Calcium Metabolism Disorders Flashcards
(27 cards)
Role of Calcium
Critical for mineralization of the skeleton
- Creates bone strength and structure
Bone is the metabolic reservoir to maintain the intra and extracellular calcium
- The remainder of calcium is in blood, extracellular fluids, muscles and other tissues
- Calcium is responsible for mediating muscle contraction, vascular contraction and vasodilatation, nerve impulse transmission, and intra and extracellular signaling
- Hormones are responsible to keep the serum Ca levels WNL to maintain homeostasis
Normal Bone Metabolism
Bone is constantly remodeling
- Balance of formation and resorption
Osteoblasts: bone formation
- Produce and secrete the unmineralized matrix of Type I collagen and proteins that then becomes mineralized
Osteoclasts: Bone resorption
- Break down mineralized bone
Phases of Bone Remodeling
Inactive
Activation
Resorption
(Reversal)
Formation
Mineralization
Inactive
Prior to initiating remodeling
Event triggers activation (loading, micro trauma, diet)
Activation
Osteoclast precursors attach to bone surface
Resorption
Osteoclasts degrade bone and free growth factors
Reversal
Osteoclasts create pits
Macrophages come in to clean up and prevent erosion
Formation
Collagen mineralized matrix (from osteoblasts) fills the cavity
Mineralization
Osteoblasts secrete matrix vesicles to increase calcium and phosphorus
Takes 3-6 months
Cortical vs Trabecular Bone Remodeling
40% of trabecular and 10% of cortical bone is remodeled annually
Bone Metabolism
Regulated by PTH, calcitonin, calcitriol (hormonal Vit D)
With low serum Ca, osteoclasts break down bone due to↑PTH
- But then the thyroid secretes calcitonin to activate osteoblasts
- PTH↑s Vit D synthesis which then↓s PTH
With high serum Ca, calcitonin blocks osteoclasts to↓ serum Ca
- Vit D helps by blocking release of Ca from kidneys
Influencers of Bone Metabolism
Skin: Impacts Vit D synthesis
Liver/Kidneys: D3 is converted into pre- hormonal form of Vit D in the liver and kidneys
Small intestine: absorbs calcium
Thyroid gland: Hormones critical for skeletal growth & bone mass - Meds to treat hypothyroidism may↑ rate of bone loss
Parathyroid gland:
Hypo: ↓s remodeling and↑bone mass too much
Hyper:↑s osteoclastic activity
Pituitary gland: secretes growth hormone
Osteoporosis
Systemic skeletal disease marked by decreased bone mineral mass and compromises in bone architecture
Clinical definition is bone density 2.5 standard deviations below the healthy adult mean as measured by DXA scan
Primary Osteoporosis Causes
Aging: After 30 years of age, osteoblastic activity decreases (net bone loss of 0.5% of total mass/year)
Menopause: Declining estrogen levels result in increased osteoclastic activity (net bone loss of 1–3% per year)
Men lose ~ 30% trabecular and ~20% cortical bone in their lifetime
Women lose ~50% trabecular and 1/3 cortical bone in their lifetime
Secondary Osteoporosis
Occurs at any age and has identifiable causes
Osteogenesis imperfecta
Hypogonadism: ↓s testosterone
Hyperthyroidism/Hypoparathyroidism
Hyperadrenalism: ↑ed cortisol
Diet-related: eating disorders
Medication-related
Hyperglycemia: alters bone metabolism
Autoimmune diseases
Neurologic diseases/diagnoses
Osteoporosis Risk Factors
Non-Modifiable:
Family history
Asian or Caucasian race
Female
Post-menopausal
Modifiable:
Nutrition
Physical Activity
Smoking
Low Body Mass
Caffeine/Alcohol Consumption
Race and Bone
Black children have greater bone mass, density, and more favorable bone architecture than white children
Osteoporosis: Less in non-Hispanic blacks compared to non-Hispanic whites, Hispanics and Asians
Muscle and Sarcopenia
Sarcopenia is a universal aging phenomenon
Operationally defined as skeletal muscle mass greater than two standard deviations below the healthy adult
Frailty
Frailty significantly associated with:
- Osteoporosis
- Hip fracture
- Recent falls
Frailty and pre-frailty are significant predictors of fractures among community- dwelling older people
Clinical Diagnosis
DXA:
T score: compared to healthy young person
-1-2.5 SD = low bone mass
>-2.5 SD = osteoporosis
Z score: sex and age matched
Fractures—particularly of hip, distal radius, vertebrae •
Kyphosis of thoracic spine
Low BMI or low lean tissue mass
Advanced age
Presence of secondary causes of low bone mass
Screening/Diagnosis
Osteoporosis Self-Assessment Tool:
- High risk: score less than –3
- Moderate risk: score 1 to –3
- Low risk: score greater than 1
SCORE Screen:
Simple Calculated Osteoporosis Risk Estimation: Score of ≥ 6 requires a DXA
FRAX: Risk Assessment
Osteoporosis Treatment
Exercise!
Drugs focus on different phases of remodeling: Decreased resorption and increased formation
Fractures
Significant reduction in fracture callus size, BMD, and mechanical strength at 3 weeks post-fracture indicating early failure of the repair process.
Quality of callus suggests a delayed healing process
Temporal changes in cellular differentiation include prolonged phase of endochondral calcification, higher phase of bone turnover.
Intervention with a first dose of bisphosphonates prior to leaving the hospital is important
Osteomalacia
Decreased absorption of calcium and phosphorus in intestine due to disturbances in vitamin D metabolism
Increased PTH secretion
Decrease in bone formation
S&S:
Soft bones - Decreased mineralization
Fractures
Non-specific bone pain: dull, achy
Bone deformities
Possible muscle weakness over time
