What is a trabecular bone score (TBS) adjusted FRAX (Fracture Risk Assessment Tool) score?

What is a TBS-Adjusted FRAX Score?

A TBS-adjusted FRAX score is a refined 10-year fracture risk calculation that incorporates trabecular bone score (TBS)—a measure of bone microarchitecture quality—into the standard FRAX algorithm to improve fracture prediction accuracy beyond what bone mineral density (BMD) and clinical risk factors alone can provide. 1

Understanding the Components

Trabecular Bone Score (TBS)

TBS is a unitless textural index that analyzes pixel gray-level variations in lumbar spine DXA images to indirectly assess trabecular bone microarchitecture 1. The technology uses FDA-approved software installed on compatible DXA systems and provides information about bone quality that BMD cannot capture:

• High TBS values = homogeneous (normal) bone texture indicating better microarchitecture
• Low TBS values = variable (weaker) bone texture indicating degraded microarchitecture
• TBS predicts fracture risk independent of BMD 1

How TBS Adjusts FRAX

The adjustment works directionally based on bone quality 1:

• Low TBS values increase the FRAX-estimated fracture risk
• High TBS values reduce the FRAX-estimated fracture risk

This adjustment has been validated in 14 prospective international cohorts 1, demonstrating that it enhances fracture risk prediction capabilities beyond standard FRAX calculations 2.

Clinical Utility and Decision-Making

When TBS-Adjusted FRAX is Most Valuable

TBS-adjusted FRAX has greatest clinical utility in patients whose fracture risk falls close to therapeutic intervention thresholds 1. Specifically:

• Patients with osteopenia (low bone mass): TBS-adjusted FRAX can be the deciding factor for treatment decisions, as it can be included directly on the DXA printout 1
• Conditions where standard FRAX underestimates risk: Type 2 diabetes and primary hyperparathyroidism benefit particularly from TBS adjustment 1
• Treatment stratification: The adjustment can change management in a modest but significant number of patients near intervention thresholds 2

Age-Related Considerations

The impact of TBS on FRAX varies substantially by age 1:

• Younger women: TBS has much greater effect on fracture risk—bone strength plays a more dominant role
• Older women: TBS has less impact on FRAX risk, likely because falls become more common with advancing age and play a greater role in fracture risk than bone microarchitecture

Technical Aspects

Measurement Details

• Obtained from the same lumbar spine DXA scan used for BMD measurement
• No additional radiation exposure required
• Uses commercially available, FDA-approved software 1
• Can be calculated even when hip BMD is unavailable 3

Reference Values

Using premenopausal women aged 20-39 as reference 4:

• Normal microarchitecture: TBS ≥1.365
• Partially degraded: TBS 1.248-1.365
• Degraded microarchitecture: TBS <1.248

Important Clinical Caveats

When to Use TBS-Adjusted FRAX

1. Borderline treatment decisions: When standard FRAX places patients near but not clearly above intervention thresholds
2. Discordant findings: When BMD appears normal but clinical suspicion for fracture risk remains high
3. Secondary osteoporosis: Particularly in diabetes and hyperparathyroidism where bone quality may be compromised despite preserved BMD

Limitations to Recognize

• Vertebral exclusions: When vertebrae must be excluded due to structural artifacts, level-specific TBS adjustments should be applied to reduce treatment reclassification 5
• Degenerative changes: Lumbar degenerative changes have statistically significant but clinically small effects on TBS-adjusted FRAX—exclusion of degenerative vertebrae is generally unnecessary 6
• Not for treatment monitoring: Changes in TBS during antiresorptive treatment are not useful indicators of antifracture effect 2
• Body composition: Current TBS methodology may have limitations across diverse body types, though adjustments for soft tissue thickness are under development 7

Practical Application

The 2020 AACE/ACE guidelines include TBS-adjusted FRAX as a diagnostic criterion for osteoporosis: T-score between -1.0 and -2.5 with high TBS-adjusted FRAX fracture probability based on country-specific thresholds qualifies for osteoporosis diagnosis 1.

This represents a significant advancement beyond BMD-only assessment, as more than 80% of fragility fractures occur in women with BMD in the osteopenia range 1, making the additional bone quality information from TBS clinically valuable for identifying at-risk patients who would be missed by BMD criteria alone.