An Update on Dual-Energy X-Ray Absorptiometry

https://doi.org/10.1053/j.semnuclmed.2009.08.001Get rights and content

Dual-energy x-ray absorptiometry (DXA) scans to measure bone mineral density at the spine and hip have an important role in the evaluation of individuals at risk of osteoporosis, and in helping clinicians advise patients about the appropriate use of antifracture treatment. Compared with alternative bone densitometry techniques, hip and spine DXA examinations have several advantages that include a consensus that bone mineral density results should be interpreted using the World Health Organization T score definition of osteoporosis, a proven ability to predict fracture risk, proven effectiveness at targeting antifracture therapies, and the ability to monitor response to treatment. This review discusses the evidence for these and other clinical aspects of DXA scanning. Particular attention is directed at the new World Health Organization Fracture Risk Assessment Tool (FRAX) algorithm, which uses clinical risk factors in addition to a hip DXA scan to predict a patient's 10-year probability of suffering an osteoporotic fracture. We also discuss the recently published clinical guidelines that incorporate the FRAX fracture risk assessment in decisions about patient treatment.

Section snippets

The Clinical Role of Bone Density Measurements

Today, BMD measurements have an important role in the evaluation of patients at risk of osteoporosis and in the appropriate use of antifracture treatment.24, 25, 26, 27 In general, the preferred method of testing is to use DXA scans to measure BMD of the lumbar spine and hip (Fig. 1).28 DXA examinations have 3 major roles, namely, the diagnosis of osteoporosis, the assessment of patients' risk of fracture, and monitoring response to treatment. The reasons for using DXA include the fact that hip

The Physical Principles of DXA Scans

DXA scanners evaluate BMD by measuring the transmission of x-rays through the body at 2 different photon energies.37 The mathematical theory of DXA, referred to as basis set decomposition, states that across a broad range of photon energies, the x-ray transmission through any physical object can be decomposed into the equivalent areal densities (g/cm2) of any 2 chosen reference materials.38 The 2 materials for DXA scanning are bone mineral (hydroxyapatite, Ca10(PO4)6(OH)2) and soft tissue.

DXA Precision Errors

Clinicians who report DXA scans will be aware that BMD measurements are affected by precision43, 44 and accuracy errors.40, 41, 42 Precision errors measure the reproducibility of BMD results in individual patients and can be demonstrated by performing repeated scans on a representative group of subjects.34 Precision is usually expressed in terms of the coefficient of variation (CV) and is typically approximately 1%-1.5% for spine and total hip BMD and 2%-2.5% for femoral neck BMD.45 DXA

Which Type of Measurement Is Best?

In addition to DXA systems for measuring the spine and hip, a variety of other types of bone densitometry equipment is also available.8, 55 These include quantitative computed tomography (QCT) measurements of the spine and hip,56, 57 peripheral DXA (pDXA) systems for measuring the forearm, heel, or hand,58 and quantitative ultrasound (QUS) devices for measurements of the heel and other peripheral sites.59 In principle, pDXA and QUS devices offer a rapid, inexpensive, and convenient method of

Results From Fracture Studies

One of the clinical advantages of DXA scans is that their ability to identify patients at risk of fracture has been assessed and proven in a large number of epidemiologic studies.29 One of the most informative of these is the Study of Osteoporotic Fractures (SOF), a study of 9704 white US women aged 65 years and over who had baseline measurements of hip, spine, forearm, and heel BMD when the study commenced in the late 1980s.30 The SOF 10-year follow-up data have confirmed the association

Appropriate Targeting of Anti-Fracture Treatments

Another advantage of spine and hip DXA (Table 2) is the proven ability to identify patients who will respond successfully to pharmaceutical treatments for preventing osteoporotic fractures. Table 3 lists the principal clinical trials of the agents proven to prevent vertebral and/or nonvertebral fractures.12, 13, 14, 15, 16, 17, 18, 19, 20, 21 It is notable that all the trials listed enrolled patients on the basis of entry criteria that included a hip or spine T score demonstrating either

