Osteomalacia and Rickets

Overview

Definition

Rickets and osteomalacia are disorders of decreased bone mineralization. Rickets affects the cartilage of the epiphyseal growth plates in children, while osteomalacia affects the sites of bone turnover in children and adults.

Epidemiology

Rickets:

• In the United States, rickets mostly affects infants who have:
  • Chronic malabsorption syndromes
  • Renal disease
  • Been exclusively breast-fed
    • Vitamin D deficiency
    • Affecting dark-skinned infants
• Worldwide prevalence depends on:
  • Nutritional deprivation
  • Lack of sun exposure

Osteomalacia:

• Mostly attributed to vitamin D deficiency
• Prevalence is growing worldwide.
• Commonly affects homebound elderly adults
• Can co-exist with rickets in children

Etiology

Rickets and osteomalacia can be caused by vitamin D, calcium, or phosphorus deficiencies. These conditions can cause be caused by abnormalities of the bone matrix deposition.

Vitamin D-related (most common):

• Vitamin D deficiency: 
  • Dietary deprivation 
  • Lack of sunlight
• Vitamin D malabsorption: 
  • Postgastrectomy
  • Gastric bypass
  • Celiac disease
  • Pancreatic insufficiency
  • Cholestyramine treatment
  • Laxative abuse
• Impaired 1-hydroxylation of 25-hydroxyvitamin D:
  • Vitamin D-dependent rickets type 1
  • X-linked hypophosphatemia 
  • Oncogenic osteomalacia
• Impaired target organ response to D3: vitamin D-dependent rickets type II

Hypophosphatemia related:

• X-linked hypophosphatemia 
• Fibrous dysplasia
• Antacid-induced osteomalacia
• Chronic metabolic acidosis
• Tenofovir
• Cadmium
• Paraproteinemia

Inhibitors of mineralization:

• Etidronate
• Fluoride
• Aluminum 
• Iron

Miscellaneous:

• Hypophosphatasia
• Axial osteomalacia
• Fibrogenesis imperfecta ossium

Pathophysiology

Rickets

• Growth plate thickness is determined by:
  • Chondrocyte proliferation and hypertrophy
  • Vascular invasion of the growth plate
• Vascular invasion requires mineralization of the growth plate cartilage.
• In the absence of mineralization, the growth plate cartilage accumulates and becomes disorganized, and the growth plate thickens.
• Accumulation of unmineralized osteoid in the metaphysis (right below the growth plate) eventually leads to bowing and deformation of bones.

Osteomalacia

• At any one time, 7% of bone surface is being actively remodeled.
• Osteoclasts resorb the old bone while osteoblasts lay down new osteoid (organic matrix).
• Osteomalacia can occur if:
  • New osteoid is qualitatively or quantitatively abnormal
  • Insufficient calcium and phosphate are in the extracellular fluid
  • Acidosis is at the calcification site (renal disease)
  • Inadequate activity of alkaline phosphatase is present
  • Calcification is influenced by drugs (e.g., aluminum, fluoride)

Clinical Presentation

Rickets

• Delay in the time of closure of fontanelles and soft skull bones (craniotabes)
• Parietal and frontal bossing
• Genu varum (toddlers) and Genu valgum (older children)
• Deformity of the spine (kyphoscoliosis) 
• Sulcus at the lower margin of the chest known as Harrison sulcus
• Bending primarily at the long bones and widening of the wrist
• Enlargement of costochondral junctions with “rib beading” (rachitic rosary)
• Problems with the dental architecture
• Greenstick fractures (linear fractures with intact periosteum)
• Short stature

Osteomalacia

• Fatigue
• Diffuse bone pain
• Muscle weakness
• Pathological fractures
• Difficulty walking/waddling gait
• Can also present with signs of hypocalcemia if associated with calcium deficiency

Diagnosis and Management

Diagnosis

History:

• Fractures
• Bone pain
• Family history
• Poor nutrition
• Homebound lifestyle
• Lack of sun exposure

Physical exam: skeletal deformities typical of rickets

Laboratory tests:

• Laboratory values vary depending on the underlying deficiencies.
• Calcipenic rickets:
  • ↑ Parathyroid hormone (PTH) 
  • ↓ Calcium
  • ↓ Phosphate
• Phosphopenic rickets:
  • PTH normal or mildly elevated
  • ↓ Phosphate
• ↓ Vitamin D (in vitamin D deficiency)
• ↑ Alkaline phosphatase levels (except in hypophosphatasia)

X-ray imaging:

• Rickets:
  • Epiphyseal widening
  • Metaphyseal cupping/fraying
  • Genu varum or valgum (bowing of long bones)
  • Rachitic rosary 
  • Craniotabes
• Osteomalacia: 
  • Osteopenia
  • Pseudofractures or “Looser zones,” a band of bone material of decreased density

Management

• Vitamin D supplementation
• Calcium-rich diet; calcium and phosphate supplementation 
• Adequate exposure to sunlight 
• Most deformities of rickets will correct with treatment.
• If severe deformities have occurred, orthopedic correction is sometimes necessary.

Differential Diagnosis

• Hyperparathyroidism: pathologically elevated PTH levels are summarized under the term hyperparathyroidism. Depending on the pathogenesis, distinctions can be made between 3 forms: primary, secondary, and tertiary hyperparathyroidism. Long-standing hyperparathyroidism can result in osteopenia and pathologic bone fractures. Treatment approach depends on the underlying cause and involves parathyroidectomy for primary hyperparathyroidism.
• Chronic renal failure: defined as an irreversible decrease of not only glomerular and tubular function, but also endocrine renal function. This condition presents as pathologically disturbed excretory and incretionary renal function. Chronic renal failure can cause hypocalcemia through decreased formation of active vitamin D, which in turn results in secondary hyperparathyroidism and leaking of calcium from the bones. Treatment is mostly supportive. 
• Blount’s disease: a pediatric orthopedic problem in which the leg is bowed outward or in a varus position. This disease is due to a dysfunctional growth of the posterior medial aspect of proximal physis of the tibia resulting in a lower limb deformity. Diagnosis is established based on clinical picture and imaging, and treatment involves bracing and sometimes surgical correction.
• Achondroplasia: a genetic disorder that results in dwarfism. In those with the condition, the arms and legs are short, while the torso is typically of normal length. Those affected have an average adult height of 131 cm (4 ft 4 in) for males and 123 cm (4 ft) for females. Diagnosis is established clinically and confirmed by genetic testing. Treatment is largely supportive.
• Osteoporosis: a loss of bone mass usually associated with aging and hormonal deficits. Most common in post-menopausal women. Presents with fragility fractures. Diagnosis is based on clinical presentation and measurements of bone mineral density. Treatment involves vitamin D and calcium supplementation and bisphosphonates.
• Osteogenesis imperfecta: a genetic disorder associated with extreme bone fragility. The disease varies in severity, but usually presents in early childhood with multiple fractures and bone deformities. Osteogenesis imperfecta is diagnosed clinically and with genetic testing. Management includes bisphosphonates and is mostly focused on fracture prevention.