Dual-energy X-ray absorptiometry (DXA)

Central DXA for diagnosis – basics for reporting

- Osteoporosis may be diagnosed in postmenopausal women and in men age 50 an older if the T-score of the lumbar spine, total hip or femoral neck is -2.5 or less

o Reference standard from which the T-score is calculated is the female, white, age 20-29 years

o The WHO diagnostic criteria may be applied to women in menopausal transition

o Distal 33% radius (non dominant arm) may be utilized in certain circumstances

Hip and/or spine cannot be measured or interpreted Hyperparathyroidism Very Obese patients (over weight limit for DXA table)

- BMD reporting in Females prior to Menopause and in Males Younger than 50

o Z-scores, not T-scores are preferredo A Z-score of -2.0 or lower is defined as the “below the expected

range for age”, and a Z-score above -2.0 is “within the expected range for age.”

o Z-scores should be population specific (age, gender, ethnicity) where adequate reference data exist

- Spine Region of interest (ROI)o Use PA L1-L4 for spine BMD measuremento Use all evaluable vertebrae and only exclude vertebrae that are

affected by local structural change or artifact.o Must have at least two valid vertebral body measurementso Anatomically abnormal vertebrae may be excluded from analysis

if: They are clearly abnormal and non-assessable There is more than a 1.0 T-score difference between the

vertebra in question and the adjacent vertebrae

- HIP ROIo Use femoral neck, or total proximal femur which ever is lowesto BMD may be measured in either hipo Insufficient data exists to determine whether mean T-scores for

bilateral hip BMD can be used for diagnosiso The mean hip BMD can be used for monitoring, with total hip

preferred

DXA background

Role of densitometry

- 50% of women and 20% of men older than 50 will have a fragility fracture in their lifetimes

- Fragility and low trauma fracture is defined as a fracture resulting from the force of a fall from a standing height or less or a bone that breaks under conditions that would not cause a normal bone to break

o Fingers, toes, skull do not count as fragility fractures- 1.5 million fragility fractures per year in the US

o 300 K hip fractures per year 24-30% excess mortality within one year Only 15% return to unassisted ambulation after 6 mo. 50% patients permanently incapacitated 20% will require long term nursing home care

o 700K spine fractures per year 5 yr excess mortality 20% Associated with back pain, reduced pulmonary function,

depression, loss of independence- In the absence of prevalent fractures, low bone mass as measured by

bone densitometry is the best predictor of future fracture risko BMD is highly correlated with bone strength by biomedical testingo Relationship between BMD and fracture risk is exponential

- Indications for BMD Measurement – National Osteoporosis Foundationo All women 65 and older and men older than 70o Younger post menopausal women and men 50-70 with risk factorso Adults with fracture after age 50o Adults with a condition associated with a high fracture risk or high

risk medicationo Anyone being considered for therapyo Postmenopausal women stopping estrogen therapy

- Treatment is effective

o Bisphosphonates (fosamax, boniva, actonel, reclast reduce hip and vertebral fractures 40-70% over five years

o Estrogen replacement: 34% reductiono Raloxifene (selective estrogen receptor modulator): 30-55%

reduction

- Indications for treatment – National Osteoporosis Foundationo Post menopausal women and men age 50 and older should be

considered for therapy Hip or vertebral fracture T score less than -2.5 at femoral neck or spine T score less than -1 and 10 yr probablility of fracture

greater than/equal to 3% at hip or major osteoporotic fracture 20% based on FRAX

- FRAX: computer based algorithm to estimate 10 yr fracture probabilityo Combination of clinical factors and BMD predict fracture risk

better than BMD or clinical factors aloneo Risk factors incorporated

Age, sex, BMI, prior fracture, parental history of fracture, tobacco use, glucocorticoids, rheumatoid arthritis, alcohol intake

o Limitations BMD from hip only site used Does not consider dose effect (alcohol, tobacco,

glucocorticoids Risk factors not included: falling, rate of bone loss, bone

turnover, parental history of non-hip fracture Applies only to untreated patients Not intended for premenopausal patients

Evaluating Technical Adequacy of the images

Spine

- Positioningo L4-L5 disc space at the iliac crests, label from the bottom upo Ribs at T12o Largest transverse process at L3o Bone density increases from L1-L3, increase in density from L3-L4

inconsistently present

- Spine Artifactso Compression fractures, osteophytes, facet sclerosis, spine surgery,

sclerotic lesions can cause a statistically significant increase in bone density

o Criteria for excluding a vertebral body They are clearly abnormal and non-assessable There is more than a 1.0 T-score difference between the

vertebra in question and the adjacent vertebrae Must have two vertebral bodies for analysis, otherwise a

distal forearm be done

Example: Degenerative change at L2-L3 should lead to exclusion of L2

Example: Compression fracture of L2 results in a clearly abnormal vertebral body

Surgical hardware at L4

Laminectomy defect at L4 and L5

Sclerotic lesion at L4

Hip

Regions of Interest in the Hip

Check positioning of hip

- goal is to have the femur internally rotated 15-20 degrees bringing the neck parallel to the plane of the table

- X-ray beam passes through the neck at a 90 degree angle, with changes in rotation the angle at which the beam passes through the neck changes, result is an apparent shortening of the neck and increased measured mineralization

- Assessment of rotation by evaluating the size and shape of the lesser trochanter

o When properly rotated the lesser trochanter is barely visibleo Proper rotation is desired on a baseline study, on follow up studies

the same degree of rotation as on prior is desired

Proper rotation

Internal Rotation

- Excessive internal rotation results in total disappearance of the lesser trochanter

External Rotation

- Excessive external rotation results in a large a pointed lesser trochanter