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Bone Densitometry

Bone densitometry is used to measure the bone mineral content and density. This measurement can indicate decreased bone mass, a condition in which bones are more brittle and more prone to break or fracture easily. Bone densitometry is used primarily to diagnose osteoporosis and to determine fracture risk. The testing procedure measures the bone density of the bones of the spine, pelvis, lower arm, and thigh.

Bone densitometry testing may be done using x-rays, dual-energy x-ray absorptiometry (DEXA or DXA) or by quantitative CT scanning using special software to determine bone density of the hip or spine. These procedures are generally done in a clinic, hospital, or free-standing radiology facility.

However, for mass screening purposes, there are portable types of bone densitometry testing. The portable testing is done using either a DEXA (or DXA) x-ray device or a quantitative ultrasound unit. Both types of portable testing may use the radius (one of the two bones of the lower arm), wrist, fingers, or heel for testing. The portable testing, while useful for general screening, is not as precise as the non-portable methods because only one bone site is tested.

Standard x-rays may detect weakened bones. However, at the point where bone weakness is obvious on standard x-rays, the bone weakness may be too far advanced for treatment to be effective. Bone densitometry testing can determine decreasing bone density and strength at a much earlier stage when treatment of the bone weakness can be beneficial.

Test results:
The bone densitometry test determines the bone mineral density (BMD). Your BMD is compared to two norms - healthy young adults (your T-score) and age-matched (your Z-score).

First, your BMD result is compared with the BMD results from healthy 25- to 35-year-old adults of your same sex and ethnicity. The standard deviation (SD) is the difference between your BMD and that of the healthy young adults. This result is your T-score. Positive T-scores indicate the bone is stronger than normal; negative T-scores indicate the bone is weaker than normal.

According to the World Health Organization, osteoporosis is defined based on the following bone density levels:
  • A T-score within 1 SD (+1 or -1) of the young adult mean indicates normal bone density.
  • A T-score of 1 to 2.5 SD below the young adult mean (-1 to - 2.5 SD) indicates low bone mass.
  • A T-score of 2.5 SD or more below the young adult mean (> - 2.5 SD) indicates the presence of osteoporosis.

Diagnostics - Bone Densitometry
In general, the risk for bone fracture doubles with every SD below normal. Thus, a person with a BMD of 1 SD below normal (T-score of -1) has twice the risk for bone fracture as a person with a normal BMD. A person with a T-score of -2 has four times the risk for bone fracture as a person with a normal BMD. When this information is known, people with a high risk for bone fracture can be treated with the goal of preventing future fractures. Severe (established) osteoporosis is defined as having a bone density that is more than 2.5 SD below the young adult mean with one or more past fractures due to osteoporosis.

 

Secondly, your BMD is compared to an age-matched norm. This is called your Z-score. Z-scores are calculated in the same way, but the comparisons are made to someone of your age, sex, race, height, and weight.

In addition to bone densitometry testing, your physician may recommend other types of tests such as blood tests, which may be used to detect the presence of renal (kidney) disease, evaluate the function of the parathyroid gland, evaluate the effects of cortisone therapy, and/or assess the levels of minerals in the body related to bone strength, such as calcium.

Other related procedures that may be used to diagnose bone problems include bone scan and x-rays of the bones. Please see these procedures for more information.

Reasons for the Procedure
Bone densitometry testing is primarily performed to identify persons with osteoporosis and osteopenia (decreased bone mass) so that the appropriate medical therapy and treatment can be implemented. Early treatment helps to prevent future bone fractures. It may also be recommended for persons who have already had a fracture and are considered at risk for osteoporosis.
The complications of broken bones resulting from osteoporosis are often severe, particularly in the elderly. The earlier osteoporosis can be identified, the sooner effective treatment can be implemented, thus most likely lessening the severity of the condition.

Bone densitometry testing may also be used:
  • To confirm a diagnosis of osteoporosis if you have already had a fracture
  • To predict your chances of fracturing in the future
  • To determine your rate of bone loss and/or monitor the effects of treatment

There may be other reasons for your physician to recommend bone densitometry.

What are the causes of osteoporosis or osteopenia?
Osteoporosis is most commonly found in postmenopausal women, where the absence of the hormone estrogen is related to the loss of bone mass. Other conditions that may cause osteoporosis or osteopenia include, but are not limited to, the following:

  • Renal (kidney) failure
  • Hyperparathyroidism (overactive parathyroid gland)
  • Prolonged immobility
  • Long-term corticosteroid therapy
  • Long-term hormone replacement therapy
  • GI (gastrointestinal) malabsorption disorder
  • Cushing's syndrome - a condition in which there is increased production of glucocorticoids by the adrenal glands due to either an adrenal gland tumor or pituitary gland hyperfunction

These conditions affect bone formation due to problems with absorption of certain substances, such as Vitamin D and calcium, which are needed to form strong bones.

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