What investigations are required for a child presenting with mild genu varum (knocked knee deformity)?

Mild Knocked Knee Deformity in Children: Investigation Approach

For a child presenting with mild knocked knee (genu valgum) deformity, clinical measurement of intercondylar distance and standing anteroposterior radiographs of both lower extremities are the essential initial investigations, with additional biochemical testing reserved for cases with atypical features or severe deformity.

Initial Clinical Assessment

Clinical measurement is the first-line investigation:

• Measure the intercondylar distance (distance between the knees when ankles are together) to quantify the severity of the valgus deformity 1
• Document the child's age, height, and growth velocity, as physiological genu valgum varies significantly with age 1
• Assess for asymmetry between limbs, which suggests pathological rather than physiological deformity 1

Important caveat: Intercondylar distance varies with age and cannot replace comprehensive orthopedic assessment, especially when limb torsion or complex deformities are present 1

Radiographic Evaluation

Standing radiographs are indicated when:

• The deformity appears moderate to severe clinically 1
• There is asymmetry between limbs 1
• The child is outside the typical age range for physiological genu valgum (typically resolves by age 7-8 years) 2, 3
• Clinical measurements alone cannot fully assess joint alignment 1

Recommended radiographic views:

• Standing anteroposterior radiographs of both lower extremities (full-length, including hips to ankles) to assess the mechanical axis and femoro-tibial angle 3
• Measure the femoro-tibial angle to objectively quantify the deformity 3
• Assess the mechanical axis of the lower extremity relative to the knee center 3

Biochemical Testing: When to Consider

Laboratory investigations are NOT routinely required for mild knocked knee deformity but should be obtained if:

• Short stature or poor growth velocity is present, suggesting rickets or metabolic bone disease 1
• Severe or progressive deformity that is atypical for physiological genu valgum 1
• Family history of metabolic bone disease 1
• Other clinical features suggesting systemic disease (e.g., bone pain, dental problems, muscle weakness) 1

If metabolic bone disease is suspected, obtain:

• Serum phosphate (low in X-linked hypophosphatemia) 1
• Serum alkaline phosphatase (elevated in active rickets) 1
• Serum calcium and PTH to evaluate for secondary hyperparathyroidism 1
• 25-hydroxyvitamin D to exclude nutritional rickets 1
• Serum creatinine to assess renal function 1

Differential Diagnosis Considerations

Physiological genu valgum:

• Most common cause in children aged 3-7 years 2, 3
• Typically bilateral and symmetric 3
• Resolves spontaneously without treatment 2
• Long-term prognosis is generally excellent, though some adults may report mild symptoms on exertion 2

Pathological causes requiring investigation:

• X-linked hypophosphatemia (XLH): Consider if short stature, dental problems, or family history present 1
• Nutritional rickets: Evaluate dietary history and vitamin D status 1
• Lateral distal femoral physeal growth disturbance: Rare but can cause progressive genu valgum, especially in overweight adolescent girls 4
• Renal tubular disorders: Consider if acidosis, glucosuria, or proteinuria present 1

When to Refer for Specialist Evaluation

Orthopedic referral is indicated for:

• Substantial limb deformities that cannot be fully assessed clinically 1
• Progressive deformity despite appropriate age 1, 4
• Asymmetric deformity between limbs 1
• Consideration of surgical intervention (corrective osteotomy or guided growth) 4

Metabolic bone disease specialist referral if:

• Biochemical abnormalities suggest rickets or other metabolic bone disease 1
• Genetic testing for hypophosphatemic rickets is needed 1

Critical Pitfalls to Avoid

• Do not obtain radiographs routinely for mild, symmetric genu valgum in children aged 3-7 years without other concerning features 2, 3
• Do not assume all knocked knee is physiological in children with short stature, poor growth, or systemic symptoms 1
• Do not delay biochemical testing when clinical features suggest metabolic bone disease, as early treatment improves outcomes 1
• Do not overlook asymmetry, which is a red flag for pathological causes 1, 4