High Yield Overview
DEXA and Bone Densitometry
Gold Standard for Bone Density Assessment
-2.5Osteoporosis T-score
-1 to -2.5Osteopenia T-score
L1-L4Spine Sites
1-2%Annual BMD Loss (Untreated)
WHO BMD Classification
Normal
PatternT-score greater than -1.0
TreatmentNo pharmacological treatment typically
Osteopenia (Low Bone Mass)
PatternT-score -1.0 to -2.5
TreatmentLifestyle measures, consider treatment if high risk
Osteoporosis
PatternT-score -2.5 or below
TreatmentPharmacological treatment indicated
Severe Osteoporosis
PatternT-score -2.5 or below + fragility fracture
TreatmentAggressive treatment, consider anabolic agents
Critical Must-Knows
- T-score: Standard deviations from young adult mean. Used for postmenopausal women and men 50 or older.
- Z-score: Standard deviations from age-matched mean. Used for premenopausal women and younger men.
- WHO Criteria: Normal T-score greater than -1, Osteopenia -1 to -2.5, Osteoporosis -2.5 or below.
- Sites measured: Lumbar spine (L1-L4), femoral neck, total hip. Use lowest T-score for diagnosis.
- FRAX: Fracture Risk Assessment Tool incorporates BMD plus clinical risk factors.
Examiner's Pearls
- "Spine BMD may be falsely elevated by osteophytes, compression fractures, or aortic calcification.
- "Femoral neck better predicts hip fracture risk than total hip.
- "FRAX uses femoral neck BMD, not spine or total hip.
- "Significant change requires more than LSC (Least Significant Change) - typically 3-5% at spine, 5-6% at hip.
- "Vertebral Fracture Assessment (VFA) can be done simultaneously to detect occult vertebral fractures.
Clinical Imaging
Imaging Gallery
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T-score vs Z-score Application
T-scores are used for postmenopausal women and men aged 50 and older. Z-scores are used for premenopausal women, men under 50, and children. A Z-score of -2.0 or below indicates BMD "below expected range for age" and warrants investigation for secondary causes of bone loss.
DEXA Principles
How DEXA Works
- Dual-Energy X-ray Beam: Two different energy levels pass through the body
- Differential Absorption: Bone and soft tissue absorb the energies differently
- Calculation: Software separates bone from soft tissue and calculates areal BMD
- Result: BMD expressed as g/cm² (areal density, not true volumetric density)
Technical Considerations
Advantages
- Gold standard for BMD measurement
- Low radiation dose (1-10 μSv)
- Quick scan time (10-20 minutes)
- Reproducible and precise
- Can monitor treatment response
Limitations
- Areal density (g/cm²) not volumetric (g/cm³)
- Affected by body size (larger bones = higher BMD)
- Artifacts from degenerative changes
- Cannot assess bone quality/microarchitecture
- 2D projection of 3D structure
Alternative Imaging Modalities
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Measurement Sites
Standard Sites
DEXA Measurement Sites
| Site | Regions | Advantages | Limitations |
|---|---|---|---|
| Lumbar Spine | L1-L4 (AP view) | Highly responsive to treatment, trabecular bone | Falsely elevated by degenerative changes |
| Femoral Neck | Narrow region of femoral neck | Best predictor of hip fracture, used for FRAX | Small region, positioning critical |
| Total Hip | Neck + trochanter + intertrochanteric | Larger region, less positioning error | Less responsive to treatment |
| Forearm (1/3 Radius) | Distal third radius | Useful when hip/spine invalid | Not standard for diagnosis |
Which T-score to Use
Diagnostic T-score Selection
For diagnosis: Use the LOWEST T-score from:
- Lumbar spine (L1-L4 mean or lowest individual vertebra)
- Femoral neck
- Total hip
- 1/3 radius (if other sites invalid)
For FRAX: Use FEMORAL NECK T-score only
For monitoring: Use the same site consistently
Factors Affecting Accuracy
Falsely ELEVATED spine BMD:
- Osteophytes (degenerative changes)
- Vertebral compression fractures
- Aortic calcification
- Sclerotic lesions (Paget, mets)
- Surgical hardware
Solution: Exclude affected vertebrae, use remaining L1-L4. If fewer than 2 vertebrae valid, use hip only.
