Biomedical Engineering Reference
In-Depth Information
with bone marrow. however, the content of the marrow transforms after birth
from red (hematopoietic, metabolically active) into yellow (fatty tissue that
is not metabolically active) marrow during skeletal maturation.
13-15
2.2.2 Trabecular bone
In contrast to the low porosity of cortical bone, trabecular (or cancellous)
bone has high porosity, with pores making up 50-90% of total trabecular
bone volume. These pores are interspersed among an orderly arranged
network of vertical and horizontal plate- and rod-like structural elements
called trabeculae, which give trabecular bone a sponge-like appearance. The
reduced matrix mass per unit volume and high porosity of trabecular bone
reduces its compressive strength to approximately one-tenth that of cortical
bone;
16
however, it has the function of providing increased surface area
for red bone marrow, blood vessels and connective tissues to be in contact
with bone. This facilitates the role of bone in hematopoiesis and mineral
homeostasis.
Trabecular bone does not have the strength of cortical bone; however, it
contributes to the mechanical role of bone by providing internal support.
This supportive role facilitates the ability of bone to evenly distribute load
and absorb energy, particularly in the vicinity of joints. It is also important
during aging, as trabecular bone is lost earlier and at a greater rate than
cortical bone.
17
This ultimately contributes to osteoporosis at skeletal sites
rich in trabecular bone, such as the femoral neck and vertebral bodies. Bone
strength at these sites is determined by the number, thickness, spacing,
distribution and connectivity of trabeculae, with the later being particularly
important.
18
For a given trabecular density, loss of connectivity has a more
deleterious effect on bone strength than the presence of thin but well connected
trabeculae.
19-21
This is supported by the finding that women with low bone
mass and vertebral fractures have four times as many unconnected trabeculae
as women without fractures, despite a similar bone mineral density.
22
2.3 Microscopic bone anatomy
2.3.1 Bone coverings
The microscopic anatomy of bone is demonstrated in Fig. 2.3. The outer
and inner surfaces of bone are covered by specialized connective tissues
called the periosteum and endosteum, respectively. The periosteum serves
as a transitional fibrous layer between cortical bone and the overlying soft
tissue or musculature. It covers the external surfaces of most bones, except
at articular surfaces, tendon insertions and the surfaces of sesamoid bones.
23
The periosteum can be divided into two distinct layers (Fig. 2.3F). The
