RADIAL DIAGNOSTICS OF OSTEOPOROSIS ? CURRENT STATE OF THE PROBLEM
Kemerovo State Medical Academy,
Kemerovo, Russia
Osteoporosis has acquired the epidemic characteristics in the modern civilization. Representatives of different specialties give the alarm relating to the incidence of this disease, which causes significant damage to health, decreases quality of life and result in high financial costs.
The main feature of osteopenia and osteoporosis is decreasing bone mineral density (BMD). BMD correlates with risk of osteoporotic fractures [9, 27]. Considering that radiologic techniques are related to low sensitivity, X-ray densitometry has acquired the highest popularity. Dual-energy X-ray absorptiometry (DXA) is considered by most researchers as the gold standard for diagnostics of osteopenic syndrome. Such techniques as single- and two-photon absorptiometry present mostly historical interest than practical significance.
Dual-energy X-ray absorptiometry is based on creation of x-radiation flow, which is divided by filter system into high- and low energy components. After tissue scanning a detector fixes radiation flows. Then acquired information is processed.
Age peak of bone mineral density and decreasing values during the following years play significant role in development of osteoporotic changes. The sooner bone mass achieves its maximal level, the higher possibility of osteoporosis in older age [13]. Accordingly, timely identification of decreasing values of bone mineral density will allow realization of correcting measures for decreasing risk of osteoporotic fractures.
Dual-energy X-ray absorptiometry is used for measuring two-dimensional BMD (g/cm2). At that, total mineral density of trabecular and cortical bone is examined.
After scanning, all BMD results are compared to the reference values, which are primarily entered into the densitometric system. Most densitometers include the data, which are fashioned according to the results of NHANES (National Health and Nutrition Examination Survey) [20]. Therefore, BMD estimation on the basis of population values for other region is not always correct. Many regions of Russian Federation have no population databases with BMD values. As result, it decreases accuracy of results. Also there are only rare studies of regional features of bone mineral density in women [18, 26, 32].
The indications for dual-energy X-ray absorptiometry are postmenopausal period, risk factors of fractures or history of osteoporotic fractures, pathology resulting in osteoporotic changes, use of medical drugs with adverse events in view of decreasing bone mass. Moreover, X-ray densitometry should be performed for patients who have plans for conduction of osteoporosis therapy.
At the present time, bone mineral density is estimated with Z- and T-criteria. Z-criterion is defined as a number of standard deviations (SD) from mean level (M) of bone mineral density for individuals of this age group. T-criterion is a number of standard deviations from the peak of bone mass in young people. In examined individuals (age < 50) BMD is within reference values, if Z-criterion exceeds -2.0. As for patients at the age of 50 and older, the conclusion about osteopenia is made with T-criterion from -1.0 to -.2.4, and about osteoporosis, if T-criterion is -2.5 or below.
X-ray densitometry is used for examination of lumbar vertebrae, proximal department or the neck of the femoral bone or forearm bones [23]. Spine densitometry includes lumbar region scanning because it contains about 25 % of total bone mass of the spine.
According to performed studies, the peak values of BMD in lumbar vertebrae are observed at the age of 20-30. After that one can observe unsteady dynamic decrease in bone mass. Pronounced decrease in BMD values is observed in women in menopause period [26, 34].
Some producers of equipment perform modernization of densitometric systems by means of software for evaluation of additional parameters. So, it is worthy to note Vertebral Fracture Assessment (VFA) favoring rapid production of an image of this skeletal region. VFA is designed for estimation of vertebral fractures [28].
In addition to spine densitometry, an examination includes estimating BMD in the femoral bone and in the middle one-third of the radial bone (as a rule, in the dominant hand).
The correlation between BMD values in the lumbar vertebrae and the femoral neck has been found (r = 0.57) [26].
In most cases the examination of forearm bones has thee characteristics of a screening test. Some authors have shown predictive significance of DXA for the radial bone in relation to the risk of fractures during 25 years [10]. Rubin M.P. (2009) offers to examine BMD in the ultradistal department of the forearm with aim of prediction of a possibility of recurrent fractures, with orientation to the cut-off value of T-criterion (it is -2.1 standard deviations) [24].
Dual-energy X-ray absorptiometry is possible with different densitometric systems. The popular systems are Lunar (UK), Hologic (USA), Norland (USA). During skeletal scanning the standardization of results is performed according to a type of a densitometer [15].
With purpose of standardization for BMD values the computer program Standart LS (Russia) was created. This program allows calculating Z-criteria in women with consideration of the regional features in dependence on the type of the densitometric system [33].
The advantages of dual-energy X-ray absorptiometry include relative availability of the technique, low radial load (0.03 mSv per scan) and high diagnostic accuracy. However one should note that presence of bone deformities, scoliotic changes, calcinosis of intervertebral discs or the aorta significantly decreases the accuracy of osteoporosis diagnostics [5, 25].
