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Metabolism bone disorder 5 mark
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Now I have enough information from authoritative textbooks. Here is a well-structured 5-mark answer on Metabolic Bone Disorders:
Metabolic Bone Disorders
Metabolic bone disorders are conditions in which abnormalities in bone mineral metabolism, bone quantity, or bone quality lead to structural weakness, deformity, or fracture. The main disorders are:
1. Osteoporosis
Definition: Decreased bone mass (osteopenia) severe enough to significantly increase the risk of fracture. Radiographically, it is defined as bone mineral density (BMD) 2.5 standard deviations or more below the mean peak bone mass of young adults (T-score ≤ -2.5).
Types:
- Primary: Postmenopausal (estrogen deficiency causes accelerated bone resorption) and Senile (age-related, "low-turnover" form)
- Secondary: Due to endocrine disorders (hyperparathyroidism, hyperthyroidism, diabetes), malabsorption, malnutrition, drugs (corticosteroids, anticonvulsants), multiple myeloma
Pathogenesis: Bone resorption exceeds bone formation. In postmenopausal women, estrogen loss increases osteoclast activity. In senile osteoporosis, osteoblast proliferative capacity is reduced. Age-related bone loss averages 0.7% per year in both sexes.
Clinical Features: Often asymptomatic until fracture occurs. Most common fracture sites: vertebrae (compression fractures), hip (neck of femur), wrist (Colles' fracture).
Diagnosis: DEXA scan (T-score), FRAX score for fracture risk assessment.
Treatment: Calcium + Vitamin D supplementation, bisphosphonates (alendronate), HRT in postmenopausal women, weight-bearing exercise.
2. Osteomalacia and Rickets
Definition: Impaired mineralization of bone matrix (osteoid). In children (growing skeleton) this is called Rickets; in adults it is called Osteomalacia.
Causes:
- Vitamin D deficiency (most common - inadequate sunlight, poor diet, malabsorption)
- Calcium deficiency
- Phosphate depletion
- Renal tubular disorders
- Inherited conditions (X-linked hypophosphatemia - excess FGF23 production)
Pathogenesis: Vitamin D (activated to 1,25-dihydroxyvitamin D3 in kidney) is essential for calcium and phosphate absorption from the gut. Deficiency leads to soft, poorly mineralized bone matrix.
Clinical Features:
- Rickets: Bow legs (genu varum), frontal bossing, rachitic rosary (costochondral beading), Harrison's sulcus, widened physes and cupped/flared metaphyses on X-ray, delayed tooth eruption
- Osteomalacia: Bone pain, muscle weakness, proximal myopathy, pseudofractures (Looser's zones)
Labs: Low serum calcium, low phosphate, elevated alkaline phosphatase (ALP), low 25-OH Vitamin D, elevated PTH (secondary hyperparathyroidism).
Treatment: Vitamin D + calcium supplementation; correction of underlying cause.
3. Hyperparathyroidism-Related Bone Disease (Osteitis Fibrosa Cystica)
Definition: Excess PTH (primary or secondary) causes accelerated bone resorption, especially subperiosteal erosions.
Features: Subperiosteal erosions of radial border of middle phalanges (pathognomonic), "salt and pepper" skull on X-ray, brown tumors (fibrous tissue replacing resorbed bone), generalized osteopenia.
In high-income countries, osteitis fibrosa cystica is now rare due to early detection of hypercalcemia.
4. Paget's Disease of Bone
Definition: A focal disorder of bone remodeling characterized by overactive, abnormal osteoclasts followed by disorganized osteoblastic repair, producing structurally weak, enlarged bone.
Pathogenesis: Abnormal osteoclasts (possibly triggered by viral infection + genetic factors - RANK gene mutations). The chaotic remodeling produces a mosaic pattern of lamellar bone ("jigsaw puzzle" pattern on histology).
Clinical Features: Bone pain, skull enlargement (hat size increase), bowing of long bones (tibia - "sabre tibia"), nerve compression (deafness due to temporal bone involvement), high-output cardiac failure (vascular bone), elevated ALP. Rare complication: secondary osteosarcoma (< 1% of cases, but 5-10% of severe polyostotic disease).
Treatment: Bisphosphonates, calcitonin.
5. Renal Osteodystrophy
Definition: Collective skeletal changes in chronic renal disease, including osteopenia, osteomalacia, secondary hyperparathyroidism, and growth retardation.
Pathogenesis: Chronic kidney disease causes:
- Decreased 1,25(OH)2 Vitamin D3 production → reduced calcium absorption → hypocalcemia
- Phosphate retention → hyperphosphatemia → further hypocalcemia
- Secondary hyperparathyroidism → increased RANKL → osteoclast activation → bone resorption
- Tubular acidosis → increased RANKL expression → osteopenia
Treatment: Phosphate binders, active Vitamin D analogues (calcitriol), cinacalcet (for secondary hyperparathyroidism), dialysis.
Summary Table
| Disorder | Bone Quantity | Mineralization | Key Lab | Key Feature |
|---|---|---|---|---|
| Osteoporosis | Decreased | Normal | Low BMD | Fragility fractures |
| Osteomalacia/Rickets | Normal/low | Defective | Low Ca, PO4; High ALP | Looser's zones / bow legs |
| Hyperparathyroidism | Decreased | Normal | High PTH, Ca | Subperiosteal erosions |
| Paget's Disease | Increased (disorganized) | Normal | High ALP | Mosaic bone pattern |
| Renal Osteodystrophy | Variable | Defective | Low Ca, High PO4, High PTH | Mixed pattern |
Sources: Robbins, Cotran & Kumar Pathologic Basis of Disease; Bailey and Love's Short Practice of Surgery, 28th Ed.; Rheumatology 2-Volume Set (Elsevier, 2022)
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