Clinical determination of bone quality: Is ultrasound an answer?

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Abstract
Progress in clinical characterization of bone relies on developing a means to clinically assessall of the important determinants of bone quality, specifically, the intrinsic material properties of a bone (stiffness and brittleness) versus the macroscopic structural properties [apparent mass density (g/cc), structural shape and distribution of cortical mass, trabecular architecture, extent of unrepaired microdamage, and defects associated with the accelerated remodeling in early menopause]. Ultrasound devices currently measure parameters related to either of only two basic properties: bone ultrasound attenuation (BUA) or the apparent velocity of wave propagation (AVU). Theory and repeated corroboration in the laboratory have shown that the velocity of sound in solids such as bone has a quantitative relationship to the elastic modulus (or stiffness) and mass density. Although no comparable physical model exists for BUA, growingin vitro andin vivo empirical evidence shows a relationship to stiffness and mass density as well. Therefore, the question of ultrasound's ability to provide additional, clinically useful information about bone quality reduces to this:Does bone quality depend significantly on bone stiffness and does stiffness depend on factors other than bone mass alone? Clinical study results provide mounting evidence of ultrasound's abilities. (1) Numerous studies compare either velocity or BUA with BMC or BMD. The correlation coefficients vary widely between studies, even when repeated by the same investigators and laboratories. Two studies demonstrated this by comparing groups of subjects who are indistinguishable by BMD at the lumbar spine, but whose mean AVU readings are significantly different. (2) Multiple studies of AVU and BUA by different investigators have shown the ability of ultrasound to distinguish, as effectively as BMC or BMD, women with osteoporotic vertebral crush deformities from normal women. Prospective studies have shown that AVU and BUA each indicated risk of future osteoporotic fractures. In a population-based, randomized, cross-sectional study of men and women, AVU discriminated between groups of subjects who had suffered low trauma fractures versus those free of fracture. Such repeated clinical evidence of the ability of BUA and AVU to detect bone fragility provides mounting evidence that ultrasound measures a clinically relevant property of bone quality in addition to and distinct from bone mass.