What causes bilateral genu valgum (knock knees) in a 10-year-old female?

Causes of Bilateral Genu Valgum in a 10-Year-Old Female

Bilateral genu valgum in a 10-year-old female is most commonly physiologic and self-resolving, but pathologic causes must be excluded, particularly metabolic bone disease (especially X-linked hypophosphatemia), obesity-related growth disturbance, and idiopathic lateral distal femoral physeal growth suppression. 1

Physiologic vs. Pathologic Differentiation

The critical first step is determining whether the deformity represents normal developmental variation or pathologic progression:

• Physiologic genu valgum peaks between ages 3-4 years and typically resolves by age 7-8 years, with the vast majority of cases requiring no intervention 1
• At age 10, persistent or progressive genu valgum should raise suspicion for pathologic causes that require evaluation and potential treatment 1

Primary Pathologic Causes

Metabolic Bone Disease

X-linked hypophosphatemia (XLH) is a critical metabolic cause to exclude:

• XLH causes severe leg bowing (both varus and valgus deformities), characterized by renal phosphate wasting, elevated FGF23 levels, hypophosphatemia, and elevated alkaline phosphatase 2, 3
• Clinical features include abnormal gait, lower limb deformity, decreased growth velocity, and disproportionate short stature 2
• Radiographs show partial fraying and irregularity of the distal femoral and proximal tibial growth plates 3
• Biochemical screening should include serum phosphate, alkaline phosphatase, vitamin D levels, parathyroid hormone, and FGF23 3
• Genetic testing for PHEX gene mutations confirms diagnosis in 70-90% of cases 2, 3

Obesity-Related Growth Disturbance

Obesity is strongly associated with idiopathic genu valgum in children over age 7:

• 71% of children with idiopathic genu valgum are obese (>95th percentile BMI), significantly higher than the general population 4
• Higher BMI predicts 9.8% of tibial valgus severity, with obesity potentially playing an etiologic role 4
• Calculate BMI and classify by CDC percentiles to assess this risk factor 4
• The mechanism involves repetitive microtrauma and compressive forces on physes sensitive to mechanical stress 5

Idiopathic Lateral Distal Femoral Physeal Growth Suppression

This rare but important cause presents specifically in overweight adolescent females:

• Progressive knock-kneed deformity develops relatively rapidly without history of trauma 5
• MRI demonstrates circular peglike disturbances of the lateral distal femoral physes 5
• This entity is particularly relevant for a 10-year-old female, as both reported cases occurred in overweight girls 5
• Etiology likely involves obesity and repetitive microtrauma superimposed on genetic factors 5

Secondary and Rare Causes

Glycogen Storage Disease Type III

• Can present with genu valgus as part of musculoskeletal manifestations 2
• Associated with hepatomegaly, hypoglycemia, hyperlipidemia, and myopathy 2
• Physical therapy assessment should include evaluation of genu valgus/recurvatum as part of musculoskeletal screening 2

Post-Traumatic or Post-Surgical

• Genu valgum can develop after removal of osteochondromas from the medial proximal tibia due to asymmetric blood flow to the proximal tibial physis 6
• This mechanism is analogous to post-traumatic tibia valgus after incomplete proximal tibia fractures 6

Diagnostic Approach

When evaluating a 10-year-old female with bilateral genu valgum, obtain:

1. Standing anteroposterior radiographs of both lower extremities to measure mechanical axis deviation, mechanical tibiofemoral angle, and assess growth plate integrity 3, 4
2. Intercondylar distance measurement to quantify deformity severity 3
3. Biochemical panel: serum phosphate, alkaline phosphatase, 25-OH vitamin D, PTH, and consider FGF23 if XLH suspected 3
4. BMI calculation and percentile classification 4
5. Skeletal maturation assessment using pelvic and peri-genu physeal closure, as this predicts 22-25% of deformity variance 4
6. MRI if rapid progression or unilateral asymmetry to evaluate for focal physeal disturbance 5

Clinical Pitfalls

• Do not assume all genu valgum at age 10 is physiologic—this age is beyond typical resolution and warrants investigation 1
• Skeletal maturation is the main predictor of overall valgus severity (explaining 22-25% of variance), not just BMI alone 4
• Untreated pathologic genu valgum leads to abnormal gait, decreased mobility, progressive joint damage, and early osteoarthritis 3, 7
• In XLH, specific treatments (phosphate supplementation and active vitamin D) can improve bowing, making early diagnosis critical 3