Broadband Ultrasound Attenuation Compared With Dual Energy X-Ray Absorptiometry In Screening For Osteoporosis In An Asian Population

* Woon CYL, Chong KC, Lim YW, Lam KS

*Department of Orthopaedic Surgery, Changi General Hospital, Singapore 529889

Address for Correspondence
Dr Chong Kian Chun
Department of Orthopaedic Surgery
Changi General Hospital, 2 Simei Street 3,
Singapore 529889
Tel: 67888833
Fax : 67880933
Email: chongkianchun@yahoo.com

Abstract

Objectives :  Quantitative ultrasound of the calcaneus is an alternative to dual-energy X-ray absorptiometry for measurement of bone mass. Various studies have demonstrated correlation between broadband ultrasound attenuation and bone mineral density in Caucasian populations. This study compares the use of broadband ultrasound attenuation (BUA) measurements with bone mineral density (BMD) for osteoporosis screening in the Asian population.
Design: A comparative study comparing calcaneal broadband ultrasound attenuation measurements and hip bone mineral density.
Materials and Methods: 32 subjects with osteoporotic hip and vertebral fractures underwent calcaneal broadband ultrasound attenuation and dual-energy X-ray absorptiometry. The results of both tests are compared.
Results: There is moderate correlation with Pearson’s correlation coefficient, r = 0.688 (p<0.0001). This is comparable to results of published series for the Caucasian population.
Conclusions: The CUBA Clinical System is a sensitive tool to screen for osteoporosis in the Asian population. Selective further testing with DEXA can be performed for individuals with low BUA measurements. This approach to screening can give potential cost savings.

J.Orthopaedics 2006;3(1)e16

Introduction:

Since the introduction of ultrasound by Langton, et al in 19841 for the evaluation of bone strength and quality, ultrasound has been used for assessment of vertebral and hip fracture risk in osteoporotic patients.^2-4 Calcaneal quantitative ultrasound (QUS) is an alternative technique to dual energy x-ray absorptiometry (DEXA) for assessing bone mass. The QUS sonometer measures velocity of sound (VOS, in ms-1) and broadband ultrasound attenuation (BUA, in dB MHz-1).¹ In contrast to DEXA, which solely measures bone mineral density, QUS gives added information regarding the microstructural properties of cancellous bone.^1,5-8 While BMD accounts for a significant proportion of bone strength,9,10 these microarchitectural properties of trabecular bone, such as strut number and thickness, trabeculae connectivity, orientation and spacing, are also important for determination of fracture risk.^11,12 Various studies have shown BUA to be moderately correlated with BMD in the Caucasian population,13-15 and it is a more significant predictor of fracture risk compared to VOS.^2,12,16,17 Osteoporotic bone, being less dense, absorbs less sound, resulting in a reduced attenuation. Normal bone, on the other hand, will have higher attenuation.  For every decrease in BUA of 1 standard deviation, the risk of hip fracture doubles.^12,16-18  

This study examines the correlation between BUA and BMD and the potential use of CUBA clinical system as a screening tool for osteoporosis in an Asian population.

Material and Methods :

In a separate study by Lim et al., 602 healthy Asian subjects of Chinese, Malay, Indian and Filipino descent had QUS performed. Left calcaneal BUA was measured with the Contact Ultrasound Bone Analyser (CUBA) clinical system, McCue Ultrasonics Ltd (Winchester, UK).19 Acoustic coupling was achieved with the use of silicone pads to increase contact area, and ultrasound gel to minimize air gaps. The study established normative data for BUA in an Asian population.

TABLE 1

Thirty-two of these patients with osteoporotic fractures of the hip and spine had additional BMD assessment with DEXA scans of the hip. The mean age was 79 years (range, 62 to 95 years)(Table 1), with 26 females and 6 male subjects. The results of calcaneal BUA and hip BMD were then compared and the association analyzed using Pearson’s correlation coefficient. A best-fit regression line, y=mx + c was then plotted to describe the relationship between BMD and BUA values.

Results :

From our series, there was a moderate correlation between left calcaneal BUA and hip BMD with Pearson’s correlation coefficient, r = 0.688 (p<0.0001). This association, displayed graphically in Figure 1, is comparable to results of the Caucasian population.14,15 Best fit was given by the linear equation hip BMD = 1.0025 (heel BUA) – 0.1417. In the Caucasian population, Greenspan et al showed that BUA of the calcaneus (CUBA) is moderately correlated with DEXA of the hip, with Pearson’s product moment correlation coefficients of 0.715.

