What does T stand for in the measurement

Bone density measurement

Bone density measurement is a quick method to determine the onset of osteoporosis and thus prevent a serious illness in good time through targeted therapeutic measures.

Why should bone density be measured?

The only way to diagnose osteoporosis early, i.e. before fractures occur, is to quantify bone density. Bone mineral density measurements "Bone mineral density tests" (BMD tests) analyze the bone density in different areas of the skeleton and, as proven by numerous studies, allow a risk statement for later fractures. Even a 10 percent reduction in bone density is accompanied by a doubling of the risk of fractures in the area of ​​the spine and tripling in the area of ​​the femoral neck. If there are already fractures, this measurement is used to confirm the diagnosis of osteoporosis and to determine the severity of the bone loss in the axial skeleton.

The bone density measurement provides the following information:

  • She discovers osteopenia or osteoporosis before fractures occur.
  • It predicts the risk of later osteoporosis.
  • It shows the rate of bone loss ("progression") in control measurements.
  • It documents the effectiveness of a treatment.
  • It increases patient and doctor compliance.

The correlation between BMD and fracture risk is well documented. The relationship between bone density (measured at the hip and lumbar spine) and femoral neck fracture is even three times more reliable than the relationship between blood cholesterol and heart attack. Bone density measurement is currently the best method to determine the risk of fractures and to document the success of the therapy.

How is a bone density measurement performed?

The bone mineral content (BMC) is measured in grams or the bone mineral density (BMD) in grams per cm² or grams per cm³. The reliability and accuracy of a measurement depends on:

  • the device type (pencil, fan or flash beam technology),
  • the regular (daily) calibration on the phantom,
  • from the cooperation of the patient (lying quietly),
  • the exact, reproducible setting by the examiner,
  • the extent of the osteoporosis (the lower the bone mass, the less accurate the measurement).

DXA stands for Dual Energy X-Ray Absorptiometry, DEXA, DXA, rarely also called QDR, DPX, DER - it is the most popular and sophisticated measurement method, the "gold standard" worldwide and in all international therapy studies. The DXA method was developed in the 1980s and its global application began in 1988. Two energy beams of different intensities are sent through the skeleton. The bone density can be calculated from the amount of radiation that has passed through the bone. Based on the measurements using two differently energetic beams, the soft tissue-related absorption portion can be determined and eliminated. The lumbar spine and hip (right and / or left) are measured. A new and promising method is the DXA measurement technology with laser support on the heel. In the meantime, the entire bone mass can also be analyzed ("Full Body DXA Scanner"). The mineral content per area (g / cm2) is then calculated within the automatically defined areas. These measurements do not only record the vertebral bodies, but also the vertebral arches and spinous processes, which also contain a considerable amount of compacta. The International Society of Clinical Densitometry (ISCD) recommends measuring at least two skeletal areas, with the diagnosis being based on the lowest T-Score value. In the area of ​​the spine, measurements are taken from L1 to L4. Important advantages of the DXA method are:

  • It is non-invasive and therefore does not represent a burden for the patient.
  • It can be carried out very quickly (5–10 minutes) with modern equipment. With the new flash beam technology, the measurement itself only takes 1-2 seconds.
  • It is inexpensive (around 40-50 euros *).
  • It has a very low radiation exposure (13 mRem, corresponding to only 1 / 10–1 / 100 of a normal x-ray). The new Flashbeam technology manages with a dose of <10μSV (1 mRem / area).
  • It measures the skeletal areas that are most sensitive to osteoporosis and at risk of fractures (lumbar spine and hips). It measures very precisely and is therefore ideal for control measurements (accuracy 2–6%, precision 1–3%).
  • It is the standard method for diagnosing osteoporosis recognized and recommended by the WHO and the DVO ("Dachverband Osteologie").
  • It was used in all major therapy studies as a method of measuring bone density.

The bone density values ​​for the lumbar vertebrae 1–4 are printed out individually and in combinations. This means that individual defective eddies can be excluded from the calculation. A long list of factors associated with changes in density in the spine or in the neighboring soft tissues can falsify the measurement results and must be taken into account in the evaluation. In the case of pronounced degenerative changes or severe scoliosis, the measurement of the lumbar spine is completely dispensed with and only the hip is taken into account. There can also be great differences in density within the proximal femur, so that only the same areas can be compared in control measurements. The only disadvantage of the DXA measurement is the integral measurement of the skeletal area to be examined.

