• 18
    Dec
  • Quantitative ultrasound of bone: calcaneus. Clinical application of QUS

 Clinical application of QUS

Although considerable effort has been made to characterize the relationship between QUS and BMD measurement of the same skeletal site, from a clinical point of view, the most impor­tant issue regarding QUS is its ability to predict fracture risk. There is ample evidence documenting the ability of calcaneal QUS to predict osteoporotic fracture risk both in women (19­34) and in men. It is important to emphasize that QUS parameters result independent predictors of osteoporotic frac­ture, even after adjustment for BMD. These studies reported a strong association of calcaneal QUS with vertebral fracture hip fracture and os­teoporotic fractures in general. Logistic regression analysis has shown that the fracture risk usually increases by 1.5-2.5 times for every 1 standard deviation reduction of each QUS parameters. Moreover it has been demonstrated that the fracture risk prediction increases with both the combination of QUS and DXA.

Correlation between QUS parameters and BMD, as measured by X-ray absorptiometry have been under investigation since QUS was first introduced for clinical practice. Correlation coeffi­cients usually range from 0.3 to 0.8 and there is general agreement that QUS and BMD interact differently with bone and that explains why the correlation between the two methods, even- though significant, is modest. Since WHO has defined Osteoporosis on the basis of BMD, a sure diagnosis can be formulated only with a technique that directly measures bone mineral density, as bone densitometry. However, the useful­ness of QUS is justified in numerous studies; in fact some Au­thors have demonstrated that QUS parameters are more pre­dictive of bone mass than the factor risk evaluation. In other wards, postmenopausal women who should be referred for further examination by DXA, could be selected better on the basis of QUS measurement than with risk factors evaluation alone. Moreover, published data on the cost-effectiveness of this approach are not sufficient to recommend a population screening with QUS. Moreover, guidelines on the position of QUS in the diagnosis and therapeutic decision are not available. In this respect, a crucial point is the evaluation of the possibility of using WHO criteria also for QUS, since it has been shown that QUS and DXA cannot always identify the same population. Some studies have shown that the cut­off of -2.5, utilised for the definition of osteoporosis with DXA, can be employed also for some QUS devices, such as the Achilles. On the other hand, for other instruments, differ­ent cut-offs have been calculated. Moreover, it has been recently demonstrated that clinical risk factors are related both to QUS and DXA parameters when expressed in terms of Z-score, and that the proportion of postmenopausal women classified as osteopenic or osteoporotic is similar, with both DXA and QUS. At present no universal cut offs are dis­posable for QUS parameters and, therefore, even though good correlations have been showed between parameters obtained by different devices, it is not possible to standardize normal range as it has been made for DXA. Moreover, QUS refer­ence phantoms for cross-calibration procedures and standard­ization methods between different devices are not available. However, QUS parameters could be considered as an impor­tant risk factor, allowing to classify a subject as at “low”, “medi­um” or “high” risk on the basis of QUS result. Ayers et al. have demonstrated that the associate evaluation of clinical risk fac­tors and QUS shows a sensitivity in the diagnosis of osteo­porosis similar or superior to the axial DXA; following this strategy a further DXA evaluation could be indicated only in pa­tients with positive risk factors and normal QUS. A strate­gy combining QUS, DXA and clinical factors for the identifica­tion of women needing an appropriate treatment has been re­cently proposed by Hans et al..
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Because of the limited experience, monitoring skeletal changes solely by QUS cannot be recommended yet. The ability of QUS to monitor change widely depends on the reproducibility of QUS parameters and on the magnitude of the response. The time period to follow individual subjects would most likely exceed those required for bone densitometry; in fact, even though some studies showed a significant increase of QUS pa­rameters in patients treated with antiresorptive drugs, it is not possible to identify, with QUS, change of bone status in­duced by these drugs, earlier than 2 years. Nevertheless, for the possible usefulness of QUS in the follow up, in order to maximize its ability to monitor change and to minimize any measurement error, adequate measurement protocols and quality assurance procedures are needed. The use of calcaneal QUS has also been proposed in sec­ondary osteoporosis, such as osteoporosis induced by corti­costeroids or associated with rheumatoid arthritis. In a recent study, the influence of corticosteroid therapy on QUS parameters during the first year after renal transplantation has been investigated. The ability of QUS to give us infor­mation on both bone mass and qualitative characteristics of bone could be utilized also in the diagnosis of different meta­bolic bone diseases, such as primary and secondary hyper- parathyroidism. QUS at calcaneus has been employed also in patients with osteogenesis imperfecta or with Sudeck’s disease of bone.

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