Discussion :

WHO defined osteoporosis as a “systemic skeletal disease characterised by low bone mass and microarchitectural deterioration of bone tissue, with a consequent increase in bone fragility and susceptibility to fracture.”20 It is desirable to identify individuals at greatest risk of fractures, as this risk can be halved with effective treatment.21 

In the United States, the National Osteoporosis Foundation recommends routine BMD screening for all women above the age of 65.22 Currently, DEXA is the gold standard for BMD measurement. However, DEXA as a population screening tool may not be cost-effective23 and is neither feasible in the UK,24 nor in the Asian population owing to limited resources and cost. In addition, bone density, being a continuous variable, is an imperfect population screening tool as it shows a large overlap in BMD of patients with fracture and those without.25 Clinical risk factors are another means of screening patients, but this has shown to be a poor discriminator of BMD of the hip and spine.26 Other means, such as QUS, should be evaluated for a more cost-friendly alternative. 

Besides cost, other advantages of QUS over DEXA are that these instruments are radiation-free, inexpensive, easy to apply, and do not require dedicated office space.15,27 

BMD measurements are predictive of risk of later fractures. A large meta-analysis involving 11 prospective cohort studies showed that measurement of BMD at any site (including proximal radius, distal radius, hip, lumbar spine and calcaneus) had similar predictive ability for fractures (relative risk 1.5, 95% CI 1.4 to 1.6) for a decrease in 1 SD in bone density, except for measurements at hip and spine, which have better predictive ability for fractures at hip and spine respectively.28   

For the Caucasian population, BUA has a greater population standard deviation compared with hip BMD.29 The diagnosis of osteoporosis is currently based on the World Health Organization (WHO) definition of BMD being more than 2.5 standard deviations below the mean for a young healthy adult woman at spine, hip or forearm.30 However, different parts of the skeleton behave differently within a particular subject, with different rates of bone accretion and loss at spine, hip and heel, and may have different BMD T-score values at these measurement sites.31 Thus the WHO criteria for BMD T-score values determined by DEXA may not be appropriate for interpretation of BUA because of the different anatomic site involved, and different measurement technology.32 As a result of this, some studies recommend the use of an adjusted BUA T-score of –2.0 SD instead.29,33 

Based on this adjusted BUA T-score reference range, only 1 case (Subject 26) would be missed in our series, giving a sensitivity of 96.9%.  

For cases with lower BUA than BMD T-scores, this may be attributed to differences in the mechanical properties of calcaneal trabecular bone, which are detected by BUA, but not by DEXA. 

The limitations of this study include the small sample size, and the inclusion of subjects with fractures only.  

Further follow-up studies are necessary to :

(1)   Establish an appropriate reference range of BUA T-scores for the Asian population, thus allowing QUS to be used as a diagnostic tool, minimizing the need for further testing with DEXA, and

(2)   Determine if such correlation between BUA and DEXA exists in Asian subjects without fractures.

Determine if QUS can be used to monitor disease progression or response to therapeutic intervention.15 This will further enhance the role of QUS in the surveillance of patients with osteoporosis.

Conclusion:

There is moderate correlation between BUA and BMD, with a Pearson’s correlation coefficient r of 0.688 (p<0.0001) in the Asian population, comparable to studies on Caucasian subjects. Our series demonstrates that the CUBA Clinical System is a sensitive tool for pre-screening for osteoporosis in an Asian population. As these instruments provide rapid measurements, do not take up dedicated office space, and are more affordable than DEXA of the hip and spine, they are possible options for initial pre-screening of patients with osteoporosis.

Further assessment of bone density and fracture risk in these individuals can then be performed with follow-up DEXA. This approach to population screening will provide potential cost savings.

Reference :

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3. Cepollaro C, Gonnelli S, Pondrelli C, Martini S, Montagnani A, Rossi S, Gennari L, Gennari C. The combined use of ultrasound and densitometry in the prediction of vertbral fracture. Br J Radiol 1997;70:691-6.

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20. NIH Consensus Development Conference. Diagnosis, prophylaxis and treatment of osteoporosis. Am J Med 1993;94:646-650.

21. Black DM, Cummings SR, Karpf DB, Cauley JA, Thompson DE, Nevitt MC, et al. Randomised trial of effect of alendronate on risk of fracture in women with existing vertebral fractures. Fracture Intervention Trial Research Group. Lancet 1996;348: 1535-1541.

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This is a peer reviewed paper 

Please cite as : Woon CYL:Broadband Ultrasound Attenuation Compared With Dual Energy X-Ray Absorptiometry In Screening For Osteoporosis In An Asian Population

J.Orthopaedics 2006;3(3)e16

URL: http://www.jortho.org/2006/3/3/e16

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