Original ArticlesBone properties as estimated by mineral density, ultrasound attenuation, and velocity
Introduction
During the last decade, dual-energy X-ray absorptiometry (DXA) has been established as the most widely used and accepted method of in vivo bone mineral analysis, and is now the technique of choice in the diagnosis of osteoporosis. Recently, first quantitative ultrasound (QUS) instruments have obtained U.S. Food and Drug Administration (FDA) approval for use in clinical diagnostics. Several manufacturers have their own QUS instruments capable of measuring ultrasound attenuation (BUA) or speed of sound (SOS). In some devices, additional parameters, such as stiffness, can be calculated as a combination of the primary parameters (BUA, SOS). The QUS technique demonstrates a level of performance comparable to DXA for the prediction of fracture risk in osteoporosis.24
Although the dependence of DXA-measured areal bone mineral density (BMD, g/cm2) on bone size is recognized, previous in vivo and in vitro QUS studies on human or bovine trabecular bone have inconsistently demonstrated the relation between BUA and bone size.5, 16, 17, 27, 32 In commercial QUS instruments, SOS is typically determined without knowledge of the true bone thickness, which inevitably makes the result size-dependent. Also, the variable thickness and composition of soft tissue around the bone can have a significant effect on both BUA and SOS.7, 15, 16 Furthermore, the relationship of femoral, spinal, and forearm BMD to the BUA and SOS of the calcaneus may be partially transmitted through the size dependence of the DXA and QUS parameters.
Bone strength is the ultimate indicator of bone quality, but unfortunately is not directly measurable in vivo. Based on theoretical and experimental analyses, ultrasound behavior in bone depends also on the structural features of the tissue, and thus may bring forth extra information regarding bone quality not obtainable using DXA.10, 18, 19, 20, 25, 29, 30 Recent experimental studies suggest that combining both DXA and QUS measurements could improve prediction of bone stiffness and strength.6, 12, 21, 22, 23, 31 However, both theoretical and experimental studies are still needed to characterize the role of bone microstructure on QUS measurements.
In this study, the dependence of DXA and QUS parameters on the size of the bovine trabecular bone sample was characterized in vitro. Bone-size-corrected DXA and QUS parameters were correlated with the true mechanical strength, obtained from the mechanical tests. In addition to in vitro measurements, interrelationships between DXA and QUS parameters of the human calcaneus were studied in vivo. We were particularly interested in the effect of calcaneal size on ultrasound results.
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Materials and methods
Bovine and human trabecular bone were investigated in vitro and in vivo, respectively. All bovine bone samples were analyzed using ultrasound, DXA, and mechanical testing and, finally, were ashed to obtain the true mineral content in bone. In the in vivo study, ultrasound and DXA measurements of the calcaneus in 34 women were conducted.
Results
When assessing reproducibility for ultrasound and DXA measurements in vitro, the coefficients of variation (cv%) were 3.0%, 0.25%, and 1.55% for BUA, SOS, and BMD, respectively. The in vitro study revealed a significant effect of bovine bone thickness on the DXA and ultrasound parameters. Volumetric BMDvol was independent of the bone thickness (Figure 2). BMD and SOS showed highly linear positive correlations with the sample thickness:
Discussion
In this study, QUS and DXA measurements were conducted on bovine and human trabecular bone in vitro and in vivo, respectively. The results revealed a significant effect of bovine bone size on the ultrasound parameters measured in vitro using a commercial, clinical instrument. By normalizing the DXA and QUS results with the bone size, we could systematically improve their ability to predict bone strength. Instead, BUA showed no significant linear correlation with either bone mineral density,
Acknowledgements
Financial support from the Kuopio University Hospital (EVO Grant 5103) is gratefully acknowledged.
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