Besides of dual-energy X-ray absorptiometry, diagnostics of osteoporotic changes is also carried out by means of quantitative computer tomography (QCT). The advantages of the technique are separate estimation of trabecular and cortical bone, 3D bone examination, prevention of overlapping of surrounding tissues. At the same time, despite of the advantage of CT densitometry, the studies of this topic are rare [1, 3, 4, 8]. The disadvantages of QCT densitometry are higher economic costs in comparison to DXA, as well as significant radiation exposure.
During quantitative computer tomography the bone mineral density (depending on software) is expressed as Houndsfield (HU), g/cm3 or mg Ca-Ha/ml (milligrams of calcium hydroxyapatite per millimeter).
CT densitometry is performed with use of calibration phantom including an equivalent of calcium hydroxyapatite. Computer preparation of the data is realized after scanning. The evaluation of scanning results is based on the values of bone mineral density in the trabecular bone. It is related to the fact that metabolic changes are more active in the bone than in the cortical region. Therefore, the highest spreading is related to CT densitometry of the spine lumbar region with prevailing trabecular bone.
For evaluation of bone mineral density in the vertebrae some tomographic scanners are equipped with special modes. So, Somatom (Siemens, Germany) includes the mode Osteo.
In CT densitometry the evaluation of bone mineral density is based on the values of trabecular bone [2]:
- The reference – BMD > 120 mg/cm3;
- Osteopenia – BMD is 80-120 mg/cm3;
- Osteoporosis – BMD < 80 mg/cm3;
The results of scanning are estimated according to the regression scale, which presents the diagram of dependence between age and level of calcium hydroxyapatite per volume unit. The regression scale corresponds to the primary reference database including the population in the manufacturing country. Therefore, each population must have its own database, which reflects regional characteristics.
The research group [30] has developed the database of population values of tridimensional bone mineral density for the population of the central region of Russia. The performed study identified the population differences from BMD values in the European population. Decreasing mineral density in trabecular bone of the vertebrae is 1.9 mg/cm3 (0.78 %) in women of reproductive age, and 2.22 mg/cm3 (0.93 %) per year in postmenopausal period [31].
The comparative assessment of QCT and DXA according to their diagnostic value for identification of osteoporosis did not result in the uniform opinion in the researchers. Some authors consider that accuracy of QCT is not higher than X-ray absorptiometry. At the same time, some studies show the advantages of QCT in comparison with DXA [11, 31].
Besides examination of the axial department of the skeleton, quantitative computer tomography can be used for densitometry of peripheral departments (Peripheral quantitative computed tomography, pQCT) with examination of femoral and radial bones [14, 19, 21, 29]. It is necessary to use the special software. The advantages of pQCT are small sizes and weight of equipment. However some researchers dispute the diagnostic value of CT-densitometry.
Bone ultrasonometry (BUS) has found its wide application for screening diagnostics of osteoporotic changes in the peripheral bones. Active practical implementation of this technique was initiated in late 80s – early 90s of the 20th century. The spheres of interest of BUS are forearm bones, the calcaneal bone, the tibial bone and finger bones. The technique is based on estimating the parameters of distribution and ultrasonic absorption by bones. During ultrasonic transmission through the bone its characteristics change themselves according to state of bone tissue.
The main parameters for bone ultrasonometry are speed of sound (SOS) and broadband ultrasound attenuation (BUA) [16].
The speed of sound (V, m/sec) means the time of ultrasonic transmission through the subject from the source of emission to the transducer. This parameter depends on elasticity and mineral density of the bone. The value of ultrasonic attenuation characterizes ultrasonic transmission through bone tissue and depends on bone mass, the number, sizes and space orientation of bone trabecules. This parameter is expressed as dB/MHz.
On the basis of the received results of ultrasonic sped and attenuation one can calculate the index of bone tissue strength.
Some studies demonstrated diagnostic significance of this technique for identification of osteoporotic changes. The inverse relationship between stiffness index (STI) and patients’ age was found [22].
The ultrasonometric system includes the in-built database, which is used for comparison of received results. As a rule, the available reference values are developed for the population of the region where the ultrasound equipment is produced. It inevitably decreases the quality of diagnostics in individuals of other population group.
The advantage of BUS is absence of ionizing radiation and moderate costs of equipment. However this technique has lower sensitivity and specificity in diagnostics of osteoporosis in comparison with dual-energy X-ray absorptiometry and quantitative computer tomography [6, 7, 17, 23]. It is necessary to note that results of ultrasonic scanning of bone tissue are not standardized and it decreases the quality of diagnostics [12].
The technique of ultrasonometry should be used only as screening diagnostics. After that other techniques are necessary for confirmation of diagnosis.
CONCLUSION
At the present time the leading place in diagnostics of osteoporotic changes is related to densitometry by means of dual-energy X-ray absorptiometry and quantitative computer tomography. For increasing the diagnostic accuracy it is necessary to develop the population standards which consider regional features, which are used for construction of the regression scale for estimation of bone mineral density.