Management of 2-Year-Old with Knee Contusion and Refusal to Weight Bear
Obtain knee radiographs immediately, as this 2-year-old meets criteria for imaging due to inability to bear weight, and consider septic arthritis or osteomyelitis as critical differential diagnoses given the age and presentation. 1
Initial Imaging Approach
Knee radiographs are the mandatory first-line imaging study for this patient because she meets ACR criteria: inability to bear weight after trauma. 1 The ACR specifically recommends radiographs as "usually appropriate" when focal tenderness, effusion, or inability to bear weight is present. 1
Minimum radiographic views required:
Critical Age-Specific Considerations
This 2-year-old is at particularly high risk for complications that older children would not face:
Children under 18 months have transphyseal vessels that allow infection to spread from metaphysis to epiphysis, making osteomyelitis more likely to involve the joint. 1 While this child is slightly older, transphyseal infection may still be underestimated in this age group. 1
Children under 2 years are significantly more likely to have septic arthritis than osteomyelitis (P = 0.0003), with the knee being one of the most commonly affected joints. 1
Septic arthritis is an orthopedic emergency because bacterial proliferation can rapidly destroy cartilage. 1
Differential Diagnosis Priority
High-priority diagnoses requiring immediate action:
Septic arthritis - Must be ruled out urgently:
- The typical triad of fever, pain, and diminished mobility occurs in only 50% of cases, so absence of fever does NOT exclude infection. 1
- Diagnosis requires arthrocentesis if clinical suspicion exists. 1
- Consider elevated inflammatory markers (ESR ≥40 mm/hour, WBC ≥12,000 cells/mm³, CRP >2.0 mg/dL). 1
Osteomyelitis - Second most common osteoarticular infection:
- Occurs twice as frequently as septic arthritis overall, but less common than septic arthritis in children under 2 years. 1
- Can spread to subperiosteal space causing abscess, leading to bone ischemia and necrosis. 1
Occult fracture - Particularly important in young children:
- Radiographs may miss subtle fractures initially. 3
- Bone contusion can present with normal X-rays but significant pain and dysfunction. 4
Management Algorithm
If radiographs are NEGATIVE:
Consider MRI without IV contrast if:
- Pain persists beyond 5-7 days 2
- Significant joint effusion present 2
- Any clinical concern for infection (even without fever) 1
- Continued inability to bear weight 2
MRI is superior to radiographs for detecting:
- Bone contusions (appear as irregular patchy T1WI low signal, T2WI isointensity/slightly high signal, with STIR showing high signal) 4
- Occult fractures 1, 2
- Soft tissue injuries 4
If radiographs show fracture:
MRI without IV contrast or CT without IV contrast may be needed to evaluate for additional bone or soft-tissue injury. 1
Conservative Management (if infection and fracture excluded):
- Control edema 5
- Avoid significant stress on healing tissues 5
- Gradual return to weight bearing as tolerated 2
- Close follow-up to ensure symptom resolution 4
Critical Pitfalls to Avoid
Do not dismiss this as simple contusion without imaging - The inability to bear weight mandates radiographic evaluation regardless of mechanism. 1
Do not wait for fever to develop before considering infection - Only 50% of pediatric osteomyelitis cases present with the classic triad of fever, pain, and diminished mobility. 1
Do not assume normal radiographs exclude significant pathology - Bone contusions, early osteomyelitis, and septic arthritis can all present with normal initial radiographs. 4, 1
Do not delay arthrocentesis if septic arthritis is suspected - This is a clinical diagnosis requiring joint aspiration, and delayed treatment can result in permanent cartilage damage. 1
Consider the unique anatomy of this age group - The presence of transphyseal vessels in young children allows infections to spread more readily between bone and joint. 1