecture and degree of
mineralization between cortical and trabecular bone. (Adapted from
Ref. 145.)

which is a thin layer of calcified mucopolysaccharides with very little collagen and low mineral con-
7
tent, remains around the perimeter of each newly formed osteon. The cement line is thought to rep-
8
resent a weak interface between the osteon and the surrounding interstitial bone. These weak
interfaces are thought to improve the fatigue properties of cortical bone by providing avenues for dis-
sipation of energy during crack propagation. 7
The bone matrix that comprises lamellar and woven bone contains another level of porosity on
the order of 5 to 10 mm that is associated with the bone cells (see Fig. 9.2a, b, c). Osteocytes, the
most common type of bone cell, are surrounded by a thin layer of extracellular fluid within small
ellipsoidal holes (5 mm minor diameter, 7 to 8 mm major diameter) called lacunae, of which there
3
are about 25,000 per mm in bone tissue. The lacunae are generally arranged along the interfaces
between lamellae. However, the lacunae also have a lower-scale porosity associated with them. Each
osteocyte has dendritic processes that extend from the cell through tiny channels (≈0.5 mm diameter,
3 to 7 mm long) called canaliculi to meet at cellular gap junctions with the processes of surrounding
∗
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cells. There are about 50 to 100 canaliculi per single lacuna and about 1 million per mm of bone.
At the highest hierarchical level (1 to 5 mm), there are two types of bone: cortical bone, which
comes as tightly packed lamellar, Haversian, or woven bone; and trabecular bone, which comes as a
highly porous cellular solid. In the latter, the lamellae are arranged in less well-organized “packets”
to form a network of rods and plates about 100 to 300 mm thick interspersed with large marrow
spaces. Many common biological materials, such as wood and cork, are cellular solids. 9
The distinction between cortical and trabecular bone is most easily made based on porosity.
Cortical bone can be defined as bone tissue that has a porosity P of less than about 30 percent or,
equivalently, a volume fraction V of greater than about 0.70 (V f = 1 − P). Volume fraction is the ratio
f
of the volume of actual bone tissue to the bulk volume of the specimen. In the field of bone
mechanics, porosity measures usually ignore the presence of lacunae and canaliculi. Porosity of adult

∗ Gap junctions are arrays of small pores in the cell membrane that make connections between the interiors of neighboring
cells, allowing direct passage of small molecules such as ions from one cell to another.