Bone Densitometry

Relevant Social Security Medical Listings

• Listing 1.04 Disorders of the Spine (Adults)
• Listing 101.04 Disorders of the Spine (Children)
• Listing 9.04 Hypoparathyroidism (Adults)
• Listing 109.04 Hypoparathyroidism (Children)
• Listing 109.09 Latrogenic Hypercorticoid State (Children)

Other Names

There are several types of bone densitometry, including Single and Dual Energy Quantitative Computed Tomography (QCT), Dual X-ray Absorptiometry (DXA)8, Dual Photon Absorptiometry (DPA), and Single-Energy Photon Absorptiometry (SPA).

Type

Objective/X-ray (Bones)

Can SSA Purchase?

Yes.

Purpose

Bone densitometry is used to:

• Determine and monitor bone mineral density in osteoporosis and metabolic bone diseases.
• Monitor bone mineral density response to treatment.

General

Bone is a form of connective tissue that has been mineralized to provide structural support for the body, and contains 99% of the body’s calcium. Bone is a living tissue that undergoes constant remodeling by resorption9 and deposition of an organic (95% collagen) protein matrix known as osteoid and an inorganic calcium mineral matrix called hydroxyapatite. Hydroxyapatite gives the bone matrix its rigidity.

Cortical type bone makes up about 75% of the skeleton, and is found in the shafts of long bones. About 20% of skeletal bone is trabecular bone that is found principally in the vertebral bodies, ribs, and ends of the long bones.
Osteoblasts are special cells responsible for producing osteoid in new bone, then laying down calcium. Adequate dietary calcium, vitamin D, and phosphorus are critical for this process. Osteoclasts are cells that increase in the resorption of bone.

The control mechanisms for bone formation and resorption are complex and involve multiple interacting endocrine factors in addition to dietary mineral intake and vitamin D. For example, parathyroid hormone (PTH) induces resorption of bone and also induces the kidneys to reabsorb more of the calcium they filter out of the blood, so that more calcium is retained for the body. The parathyroid glands secrete PTH when blood calcium levels fall. On the other hand, a hormone called calcitonin is produced by the parafollicular cells of the thyroid gland and inhibit bone resorption by inhibiting osteoclasts. Growth hormone stimulates skeletal growth, thyroid hormone has an effect similar to PTH, and glucocorticoid (steroid hormones like cortisol) hormones produced by the adrenal glands have complex effects that result in overall bone resorption with decreased bone formation.

The most common type of bone loss is osteoporosis, especially in post- menopausal females not taking estrogen hormone replacement therapy. Osteoporosis is loss of bone mass per unit volume of bone, i.e., a decrease in bone density with maintenance of a normal ratio of organic and mineral elements. Osteomalacia refers to defective mineralization of bone. Osteopenia is a general term referring to any decrease in bone mass below normal, whether osteoporosis or osteomalacia. However, some authorities use it specifically to refer to loss of bone mass related to decreased osteoid synthesis in relationship to osteoid resorption.

From the above brief comments, it can be seen that there are many disorders that can affect bone density in direct and indirect ways.

Disorders associated with osteoporosis include physical factors (e.g., immobilization, space flight, radiation); structural factors (e.g., aging, osteogenesis imperfecta); pharmacologic factors (e.g., steroids, alcoholism, heparin); endocrine factors (e.g., acromegaly, hyperparathyroidism, hyperthyroidism, Addison’s disease, Cushing’s syndrome, pregnancy, deficient estrogen hormones either from menopause or removal of the ovaries surgically); metabolic factors (e.g., protein or calcium deficiency, diabetes mellitus, vitamin C deficiency); and other conditions such as ulcerative colitis.

Disorders associated with osteomalacia include endocrine factors (e.g., pregnancy, hypothyroidism, hyperparathyroidism); toxic factors (e.g., heavy metal poisoning such as cadmium and beryllium); renal disease; metabolic factors (e.g., decreased vitamin D, cirrhosis of the liver, hemodialysis, decreased phosphates); pharmacologic factors (e.g., laxative abuse, antacids, diphenylhydantoin, primidone, diazepam, cholestyramine); malabsorption (e.g., inflammatory bowel disease, gastrectomy, steatorrhea, pancreatic insufficiency); and other miscellaneous factors (e.g., inadequate sun exposure, obstruction of the bile system, systemic lupus erythematosus, multiple myeloma).

