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 establish the diagnosis and rule out infection or crystal disease, particularly given the atraumatic presentation with significant effusion of unknown etiology. 1

Initial Diagnostic Approach

The patient has already received appropriate initial imaging with plain radiographs showing knee effusion without obvious fracture. 2 This satisfies the American College of Radiology's recommendation for first-line imaging in patients meeting Ottawa knee rule criteria (inability to bear weight, focal tenderness, and effusion). 1

Rationale for Arthrocentesis

For atraumatic effusions of unknown etiology, diagnostic arthrocentesis is specifically recommended to establish diagnosis and rule out infection or crystal disease. 1 While this case involves a mechanical fall, the absence of fracture on radiographs and presence of significant effusion warrants fluid analysis to:

• Rule out septic arthritis, which is an orthopedic emergency requiring urgent intervention 1
• Exclude crystal arthropathy that may have been precipitated by minor trauma 1
• Identify lipohemarthrosis, which would indicate an occult intra-articular fracture despite negative radiographs 2
• Provide immediate symptomatic relief, though this benefit is temporary (lasting approximately one week) 3

Why Other Options Are Inappropriate

Lower Extremity CT Angiogram (Option B)

• CT angiography has no role in routine knee trauma evaluation with normal vital signs and no vascular compromise 2
• There are no clinical indicators of vascular injury in this presentation 2

Lower Extremity CT Scan (Option C)

• CT is not routinely used as initial imaging for acute knee trauma 2
• CT may be considered as a next study only when radiographically occult fractures are suspected, but this requires clinical suspicion beyond simple effusion 2
• The patient already has radiographs showing no fracture, making CT premature at this stage 1

Lower Extremity Doppler Ultrasound (Option D)

• Ultrasound is not routinely used for initial evaluation of acute knee trauma 2
• Doppler would only be indicated if there were clinical signs of vascular compromise, which are absent 2

Clinical Decision Algorithm

Step 1: Plain radiographs obtained ✓ (already completed, showing effusion without fracture) 2, 1

Step 2: Perform diagnostic arthrocentesis because:

• Significant effusion present 1
• Etiology uncertain despite trauma history 1
• Need to exclude infection (septic arthritis) 1
• Need to identify lipohemarthrosis suggesting occult fracture 2

Step 3: Based on aspirate findings:

• If lipohemarthrosis present: Consider CT for occult fracture characterization 2
• If inflammatory/infectious: Urgent orthopedic consultation 1
• If hemorrhagic without fat: Consider MRI at 5-7 days if symptoms persist or mechanical symptoms develop 1

Critical Pitfalls to Avoid

• Never delay aspiration when infection is suspected, as septic arthritis requires urgent intervention 1
• Do not skip radiographs before aspiration, even when effusion is clinically obvious, as they are essential to rule out fractures 1
• Avoid ordering advanced imaging (CT, MRI) without first obtaining synovial fluid analysis when the diagnosis remains uncertain 1
• Remember that aspiration provides only temporary symptomatic improvement (approximately one week) due to early re-accumulation, but remains valuable for diagnosis 3

Synovial Fluid Analysis Priorities

When performing arthrocentesis, send fluid for:

• Cell count with differential (to assess for infection) 1
• Gram stain and culture (if infection suspected) 1
• Crystal analysis (to exclude gout or pseudogout) 1
• Gross appearance assessment for lipohemarthrosis (fat droplets indicating occult fracture) 2