204
UNIT TWO
Osteopenia and osteoporosis are assessed
by measuring bone mineral density (BMD). This
is most often done in the hip bone and lower
spine with a technique called dual-energy X-ray
absorptiometry. Osteopenia is defined as BMD
at least 1 to 2.5 standard deviations below the
mean. Osteoporosis is deF
ned as BMD at least 2.5
standard deviations below the mean for young
adults. These measurements produce T values.
Another measurement, a Z value, compares BMD
to other individuals of a person’s age and is used
to assess skeletal health in individuals under age
sixty-F
ve.
People approaching retirement age are not
the only ones who should be concerned about
osteopenia and osteoporosis, because these con-
ditions can be delayed or prevented if dealt with
early. Researchers think that what puts people
at risk is failing to attain maximal possible bone
density by age thirty. To keep bones as strong as
possible for as long as possible, it is essential to
get at least thirty minutes of exercise daily (some
weight bearing), consume enough daily calcium
(1,000–1,200 mg) and vitamin D (200 IU), and
not smoke. There is much you can do to promote
skeletal health—at any age.
A “fragility fracture” is a telltale sign of dan-
gerously low bone density. This is a fracture
that happens after a fall from less than standing
height, which a strong, healthy skeleton could
resist. ±ragility fractures occur in 1.5 million peo-
ple in the United States each year, yet despite
this warning sign, only one quarter to one third
of them are followed up with bone scans and
treatment to build new bone tissue. Since 1995,
five new drugs have become available to treat
osteoporosis. One class, the bisphosphonates,
builds new bone. They are taken once a week or
once a month.
Osteopenia and osteoporosis are com-
mon. The Surgeon General estimates that half
of people over age fifty have either condition,
which amounts to thirty-F
ve million people with
osteopenia and another ten million people with
osteoporosis. Screening is advised for all individu-
als over age sixty-F
ve, as well as for those with risk
factors. The most telling predictor is a previous
fragility fracture. Other risk factors include genetic
predisposition, low dietary calcium, lack of exer-
cise, smoking, drinking alcohol, recent weight loss,
recent height loss (this could be an asymptomatic
compression fracture), and older age.
S
keletal health is a matter of balance. Before
age thirty, cells that form new bone tissue
(osteoblasts) counter cells that degrade it
(osteoclasts), keeping living bone in a constant
state of remodeling. Over time, the balance shifts
so that bone is lost, especially in women past
menopause due to falling estrogen levels. This
imbalance may progress to osteopenia (“low
bone mass”) and, eventually, the more severe
osteoporosis (“porous bones”).
Osteopenia and osteoporosis are a contin-
uum in the breakdown of the microarchitecture
of bone tissue. By the time the declining bone
mass is considered osteoporosis, trabeculae are
lost and the bones develop spaces and canals,
which enlarge and F
ll with F
brous and fatty tis-
sues. Such bones easily fracture and may sponta-
neously break because they are no longer able to
support body weight. Sections of the backbone
(vertebrae) may collapse or the distal portion of
a forearm bone (radius) snap as a result of minor
stress. Most common is hip fracture, which hap-
pens to 200,000 senior citizens in the United
States each year. The femur may begin to frac-
ture from a minor movement before the fall that
seems to be the cause of the break.
7.2
CLINICAL APPLICATION
Osteopenia and Osteoporosis: Preventing “Fragility Fractures”
Biomineralization—the combining of minerals with organic mol-
ecules, as occurs in bones—is seen in many animal species. Ancient
Mayan human skulls have teeth composed of nacre, also known as
“mother-of-pearl” (found in clam shells) attached to human tooth
roots. The Mayan dentists knew that the human body could some-
how incorporate a biomineral from another species. Today, nacre is
used to F
ll in bone lost in the upper jaw. The nacre not only does not
evoke rejection by the immune system, but it also stimulates the per-
son’s osteoblasts to produce new bone tissue.
PRACTICE
12
Name the major functions of bones.
13
Distinguish between the functions of red marrow and yellow
marrow.
14
Explain regulation of the concentration of blood calcium.
15
List the substances normally stored in bone tissue.
The human body requires calcium for a number of vital
metabolic processes, including muscle cell contraction,
nerve impulse conduction, and blood clot formation. When
the blood is low in calcium, parathyroid hormone stimulates
osteoclasts to break down bone tissue, releasing calcium salts
from the extracellular matrix into the blood. On the other
hand, very high blood calcium inhibits osteoclast activity,
and calcitonin from the thyroid gland stimulates osteoblasts
to form bone tissue, storing excess calcium in the extracellu-
lar matrix
(f g. 7.13)
. This response is particularly important
in developing bone matrix in children. The details of this
homeostatic mechanism are in chapter 13, pp. 501–503.
In addition to storing calcium and phosphorus (as cal-
cium phosphate), bone tissue contains smaller amounts of
magnesium, sodium, potassium, and carbonate ions. Bones
also accumulate certain harmful metallic elements such as
lead, radium, and strontium, which are not normally present
in the body but sometimes accidentally ingested.
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