An Update on Dual-Energy X-Ray Absorptiometry
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)
- et al.
Epidemiology and outcomes of osteoporotic fractures
Lancet
(2002) - et al.
Randomised trial of the effect of alendronate on risk of fracture in women with existing vertebral fractures
Lancet
(1996) - et al.
Technical principles of dual-energy x-ray absorptiometry
Semin Nucl Med
(1997) - et al.
Importance of precision in bone density measurements
J Clin Densitom
(2001) - et al.
Measurement of vertebral bone mineral density at the spine and proximal femur by volumetric quantitative computed tomograph and dual-energy X-ray absorptiometry in elderly women with and without vertebral fractures
Bone
(2002) - et al.
Discordance in patient classification using T-scores
J Clin Densitom
(1999) - et al.
Bone density at various sites for prediction of hip fractures
Lancet
(1993) - et al.
Peripheral or central densitometry: does it matter which technique we use?
J Clin Densitom
(2001) Diagnosis of osteoporosis and assessment of fracture risk
Lancet
(2002)- et al.
A meta-analysis of previous fracture and subsequent fracture risk
Bone
(2004)
A family history of fracture and fracture risk: a meta-analysis
Bone
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
Intervention thresholds for osteoporosis in the UK
Bone
Bone densitometry: choosing the proper site to measure
J Clin Densitom
Scientific flaws of the WHO T-score definition of osteoporosis
J Clin Densitom
A new approach to the development of assessment guidelines for osteoporosis
Osteoporos Int
Fast facts on osteoporosis
The cost of treating osteoporotic fractures in the United Kingdom female population
Osteoporos Int
Cost-equivalence of different osteoporotic fractures
Osteoporos Int
Population based study of survival after osteoporotic fractures
Am J Epidemiol
Mortality after all major types of osteoporotic fractures in men and women: an observational study
Lancet
Noninvasive assessment of bone mineral and structure: state of the art
J Bone Miner Res
Effect of intermittent cyclical etidronate therapy on bone mass and fracture rate in women with postmenopausal osteoporosis
N Engl J Med
Assessment of fracture risk and its application to screening for postmenopausal osteoporosis: technical report series 843
International Osteoporosis Foundation: an update on the diagnosis and assessment of osteoporosis with densitometry
Osteoporos Int
Effect of alendronate on risk of fracture in women with low bone density but without vertebral fractures: results from the Fracture Intervention Trial
JAMA
Effects of risedronate treatment on vertebral and non-vertebral fractures in women with postmenopausal osteoporosis
JAMA
Effect of risedronate treatment on hip fracture risk in elderly women
N Engl J Med
Effects of oral ibandronate administered daily or intermittently on fracture risk in postmenopausal osteoporosis
J Bone Miner Res
Once-yearly zoledronic acid for treatment of postmenopausal osteoporosis
N Engl J Med
Reduction of vertebral fracture risk in postmenopausal women with osteoporosis treated with raloxifene: results from a 3-year randomised clinical trial
JAMA
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
The effects of strontium ranelate on the risk of vertebral fracture in women with postmenopausal osteoporosis
N Engl J Med
Strontium ranelate reduces the risk of nonvertebral fractures in postmenopausal women with osteoporosis: TROPOS study
J Clin Endocrinol Metab
Strontium ranelate reduces the risk of vertebral and nonvertebral fractures in women eighty years of age and older
J Bone Miner Res
FRAX-WHO Fracture Risk Assessment Tool
Assessment of fracture risk
Osteoporos Int
Clinician's guide to prevention and treatment of osteoporosis
FRAX and the assessment of facture probability in men and women from the UK
Osteoporos Int
Case finding for the management of osteoporosis with FRAX—assessment and intervention thresholds for the UK
Osteoporos Int
Updated data on proximal femur bone mineral levels of US adults
Osteoporos Int
Meta-analysis of how well measures of bone mineral density predict occurrence of osteoporotic fractures
BMJ
BMD at multiple sites and risk of fracture of multiple types: long-term results from the Study of Osteoporotic Fractures
J Bone Miner Res
Predictive value of BMD for hip and other fractures
J Bone Miner Res
Treatment of postmenopausal osteoporosis
N Engl J Med
Monitoring skeletal change by radiological techniques
J Bone Miner Res
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2021, Journal of Clinical DensitometryThe effect of interbody fusion cage design on the stability of the instrumented spine in response to cyclic loading: an experimental study
2018, Spine JournalCitation 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.