DXA is the clinical gold standard for diagnosing osteopenia and osteoporosis. It does this by measuring bone mineral density (BMD) at the spine and hip and reporting results as T-scores. While T-scores are essential for diagnosis, they do not fully explain fracture risk. Many people with similar BMD values have very different fracture outcomes. A growing set of DXA-derived “expanded indices” now capture bone texture, geometry, and estimated strength, improving fracture risk estimates beyond traditional to T-scores.

These expanded indices do not replace diagnostic T-scores, which remain the basis for defining osteopenia and osteoporosis, but they may help refine fracture risk stratification, particularly in osteopenia or borderline cases and inform therapy decisions in patients with discordant BMD and clinical risk factors. Unfortunately, ‘vendor-specific’ implementations, and the need for standardization and reference data, currently limit the application of these indices along with incomplete integration into guidelines and reimbursement frameworks. However, that doesn’t decrease their importance!

These expanded indices include:

1. Trabecular Bone Score (TBS)
2. Hip Structural Analysis (HAS) & Hip Axis Length (HAL)
3. Bone Strength – Finite Element (FE) Modeling
4. Bone Strain Index (BSI)
5. DXA-based 3D Reconstructions and 3D FE Models

1. Trabecular Bone Score (TBS): Bone Quality, Not Just Density

TBS is calculated from routine lumbar spine DXA images and reflects the quality of trabecular bone microarchitecture.

• Low TBS indicates degraded bone structure
• TBS predicts spine and hip fractures independently of BMD
• When added to tools like FRAX (Fracture Risk Assessment Tool), TBS improves fracture risk prediction
• TBS can reclassify some patients with osteopenia into a higher-risk category
• TBS-adjusted FRAX is already integrated into many DXA systems and supported by expert guidelines

2. Hip Geometry: How Bone Shape Affects Strength

DXA images also contain information about bone geometry that can be quantified through Hip Structural Analysis (HSA).

HSA estimates:

• Cross-sectional area (CSA)
• Section Modulus and Cross-Sectional Moment of Inertia (CSMI)
• Cortical thickness estimates and buckling ratio

These measurements help explain why two people with the same hip BMD may have very different fracture risks and can help distinguish different types of hip fractures.

A related measure, Hip Axis Length (HAL), reflects the length of the femoral neck. A longer HAL is associated with higher hip fracture risk, even when BMD is the same.

3. Bone Strength Estimates: Moving Toward Mechanics

DXA images can also be used to estimate bone strength using computer models.

• Finite element (FE) analysis estimates how much force a bone can withstand during a fall
• These estimates improve hip fracture prediction beyond BMD alone, especially in older or high-risk populations

4. Bone Strain Index (BSI)is another DXA-derived index that represents the average strain distribution within the lumbar spine or proximal femur

• Higher BSI indicates higher mechanical strain for a given load
• BSI predicts vertebral fractures independently of BMD
• BSI may be useful in certain secondary bone diseases

5. DXA-based 3D reconstructions and 3D FE Models

 These 3D hip images show promising accuracy for predicting incidents of hip fractures and my soon become more widely available. Statistical shape modeling of DXA hip shape is being explored as an independent predictor of hip fracture beyond BMD and standard geometry.  

What This Means Clinically

• T-scores remain essential for diagnosing osteopenia and osteoporosis
• Expanded DXA indices do not replace diagnosis
• They help refine fracture risk, especially when:

• BMD is borderline
• Clinical risk factors and BMD do not agree
• Treatment decisions are uncertain

• Current Limitations

• Some indices are vendor-specific
• Standardization and reference data are still evolving
• Integration into guidelines and reimbursement is incomplete

Bottom Line - DXA provides more than just BMD. When used thoughtfully, expanded DXA-derived indices can improve fracture risk assessment and support better clinical decision-making—particularly in patients where BMD alone does not tell the full story.