Sometimes it is not possible to determine exactly whether calcium structures (e.g. aortic calcium, calcified lymph nodes or muscle parts, spondylophytes) or other absorbing substances (metal closures, radiopaque contrast media, calcium tablets) are included in the measurement. These "pitfalls" can be avoided by taking a previous x-ray of the lumbar spine. New device developments can also measure in side projection and, thanks to higher image resolution, even show the structure of the vertebral bodies and the hip in detail.

T-value and Z-value

Two terms are clinically important when evaluating the DXA measurement:

Z-Score (Z-Score): Comparison of the bone density of the measured person with "normal persons of the same age and of the same sex" ("age and sex matched" controls).

T-Score (T-Value): Comparison of the density values ​​of the measured person with those of a normal young adult (20-30 years old) (comparison with the "maximum bone density").

Since the BMD decreases with age in all skeletal areas, the T-value is lower than the Z-value in all measured patients who are older than 30 years, the differences increasing with age. By definition, the diagnosis of osteoporosis is based on a T-value of <-2.5 SD (standard deviation).

Who should go for a bone density measurement?

Just a few years ago, the diagnosis of osteoporosis was based on medical history, x-rays, and clinical symptoms, particularly evidence of fractures. The clinical relevance of the quantitative measurement is based on two important assumptions: that bone density correlates with the risk of fractures and that modern forms of treatment can increase bone mass again.

In fact, with the introduction of the measurement, the diagnosis of osteoporosis can be made at an early, asymptomatic phase. Low bone density is recognized as the most important risk factor for osteoporosis-associated fractures, comparable to blood pressure or cholesterol level as reliable risk factors for impending cardiovascular diseases. Nevertheless, bone density measurements are still not recognized as part of a preventive program. This test has long been recognized in its clinical significance by health-conscious people. The measurement is cheap, quick and reliable to carry out. It considerably simplifies the diagnosis and assessment of the course of osteoporosis.

Indications for bone density measurement

The DXA method is currently recommended for all women with evidence of multiple risks, e.g. if the woman enters menopause early, does not receive estrogen replacement in postmenopause, or has a family history of osteoporosis. According to the guidelines of the National Osteoporosis Foundation (NOF, USA), a measurement is recommended for the following women:

  • All women> 65 years (regardless of other risk factors)
  • All postmenopausal women with fractures
  • All women who are considering osteoporosis therapy and whose decision depends on the result of a bone density measurement
  • All women who are undergoing prolonged hormone therapy.

Additional indications:

  • Age-related decrease in height
  • Back pain of unclear origin
  • Slim smokers
  • Previous fractures
  • Joint diseases with restricted mobility
  • Long-term therapy (> 6 months) with bone-damaging drugs such as corticosteroids, marcoumar or anti-epileptic drugs; Hyperthyroidism and hyperparathyroidism (hyperthyroidism or overactive parathyroid glands)
  • Transplant patients
  • Chronic diseases and operations in the gastrointestinal area
  • Anorexia nervosa (anorexia)
  • Chronic renal failure

Bone density measurement using DXA is currently the only reliable method to document the success of therapy in osteoporosis. An annual measurement also increases the compliance of both the patient and the doctor. Large clinical studies under therapy with bisphosphonates have documented significant increases in bone density after only three months in the area of ​​the vertebral bodies and after one year in the hip area. Six-monthly measurements should be carried out in high-risk patients, e.g. under corticosteroid therapy.

Bone density measurement as a health insurance benefit

Incomprehensibly, the costs of 40–50 * euros for the measurement are usually only reimbursed by the cash registers after the first break **. We are repeatedly asked by patients who are interested in a measurement. And what about assumption of costs. We try to give an answer: “Bone density measurement as a health insurance benefit

Page updated: 04/18/2021

© Text: Bone Density Measurement | Prof. Dr. Reiner Bartl | published in Orthoprof. | With the kind permission of Edition Nymphenburg
* Costs updated afterwards
** Please inquire about the current status of this requirement, it may have been changed or adapted