Biology Reference
In-Depth Information
1996
). Fractures often begin at either the weakest area in the bone, or the area where the force
applied first overcomes the strength of the bone. Fractures can be large and noticeable or as
small as being visible only under a microscope.
Biomechanics and Biomaterials of Human Bone
It is just as important to understand the properties of bone as a tissue as it is to understand
basic biomechanics.
Bone Tissue Structure
Vertebrate skeletal systems contain two types of bone, cortical or compact and cancellous
or spongy (
Harkess et al., 1984
). Cortical bone is stiff and more dense, while cancellous bone
is porous and lightweight with a characteristic fragile honeycomb appearance. While the
makeup of each type of bone is identical, cortical and cancellous bone differ greatly in reac-
tion to force due to their construction. Cortical bone has a higher Young's modulus, indi-
cating greater stiffness (
Nordin and Frankel, 1989
). It can withstand a greater amount of
axial compression than tension before failure. Cancellous bone is less stiff and can withstand
a greater amount of axial tension than cortical bone can. Cortical bone fails when strain
exceeds 2%, while cancellous bone can withstand up to 7% axial tensile load (
Nordin and
Frankel, 1989
).
Bone Histology
A basic understanding of histology is important in order to understand how bone
responds to stress, even on a microscopic level. Bone is composed of cells and an extracellular
matrix. Bone cells include osteoblasts, osteoclasts, and osteocytes (
Bouvier, 1989
). Osteo-
blasts are cuboidal cells that are responsible for the secretion of bone matrix. Osteoclasts
are larger, multinucleated cells responsible for the absorption of bone. Osteocytes are osteo-
blasts that have finished their function of secreting bone matrix and have become trapped
within hardened matrix, and are now responsible for bone maintenance. The circular struc-
tures that house the osteocytes are known as osteons. While most trauma studies deal mainly
with the macroscopic response of bone to trauma, a good foundation in bone histology can
deepen the understanding of bone's response to trauma. Refer to Trammell and Kroman
(Chapter 13), this volume for more information on histology; and for a more comprehensive
overview of bone histology see
Crowder and Stout (2012)
.
Bone's Response to Chronic Stress
Another unique aspect of bone is its ability to remodel in response to chronic stress. These
patterns are referred to as
musculoskeletal markers of stress
(MSM) and are often utilized by
anthropologists to investigate activity patterns. The theory behind stress markers involves
the physiology driving muscle and bone interaction (
Weiss, 2004
). As muscles in the body
are more frequently used, overused, or used in unusual ways, increased stress is put on
the periosteum (nutritive fibrous covering of bone where muscle tendons initially attach)
and the bony cortex. As per
Wolff's law (Wolff, 1892)
, bone is a living responsive material
and therefore it remodels in response to stress, forming larger, rougher areas of muscle
attachment (
Woo et al., 1981
). While the type of stress that induces increased muscle markers
