What are the causes of genu varum (bowleggedness)?

Causes of Genu Varum (Bowleggedness)

Genu varum (bowleggedness) is primarily caused by physiologic, pathologic, and metabolic conditions, with the most common pathologic cause being Blount's disease (infantile tibia vara), while metabolic causes like X-linked hypophosphatemia and vitamin D disorders should be considered in persistent cases.

Physiologic Causes

• Physiologic bowing is the most common cause of genu varum in children under 2 years of age and typically resolves spontaneously with growth 1
• Despite being called "physiologic," recent evidence suggests that even seemingly normal bowing may be associated with underlying vitamin D disorders in toddlers 2
• Physiologic bowing typically appears as children begin walking and is often associated with internal tibial rotation relative to the femur 3

Pathologic Causes

• Idiopathic tibia vara (Blount's disease) is the most common pathologic cause of genu varum, predominantly found in obese children with early walking onset 4
• Focal fibrocartilaginous dysplasia is a rare disorder that can cause genu varum but usually requires no treatment 1
• Traumatic causes include physeal injuries that affect normal bone growth 4
• Infections affecting the growth plates can lead to asymmetric growth and subsequent bowing 4
• Genetic predisposition can contribute to the development of genu varum 4

Metabolic and Systemic Causes

• X-linked hypophosphatemia (XLH) is a significant cause of genu varum, characterized by renal phosphate wasting and elevated FGF23 levels 5
• Vitamin D disorders, including vitamin D-resistant rickets and nutritional rickets, can cause genu varum 1, 2
• Renal osteodystrophy can lead to bowed legs and short stature 1
• Osteogenesis imperfecta can result in genu varum as part of its skeletal manifestations 1
• Achondroplasia commonly presents with genu varum 1, 6
• Metaphyseal chondrodysplasia can cause progressive bowing of the lower extremities 6

Clinical Features and Assessment

• Genu varum may present with varying degrees of severity, from mild cosmetic concerns to significant functional impairment 7
• Assessment should include measurement of intercondylar distance to quantify the severity of the deformity 5
• Radiographic evaluation with standing films of the entire lower extremities is necessary to determine the exact location of the deformity (proximal tibia vs. distal femur) 1
• In children with XLH, radiographs typically show severe leg bowing, partial fraying, and irregularity of the distal femoral and proximal tibial growth plates 5
• Foot deformities can affect the mechanical axis deviation and should be considered during evaluation of genu varum 6

Diagnostic Considerations

• Biochemical evaluation should be considered in persistent cases to rule out metabolic causes:
  • Serum phosphate, alkaline phosphatase, vitamin D levels, and parathyroid hormone should be assessed 5, 2
  • In XLH, hypophosphatemia, elevated alkaline phosphatase, and elevated FGF23 levels are characteristic findings 5
• Genetic testing may be necessary in suspected hereditary conditions like XLH, where PHEX gene mutations are found in approximately 70-90% of cases 5
• Distinguishing between physiologic and pathologic genu varum is crucial for appropriate management 4

Associated Complications

• Untreated genu varum can lead to:
  • Abnormal gait and decreased mobility 5
  • Progressive joint damage and early osteoarthritis 7
  • Decreased growth velocity and disproportionate short stature in cases like XLH 5
  • Potential for pseudofractures in adults with metabolic causes 5

Management Considerations

• Treatment approach depends on the underlying cause, severity of deformity, age of the patient, and functional impairment 7
• For metabolic causes like XLH, specific treatments addressing the underlying disorder (phosphate supplementation and active vitamin D) can improve the bowing 5
• Surgical intervention may be necessary for severe or persistent deformity, with the goal of restoring the mechanical axis of the limb 1
• Regular monitoring of growth and development is essential in children with genu varum 5

Pitfalls and Caveats

• Assuming all cases of genu varum in toddlers are physiologic without appropriate biochemical evaluation can lead to missed diagnoses of vitamin D disorders or other metabolic conditions 2
• Focusing solely on the knee without evaluating the entire lower extremity can miss deformities at other levels that contribute to the overall alignment 1
• Failure to consider foot deformities when evaluating genu varum can lead to incomplete assessment of the mechanical axis 6
• Nutritional rickets and XLH may coexist, and diagnosis of XLH should be considered if phosphate levels do not improve with vitamin D supplementation 5