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Skeletal structure 115

this philosophical and descriptive understanding of the role of mechanics
in skeletal growth. As the twentieth century progressed, biology increas-
ingly reduced the organism to the molecular level, and the interest in
mechanics and other biophysical factors waned. In recent years, the emer-
gence of several new technologies has fostered a reexamination of the old
questions relating to the mechanical regulation of tissue growth and adap-
tation. The first of these is computer-based structural modeling, which
allows a more valid analysis of effects of physical forces within complex
skeletal geometries; the second is molecular biology, which localises indi-
vidual gene expression and protein synthesis under different mechanical
forces; and the third is the tremendous advances in imaging technologies
that enable scientists to identify microstructural characteristics of tissues
and the role of cells in constructing and maintaining skeletal strength. In
this essay, we call on our current understanding of the role of mechanical
forces in skeletal biology to highlight the interaction between the physical
and biological sciences.

7.2 Form and function in bone

The musculoskeletal system consists of bones, blood vessels, nerves, liga-
ments, tendons, muscles, and cartilage, which work together to perform
the structural and kinematic functions of the organism. These musculos-
keletal tissues all have a composite structure of cells embedded in a matrix
produced by the cells themselves.

7.2.1 Bone structure
The geometry and structure of a bone consist of a mineralised tissue pop-
ulated with cells. This bone tissue has two distinct structural forms: dense
cortical and lattice-like cancellous bone, see Figure 7.2(a). Cortical bone is
a nearly transversely isotropic material, made up of osteons, longitudinal
cylinders of bone centred around blood vessels. Cancellous bone is an
orthotropic material, with a porous architecture formed by individual
struts or trabeculae. This high surface area structure represents only 20 per
cent of the skeletal mass but has 50 per cent of the metabolic activity. The
density of cancellous bone varies significantly, and its mechanical behavi-
our is influenced by density and architecture. The elastic modulus and
strength of both tissue structures are functions of the apparent density.