Osteoporosis Flashcards
What are the 7 important functions of bones?
- Support
- Protection
- Movement
- Mineral and growth factor storage
- Blood cell formation
- Triglyceride (fat) storage
- Hormone production
What are the 2 layers of bone?
- Cortical/compact bone on the outside
- Spongy/cancellous bone in the middle with trabeculae
What are the basic cell types that form different types of bone tissue?
- Osteogenic stem cells: progenitor cells,
differentiate into osteoblasts - Osteoblasts: secrete osteoid
- Osteocytes: (imbedded osteoblasts) sense stress and strain, communicate with osteoblasts & clasts
- Bone lining cells: periosteal and endosteal cells, maintain matrix
- Osteoclasts: resorb bone
What are the organic components of bone?
- Cells & osteoid
- Sacrificial bonds in/between collagen molecules (provide resilience)
What is the osteoid made up of?
- Ground substance (proteoglycans & glycoproteins)
- Collagen fibres
- Provides tensile strength & flexibility (1/3 of matrix)
How do sacrificial bonds provide resilience?
- Stretch & break easily on impact to dissipate energy and prevent fracture
- If no addition trauma, bonds re-form
What are the inorganic components of bone?
Hydroxyapatities
- Mineral salts
- 65% bone mass
- Mainly of tiny calcium phosphate crystals in/around collagen fibres
- Responsible for hardness & resistance to compression
What hormones regulate bone growth?
- Growth hormone: Stimulating epiphyseal plate activity
- Thyroid hormone: Modulates activity of growth hormone, ensures proper proportions
- Testosterone & estrogens: Promote adolescent growth spurts, end growth by inducing epiphyseal plate closure
How is bone homeostasis maintained?
- Recycle 5-7% bone mass each week
- Older bone becomes more brittle (calcium salts crystallise)
- Bone modelling & repair
What does bone remodelling involve?
- Bone deposit & bone resorption
- Occurs at surfaces of periosteum & endosteum
- Remodelling units: Adjacent osteoblasts & clasts
What does bone resorption involve?
Osteoclasts
- Secret lysosomal enzymes that digest matrix & protons
- Acitdity converts calcium salts to soluble forms
- Phagocytize demineralized matrix & dead osteocytes
- Transcytosis allow release into interstitial fluid and then into blood
What happens to osteoclasts once resorption is complete?
Undergo apoptosis
What proteins are involved with osteoclast activation?
PTH & T cell-secreted proteins
What are the functions of calcium?
- Nerve impulse transmission
- Muscle contraction
- Blood coagulation
- Secretion by glands and nerve cells
- Cell division
How is calcium stored?
- 1200-1400g, 99% stored as bone minerals
- Amount in blood tightly regulated
- Intestinal absorption requires Vitamin D metabolites
- Dietary intake required
What hormones control blood calcium levels?
- Parathyroid hormone (PTH): Removes calcium from bone regardless of bone integrity
- Calcitonin: In high doses lowers blood calcium levels temporarily
What is involved in the negative feedback mechanism when blood calcium decreases?
- PTH secreted by PTH glands
- Stimulates clasts, inhibits blasts
- Blood calcium increases
- Calcitonin secreted by T gland
- Stimulates blasts, inhibits clasts
- Blood calcium decreases
What is osteoporosis?
- Reduction of bone mineral density
- Increase in fracture risk due to bone loss
- Loss of microarchitecture of bone
What bone density levels indicate osteopenia & osteoporosis?
- Normal: BMD 0-1 SD below young healthy adult mean
- Osteopaenia: < 1-2.5 SD below young adult mean
- Osteoporosis: < 2.5 SD below young adult mean
- Measured by T score
What does loss of microarchitecture of bone include?
- Total bone mass,
- Trabecular volume
- Trabecular connectivity/separation
- Trabecular plate thickness & number
What are the primary risk factors for osteoporosis?
- Postmenopausal women: 30% 60-70yrs, 70% by 80yrs
- Men when older
- Decreased sex hormone production
What are the secondary risk factors for osteoporosis?
- Petite body form
- Insufficient exercise to stress bones
- Diet poor in calcium and protein
- Low Vit D/C
- Smoking
- Hormone-related conditions: Hyperthyroidism, low thyroid-stimulating hormone
- Diabetes mellitus
- Immobility
- Males with prostate cancer taking androgen-
suppressing drugs - Genetics
- Other pathologies
What pathologies can be risk factors for osteoporosis?
- Rheumatoid arthritis,
- Coeliac disease and
- Chronic lung diseases
- Chronic renal failure
- Chronic liver failure
- *Most involve being on corticosteroid medication (increases loss of bone)
How is osteoporosis diagnosed?
- DXA or bone densitometry: Detects bone loss before fracture
- Previously based on XR, but unable to detect until >30% lost
- Calcanea US (very inaccurate)