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Bone Mineral Density

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Bone Mineral Density, Bone Densitometry, T-Score, Z-Score

  • See Also
  1. Osteoporosis
  1. Patient is indecisive on starting Estrogen Replacement
  2. Direct management of asymptomatic Hyperparathyroidism
  3. Incidental radiographic evidence of bone loss
  4. Signs of Osteoporosis in female patients
    1. Height loss
    2. Osteoporotic Fracture
  5. Patients at high risk for Osteoporosis
    1. See Osteoporosis Evaluation for risk factors
    2. Longterm Corticosteroids (Prednisone 7.5 mg/day)
      1. AACE: Corticosteroid use for over 1 month
      2. Medicare: Corticosteroid use for over 3 months
  6. Monitor benefits of current Osteoporosis Management
    1. Bisphosphonates
    2. Estrogen Replacement Therapy
  • Imaging Modalities
  1. Dual Energy XRAY Absorptiometry (DEXA)
  2. Quantitative Computed Tomography (QCT)
  3. Calcaneal Ultrasonography
  • Score Types
  1. General
    1. Scores are not interchangeable across imaging types
    2. Scores are based on standard deviations below mean
      1. Example: T-Score of -2 is two S.D. below the mean
  2. T-Score (WHO standard BMD measure)
    1. Mean based on average premenopausal woman
    2. Fracture risk increases 2-3x per T unit
      1. T-Score is best indicator of Fracture risk
      2. Example: T-Score of -2 confers Fracture risk 4-6x
  3. Z-Score
    1. Mean based on women of similar Osteoporosis risk
    2. Accounts for age, weight, and ethnicity
    3. Fracture risk increases 2x per Z unit
      1. Measures Fracture risk for remaining lifespan
      2. Example: Z-Score of -2 confers Fracture risk 4x
  • Interpretation
  • T-Score or Z-Score
  1. Normal: <1 Standard Deviation (SD) below the mean (T Score >=-1.0)
  2. Osteopenia: 1.0 to 2.5 SD below the mean (T Score -1.0 to -2.5)
  3. Osteoporosis: >2.5 SD below the mean (T Score <-2.5)
  • Prognosis
  1. Risk of Fracture also related to scoring above
  2. Bone Mineral Density more than 2 SD below mean
    1. Risk of Fracture increases exponentially
  3. Each 10% decrease in spinal Bone Mineral Density
    1. Associated with 2 times risk of Vertebral Fracture
  4. Each 15% decrease in hip Bone Mineral Density
    1. Associated with 2.5 times risk of Hip Fracture
  5. References
    1. Sturtridge (1996) Can Med Assoc J 155:924 [PubMed]
  • References
  1. Blake (1998) Endocrinol Metab Clin North Am 27:267-88 [PubMed]
  2. Bracker (1998) Postgrad Med 104(4):77-86 [PubMed]
  3. Brunader (2002) Am Fam Physician 65(7):1357-64 [PubMed]
  4. Genant (1998) Radiographics 18:913-8 [PubMed]