Bone density measurement
As bone densitometry , and bone densitometry , medical-technical methods are referred to for determining bone density or the Kalksalzgehaltes the bone serve. Dual X-ray absorptiometry (DXA) is standard, but there are also other methods such as quantitative computed tomography or a standardized ultrasound examination.
The method is used to diagnose and control osteoporosis and other bone metabolism disorders with an increased risk of bone fractures . This mainly affects women after the menopause , men over 50 years of age, smokers, alcoholics and people with malnutrition or reduced vitamin D levels. Certain diseases such as hyperthyroidism or drugs such as corticoids also promote the loss of substance in the bones.
T value
The various methods and devices cannot be compared with one another. Therefore, no absolute density should (except in the finding qCT or pQCT be specified), or areal density, but the deviations from (age and gender) normal in multiples of a standard deviation (a so-called T-value , engl. T-score, dimensionless ).
According to the current definition of the WHO , osteoporosis is present if the measured value of the bone density measurement is at least 2.5 standard deviations below the average of 30-year-old healthy people of the same sex ( peak bone mass ), i.e. that is, a T value ≤ -2.5 is present. Between –1 and –2.5 standard deviations, there is talk of reduced bone density, or osteopenia . From a T score of −2.6, there is a significantly reduced bone density and there is an increased risk of bone fractures osteoporosis .
| Degree | T value | Fractures | Classification of the WHO |
|---|---|---|---|
| +1 to -1 | No fractures | Normal findings | |
| 0 | -1 to -2.5 | No fractures | Osteopenia |
| 1 | from –2.5 | No fractures | preclinical osteoporosis |
| 2 | from –2.5 | 1 to 3 vertebral body fractures | manifest osteoporosis |
| 3 | from –2.5 | multiple vertebral body fractures, often extraspinal fractures as well | Advanced osteoporosis |
Z-Value
The relation of the T-value to the peak bone mass brings with it the problem that with increasing age, ever larger proportions of the population would have to be regarded as "sick"; almost 50% of women aged 70. Therefore, a value is also given that relates to healthy men or women of the same age, the Z value.
A normal Z value (> –1) indicates that the bone density is age-typical. Old age is not a disease and cannot be treated; and in the case of very old people, the increased risk of fractures is typical of their age. Osteologists therefore advise against drug therapies ( hormones , calcium , bisphosphonates, etc.) if the T value is low but the Z value is normal and recommend preventive measures such as gymnastics, avoiding sedatives , eliminating tripping hazards at home, suitable visual aids and crutches.
Cost aspects
In Germany , around 800 measuring stations are available in medical practices and hospitals. It is controversial whether comparably good predictions of the risk of fractures are possible even without measurement using an apparatus, only on the basis of anamnestic information. Before the year 2000, doctors (according to the statutory health insurance companies) made and billed a large number of bone density measurements without any clear benefit.
As a result of a new regulation since April 1, 2000, the costs of the examination are only borne by the statutory health insurance if the patient has already suffered a bone fracture - for no external reason - and if there is a high degree of suspicion of osteoporosis (= reduced calcium salt content). According to the GOÄ (official fee schedule for doctors), private patients and self-payers pay between 18 and 32 euros and usually other surcharges. This amount has remained constant in the western part of Germany since 1996; in the new federal states it was only brought into line with the western level on January 1, 2007 by means of the federal government's fee adjustment regulations.
The requirements that must be met in order to be able to prescribe an osteodensitometry (bone density measurement) at the expense of the statutory health insurance (GKV) were relaxed again in February 2013. The Federal Joint Committee (G-BA) has come to the conclusion that osteodensitometry can in future be performed at the expense of the statutory health insurance if, on the basis of specific findings, there is a specific drug treatment intention. A clinically recognizable fracture without adequate trauma, for example a vertebral body, is still regarded as such a finding. However, such a constellation no longer has to be present. This is especially true if the measurement of bone density is to be repeated for the purpose of checking the ongoing therapy.
Measurement method
The common methods for osteodensitometry use the density-dependent attenuation of X-rays . There are dedicated devices that use either radioactive sources or X-ray tubes, or the examination is carried out on computer tomographs .
Dual X-ray Absorptiometry (DXA / DEXA)
While conventional digital X- ray methods only use one X-ray source, the dual-energy X-ray absorptiometry (DXA or DEXA) uses two energetically slightly different X-ray sources at the same time. Materials with different densities show different attenuation characteristics depending on the energy of the X-ray radiation. For each measuring point in the X-ray image, there are two attenuation values for the two X-ray energies used in the DEXA method. Accordingly, in comparison to the conventional X-ray method, not only can the general weakening throughout the entire body be measured, but different materials can also be more precisely distinguished. What is important here is as great a difference as possible in the respective density.
When used on humans, a distinction is made between three types of tissue: bone, muscle and fat tissue. However, only two measured values are available. In order to differentiate between these, additional assumptions have to be made which, depending on the area of application, lead to more or less large measurement errors. The DXA method is suitable, for example, for determining the body composition from bone, fat and muscle mass. It does not provide any information about the three-dimensional geometry of the measuring object and thus no density values in the physical sense ( SI unit of density: kg / m³), but an area-projected mass (SI unit: kg / m², also referred to as area density ).
Quantitative Computed Tomography (QCT / pQCT)
Quantitative computed tomography (QCT, qCT) and peripheral quantitative computed tomography (pQCT) are special forms of computed tomography , an imaging process based on X-rays. In conventional CT methods, the exact density of each individual volume element (so-called voxel ) is only determined as a gray value, which can only be specified as a bone density value after calibration.
