What is the best initial management for a patient with a swollen and painful knee after a mechanical ground-level fall, with normal vital signs, limited range of motion, and a knee effusion on x-ray?

Management of Post-Traumatic Knee Effusion

The correct answer is A: knee joint aspiration for fluid analysis should be performed in this patient to rule out septic arthritis (an orthopedic emergency), exclude crystal arthropathy precipitated by trauma, and identify lipohemarthrosis indicating an occult intra-articular fracture despite negative radiographs. 1

Rationale for Arthrocentesis

Diagnostic arthrocentesis is the appropriate next step after plain radiographs show effusion without obvious fracture in a post-traumatic knee presentation. 1, 2 The American College of Radiology recommends this approach with high strength of evidence for several critical reasons:

• Septic arthritis must be urgently excluded, as it represents an orthopedic emergency requiring immediate intervention, even in trauma settings where infection may not be the primary consideration. 1, 2

• Crystal arthropathy (gout or pseudogout) can be precipitated by minor trauma and must be ruled out, as management differs significantly from purely mechanical injury. 1

• Lipohemarthrosis detection indicates occult intra-articular fracture that may not be visible on initial radiographs, which would change management and potentially require CT imaging for fracture characterization. 1, 2

Synovial Fluid Analysis Priorities

When performing arthrocentesis in this clinical scenario, send fluid for:

• Cell count with differential to assess for infection (high strength of evidence). 1
• Gram stain and culture if infection is suspected based on clinical presentation. 1
• Crystal analysis to exclude gout or pseudogout. 1
• Gross appearance assessment for lipohemarthrosis (fat globules indicating occult fracture). 1

Why Other Options Are Incorrect

• CT angiography (Option B) has no role in routine knee trauma evaluation with normal vital signs and no evidence of vascular compromise. 1

• CT scan (Option C) is not routinely used as initial imaging for acute knee trauma and would only be considered if lipohemarthrosis is identified on arthrocentesis. 1

• Doppler ultrasound (Option D) is not routinely used for initial evaluation of acute knee trauma. 1

Clinical Algorithm

The American College of Radiology recommends this step-wise approach with high strength of evidence:

1. Plain radiographs first (already completed, showing effusion without fracture). 1, 2
2. Diagnostic arthrocentesis for fluid analysis (the current decision point). 1, 2
3. If lipohemarthrosis present: Consider CT for occult fracture characterization. 1
4. If inflammatory or infectious findings: Urgent orthopedic consultation. 1
5. If negative workup with persistent symptoms: MRI at 5-7 days for internal derangement evaluation. 2

Important Caveats

• While one research study showed aspiration provided only temporary symptomatic improvement in traumatic effusions 3, the primary indication here is diagnostic, not therapeutic—to exclude life-threatening infection and identify occult fractures that would alter management. 1, 2

• Do not skip arthrocentesis even when infection seems unlikely in a mechanical fall, as septic arthritis can present without fever or systemic symptoms. 2

• The inability to bear weight in this patient is a red flag that increases the likelihood of significant intra-articular pathology requiring definitive diagnosis. 1, 4