T-scores and Z-scores
T-score
T-score Definition
T-score = (Patient BMD - Young Adult Mean BMD) / SD of Young Adult
- Compares patient to healthy young adult (peak bone mass)
- Each 1 SD decrease roughly doubles fracture risk
- Used for postmenopausal women and men 50 or older
- Basis for WHO diagnostic criteria
Z-score
Z-score Definition
Z-score = (Patient BMD - Age-Matched Mean BMD) / SD of Age-Matched Population
- Compares patient to others of same age and sex
- Used for premenopausal women, men under 50, children
- Z-score -2.0 or below = "below expected range for age"
- Prompts investigation for secondary causes
WHO Diagnostic Criteria
WHO BMD Classification
| Category | T-score | Fracture Risk | Management |
|---|---|---|---|
| Normal | Above -1.0 | Low | Reassurance, lifestyle advice |
| Osteopenia | -1.0 to -2.5 | Increased | FRAX assessment, lifestyle, consider treatment if high risk |
| Osteoporosis | -2.5 or below | High | Pharmacological treatment indicated |
| Severe Osteoporosis | -2.5 or below + fracture | Very High | Aggressive treatment, anabolic agents |
Clinical Example: DXA Lumbar Spine Report
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FRAX Assessment
Fracture Risk Assessment Tool
FRAX calculates 10-year probability of:
- Major osteoporotic fracture (spine, hip, humerus, wrist)
- Hip fracture alone
FRAX Inputs
Mnemonic
FRAX HELPSFRAX Risk Factors
F
Fracture
Prior fragility fracture
R
Rheumatoid
Rheumatoid arthritis
A
Alcohol
3 or more units per day
X
X-ray (BMD)
Femoral neck BMD (optional)
H
Hereditary
Parent with hip fracture
E
Estrogen/Low BMI
Early menopause, BMI
L
Longevity
Age
P
Prednisone
Glucocorticoids (current or past)
S
Smoking
Current smoking
Memory Hook:FRAX HELPS predict fracture risk
Treatment Thresholds
FRAX Treatment Thresholds (Example - Guidelines Vary)
Consider treatment if:
- 10-year hip fracture probability 3% or greater
- 10-year major osteoporotic fracture probability 20% or greater
- T-score -2.5 or below (osteoporosis)
- Fragility fracture regardless of BMD
Note: Thresholds vary by country and guideline. Australian guidelines may use different cut-offs.
Treatment Monitoring
When to Repeat DEXA
DEXA Monitoring Intervals
| Situation | Recommended Interval | Rationale |
|---|---|---|
| Stable on treatment | 2-3 years | Assess maintenance of BMD |
| New treatment initiation | 1-2 years | Confirm treatment response |
| Glucocorticoid initiation | 12 months | Rapid bone loss with steroids |
| Postmenopausal, no treatment | 2-5 years based on T-score | Monitor for progression |
Least Significant Change (LSC)
Meaningful Change
LSC = Minimum change needed to be confident change is real (not measurement error)
Typical LSC values:
- Lumbar spine: 3-5%
- Total hip: 4-5%
- Femoral neck: 5-6%
A change must exceed the LSC to be considered significant
Example: If spine BMD changes from 0.850 to 0.870 g/cm² (2.4% increase), this may be within measurement error if LSC is 3%.
