Fracture Risk Assessment and DEXA

Fracture risk assessment is the structured estimation of how likely a person is to break a bone, combining a measurement of bone mineral density with clinical risk factors. Dual-energy X-ray absorptiometry (DXA, often written DEXA) is the standard tool for measuring bone density, and the T-score it yields, together with risk calculators such as FRAX, frames how osteoporosis is identified and quantified.

Definition

Fracture risk assessment is the estimation of an individual's probability of fragility fracture, typically using dual-energy X-ray absorptiometry to measure bone mineral density (expressed as a T-score) alongside clinical risk factors, sometimes combined in an absolute-risk algorithm.

Scope

This entry covers how bone density is measured by DXA, how the T-score defines osteoporosis densitometrically, how density relates to fracture risk, and how absolute-risk tools integrate density with clinical factors. It is reference-educational; it explains the concepts and evidence behind risk assessment and does not provide thresholds for individual treatment decisions.

Core questions

• How does DXA measure bone mineral density?
• What do the T-score and Z-score mean?
• How well does bone density predict fracture?
• Why combine density with clinical risk factors?
• What does an absolute fracture-probability tool add?

Key concepts

• Dual-energy X-ray absorptiometry (DXA/DEXA)
• Bone mineral density
• T-score and Z-score
• WHO densitometric definition of osteoporosis
• Clinical risk factors for fracture
• Absolute (10-year) fracture probability
• FRAX algorithm

Mechanisms

DXA estimates areal bone mineral density by measuring the differential attenuation of two X-ray energies passing through bone and soft tissue, usually at the hip and lumbar spine. The result is compared with a young-adult reference population to yield a T-score, the number of standard deviations from the young-adult mean; the WHO framework defines osteoporosis as a T-score at or below -2.5 (Kanis, 1994). Bone density predicts fracture in a graded way — risk rises as density falls — but a single density measure has limited sensitivity, because many fractures occur in people above the osteoporotic threshold (Marshall et al., 1996). Absolute-risk tools such as FRAX therefore combine density with independent clinical risk factors (age, prior fracture, parental hip fracture, smoking, glucocorticoid use, and others) to estimate a person's 10-year probability of major osteoporotic and hip fracture (Kanis et al., 2008).

Clinical relevance

Fracture risk assessment is the bridge between measuring bone and estimating who is likely to fracture, and it underlies how osteoporosis is recognized and how the value of intervention is judged. This entry describes the methods and their evidence base for educational reference; it does not specify diagnostic cut-offs or treatment thresholds for individual patients.

Epidemiology

Because the relationship between density and fracture is continuous, most fragility fractures in a population arise among people with density values above the osteoporotic threshold, which is the rationale for combining density with clinical risk factors rather than relying on density alone (Marshall et al., 1996; Kanis et al., 2008).

Evidence & guidelines

The densitometric definition of osteoporosis derives from a WHO report (Kanis, 1994), the predictive value of bone density rests on meta-analysis of cohort data (Marshall et al., 1996), and absolute-risk estimation is exemplified by the FRAX algorithm (Kanis et al., 2008); clinical reviews integrate these into contemporary assessment (Compston et al., 2019).

History

Bone densitometry matured in the 1980s and 1990s with dual-energy absorptiometry, and the 1994 WHO report standardized the T-score-based definition of osteoporosis. Recognition that density alone underestimates many fractures led, in the 2000s, to absolute-risk models such as FRAX that integrate clinical risk factors with density.

Debates

Should fracture risk be based on bone density or absolute probability?

Density defines osteoporosis but predicts only part of fracture risk; absolute-risk tools that add clinical factors capture more risk, and how best to combine the two for decision-making remains an active methodological question.

Key figures

• John A. Kanis
• L. Joseph Melton
• Olof Johnell
• Eugene McCloskey

Seminal works

• kanis-1994
• marshall-1996
• kanis-2008-frax

Frequently asked questions

What is a T-score?

A T-score is the number of standard deviations a person's bone mineral density differs from the mean of a healthy young-adult reference; the WHO framework defines osteoporosis as a T-score of -2.5 or lower.

If my bone density is normal, can I still fracture?

Yes; bone density predicts risk only partially, and because the relationship is continuous many fragility fractures occur in people whose density is above the osteoporotic threshold, which is why clinical risk factors are also assessed.

What does FRAX do?

FRAX is an algorithm that combines bone density with clinical risk factors such as age and prior fracture to estimate a person's 10-year probability of major osteoporotic and hip fracture.