Not all disorders affecting bone are associated with a loss of bone mass. Osteopetrosis is a rare disorder beginning in childhood that results in increased bone mass.

Whatever the cause of bone abnormality, bone densitometry can provide valuable information about quantity of the mineral content of the patient’s bones that cannot be determined from plain x-ray films. Advanced bone loss can be seen on plain x-rays, and is usually called osteopenia, demineralization, or osteoporosis by the interpreting physician. In the context of plain x-ray interpretation these terms all mean the same thing.

Technique

Testing does not require administration of intravenous x-ray contrast material. The patient should continue whatever medications they are taking, and there are no pretest dietary restrictions. The study requires only about 10-20 minutes.

The specific bones x-rayed depend on the type of test utilized (QCT, DXA, SPA, DPA, etc.).

SPA is the oldest type of test (1960’s) and used a radioactive isotope to produce a beam that penetrated a bone and passed on to a detector. The radius was most often used. This test is essentially obsolete.

DPA was the next available test that used a radioactive isotope and two peaks of energy—one preferentially absorbed by soft tissue and the other by bone. The soft tissue result would be subtracted from the total to give the bone component, usually for the hip and spine. This test is essentially obsolete.

DXA can be done more quickly than SPA or DPA and involves very small doses of x-rays to penetrate bone—less than a chest x-ray. It is also fast, requiring only about 10 minutes. DXA is useful for following changes in bone density and can be done on the hip, spine or total body. There are also portable (pDXA) machines that measures bone density in the radius, fingers or heel bone (but not hip or spine).

QCT of the spine can be performed on most computed tomography scanners and usually involves x-ray measurements of four vertebral bodies (T12 – L4). If the patient has emphysema and hyperinflated lungs, T12 should be left out of the study. Also, the endplates of vertebra should not be x-rayed and care must be taken to avoid x-raying residual contrast material from other medical studies (e.g., barium in the intestine from prior barium enema) as this would render the results inaccurate. Compression fractures from prior collapse of weakened osteoporotic vertebral bodies must be left out of the study, because their bone density readings will not be characteristic of the spine in general. QCT can also be used on the hip or total body. QCT requires about 10 times the radiation dose as DXA, but provides true volume-density measurements of bone mass that cannot be done with other methods, as well as three-dimensional imagery. However, as noted above, there are limitations in regard to which bones can be used.

Another type of portable bone densitometry uses ultrasound beamed through the calcaneus (heel). By knowing the speed of the sound through tissue and the amount of attenuation through the heel bone, the bone density can be estimated.

Interpretation

Race, sex, and age all influence bone mass. Bone mass increases through the adolescent years as bone formation exceeds bone resorption. After bone growth is finished, bone mass tends to stay constant and then begins to decline in about the fourth decade of life. By time a person is 80 years old, their bone mass may be reduced by half.

Black males have the highest bone density at any age, followed by white males, black females, and white females. All of these factors must be taken into account in determining whether there is normal bone mass for a particular individual. For example, at age 30 a normal black male may have a radius bone mass mineral content exceeding 0.9 grams/cm³. A normal white female of the same age would have normal radius bone mass at 0.75 grams/cm³. Additionally, normal values also vary for the type of bone tested (e.g., radius vs. spine) and the specific type of x-ray technique performed. Furthermore, normal predicted values must be interpreted as a range of possible values so that there is no clear cut-off point between normal and abnormal. Normal reference values should be stated for the facility performing the test. Repeated testing may be performed in order to monitor bone mineral content over a period of time and evaluate the response to treatment.

DXA gives accurate results within 1 – 2% of actual bone density and so is considered the “gold standard” for densitometry. Portable DXA is also accurate, but such testing of peripheral bones may need follow-up with more extensive measurements, particularly if the results of such screening tests are abnormal.

However, measurement of one peripheral bone might be misleading about the true extent of osteoporosis since bone loss tends not to be uniform over the body in the earlier stages of menopause, and is not maximally uniform until age 70. The recommendation has been made by some authorities that up until age 65 densitometry be made on the spine; after that, any of the usually tested bones would be sufficiently accurate. Of course, multiple bones could be selected, as in Example 1 below. Another source of possible error is degenerative osteoarthritic changes in the spine, which could produce a result that is higher than is actually the case for the vertebral bodies.