Expected Treatment Responses
BMD Changes with Treatment
| Treatment | Spine BMD Change (3 years) | Hip BMD Change |
|---|---|---|
| Alendronate | +5-8% | +3-4% |
| Zoledronic acid | +6-7% | +5-6% |
| Denosumab | +8-10% | +5-6% |
| Teriparatide | +10-15% | +3-4% |
| Romosozumab | +13-15% | +5-7% |
Vertebral Fracture Assessment
VFA (Vertebral Fracture Assessment)
DEXA machines can perform low-dose lateral spine imaging:
- Identifies vertebral compression fractures
- Many vertebral fractures are asymptomatic (only 30% come to clinical attention)
- Presence of vertebral fracture increases future fracture risk regardless of BMD
Indications for VFA
When to Order VFA
- T-score -1.0 or below (osteopenia or osteoporosis)
- Age 70 or older (women) or 80 or older (men)
- Height loss of 4 cm or more (historical) or 2 cm or more (prospective)
- Previous vertebral fracture
- Glucocorticoid treatment
Genant Classification of Vertebral Fractures
Vertebral Fracture Grading
| Grade | Height Loss | Description |
|---|---|---|
| Grade 0 | Less than 20% | Normal |
| Grade 1 (Mild) | 20-25% | Mild compression |
| Grade 2 (Moderate) | 25-40% | Moderate compression |
| Grade 3 (Severe) | Greater than 40% | Severe compression |
Special Populations
Premenopausal Women and Young Men
- Use Z-score, not T-score
- Z-score -2.0 or below = "below expected range for age"
- Investigate for secondary causes:
- Glucocorticoids
- Hypogonadism
- Hyperthyroidism
- Malabsorption
- Eating disorders
Children and Adolescents
- DEXA can be used but interpretation differs
- Use Z-scores adjusted for age, sex, and bone age
- Spine and whole body (excluding head) preferred sites
- Diagnosis is "low bone mineral content/density for chronological age"
Glucocorticoid-Induced Osteoporosis
- Fracture risk higher at any given T-score
- Treatment thresholds lower (some guidelines: T-score -1.0 or below)
- Monitor more frequently (baseline, 12 months)
- FRAX may underestimate risk
Exam Viva Scenarios
Practice these scenarios to excel in your viva examination
VIVA SCENARIOStandard
DEXA Interpretation
EXAMINER
"A 65-year-old postmenopausal woman has a DEXA showing spine T-score -1.8 and femoral neck T-score -2.6. How do you interpret this and what is your management?"
EXCEPTIONAL ANSWER
This patient has osteoporosis based on the WHO criteria. The femoral neck T-score of -2.6 is below -2.5, meeting the diagnostic threshold for osteoporosis. We use the lowest T-score for diagnosis - in this case the femoral neck. The spine T-score of -1.8 falls in the osteopenia range. The discrepancy between sites is common; the spine may be falsely elevated due to degenerative changes, which are common at age 65. For management, I would first assess for secondary causes with basic investigations including calcium, vitamin D, PTH, renal and liver function, thyroid function, and consider myeloma screen. I would calculate her FRAX score using the femoral neck BMD to quantify her 10-year fracture risk. Given she has osteoporosis, pharmacological treatment is indicated - first-line would typically be an oral bisphosphonate such as alendronate. I would also recommend calcium and vitamin D supplementation, weight-bearing exercise, and fall prevention strategies.
KEY POINTS TO SCORE
Diagnosis based on lowest T-score (femoral neck -2.6 = osteoporosis)
Spine may be falsely elevated by degenerative changes
FRAX uses femoral neck BMD
Pharmacological treatment indicated for osteoporosis
COMMON TRAPS
✗Using spine T-score when it's likely falsely elevated
✗Not calculating FRAX
✗Forgetting to investigate secondary causes
LIKELY FOLLOW-UPS
"What secondary causes of osteoporosis would you investigate?"
"When would you repeat the DEXA?"
"What is the difference between T-score and Z-score?"
VIVA SCENARIOStandard
Monitoring Treatment
EXAMINER
"A patient on alendronate for 2 years has a follow-up DEXA showing spine BMD increased from 0.780 to 0.810 g/cm². Is this a significant improvement?"