Choice of Reference Ranges

Over the last 15 years the interpretation of DXA scans has been guided by the WHO T score definition of osteoporosis (Table 1). However, care is necessary in the choice of reference data for the calculation of T score values if scan results are to be interpreted reliably. For consistency, most guidelines on patient treatment recommend the use of the Third National Health and Nutrition Examination Survey (NHANES III) reference database for T score derivation in the hip.28 This recommendation

Interpretation of T Scores Using the WHO Criteria

As explained above, there is widespread consensus that spine, hip, and forearm DXA measurements should be interpreted using the WHO T score definition of osteoporosis. However, the WHO definition should not be used to interpret QCT or QUS measurements, or pDXA results at sites other than the 33% radius.34 The reason for this rule can be understood from Figure 5. When the reference ranges for different types of bone density measurement are plotted as graphs of mean T score against age, the

The WHO FRAX Fracture Risk Algorithm

Views on the best way of using information from DXA scans to advise patients about the use of antifracture treatment continue to evolve.2, 24, 69, 70 As emphasized above, the real clinical value of BMD examinations lies in the information they provide about fracture risk. An important limitation of the WHO T score approach to making decisions about treatment is that age as well as BMD is an important factor in determining the likelihood of a patient having a fracture within the next 5 or 10

New Treatment Guidelines Incorporating FRAX

The launch of the FRAX website in 2008 was followed by the publication of new guidelines with recommendations on how estimates of 10-year fracture probability should be incorporated into decisions about patient treatment.25, 27 Most commentators have noted that treating patients solely according to their fracture risk results in fewer younger individuals receiving treatment because, although they might have a low T score, their short-term risk of fracture is small. Instead, treatment is

Monitoring Response to Treatment

Verifying response to treatment using follow-up DXA scans is widely believed to have a beneficial role in encouraging patients to continue taking their medication, and also in identifying nonresponders who may benefit from a different treatment regimen. Central DXA has a number of advantages as a technique for monitoring patients' response, of which one of the most important is the good precision of BMD measurements (see discussion of precision errors above). A second requirement for effective

Postscript

The evolution of ideas about the clinical role of bone densitometry has some way to go before they can be regarded as well grounded in science. One important question is the scientific rationale for the continued use of T scores.93 The FRAX assessment tool makes it abundantly clear that a low BMD result is best regarded as just another clinical risk factor for fracture (Table 4). Given the magnitude of the soft-tissue accuracy errors (±0.5 T score units) and the discordant T scores between

References (93)

  • J.A. Kanis et al.

    A family history of fracture and fracture risk: a meta-analysis

    Bone

    (2004)
  • H.A. Fink et al.

    Differences in site-specific fracture risk among older women with discordant results for osteoporosis at hip and spine: study of osteoporotic fractures

    J Clin Densitom

    (2008)
  • J.A. Kanis et al.

    Intervention thresholds for osteoporosis in the UK

    Bone

    (2005)
  • K.G. Faulkner

    Bone densitometry: choosing the proper site to measure

    J Clin Densitom

    (1998)
  • G.M. Blake

    Scientific flaws of the WHO T-score definition of osteoporosis

    J Clin Densitom

    (2008)
  • J.A. Kanis et al.

    A new approach to the development of assessment guidelines for osteoporosis

    Osteoporos Int

    (2002)
  • Fast facts on osteoporosis

  • P. Dolan et al.

    The cost of treating osteoporotic fractures in the United Kingdom female population

    Osteoporos Int

    (1998)
  • L.J. Melton et al.

    Cost-equivalence of different osteoporotic fractures

    Osteoporos Int

    (2003)
  • C. Cooper et al.

    Population based study of survival after osteoporotic fractures

    Am J Epidemiol

    (1993)
  • J.R. Centre et al.

    Mortality after all major types of osteoporotic fractures in men and women: an observational study

    Lancet

    (1999)
  • H.K. Genant et al.

    Noninvasive assessment of bone mineral and structure: state of the art

    J Bone Miner Res

    (1996)
  • T. Storm et al.

    Effect of intermittent cyclical etidronate therapy on bone mass and fracture rate in women with postmenopausal osteoporosis

    N Engl J Med

    (1990)
  • Assessment of fracture risk and its application to screening for postmenopausal osteoporosis: technical report series 843

    (1994)
  • J.A. Kanis et al.

