Chapter 6
Bones and Skeletal Tissues
189
6
Wolff’s law also explains the featureless bones of the fetus
and the atrophied bones of bedridden people—situations in
which bones are not stressed.
How do mechanical forces communicate with the cells respon-
sible for remodeling? Deforming a bone produces an electrical
current. Because compressed and stretched regions are oppositely
charged, it has been suggested that electrical signals direct remod-
eling. Tis principle underlies some of the devices used to speed
bone repair and heal fractures. Fluid flows within the canaliculi
also appear to provide stimuli that direct the remodeling process.
Te skeleton is continuously subjected to both hormonal influ-
ences and mechanical forces. At the risk of constructing too large a
building on too small a foundation, we can speculate that:
Hormonal controls determine
whether
and
when
remodeling
occurs in response to changing blood calcium levels.
Mechanical stress determines
where
remodeling occurs.
For example, when bone must be broken down to increase
blood calcium levels, P±H is released and targets the osteoclasts.
As a result of these mechanical stressors, long bones are
thickest midway along the diaphysis, exactly where bending
stresses are greatest (bend a stick and it will split near the mid-
dle). Both compression and tension are minimal toward the
center of the bone (they cancel each other out), so a bone can
“hollow out” for lightness (using spongy bone instead of com-
pact bone) without jeopardy.
Wolff’s law also explains several other observations:
Handedness (being right or le² handed) results in the bones
of one upper limb being thicker than those of the less-used
limb. Vigorous exercise of the most-used limb leads to large
increases in bone strength
(Figure 6.14)
.
Curved bones are thickest where they are most likely to buckle.
Te trabeculae of spongy bone form trusses, or struts, along
lines of compression.
Large, bony projections occur where heavy, active muscles
attach. Te bones of weight li²ers have enormous thicken-
ings at the attachment sites of the most-used muscles.
Load here
(body weight)
Head of
femur
Compression
here
Point of
no stress
Tension
here
Figure 6.13
Bone anatomy and bending stress.
Body weight
transmitted to the head of the femur (thigh bone) threatens to bend
the bone along the indicated arc, compressing it on one side (con-
verging arrows on right) and stretching it on the other side (diverging
arrows on left). Because these two forces cancel each other internally,
much less bone material is needed internally than superficially.
Cross-
sectional
dimension
of the
humerus
Added
bone matrix
counteracts
added stress
(b)
(a)
Serving arm
Nonserving arm
Figure 6.14
Vigorous exercise can strengthen bone.
The
brown rings in (b) represent differences in the cross-sectional areas
of the serving and nonserving arms of a professional tennis player.
Average increases in bone rigidity and strength of 62% and 45%,
respectively, were recorded in the serving arms. The structural
changes were more pronounced in those who began training at an
early age.
Source:
C. B. Ruff, “Gracilization of the Modern Human
Skeleton,”
American Scientist
94(6): p. 513, Nov–Dec 2006.
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