In contrast to conventional CT, the QCT / pQCT method determines the physical density as mass / volume of each voxel very precisely. To ensure this, the measuring system has to compensate for non-linearities and drifts of the X-ray tube and X-ray detectors during the measurement. While QCT systems are usually designed to measure the entire body (whole body scanners), the cheaper and more compact pQCT systems are limited to the periphery, such as arms, legs or even the head.
In addition to the precise determination of the local bone density, the cross-section of the bone geometry is also recorded in computed tomographic processes. Mechanical parameters of the bone, such as the stress strain index (SSI), can be calculated from the combination of material properties (e.g. density) and the material distribution (structural properties) over the cross section determined in this way . Thus, in addition to the bone density, a measure for the mechanical bone strength can be calculated, which results from the combination of material properties, geometry and direction of the force.
In contrast to the DXA method (Dual-Energy X-ray Absorptiometry), which measures an area-projected mass (kg / m²), QCT / pQCT methods indicate the physical density (kg / m³) of each volume element ( voxel ). The measured values from the QCT / pQCT method represent mechanical parameters such as bone strength or flexural strength relatively precisely. In addition, with the QCT / pQCT method, the mineral content of the different bone components such as the cortex (bone wall) and cancellous bone (spongy structure inside the bone in the area near the joint, made up of so-called trabeculae ) can be analyzed separately. Due to the increased bone metabolism function in the area of the cancellous bone, pathological changes such as those that occur in osteoporosis , for example, can be recognized earlier and more clearly than with the averaging over the entire bone cross-section, which is effectively carried out in the DXA method. The composition of the body (muscle, fat and bone mass), on the other hand, can only be determined locally and not across the entire body as in the DXA / DEXA method.
In both pQCT and DXA, roughly the same radiation dose of around 1–2 μSv is used. The (central) QCT works with considerably higher radiation doses (25–60 μSv). The annual natural background pollution is around 2400 μSv, the pollution from a transatlantic flight is around 50 μSv.
Sonography
Special ultrasound devices that determine the bone density based on the transit times and reflections of the sound in the extremities are also available. However, their usefulness for this purpose is controversial.
See also
Web links
- Julian Meßler: On the influence of strontium on bone density measurement and the bone mineral in vivo: mathematical-theoretical calculations and experimental investigations . Dissertation . University of Bonn, Faculty of Medicine, 2005. (online)
literature
- German Society for Osteology V. (DGO): Guideline prophylaxis, diagnosis and therapy of osteoporosis in adults. S3 guideline of the umbrella association of German-speaking scientific osteological societies, 2014. Accessed on February 13, 2019.
Individual evidence
- ↑ tk.de: Bone Density Measurement .
- ↑ Bone density measurement in the future for other indications, health insurance benefits , press release of the G-BA from March 21, 2013.
- ↑ a b W. A. Calendar: Computed Tomography - Fundamentals, System Technology, Image Quality, Applications. 2nd Edition. Publicis, ISBN 3-89578-216-5 , 2005.
- ↑ P. Schneider, C. Reiners: Quantitative determination of bone mass: current status and pitfalls of the methods. In: Med world. Volume 49, 1998, pp. 157-163.
- ↑ H. Sievänen, I. Vuori: Peripheral Quantitative Computed Tomography in human Long Bones: Evaluation of in vitro and in vivo Precision. In: J Bone Miner. Res. Vol. 13, 1998, pp. 871-882. PMID 9610752 .
- ^ MB Leonard, J. Shults, DM Elliot, VA Stallings, BS Zemel: Interpretation of whole body dual X-ray absorption measures in children: comparison with peripheral quantitative comuted tomography. In: Bone. Vol. 34, 2004, pp. 1044-1052.
- ↑ T. Jämsä, P. Jalovaara, Z. Peng, HK Väänänen, J. Tuukkanen: Comparison of three-point bending test and peripheral quantitative computed tomography analysis in the evaluation of the strength of mouse femur and tibia. In: Bone. 23 (2), Aug 1998, pp. 155-161. PMID 9701475 .
- ↑ H. Schießl, J. Willnecker: New insights about the relationship between bone strength and muscle strength. Pediatric Osteology, Proceedings of the 2nd International Workshop, Cologne, Germany, 3-5 October 1997, New aspects of bone and muscle diagnostics
- ↑ H. Schießl, HM Frost, WSS Jee: Estrogen and Bone-Muscle Strength and Mass Relationships. In: Bone. Vol. 22, 1998, pp. 1-6. PMID 9437507 .
- ↑ P. Schneider et al .: Importance of two different bone density measurement methods for determining the mineral content on the peripheral and axial skeleton. In: Z. Orthop. Vol. 130, 1992, pp. 16-21.
- ↑ MJ Braun et al .: Clinical evaluation of a high-resolution new peripheral quantitative computized tomography (pQCT) scanner for the bone densitometry at the lower limbs. In: Physics in Medicine and Biology Vol. 43, 1998, pp. 2279-2294. PMID 9725604 .
- ↑ CF Njeh et al .: Radiation exposure in bone mineral density assessment. In: Appl Radiat Isot. 50, 1999, pp. 215-236. PMID 10028639 .
- ^ United Nations Scientific Committee on the Effects of Atomic Radiation, "UNSCEAR 2000 Report to the General Assembly — Volume 1"; 2000.
- ^ United Nations Scientific Committee on the Effects of Atomic Radiation, "UNSCEAR 2000 Report to the General Assembly — Annex B: Exposures from natural radiation sources", 2000.