Is an X-ray (X-radiation) warranted as the initial test for a patient presenting with a pop in the back of the knee?

X-ray for Patient with Pop in Back of Knee

X-ray is warranted as the initial test for a patient presenting with a pop in the back of the knee, especially if there is focal tenderness, effusion, or inability to bear weight. 1

Initial Assessment Algorithm

When to Order Knee X-rays:

• Order radiographs if any Ottawa knee rule criteria are present:

  • Age >55 years 2
  • Focal tenderness at patella or fibular head 1
  • Inability to bear weight for 4 steps immediately after injury 1, 3
  • Inability to flex knee to 90 degrees 1, 3

• Additionally, radiographs should be obtained regardless of Ottawa criteria if the patient has:

  • Gross deformity 1
  • Palpable mass 1
  • Penetrating injury 1
  • Prosthetic hardware 1
  • Altered mental status 1
  • Neuropathy 1
  • History suggesting increased risk of fracture 1

Recommended X-ray Views:

• Minimum of two radiographs should be obtained:
  • Anteroposterior view 1
  • Lateral view (with knee at 25-30 degrees flexion) 1
• Additional views to consider based on clinical suspicion:
  • Patellofemoral view for suspected patellar fractures or subluxation/dislocation 1
  • Cross-table lateral view to visualize lipohemarthrosis (indicating intra-articular fracture) 1
  • Internal and external oblique views for better visualization of tibial plateau 1

Clinical Reasoning

A "pop" in the back of the knee is concerning for potential ligamentous injury, meniscal tear, or fracture. X-ray is the appropriate initial imaging modality to rule out fracture before considering advanced imaging 1, 4. The American College of Radiology strongly recommends radiographs as the first-line imaging study for acute knee trauma 1.

Studies have shown that following the Ottawa knee rules can reduce unnecessary radiographs by 35-41% without missing fractures 3, 5. The Ottawa knee rules have demonstrated 100% sensitivity for detecting knee fractures 5.

Follow-up Imaging Considerations

If radiographs are negative but clinical suspicion remains high for internal derangement:

• MRI without contrast is the appropriate next imaging study for suspected meniscal or ligamentous injuries 1
• CT may be considered for better characterization of suspected occult fractures 1

Common Pitfalls to Avoid

• Failing to obtain appropriate radiographic views: Standard AP and lateral views may miss certain fractures, such as vertical patellar fractures (best seen on sunrise views) or tibial plateau fractures (may require tunnel views) 6, 7
• Not considering the mechanism of injury: A "pop" sound is often associated with ACL tears or patellar dislocations, which may have associated avulsion fractures 7
• Relying solely on radiographs when clinical suspicion for soft tissue injury is high: Negative radiographs do not rule out significant internal derangement 1

Remember that physician judgment should supersede clinical guidelines when appropriate 1. The primary goal is to identify fractures that require immediate intervention while avoiding unnecessary radiation exposure.