Fracture Risk Explained: FRAX, Fragility and Bone Quality

What is FRAX and what does it actually measure?

FRAX is a fracture-risk calculator that estimates your 10-year probability of breaking a bone — not a bone-density scan. It gives two numbers: the 10-year probability of a hip fracture, and the 10-year probability of a major osteoporotic fracture (MOF), which covers a clinical spine, hip, forearm or shoulder (humerus) fracture FRAX / University of Sheffield, Osteoporosis Research Ltd.

The tool was developed in 2008 by what was then the WHO Collaborating Centre for Metabolic Bone Diseases at Sheffield. It integrates a person's clinical risk factors and, optionally, a femoral-neck bone-mineral-density (BMD) result, to compute those 10-year probabilities NOGG 2024 Guideline. The Royal Osteoporosis Society describes this kind of fracture-risk assessment plainly: a questionnaire that estimates the 10-year chance of breaking a bone from factors such as family history, medical conditions, previous fractures, body weight and lifestyle, which a doctor then uses to help decide whether osteoporosis medication is needed Royal Osteoporosis Society.

The key idea is that a fracture happens when the load on a bone exceeds its strength — and strength depends on far more than a single density reading. FRAX exists to fold those other factors into one estimate.

What information does FRAX use to calculate risk?

FRAX combines a set of clinical risk factors, most of which need nothing more than a short questionnaire. According to the NOGG guideline, the inputs are age, sex, body mass index (from your weight and height), a prior fracture, a parent's hip fracture, current smoking, oral glucocorticoid (steroid) use, rheumatoid arthritis, other secondary causes of osteoporosis, and alcohol intake NOGG 2024 Guideline.

Crucially, a femoral-neck BMD value is an optional input, not a requirement. You can run FRAX without a scan to get a first estimate, and add a density result later to sharpen it NOGG 2024, Section 3. That is why FRAX can be used as an early triage step: it flags who might benefit from a bone-density scan, rather than assuming everyone needs one straight away.

If you want to understand what that density result itself means once you have one, our guide to DEXA scan results and T-scores walks through how the numbers are classified.

How is a FRAX result used in the UK?

In the UK, a FRAX probability only becomes a decision once it is compared with NOGG's age-adjusted thresholds. The National Osteoporosis Guideline Group sets a tiered system that turns a percentage into a next step NOGG 2024 Guideline.

Broadly, UK management reads a FRAX major-osteoporotic-fracture probability against four reference points:

• Below the lower assessment threshold — reassurance and lifestyle advice.
• Around an upper assessment threshold — a bone-density (DXA) measurement is recommended to refine the estimate.
• Above the intervention threshold — treatment is considered.
• Above the very-high-risk threshold — referral to a specialist is considered.

These thresholds are age-dependent, which is one of the most misunderstood parts of FRAX NOGG 2024 Guideline. The same MOF percentage can sit below the treatment line at one age and above it at another, because the background risk of fracturing changes as we get older. That is why a FRAX number is never self-explanatory: it has to be interpreted by a clinician against the right threshold for your age. The Royal Osteoporosis Society frames the tool the same way — the probability is there to help a doctor decide whether medication is warranted, not to hand you a verdict on its own Royal Osteoporosis Society.

Can you break a bone even if your bone density is 'normal'?

Yes — and it is more common than most people expect. A prospective study found that more than 70% of women who go on to sustain a fracture have osteopenia or 'normal' bone mineral density rather than diagnosable osteoporosis, and that adding a measure of microstructural deterioration identified at-risk women whom density alone missed Journal of Bone and Mineral Research, 2020.

The reason is that bone strength has two components. One is quantity — how much mineral is packed into the bone, which is what density measures. The other is quality: the internal microarchitecture, the way the cortical shell and the trabecular lattice inside are arranged. Two bones can share the same density number but differ in how well that material is organised, and therefore in how easily they break.

This is exactly why fracture-risk assessment does not rely on a density number alone. FRAX already captures part of the quality signal indirectly — a previous fracture, for example, tells you something a scan cannot. But it has fuelled interest in measuring bone quality more directly. If you are curious about what a standard density scan can and cannot reveal, our explainer on what a DEXA scan shows is a good companion to this.

How does REMS add a Fragility Score, and how is that different from density?

REMS produces a Fragility Score — a bone-quality measure that is calculated independently of density. REMS (radiofrequency echographic multi-spectrometry) is a radiation-free ultrasound method for assessing the spine and hip. Alongside a bone-density estimate, it derives a Fragility Score that reflects skeletal microarchitecture, reported on a scale from 0 (normal) to 100 (maximum fragility) Diagnostics (Basel), 2025.

Because the Fragility Score is designed to be independent of BMD, it aims to capture the "quality" side of the strength equation that a density number can miss. In one study it behaved as a BMD-independent marker of bone quality and showed statistically significant differences between patients who had fractured and those who had not Journal of Clinical Medicine, 2022. In a follow-up cohort, the score predicted incident fragility fractures over a five-year window and, in that group, showed higher predictive ability than the T-score alone Aging Clinical and Experimental Research, 2023.

It is important to keep this in proportion. This evidence is emerging, comes largely from observational studies, and does not displace FRAX or DEXA in UK guidance — it sits alongside them as a way to add information about bone quality. The honest summary is that the Fragility Score is a promising signal that may help flag people who look reassuring on density but are not, rather than a settled replacement for existing tools. For a fuller comparison of the two technologies, see our guide to REMS vs DEXA scans.

Screen My Bones delivers REMS scans across the UK through mobile assessment days, giving a radiation-free look at both bone density and bone quality. If you would like to check when a scan is available near you, you can book a REMS assessment.

---

This article is part of our screening technology series.