What is the appropriate work‑up and management for a 2‑year‑old child with three days of inability to fully extend the right knee and limp, without known injury, swelling, or tenderness?

Immediate Work-Up and Management

Start with plain radiographs of the right knee (AP and lateral views) as the initial imaging study, even though this 2-year-old lacks focal tenderness, swelling, or effusion 1. The ACR Appropriateness Criteria specifically note that children between 5 and 12 years fall outside the standard Ottawa and Pittsburgh rule age ranges, and radiographs may be beneficial despite lack of clinical symptoms 1. A 2-year-old falls well below this threshold, making imaging even more justified given the persistent inability to extend the knee and limp lasting 3 days.

Clinical Reasoning

The presentation of inability to fully extend the knee with limping in a toddler without known injury raises several diagnostic possibilities:

• Toddler's fracture (spiral tibial fracture) - most likely given age and presentation
• Occult knee or distal femur fracture
• Hip pathology (referred pain to knee is common in children)
• Septic arthritis or osteomyelitis (though less likely without fever, swelling, or systemic symptoms)

The 3-day duration and lack of improvement warrant imaging rather than observation alone.

Diagnostic Algorithm

Step 1: Obtain knee radiographs (AP and lateral)

• If the child cannot cooperate for isolated knee films, obtain full lower extremity radiographs including hip, femur, tibia, and ankle 1
• Remember that knee pain in children often originates from hip pathology 2, 3

Step 2: If radiographs are negative but symptoms persist:

• Obtain MRI of the knee without IV contrast 1. The ACR guidelines specifically state this is "usually appropriate" for skeletally immature children after negative radiographs to evaluate for occult fractures or internal derangement 1
• Consider hip radiographs if not already obtained, as hip pathology commonly presents as knee symptoms in this age group

Step 3: Consider hip ultrasound if:

• Any limitation of hip range of motion is detected
• The child has any systemic symptoms
• Radiographs remain unrevealing

Management Based on Findings

If toddler's fracture is identified:

• Immobilize with a controlled ankle motion (CAM) boot or short leg back slab 4
• These are preferred over casting due to fewer complications and ease of removal by family
• Duration: typically 3-4 weeks
• No orthopedic follow-up needed for most uncomplicated cases 4

If radiographs are negative but clinical suspicion remains high:

• Consider presumptive treatment as toddler's fracture with immobilization 4
• Arrange MRI within 1-2 weeks if no improvement
• Toddler's fractures can be radiographically occult initially

Critical Red Flags to Assess

Even without current fever or swelling, evaluate for:

• Septic arthritis risk factors (though less likely here):
  • Temperature >101.3°F (38.5°C)
  • Complete refusal to bear weight
  • ESR >40 mm/hour
  • WBC >12,000/mm³
  • CRP >20 mg/L 2

If any of these are present, urgent joint aspiration is required.

Common Pitfalls

• Don't assume knee pathology just because the child points to the knee - hip pathology frequently refers pain to the knee in children 2, 3
• Don't rely solely on Ottawa or Pittsburgh rules in toddlers - these were not validated for children under 5 years old 1
• Don't delay imaging for 3+ days of symptoms - the persistence of symptoms beyond 48-72 hours warrants radiographic evaluation
• Don't miss occult fractures - toddler's fractures are frequently subtle or invisible on initial radiographs 4

The absence of swelling and tenderness does NOT rule out significant pathology in a 2-year-old who cannot fully extend the knee after 3 days.