When is imaging warranted in pediatric knee injuries?

Imaging for Pediatric Knee Injuries

Radiographs should be obtained in pediatric knee injuries when there is focal tenderness, effusion, or inability to bear weight. 1

Clinical Decision Rules for Knee Imaging

The American College of Radiology (ACR) provides clear guidelines for when imaging is warranted in pediatric knee injuries:

Initial Imaging Criteria

• Radiographs are indicated as first-line imaging when any of these are present:
  • Focal tenderness
  • Joint effusion
  • Inability to bear weight 1, 2
  • Inability to flex the knee to 90 degrees 3
  • Gross deformity
  • Palpable mass
  • Penetrating injury
  • Presence of prosthetic hardware
  • Unreliable clinical examination due to multiple injuries
  • Altered mental status
  • Neuropathy 1

Radiographic Protocol

When radiographs are indicated, a minimum of two views should be obtained:

• Anteroposterior (AP) view
• Lateral view (with knee at 25-30 degrees flexion)

Additional views that may be helpful in specific situations include:

• Patellofemoral view (for suspected patellar fractures or subluxation)
• Internal and external oblique views 1

Advanced Imaging After Initial Radiographs

If radiographs do not show a fracture but clinical suspicion remains high:

• MRI without IV contrast is the preferred next imaging study for skeletally immature children with suspected occult fractures or internal derangement 1, 2

• CT may be considered for better characterization of complex fractures already identified on radiographs, but is not routinely used as initial imaging 1

Efficiency of Clinical Decision Rules

Applying clinical decision rules can significantly reduce unnecessary imaging:

• Using the criteria of inability to bear weight and inability to flex to 90 degrees could reduce radiographs by up to 73% without missing fractures 3
• The ability to take four weight-bearing steps in the emergency department alone could potentially reduce radiographs by 53% 1

Special Considerations for Pediatric Patients

Children have unique injury patterns due to:

• Open growth plates (physes)
• Apophyses that are relative weak points
• Different biomechanical forces compared to adults 4

Common pediatric-specific knee injuries that may require imaging include:

• Physeal fractures
• Osteochondral injuries
• Patellar and patellar tendon injuries 4

Pitfalls to Avoid

• Don't rely solely on point tenderness - Research has shown this is not a good predictor of knee fracture in children 3
• Don't overlook the importance of weight-bearing ability - This is one of the most reliable indicators for fracture risk
• Don't forget to consider developmental variations - The appearance of the knee changes during skeletal maturation, which can lead to misinterpretation of normal findings as pathological 5

When to Consider Advanced Imaging Without Initial Radiographs

In cases of significant trauma (e.g., motor vehicle accidents, knee dislocations), both radiographs and CTA of the lower extremity with IV contrast may be appropriate as initial imaging to evaluate for vascular injuries 1, 2

Remember that clinical judgment should always supersede clinical guidelines when appropriate 1. The physician's assessment of the specific situation remains paramount in determining the need for imaging.