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What is the difference between osteoporosis and osteomalacia?

Osteoporosis

  • Brittle bone
  • Imbalance between resorption and bone building processes
  • Bone matrix is appropriately mineralized
    • Normal bone, just less of it
    • Because the balance is tipped more towards resorption (which affects both the mineral and organic components of bone equally) so amount of bone matrix relative to bone mineral is normal
  • Lab results
    • Low bone mineral density
    • Normal Ca
    • Normal PO4
  • 4 high yield example causes for osteoporosis where there is an imbalance between resorption and building
    • Woman undergoes menopause, estrogen (which causes osteoclast apoptosis, increases OPG and decreases RANKL) decreases, results in increase in bone resorption
    • Glucocorticoids inhibit OPG production and also stimulate RANKL production, results in increase in bone resorption
    • Thin individuals or small frame increase risk for osteoporosis possibly due to a lower peak bone mineral density or sub-optimal mechanical weight load on bones (weight on bones is needed for bone remodeling), balance is therefore tipped towards osteoclastic activity
    • Immobile individuals are not placing any mechanical load on their bones which is needed for healthy bone remodelling, balance is tipped towards osteoclastic activity

Osteomalacia

  • Soft bone
  • Insufficient Ca and phosphate to mineralize newly formed osteoid (typically from vitamin D deficiency)
  • Bone matrix is inappropriately mineralized
    • Abnormal bone, normal amount of bone
    • More bone matrix (i.e. osteoid), less bone mineral (i.e. Ca, PO4)
  • Lab results
    • Low bone mineral density
    • Low Ca (consequently high PTH)
    • Low PO4
  • Why is bone soft in osteomalacia?
    • Well to understand this, let’s understand what is bone
    • Bone is made up of an organic and inorganic component
      • Organic: osteoid also known as bone matrix (primarily made of type I collagen) – gives bone resistance to stretching
      • Inorganic: hydroxyapatite crystals (primarily Ca and PO4) – gives bone resistance to compression
    • In osteomalacia, we have bone that is deficient in the inorganic component which we say gives the bone its hardness and resistance to compression, therefore the bone becomes soft!