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Neil Roach, MD

OsteOpOrOsis

1. What is osteoporosis? How does it differ from osteomalacia and osteopenia?

•

Osteoporosis is characterized by low bone mass and microarchitectural deterioration of bone tissue, leading to increased

bone fragility and an increased risk of fracture. The bone has normal mineralization and histologic characteristics.

•

Osteomalacia is characterized by incomplete mineralization of normal osteoid tissue. Osteomalacia in a growing child

is also known as rickets.

•

Osteopenia is a descriptive term for decreased calcification or density of bone. Osteopenia if not due to a particular

cause, pathophysiology, or disease.

2. What are potential causes for generalized osteoporosis?

Arthritis may be either primary or secondary. Any patient who presents with osteoporosis should be evaluated

for secondary causes before being diagnosed with primary osteoporosis. Causes of primary osteoporosis include

involutional osteoporosis, juvenile osteoporosis, and idiopathic osteoporosis (premenopausal women or middle-aged

men). Secondary osteoporosis has a wide differential diagnosis, including:

•

Endocrine diseases (e.g., hypogonadism, ovarian agenesis, hyperadrenocorticism, hyperthyroidism,

hyperparathyroidism, diabetes mellitus, and acromegaly)

•

Gastrointestinal diseases (e.g., subtotal gastrectomy, malabsorption syndromes, chronic obstructive jaundice, primary

biliary cirrhosis, severe malnutrition, and anorexia nervosa)

•

Bone marrow disorders (e.g., multiple myeloma, systemic mastocytosis, and disseminated carcinoma)

•

Connective tissue diseases (e.g., osteogenesis imperfecta, homocystinuria, Ehlers-Danlos syndrome, and Marfan syndrome)

•

Miscellaneous causes (e.g., immobilization, chronic obstructive pulmonary disease, chronic alcoholism, long-term

heparin therapy, and rheumatoid arthritis)

3. What are some causes for regional or localized osteoporosis?

A focal area of bone loss can be highly localized (also known as a lytic lesion). Metastasis, multiple myeloma, and

osteomyelitis are the most common causes of lytic lesions. Larger areas of bone loss, usually without violation of

cortical bone, may be caused by a stroke or immobility of an extremity. Another cause for regional osteoporosis is reflex

sympathetic dystrophy syndrome, or Sudeck atrophy. Reflex sympathetic dystrophy is a multifactorial disorder. The

underlying cause is typically trauma, followed by regional or localized osteoporosis.

4. In what demographic groups is generalized osteoporosis most prevalent?

Bone loss starts to occur in men and women around 35 years old. Women have a greater rate of bone loss. Whites and

Asians are more likely to develop osteoporosis than blacks. A white or Asian woman has the greatest risk of developing

osteoporosis.

5. Are conventional radiographs sensitive enough to diagnose osteoporosis?

No. A large percentage (30% to 40%) of bone must be lost to appreciate a change on radiographic examination. There is

also marked intrareader and inter-reader variability for this diagnosis on radiographs.

6. What radiographic features are useful in diagnosing osteoporosis?

Decreased bone density, prominence of vertebral body end plates, and cortical thinning can be seen with osteoporosis,

but are subjective. Accentuation of trabecular stress lines from resorption of secondary trabecula is another radiographic

sign of osteoporosis; this is best appreciated around the femoral neck.

7. Is computed tomography (CT) more sensitive than conventional radiographs for

evaluating bone mineral density (BMD)?

Yes. Quantitative CT can be performed using software that is added to a standard CT scanner to measure

BMD. Other methods of assessment based on relative absorption of x-rays using a less expensive scanner are

preferred.

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OsteOpOrOsis

8. What are other quantitative methods of measuring BMD?

Single-photon absorptiometry and dual-photon absorptiometry were the early techniques for measuring BMD. Both

used radioactive sources to produce photons. These units have essentially been replaced with dual x-ray absorptiometry

(DXA) units. DXA uses the same principles as dual-photon absorptiometry except that the radionuclide source is

replaced with an x-ray tube. Two distinct energy x-ray beams are used (usually 70 kVp and 140 kVp). DXA is the

examination of choice for the diagnosis and follow-up of osteoporosis.

