TOACS FCPS Station · Cover-Up Test for Blount Disease

📷 Clinical Imaging – Cover-Up Test (Blount Disease)

Radiograph showing the cover-up test (Langenskiöld) used to differentiate Blount disease from physiologic genu varum – the distal femur overlaps the tibia in Blount disease

Figure 1 · Cover-Up Test · Blount Disease vs Physiologic Genu Varum

🔍 Key features of the Cover-Up Test:

✓ Lateral knee radiograph – with knee extended
✓ Assesses femoral-tibial relationship
✓ Positive cover-up test – distal femur overlaps the tibia
✓ Indicates tibial torsion or Blount disease
✓ Helps differentiate Blount from physiologic bowing

📋 Clinical scenario (examiner prompt)

A 2‑year‑old child is brought to the clinic because of bowlegs (genu varum) that have been progressive over the past 6 months. The child is obese (BMI >95th percentile) and began walking early. On examination, there is lateral thrust on walking and a varus deformity of both knees. A lateral radiograph of the knee shows a "cover-up" sign – the distal femoral condyle overlaps the proximal tibia (see image). The metaphyseal-diaphyseal angle (MDA) is 16°.

Progressive genu varum Obesity, early walking Positive cover-up test Metaphyseal-diaphyseal angle 16°

🧑‍⚕️ Examiner tasks · TOACS

1. Identify the diagnosis from the clinical image and context.

2. Describe the clinical features (progressive genu varum, obesity, lateral thrust, metaphyseal-diaphyseal angle >11°).

3. Explain the cover-up test (lateral radiograph, femoral overlap of tibia, indicates Blount disease).

4. Discuss management (bracing vs surgery – guided growth or osteotomy).

⚠️ Key concept: The cover-up test (Langenskiöld) is performed on a lateral knee radiograph. In Blount disease, the distal femoral condyle overlaps the tibial metaphysis (positive cover-up sign) due to tibial torsion and medial plateau depression. This helps differentiate Blount disease from physiologic genu varum, which has a negative cover-up test and resolves by age 2-3 years. Blount disease is a disorder of the proximal tibial physis causing progressive varus, internal rotation, and lateral thrust. Management: Knee-ankle-foot orthosis (KAFO) for mild cases in children <3 years; guided growth (hemiepiphysiodesis) or high tibial osteotomy for severe or older children.

🎯 Expected answers (for examiners)

• Diagnosis: Infantile Blount disease (tibia vara)
• Clinical features: Progressive genu varum in a child >2 years; obesity; early walking; lateral thrust on gait; internal tibial torsion; metaphyseal-diaphyseal angle (MDA) >11° (normal <9°)
• Cover-up test (Langenskiöld): Lateral knee radiograph with knee extended. Positive if the distal femoral condyle overlaps the tibial metaphysis (due to tibial torsion). Negative in physiologic genu varum (no overlap).
• Classification (Langenskiöld): Stages I-VI based on radiographic severity (metaphyseal changes, physeal irregularity, beaking, fragmentation).
• Management: Stage I/II in children <3 years → bracing (KAFO) with medial upright. Stage III+ or >3 years → surgery (guided growth – tension band plate, or high tibial osteotomy).

📌 Cover-up test – key points:
• Purpose: Differentiate Blount disease from physiologic bowing
• Technique: Lateral knee radiograph, knee extended
• Positive: Distal femur overlaps the tibia → Blount disease
• Negative: No overlap → physiologic genu varum
• Associated: Tibial torsion, varus, lateral thrust
• Treatment: Bracing (KAFO) vs guided growth/osteotomy

⚡ Quick FCPS‑style MCQ

A 2.5-year-old obese child with progressive genu varum has a positive cover-up test on lateral knee radiograph. The most likely diagnosis is:

A. Physiologic genu varum B. Blount disease (tibia vara) C. Rickets D. Genu valgum

Feature	Blount Disease	Physiologic Genu Varum
Age of onset	Usually >2 years	Infancy (0-2 years)
Progression	Progressive	Resolves with growth
Obesity	Common	Not typical
Lateral thrust	Present	Absent
Cover-up test	Positive (femoral overlap)	Negative
MDA (metaphyseal-diaphyseal angle)	>11°	<9°
Treatment	Bracing or surgery	Observation

🦵 TOACS FCPS Station · Cover-Up Test to Differentiate Blount Disease

📷 Clinical Imaging – Cover-Up Test (Blount Disease)

🧑‍⚕️ Examiner tasks · TOACS

📌 Topic summary · Cover-Up Test for Blount Disease