The T-score is important in interpreting densitometry results and is the number of standard deviations (SD) above or below the average expected for a normal young adult, which in turn is based on peak density values at age 20, adjusted for sex and race. For every SD below normal, the risk of fracture approximately doubles, e.g., a SD of -1.0 doubles the risk, -2.0 SD has 4 times the risk, -3.0 SD would carry a risk 8 times higher than normal and so on. Put another way, every SD below normal represents a loss of about 10 – 12% of bone density. The scoring rules are built by the manufacturer into the computer software that interprets the results for a particular type of machine and personally calculated in some way by the radiologist.

The Z-score is a result based on the number of SD’s above or below the average expected for the patient’s age. High negative scores, those -1.5 or more, suggest osteoporosis secondary to some process other than normal aging—such as bone loss from metabolic disease (e.g., hypoparathyroidism) or the chronic use of corticosteroid drugs.

World Health Organization Definitions of Osteoporosis Based on Bone Density Levels (T-Score)

Normal.
Bone Density is within 1 SD (+1 or -1) of the young adult mean.

Low Bone Mass.
Bone density is 1 to 2.5 SD below the young adult mean (-1 to -2.5 SD).

Osteoporosis.
Bone density is 2.5 SD or more below the young adult mean (> -2.5 SD).

Severe (established) osteoporosis.
Bone density is more than 2.5 SD below the young adult mean and there has been one or more osteoporotic fractures.

Table 1.05-1 shows the relationship between percentile ranking and Z-Score when comparing people of the same age and gender. Some machines include the effects of race on results. Since the Z-Score is based relatively on values for others of the same age, etc., it does not provide a very good absolute estimate of bone strength. For example, elderly white women tend to have much thinner bone than elderly black women, so an elderly white woman could actually have a better Z-Score and yet objectively have weaker bone than someone of another race.

Table 1.05-1

Calculating T- and Z-Scores
The following formula calculates the Z-Score based on bone mass density (BMD) and standard deviation (SD). The “expected BMD” and SD are those expected for age, gender, and possibly race.

Z-Score = (Patient’s BMD – Expected BMD) / SD

The following formula calculates the T-Score. The “expected BMD” and SD are those expected for a young, healthy person, rather than those for other people the same age as the individual tested. While most reference BMDs used apply to the same race and gender, a T-Score could be calculated that crosses these categories.

T-Score = (Patient’s BMD – Reference BMD) / SD

Converting T- and Z-Scores

Z-score = T-score – Reference T-score
T-score = Z-score + Reference T-score

To use the above conversion equations, the gender, age, race, and skeletal site must be known. This information is available from the National Health and Nutrition Examination Survey (NHANES) datasets, as in Table 1.05-2.

Table 1.05-2: T-Score at Total Hip

For example:

A 25-year-old white female with a T-Score of 0.00 has a Z-Score of 0.00 (Z-Score = 0.00 – 0.00).

A 65-year-old white man with T-score of -0.3 has Z-score of +0.3 (Z-Score = -0.3 – (-0.6)).

A 55-year-old black female with a Z-Score of -1.5 has a T score of -2.0 (T-Score = -1.5 + (-0.50)).

Comments

There are now data showing a relationship between the severity of osteoporosis and the severity of ischemic heart disease, as measured directly by exercise tolerance (stress testing with echocardiography). This suggests that when one of these disorders is present, the presence of the other should be considered a possibility. Current evidence suggests that cardiac ischemia risk increases 22% for every point (one standard deviation) decrease in T score. T scores in the -3 to -4 range, for example, were associated with significantly decreased exercise tolerance. Osteoporosis has also been associated with increased risk of stroke.

Examples

Example 1
DXA Bone Densitometry
Date: xx/xx/xx
History: 80 Y/O W/F with menopause in her late 40’s and has been on hormone replacement therapy about 10 years. There is no history of family osteoporosis. There is a history of bilateral wrist fractures. She takes Citracal with vitamin D, 630 mg daily. She measures about 1 ½ inches shorter than her tallest height.

The bone density of the spine was 90% of normal and the hip 83%. This is comparing the results to those of young individuals when bone mass is at a peak value.

The bone density test results are significant for bone loss between 1.0 and 2.5 standard deviations below normal (10% to 25%) in one or more of the areas tested. Compared to other people the same age as the patient, there is very little or no significant bone loss.

Bone Density Measurements

Recommendations
Spine normal, but hip is slightly lowered in density. Patient is to get calcium 1500 mg and vitamin D 400 IU daily in her diet and supplements combined. We will re-scan in one year to compare readings.