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)
What are the sites of BMD measurement?
- Femoral neck (best site for predicting hip fractures)
- Vertebra (best site for predicting spinal fractures)
- Radius
How does risk increase once a person with osteoporosis has a fracture?
- Women with # spine >4x more likely to have another fracture within a year
- > 2 osteoporotic fractures = 9x more likely to have another fracture
- > 3 osteoporotic fractures = 11x more likely to have another fracture
How can fractures affect mortality & QOL?
- Osteoporotic hip fractures often result in THR
- 20-25% will be permanently disabled
- 50% never regain former independence,
and will require long-term nursing care - 1 in 3 return to activity and independence
- 12-15% mortality within 6 months
- 25% mortality at 12 months
What are the traditional treatments for osteoporosis?
- Calcium
- Vitamin D supplements
- Weight-bearing exercise
- Hormone replacement therapy
What is the function of hormone replacement therapy?
- Slows bone loss but does not reverse it
- Controversial due to increased risk of heart attack, stroke, and breast cancer
- Some take estrogenic compounds in soy as substitute
What drugs are used for osteoporosis?
- Bisphosphonates: Decrease # risk by 50% (Decrease clast activity/number)
- Selective estrogen receptor modulators:
Mimic estrogen without targeting breast & uterus - PTH
- Denosumab: Inhibits clasts, injections every 6 months
How can osteoporosis be prevented?
- Maximise peak bone mass (nutrition, milk, limit alcohol, exercise no smoking)
- Limit gradient of decline
- Prevent falls & fractures
What are the functions of vitamin D?
- Regulation of plasma calcium and bone
mineralisation - Maintains function of NMJs
- Effects on other systems (immune, oncogenesis)
What are the consequences of vitamin D deficiency?
- Increased risk of fracture
- Independent risk factor for falls in residential facilities
- More common with increasing age, esp. when limited sun exposure
What are the main sources of vitamin D?
- Sunlight acting on the skin
- Also found in some foods (fatty fish, eggs, some low-fat milk)
How much sun do you need each day in ACT/NSW?
- June-July: 2-3 hours per week
- Summer: Few minutes on most days
What is Wolff’s law for exercise?
Bone strength increases and decreases as the functional forces on the bone increase and decrease
What exercise is recommended to prevent osteoporosis?
- Weight bearing exercise reduces bone loss,
maintains muscle strength & proprioception. - Walking with stairs, jogging, weights
- Consider falls, balance, power
What are the general principles of exercise for osteoporosis?
- Long-term (forever)
- Ongoing but changing
- Site specific
- Produce tension or force (include partial & NWB)
- Progressive overload
- Reversibility
- Initial values (lower start = more change)
- Diminished returns
- Intermittent & dynamic
- High magnitude, high strain rate
- Low reps
What are examples of weight bearing aerobic exercise?
- Brisk walking, stair climbing (not leisurely walking)
- Graded exercise
- Avoid flexion, twisting & asymmetrical loading
