Metabolic Bone Disorders

Definition | Types | Aetiology | Pathophysiology | Risk Factors | Signs and Symptoms | Investigations | Management | Example Management | References

Definition

Metabolic bone disorders are a group of conditions that result in abnormalities of bone strength, structure, or metabolism due to defects in bone formation or resorption, mineral metabolism, or bone matrix quality. These disorders can lead to fractures, deformities, and other skeletal issues.

Types

Common metabolic bone disorders include:

• Osteoporosis
• Osteomalacia
• Rickets
• Paget’s disease of bone
• Hyperparathyroidism-related bone disease

Aetiology

The causes of metabolic bone disorders vary depending on the specific condition:

• Osteoporosis: Ageing, hormonal changes, genetic factors, inadequate intake of calcium and vitamin D, sedentary lifestyle.
• Osteomalacia and Rickets: Vitamin D deficiency, disorders of vitamin D metabolism, chronic kidney disease, certain medications.
• Paget’s Disease of Bone: Genetic factors, possible viral infection.
• Hyperparathyroidism-related Bone Disease: Primary hyperparathyroidism (parathyroid adenoma), secondary hyperparathyroidism (chronic kidney disease), tertiary hyperparathyroidism.

Pathophysiology

The pathophysiology of metabolic bone disorders involves disruptions in the normal processes of bone remodelling, which include bone resorption by osteoclasts and bone formation by osteoblasts:

• Osteoporosis: Increased bone resorption relative to bone formation, leading to reduced bone mass and structural deterioration.
• Osteomalacia and Rickets: Defective mineralisation of the bone matrix due to insufficient levels of calcium, phosphate, or vitamin D.
• Paget’s Disease of Bone: Abnormal bone remodelling with excessive bone resorption followed by disorganised bone formation, resulting in structurally abnormal bone.
• Hyperparathyroidism-related Bone Disease: Excessive production of parathyroid hormone (PTH) increases bone resorption, leading to bone loss and elevated calcium levels in the blood.

Risk Factors

• Advanced age
• Family history of metabolic bone disorders
• Female gender, particularly postmenopausal women
• Low body weight or body mass index (BMI)
• Vitamin D deficiency
• Calcium deficiency
• Physical inactivity
• Smoking
• Excessive alcohol consumption
• Use of certain medications (e.g., glucocorticoids)
• Chronic kidney disease

Signs and Symptoms

Symptoms of metabolic bone disorders can vary depending on the specific condition:

• Osteoporosis: Often asymptomatic until a fracture occurs; may include back pain, loss of height, stooped posture.
• Osteomalacia: Bone pain and tenderness, muscle weakness, fractures, difficulty walking.
• Rickets: In children, delayed growth, bone pain, skeletal deformities such as bowed legs or thickened wrists and ankles.
• Paget’s Disease of Bone: Bone pain, skeletal deformities, fractures, arthritis in affected joints.
• Hyperparathyroidism-related Bone Disease: Bone pain, fractures, kidney stones, fatigue, depression, muscle weakness.

Investigations

• Bone densitometry (DEXA scan): To measure bone mineral density (BMD) and diagnose osteoporosis.
• Serum calcium, phosphate, and alkaline phosphatase levels: To assess bone metabolism and detect abnormalities.
• Vitamin D levels: To check for deficiency.
• Parathyroid hormone (PTH) levels: To diagnose hyperparathyroidism.
• X-rays: To identify fractures, bone deformities, and signs of metabolic bone disease.
• Bone biopsy: In rare cases, to confirm the diagnosis of certain bone disorders.

Management

Primary Care Management

• Lifestyle modifications: Encouraging a diet rich in calcium and vitamin D, regular weight-bearing and muscle-strengthening exercises, smoking cessation, and limiting alcohol intake.
• Calcium and vitamin D supplementation: To maintain adequate levels and support bone health.
• Medications for osteoporosis:
  • Bisphosphonates (e.g., alendronate, risedronate) to reduce bone resorption.
  • Selective oestrogen receptor modulators (SERMs) (e.g., raloxifene) for postmenopausal women.
  • Denosumab: A monoclonal antibody that inhibits osteoclast formation and function.
• Monitoring: Regular follow-up to assess treatment adherence, side effects, and bone mineral density.
• Education: Providing information on the importance of lifestyle changes, medication adherence, and fall prevention strategies.

Specialist Management

• Referral to endocrinologist or rheumatologist: For complex cases or if the patient has secondary causes of metabolic bone disease.
• Treatment for specific disorders:
  • Osteomalacia and Rickets: High-dose vitamin D and calcium supplementation.
  • Paget’s Disease of Bone: Bisphosphonates or calcitonin to control bone turnover.
  • Hyperparathyroidism: Parathyroidectomy for primary hyperparathyroidism, medical management for secondary hyperparathyroidism.
• Management of complications: Addressing fractures, deformities, and other complications of metabolic bone disorders.
• Genetic counselling: For patients with hereditary bone disorders.

Example Management for Metabolic Bone Disorders

A postmenopausal woman diagnosed with osteoporosis presenting with a low bone mineral density (BMD) on a DEXA scan should be managed with lifestyle modifications such as a diet rich in calcium and vitamin D, regular weight-bearing exercises, and smoking cessation. Pharmacological treatment may include bisphosphonates (e.g., alendronate) to reduce bone resorption. Regular monitoring of bone mineral density and serum calcium and vitamin D levels is essential to assess the effectiveness of treatment and detect potential side effects. The patient should be educated on the importance of medication adherence, fall prevention strategies, and maintaining a healthy lifestyle.

References

1. NICE. (2024). Osteoporosis: Assessing the Risk of Fragility Fracture. Retrieved from NICE
2. NHS. (2023). Osteoporosis. Retrieved from NHS
3. National Osteoporosis Foundation. (2022). Clinician’s Guide to Prevention and Treatment of Osteoporosis. Retrieved from National Osteoporosis Foundation
4. Royal Osteoporosis Society. (2021). Managing Osteoporosis and Preventing Fragility Fractures. Retrieved from Royal Osteoporosis Society
5. National Institute of Arthritis and Musculoskeletal and Skin Diseases (NIAMS). (2022). Osteomalacia. Retrieved from NIAMS
6. Paget’s Association. (2023). Paget’s Disease of Bone. Retrieved from Paget’s Association
7. British Thyroid Foundation. (2022). Parathyroid Disease. Retrieved from British Thyroid Foundation
8. Endocrine Society. (2021). Clinical Practice Guidelines for the Management of Primary Hyperparathyroidism. Retrieved from Endocrine Society

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