    International Osteoporosis Foundation: an update on the diagnosis and assessment of osteoporosis with densitometry

    Osteoporos Int

    (2000)
  • S.R. Cummings et al.

    Effect of alendronate on risk of fracture in women with low bone density but without vertebral fractures: results from the Fracture Intervention Trial

    JAMA

    (1998)
  • S.T. Harris et al.

    Effects of risedronate treatment on vertebral and non-vertebral fractures in women with postmenopausal osteoporosis

    JAMA

    (1999)
  • M.R. McClung et al.

    Effect of risedronate treatment on hip fracture risk in elderly women

    N Engl J Med

    (2001)
  • C.H. Chesnut et al.

    Effects of oral ibandronate administered daily or intermittently on fracture risk in postmenopausal osteoporosis

    J Bone Miner Res

    (2004)
  • D.M. Black et al.

    Once-yearly zoledronic acid for treatment of postmenopausal osteoporosis

    N Engl J Med

    (2007)
  • B. Ettinger et al.

    Reduction of vertebral fracture risk in postmenopausal women with osteoporosis treated with raloxifene: results from a 3-year randomised clinical trial

    JAMA

    (1999)
  • R.M. Neer et al.

    Effect of recombinant human parathyroid hormone (1-34) fragment on spine and non-spine fractures and bone mineral density in postmenopausal osteoporosis

    N Engl J Med

    (2001)
  • P.J. Meunier et al.

    The effects of strontium ranelate on the risk of vertebral fracture in women with postmenopausal osteoporosis

    N Engl J Med

    (2004)
  • J.Y. Reginster et al.

    Strontium ranelate reduces the risk of nonvertebral fractures in postmenopausal women with osteoporosis: TROPOS study

    J Clin Endocrinol Metab

    (2005)
  • E. Seeman et al.

    Strontium ranelate reduces the risk of vertebral and nonvertebral fractures in women eighty years of age and older

    J Bone Miner Res

    (2006)
  • FRAX-WHO Fracture Risk Assessment Tool

  • J.A. Kanis et al.

    Assessment of fracture risk

    Osteoporos Int

    (2005)
  • Clinician's guide to prevention and treatment of osteoporosis

  • J.A. Kanis et al.

    FRAX and the assessment of facture probability in men and women from the UK

    Osteoporos Int

    (2008)
  • J.A. Kanis et al.

    Case finding for the management of osteoporosis with FRAX—assessment and intervention thresholds for the UK

    Osteoporos Int

    (2008)
  • A.C. Looker et al.

    Updated data on proximal femur bone mineral levels of US adults

    Osteoporos Int

    (1998)
  • D. Marshall et al.

    Meta-analysis of how well measures of bone mineral density predict occurrence of osteoporotic fractures

    BMJ

    (1996)
  • K.L. Stone et al.

    BMD at multiple sites and risk of fracture of multiple types: long-term results from the Study of Osteoporotic Fractures

    J Bone Miner Res

    (2003)
  • O. Johnell et al.

    Predictive value of BMD for hip and other fractures

    J Bone Miner Res

    (2005)
  • R. Eastell

    Treatment of postmenopausal osteoporosis

    N Engl J Med

    (1998)
  • C.C. Gluer

    Monitoring skeletal change by radiological techniques

    J Bone Miner Res

    (1999)
  • Cited by (93)

    • The applicability of bone mineral density for adult age estimation

      2023, Methodological and Technological Advances in Death Investigations: Application and Case Studies
    • The effect of interbody fusion cage design on the stability of the instrumented spine in response to cyclic loading: an experimental study

      2018, Spine Journal
      Citation Excerpt :

      The BLL cages showed a significantly higher response hysteresis (100%, p<.01) and rate of subsidence (10%–30%) compared with the ACS and the UOL cage designs. The measured BMD values of the donors' spines (Table 1) were in agreement with the values previously reported for human subjects spanning the age range employed for this study [57–59]. Fig. 7 presents the change in measured cage-bone interface stiffness (Scage) versus BMD values at the 10th and 5000th cycle of loading.

    View all citing articles on Scopus
    View full text