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U N I T 1 2
Musculoskeletal Function
Osteopenia
Osteopenia is a condition that is common to all meta-
bolic bone diseases. It is characterized by a reduction
in bone mineral density greater than expected for age,
race, or gender, and occurs because of a decrease in bone
formation, inadequate bone mineralization, or excessive
bone deossification.
1
Osteopenia
is not a diagnosis but
a term used to describe an apparent loss of bone density
seen on x-ray studies.
2
Approximately 60% of bone is
mineral content, approximately 30% is organic matrix,
and the remainder is living bone cells. Osteopenia can
involve a decrease in bone matrix due to an imbalance
between bone formation and destruction, or a decrease
in mineralization. The major causes of osteopenia are
osteoporosis, osteomalacia, malignancies such as mul-
tiple myeloma, and endocrine disorders such as hyper-
parathyroidism and hyperthyroidism.
Osteoporosis
Osteoporosis is a metabolic bone disease characterized
by decreased bone density (i.e., increased porosity) and
strength in which both the bone matrix and mineraliza-
tion are decreased.
3–6
The World Health Organization
has defined osteoporosis as a bone mineral density
(BMD) value greater than 2.5 standard deviations (SD)
below the mean for a young adult reference population.
7
The most useful methods of estimating fracture risk are
BMD testing and consideration of clinical risk factors
for fracture.
8
Fracture risk assessment tests, such as
the International Osteoporosis Foundation one-minute
osteoporosis risk test, are available online to estimate
the fracture probability.
9
Osteoporosis can occur as the result of a number of
disorders, but is most often associated with the aging
process. In the United States alone, osteoporosis affects
approximately 10 million persons aged 50 years or
older, and an additional 34 million have low bone mass
(osteopenia) and are potentially at risk for development
of osteoporosis and its complications.
10
Etiology and Pathogenesis
The cause of osteoporosis remains largely unknown, but
most data suggest an imbalance between bone resorp-
tion and formation such that bone resorption exceeds
bone formation. Although both of these factors play a
role in most cases of osteoporosis, their relative contri-
bution to bone loss may vary depending on age, gen-
der, genetic predisposition, activity level, and nutritional
status.
Under normal conditions, bone mass increases
steadily during childhood, reaching a peak in the young
adult years. The peak bone mass, or BMD, is an impor-
tant determinant of the subsequent risk for osteoporo-
sis. It is determined in part by genetic factors, hormone
(estrogen) levels, exercise, calcium intake and absorp-
tion, and environmental factors (Chart 44-1). Genetic
factors are linked, in largest part, to the maximal
amount of bone in a given person, referred to as
peak
bone mass.
Race is a key determinant of BMD and the
risk of fractures. Incidence rates obtained from stud-
ies among racial and ethnic groups demonstrate that
although women have higher fracture rates compared
with men overall, these differences vary by race and age.
White and Asian women had higher rates for all age
groups older than 50 years.
10
The highest BMD values
and lowest fracture rates have been reported for black
women.
10
Body size is another factor affecting the risk of
osteoporosis and risk of fractures. Women with smaller
body builds are at increased risk of hip fracture because
of lower hip BMD.
Hormonal factors play a significant role in the devel-
opment of osteoporosis, particularly in postmeno-
pausal women.
11
Postmenopausal osteoporosis, which
is caused by an estrogen deficiency, is manifested by a
loss of cancellous bone and a predisposition to frac-
tures of the vertebrae and distal radius. The loss of bone
mass is greatest during early menopause, when estrogen
levels are withdrawing. Several factors appear to influ-
ence the increased loss of bone mass associated with an
estrogen deficiency, including an increased secretion of
cytokines by monocytes and bone marrow cells. These
cytokines stimulate osteoclast recruitment and activity
by increasing the levels of RANK ligand (RANKL) while
CHART 44-1
Risk Factors Associated with
Osteoporosis*
Personal Characteristics
Advanced age
Female
Gender
Ethnicity (white or Asian)
Small bone structure/low body weight
Postmenopausal
Family history
Lifestyle
Sedentary
Calcium/Vitamin D deficiency
High-protein diet
Excessive alcohol intake
Excessive caffeine intake
Smoking
Drug and Disease Related
Aluminum-containing antacids
Anticonvulsants
Heparin
Corticosteroids or Cushing disease
Gastrectomy
Celiac disease
Diabetes mellitus
Anorexia nervosa/female athlete triad
Hyperthyroidism
Hyperparathyroidism
Rheumatoid arthritis
*Not exclusive