0.2

20 25 30 35 40 45 50

Age

55 60 65

80 85

70 75

Total

0.4

BMD

0.6

0.8

1.0

1.2

1.4

1.6

0.2

25 30 35 40 45 50

Age

55 60 65

80 85

70 75

Total

0.4

BMD

0.6

0.8

1.0

1.2

1.4

1.6

Figure 39-1.

A and B, Normal lumbar spine DXA scan in a 47-year-old woman. C and D, Normal right hip DXA scan in the same patient.

Key Points: Osteoporosis

1. Osteoporosis is the most common skeletal disorder worldwide.

2. Osteoporosis can start at any age.

3. Osteoporosis progresses silently over a long period.

4. Osteoporosis is preventable with therapy.

OsteOpOrOsis

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mUsCULOsKeLetAL rADiOLOGY

9. What are the units of measurement of BMD?

BMD is measured in grams divided by area: g/cm

. Technically, it is not a density (weight/volume) being measured, but a

weight divided by area.

10. What sites of the skeleton are routinely assessed on a DXA scan?

The lumbar spine, from L1 or L2 to L4, and the proximal femur (regions of interest are the femoral neck, trochanteric

region, and Ward triangle) are routinely assessed on a DXA scan. Ward triangle is a site at the proximal femur where

bone mineral loss is thought to occur first (

Figs. 39-1

and

39-2

11. How can large osteophytes and sclerotic changes in a patient with lumbar

degenerative disease affect bone densitometry assessment?

On an anteroposterior evaluation of the lumbar spine, the osteophytes and other degenerative changes in the vertebrae

and posterior elements can cause pseudoelevation of the BMD. This pseudoelevation can result in the BMD of an

osteoporotic patient appearing falsely normal. This is one reason that BMD measurements are routinely obtained in two

separate locations.

0.2

20 25 30 35 40 45 50

Age

55 60 65

80 85

70 75

Total

0.4

BMD

0.6

0.8

1.0

1.2

1.4

1.6

0.4

25 30 35 40 45 50

Age

55 60 65

80 85

70 75

Total

BMD

0.6

0.8

1.0

1.2

1.4

1.6

Figure 39-2.

A and B, Osteoporosis in a 65-year-old woman. The t score is −2.9. C and D, Osteopenia of the right hip with a t score of

−1.8 in the same patient.

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12. What are the World Health Organization criteria for the diagnoses of normal BMD,

osteopenia, and osteoporosis?

•

Normal is a BMD within 1 SD of the young adult reference mean.

•

Osteopenia is a BMD 1 to 2.5 SD below the young adult reference mean.

•

Osteoporosis is a BMD more than 2.5 SD below the young adult reference mean.

13. What are t and z scores?

•

The t score is the number of standard deviations above or below the young adult mean. The young adult mean is the

expected normal value for the patient compared with others of the same sex and ethnicity in a reference population

the manufacturer builds into the DXA software. It is approximately what the patient should have had at his or her peak

bone density at about age 20.

•

The z score is the number of standard deviations of the patient’s bone density above or below the values expected

for the patient’s age. By comparing the patient’s BMD with the expected BMD for his or her age, the z score can help

classify the type of osteoporosis. Primary osteoporosis is age-related osteoporosis in which no secondary causes are

found.

iBliography

[1] National Osteoporosis Foundation, Physicians Resource Manual on Osteoporosis: A Decision-making Guide, second ed., National

Osteoporosis Foundation, Washington, DC, 1991.

[2] World Health Organization, Assessment of fracture risk and its application to screening for postmenopausal osteoporosis: report of a WHO

study group, WHO Technical Report Series 843 (1994) 1–129.

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Osteoporosis

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