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 best initial management for this patient is knee joint aspiration for fluid analysis (Option A). 1

Rationale for Arthrocentesis in This Clinical Scenario

Diagnostic arthrocentesis is the critical next step after plain radiographs show effusion without obvious fracture. 1 This patient meets multiple criteria that mandate aspiration:

Primary Indications for Aspiration

• Septic arthritis must be urgently excluded, as it represents an orthopedic emergency requiring immediate intervention, and even minor trauma can precipitate joint infection in susceptible patients. 1, 2

• Crystal arthropathy (gout or pseudogout) can be precipitated by minor trauma and must be ruled out through synovial fluid crystal analysis. 1

• Lipohemarthrosis detection is essential, as the presence of fat globules in the aspirate indicates an occult intra-articular fracture despite negative radiographs—this finding would necessitate further imaging with CT. 1, 2

Synovial Fluid Analysis Protocol

When performing arthrocentesis in this patient, the following studies should be ordered:

• Cell count with differential to assess for infection (septic arthritis typically shows >50,000 WBC/mm³ with >75% neutrophils). 1

• Gram stain and culture to identify bacterial pathogens if infection is suspected. 1

• Crystal analysis under polarized microscopy to exclude gout (monosodium urate crystals) or pseudogout (calcium pyrophosphate crystals). 1

• Gross appearance assessment for lipohemarthrosis—a fat-fluid level or creamy supernatant after centrifugation indicates occult fracture. 1

Why Other Options Are Incorrect

CT Angiogram (Option B)

• CT angiography has no role in routine knee trauma evaluation when vital signs are normal and there are no signs of vascular compromise (absent pulses, pallor, paresthesias, paralysis). 1

CT Scan (Option C)

• CT is not routinely used as initial imaging for acute knee trauma and should only be considered after arthrocentesis if lipohemarthrosis is identified, suggesting an occult fracture requiring characterization. 1

Doppler Ultrasound (Option D)

• Ultrasound is not routinely used for initial evaluation of acute knee trauma. 1 While ultrasound can confirm effusion presence and guide aspiration, it does not replace the diagnostic value of arthrocentesis for fluid analysis. 3

Clinical Decision Algorithm

The American College of Radiology recommends this stepwise approach:

1. Plain radiographs first (already completed—shows effusion, no obvious fracture). 1, 2

2. Diagnostic arthrocentesis next to establish diagnosis and rule out infection, crystal disease, or occult fracture. 1, 2

3. If lipohemarthrosis is present, proceed to CT for occult fracture characterization. 1

4. If inflammatory or infectious findings are present, obtain urgent orthopedic consultation. 1

Common Pitfalls to Avoid

• Never delay aspiration when infection is suspected—septic arthritis requires urgent intervention, and even low-grade fever or systemic symptoms warrant immediate aspiration. 2

• Do not assume all post-traumatic effusions are simple hemarthrosis—the differential includes septic arthritis, crystal arthropathy precipitated by trauma, and occult fractures. 1

• Aspiration provides only temporary symptomatic relief (lasting approximately one week due to re-accumulation), but its primary value is diagnostic, not therapeutic. 4