EXCEPTIONAL ANSWER
To determine if this is a significant change, I need to calculate the percentage change and compare it to the Least Significant Change (LSC) for spine DEXA. The change is (0.810-0.780)/0.780 = 3.8% increase. The typical LSC for lumbar spine is 3-5% depending on the center's precision assessment. If this center's LSC is 3%, then a 3.8% change exceeds it and represents a statistically significant improvement. If the LSC is 5%, the change is within measurement error and we cannot be confident it's a true improvement. However, even if the change doesn't reach significance, stability or minimal improvement on bisphosphonates is actually a good response - we're preventing the 1-2% annual loss expected without treatment. I would continue alendronate, ensure adherence and adequate calcium/vitamin D, and repeat DEXA in another 2-3 years. If BMD continues to decline despite treatment, I would reassess adherence, consider alternative agents, and investigate secondary causes.
KEY POINTS TO SCORE
Calculate percentage change (3.8%)
Compare to LSC (typically 3-5% for spine)
Stability on treatment is actually a good response
Continue treatment and monitor
COMMON TRAPS
✗Expecting large BMD gains on bisphosphonates
✗Not knowing about LSC concept
✗Stopping treatment for small changes
LIKELY FOLLOW-UPS
"What BMD changes would you expect with different treatments?"
"How long should bisphosphonate treatment continue?"
"What would prompt you to change treatment?"
VIVA SCENARIOAdvanced
T-score vs Z-score
EXAMINER
"A 35-year-old woman with rheumatoid arthritis on prednisolone has a spine T-score of -2.0. What is your interpretation?"
EXCEPTIONAL ANSWER
For this 35-year-old premenopausal woman, I should actually focus on the Z-score rather than the T-score for initial interpretation. T-scores compare to young adult peak bone mass and are designed for postmenopausal women and men over 50. The Z-score compares her to other women her age and is more appropriate here. If her Z-score is -2.0 or below, this indicates her BMD is 'below expected range for age' and warrants investigation. However, in her case, we already know the likely cause - glucocorticoid-induced osteoporosis from prednisolone for her RA. Glucocorticoids cause rapid bone loss, particularly in the first 6-12 months. Despite being premenopausal, guidelines recommend treatment for glucocorticoid-induced osteoporosis at lower thresholds than primary osteoporosis. She should be on calcium and vitamin D, and bisphosphonate therapy is typically indicated for patients on moderate-to-high dose glucocorticoids expected to continue for 3 months or more, regardless of DEXA results. Her FRAX risk may also be underestimated as FRAX doesn't fully account for glucocorticoid dose.
KEY POINTS TO SCORE
Z-score appropriate for premenopausal women
Glucocorticoid-induced osteoporosis has lower treatment thresholds
Treatment often indicated regardless of BMD in steroid users
FRAX underestimates risk with glucocorticoids
COMMON TRAPS
✗Using T-score alone for premenopausal women
✗Not recognizing glucocorticoid-induced osteoporosis guidelines
✗Waiting for T-score -2.5 before treating
LIKELY FOLLOW-UPS
"What is the pathophysiology of glucocorticoid-induced osteoporosis?"
"What treatment would you choose?"
"How often would you monitor her DEXA?"
DEXA Exam Day Cheat Sheet
High-Yield Exam Summary
T-score Thresholds
- •Normal: Greater than -1.0
- •Osteopenia: -1.0 to -2.5
- •Osteoporosis: -2.5 or below
- •Severe: -2.5 or below + fragility fracture
Score Selection
- •T-score: Postmenopausal women, men 50+
- •Z-score: Premenopausal women, men under 50
- •Use LOWEST T-score for diagnosis
- •Use FEMORAL NECK for FRAX
Common Artifacts
- •Spine elevated by: osteophytes, fractures, aortic calcification
- •Solution: Exclude affected vertebrae
- •If fewer than 2 vertebrae valid, use hip only
- •LSC: spine 3-5%, hip 5-6%
Monitoring
- •Repeat DEXA: 2-3 years on treatment
- •Change must exceed LSC to be significant
- •Stability is success on bisphosphonates
- •VFA: detects occult vertebral fractures