Biomedical Engineering Reference
In-Depth Information
osteocytes in lacunae lying between calcified lamellae. As in osteons,
canaliculi present in trabeculae provide connections between osteo-
cytes. However, because each trabecula is only a few cell layers thick,
each osteocyte can exchange nutrients with nearby blood vessels.
Thus, no central canal is necessary. The Young's modulus of spongy
bone is only about 1 GPa, and the corresponding porosity is between
30% and 90%.
1.2.2 Bone Types Based on Microscopic Observation
On the basis of microscopic observation of matrix arrangement, bones can
be divided as follows:
•
Lamellar bone.
The lamellar bone is a normal type of adult mammalian
bone composed of thin plates (lamellae). Whether spongy or compact,
it is composed of parallel lamellae of bony tissues in the former and
concentric lamellae around a vascular canal in the latter. Lamellar
organization reflects a repeating pattern of collagen fibroarchitec-
ture (Figure 1.2a).
•
Woven bone.
Woven bone is immature bone containing collagen fibers
arranged in irregular random arrays. It has smaller amounts of min-
eral substance and a higher proportion of osteocytes than lamellar
bone (see Figure 1.2b). Woven bone is temporary and is eventually
converted to lamellar bone; this type of bone is also pathological
tissue in adults, except in a few places such as areas near the sutures
of the flat bones of the skull, tooth sockets, and the insertion sites of
some tendons.
1.2.3 Bone Types Based on Geometric Shape
On the basis of geometric shape, bones are classified into six classes: long
bones, short bones, flat bones, irregular bones, pneumatic bones, and sesa-
moid bones.
•
Long bones.
Long bones include some of the longest bones in the
human body, such as the femur, humerus, and tibia, but they are
also some of the smallest, including the metacarpals, metatarsals,
and phalanges. These bones typically have an elongated shaft and
two expanded ends, one on either side of the shaft (see Figure 1.3).
The shaft is known as the diaphysis and the ends are called
epiphyses. Normally, the epiphyses are smooth and articular. The
shaft has a central medullary cavity where the bone marrow lies.
Long bones that have a longitudinal emphasis are used as levers.
The upper extremity bones are lighter, as they are used for a greater
