Biomedical Engineering Reference
In-Depth Information
Bones can be either woven or lamellar. The fibers of woven
bones are randomly aligned and as the result have a low strength. In
contrast, lamellar bones have parallel fibers and are much stronger.
Woven bones are put down rapidly during growth or repair [574]
but as growth continues, they are often replaced by lamellar bones.
The replacement process is called “secondary bone formation” and
described in details elsewhere [575 and references therein]. In
addition, bones might be long, short, flat, and irregular. The sizes and
shapes of bones reflect their function. Namely, broad and flat bones,
such as scapulae, anchor large muscle masses, flat skull bones protect
the brain, ribs protect the lungs, pelvis protects other internal organs,
short tubular bones in the digits of hands and feet provide specific
grasping functions, hollow and thick-walled tubular bones, such as
femur or radius, support weight and long bones enable locomotion
[576, 577]. Long bones are tubular in structure (e.g., the tibia). The
central shaft of a long bone is called the diaphysis and has a medullar
cavity filled with bone marrow (Fig. 1.10). Surrounding the medullar
cavity is a thin layer of cancellous bone that also contains marrow.
The extremities of the bone are called the epiphyses and are mostly
cancellous bone covered by a relatively thin layer of compact bone.
Short bones (e.g., finger bones) have a similar structure to long bones,
except that they have no medullar cavity. Flat bones (e.g., the skull and
ribs) consist of two layers of compact bone with a zone of cancellous
bone sandwiched between them. Irregular bones (e.g., vertebrae) do
not conform to any of the previous forms. Thus, bones are shaped in
such a manner that strength is provided only where it is needed. All
bones contain living cells embedded in a mineralized organic matrix
that makes up the main bone material [576-578]. The structure of
bones is most easily understood by differentiating between seven
levels of organization because bones exhibit a strongly hierarchical
structure (Fig. 1.11) [432, 454, 533, 544, 551-556, 562-567, 569-
572, 579-584].
The mechanical properties of bones reconcile high stiffness and
high elasticity in a manner that is not yet possible with synthetic
materials [584]. Cortical bone specimens have been found to
have tensile strength in the range of 79-151 MPa in longitudinal
direction and 51-56 MPa in transversal direction. Bone's elasticity
is also important for its function giving the ability to the skeleton to
withstand impact. Estimates of modulus of elasticity of bone samples
are of the order of 17-20 GPa in longitudinal direction and of 6-13
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