Bone Densitometry: Treatment, Effect & Risks

Bone densitometry is used to assess the risk of osteoporosis as well as bone fractures. Different measurement methods allow an assessment of bone strength and structure by determining the calcium salt content in the examined bone.

What is bone densitometry?

Schematic representation of the density of healthy bone and bone with osteoporosis. Click to enlarge. Bone densitometry (osteodensitometry) is used to indirectly determine the stability and quality of the bones under examination in each case via the calcium hydroxyapatite content determined. Various methods are available for measuring bone density, which differ in terms of their significance. All methods of bone density measurement use radiation that penetrates the bone (including X-rays, ultrasound), whereby the respective radiation exposure is less than that of an X-ray thorax (X-ray examination of the chest). Bone density measurements are generally performed in cases of osteoporosis or a suspicion of osteoporosis for early detection and follow-up, since the relationship between calcium salt content and bone matrix is reduced in osteoporosis. In certain diseases of the gastrointestinal tract (including Crohn’s disease, malabsorption), long-term use of cortisone, hyperthyroidism (hyperthyroidism), and in organ transplant patients, regular bone density measurements are also recommended because of the increased risk of osteoporosis.

Function, effect, use, and goals

Bone densitometry is primarily used for (early) detection of osteoporosis (progressive reduction of bone substance) and osteopenia, which is characterized by reduced bone density compared with the age-specific normal value and is considered an optional precursor of osteoporosis. The course of osteoporosis can also be monitored by regular bone density measurements. Bone density measurements can also be used to determine the individual risk of bone fractures. All available measurement methods use radiation that is absorbed differently depending on the specific bone density or mineral salt content. The extent of radiation absorption by the mineral salts present in the bone allows statements to be made about bone density by determining the deviation from an age-specific standard value. A reliable and most frequently used method for assessing the long-term success of osteoporosis therapy is DXA or DEXA (Dual Energy X-Ray Absorptiometry). Here, two images are taken with energetically different X-ray sources so that the proportion of soft tissue (fat, muscle, connective tissue) in the X-ray absorption can be determined and subtracted accordingly. As a rule, the measurement is performed at the hip joint or at the lumbar spine, because the most meaningful results can be expected there. The area-projected mass (two-dimensional areal density) determined in the course of DXA is used in particular to assess the risk of bone fractures close to the hip (including fractures of the neck of the femur) and of vertebral body fractures (including those in the lumbar spine). In addition, bone density can be determined by quantitative computed tomography (QCT). This procedure is a special form of computed tomography in which three-dimensional X-ray images of the lumbar spine are taken. This ensures differentiation between the bone density of the outer layer of bone (cortical bone) on the one hand, and the bone tubercle (trabecular bone) on the other. Since metabolic activity is higher in the trabeculae than in the outer layer of bone, the procedure allows statements to be made about changes in bone metabolism. This in turn makes it possible to assess the risk of fractures and the rate of progression at which bone substance regresses in osteoporosis. In peripheral quantitative computed tomography (pQCT), bone density is measured on the forearm rather than the lumbar spine. In contrast to DXA, qualitative computed tomography can only determine the composition of bone, muscle and fat tissue locally. Qualitative ultrasound (QUS) is another method for determining the density of peripheral bones. Here, the bone to be examined is sonographically sonicated.The sound absorption and speed with which the sound passes through the bone allow conclusions to be drawn about the bone condition. Since bone density in the axial skeleton cannot yet be determined by this osteodensitometry method, its use for the diagnosis and monitoring of osteoporosis is currently unsuitable.

Risks, side effects, and hazards

With the exception of qualitative ultrasound, all methods of bone densitometry involve the use of X-rays and, accordingly, different levels of radiation exposure to the human organism depending on the particular method used. For example, the radiation exposure for a DXA is about one to six µSv, which is many times lower than the annual average exposure to earth radiation of about two mSv (1 mSv = 1000 µSv). At one to five mSv, qualitative computed tomography correlates with a comparatively high radiation exposure. From 100 mSv annually, there is a statistically verifiable increased risk of cancer. Considered in isolation, a routine X-ray examination is accordingly generally low-risk, but frequent and unnecessary X-rays should be avoided. In the presence of pregnancy, bone densitometry with X-rays is contraindicated, as even low levels of radiation exposure can affect the genesis of the unborn child.

Typical and common bone diseases

  • Osteoporosis
  • Bone pain
  • Bone fracture
  • Paget’s disease