Metabolic Bone Disorders
Definition | Aetiology | Pathophysiology | Risk Factors | Signs and Symptoms | Investigations | Management
Definition
Metabolic bone disorders are a group of conditions affecting bone strength and structure due to imbalances in bone metabolism, commonly involving calcium, phosphate, vitamin D, or hormonal regulation.
Aetiology
- Osteoporosis: reduced bone mineral density due to ageing, menopause, corticosteroid use.
- Osteomalacia/Rickets: defective bone mineralisation due to vitamin D deficiency.
- Paget’s disease of bone: excessive bone turnover leading to structurally weak bone.
- Hyperparathyroidism: excessive PTH causing bone resorption and calcium loss.
- Renal osteodystrophy: secondary to chronic kidney disease affecting calcium-phosphate metabolism.
Pathophysiology
- Disruption in bone remodelling due to hormonal, nutritional, or metabolic imbalances.
- Osteoporosis results from increased osteoclast activity and decreased bone formation.
- Osteomalacia involves defective mineralisation of the bone matrix due to low vitamin D.
- Paget’s disease leads to disorganised bone architecture due to excessive osteoclastic and osteoblastic activity.
Risk Factors
- Advanced age.
- Postmenopausal status (oestrogen deficiency).
- Chronic kidney disease.
- Vitamin D or calcium deficiency.
- Long term corticosteroid use.
- Family history of metabolic bone disorders.
Signs and Symptoms
- Osteoporosis: fractures (hip, vertebrae, wrist), kyphosis.
- Osteomalacia: bone pain, muscle weakness, waddling gait.
- Paget’s disease: bone pain, skull enlargement, hearing loss.
- Hyperparathyroidism: bone pain, nephrolithiasis, abdominal pain.
- Renal osteodystrophy: skeletal deformities, bone pain, growth retardation.
Investigations
- Serum calcium, phosphate, PTH: assess for hyperparathyroidism or vitamin D deficiency.
- Vitamin D: low in osteomalacia and rickets.
- Bone specific alkaline phosphatase: elevated in Paget’s disease.
- DEXA scan: assesses bone mineral density for osteoporosis.
- Bone biopsy: used in cases of unclear metabolic bone disease.
- Radiographs: show characteristic changes (e.g., Looser’s zones in osteomalacia, lytic and sclerotic lesions in Paget’s disease).
Management
1. Osteoporosis:
- Calcium and vitamin D supplementation: essential for bone health.
- Bisphosphonates: first line therapy to reduce fracture risk.
- Denosumab: monoclonal antibody for severe osteoporosis.
2. Osteomalacia/Rickets:
- Vitamin D and calcium replacement: primary treatment.
- Sunlight exposure: promotes endogenous vitamin D production.
3. Paget’s Disease:
- Bisphosphonates: reduce abnormal bone turnover.
- Pain management: NSAIDs or analgesics for bone pain.
4. Hyperparathyroidism:
- Parathyroidectomy: definitive treatment for primary hyperparathyroidism.
- Calcimimetics (e.g., cinacalcet): used in secondary hyperparathyroidism.
5. Referral:
- Endocrinology: for persistent or complex cases.
- Rheumatology: if associated with systemic conditions.
- Nephrology: in renal osteodystrophy.
- Orthopaedics: for fractures or significant deformities.