What is the appropriate work‑up for an adult with acute right‑knee swelling that has spread to the entire leg?

Work-Up for Right Knee Swelling and Pain Spreading to Entire Leg

Obtain plain radiographs of the knee immediately as the first imaging study, then perform urgent arthrocentesis with synovial fluid analysis to rule out septic arthritis, which is an orthopedic emergency requiring immediate intervention. 1, 2

Immediate Clinical Assessment

The spread of knee swelling to the entire leg is a red flag that demands urgent evaluation for several life- and limb-threatening conditions:

• Assess for signs of infection: fever, warmth, erythema, systemic symptoms, or recent bacteremia suggest septic arthritis requiring emergent intervention 2
• Evaluate vascular status: check distal pulses, capillary refill, skin temperature, and color to exclude acute limb ischemia or deep vein thrombosis 1
• Document ability to bear weight: inability to bear weight for 4 steps mandates imaging 1, 2
• Check for focal bony tenderness: tenderness at the malleoli, tibial plateau, or patella suggests fracture 1
• Measure joint effusion: significant effusion combined with inability to flex knee to 90 degrees requires immediate radiographic evaluation 1, 2

Initial Imaging Protocol

Plain radiographs are mandatory as the first-line study to exclude fractures, tumors, osteomyelitis, hardware complications, and chronic bone changes before proceeding with any other imaging or intervention 1, 2:

• Obtain at least two views (anteroposterior and lateral) of the knee 1
• Radiographs may show: joint effusion, soft tissue swelling, periosteal reaction (suggesting osteomyelitis), gas in soft tissues (indicating necrotizing infection), or fractures 1
• Early osteomyelitis (<14 days) may show only soft tissue swelling on radiographs, but obtaining them is still essential to guide interpretation of subsequent advanced imaging 1

Urgent Arthrocentesis

If infection is suspected based on clinical presentation, perform arthrocentesis immediately after obtaining radiographs, unless surgery is immediately planned 1, 2:

• Joint aspiration is the reference standard for diagnosing septic arthritis; a positive culture confirms the diagnosis 1
• Image-guided aspiration (ultrasound, CT, or fluoroscopy) is preferred to confirm proper needle placement and avoid contaminating adjacent tissues 1
• Send synovial fluid for: cell count with differential, Gram stain, culture, and crystal analysis 2
• Do not delay aspiration if infection is suspected, as septic arthritis can lead to rapid joint destruction and systemic sepsis 2, 3

Laboratory Studies

• Obtain ESR and CRP: elevated inflammatory markers increase suspicion for infection, though they are nonspecific 2
• Blood cultures if febrile or if acute symptom onset suggests bacteremia 2

Advanced Imaging Based on Initial Findings

If Radiographs Show Fracture or Hardware Complications:

• CT without IV contrast is appropriate to further characterize fracture extent, assess for hardware loosening, evaluate for nonunion, or detect osteolysis 1

If Radiographs Are Normal or Show Only Effusion/Soft Tissue Swelling:

MRI without IV contrast is the next appropriate study if infection or internal derangement is suspected 1, 2:

• MRI has high sensitivity and specificity for detecting osteomyelitis, soft tissue abscesses, septic arthritis, and internal derangement 1
• MRI can identify: bone marrow edema (early osteomyelitis), cortical erosions, soft tissue abscesses, extent of infection, meniscal tears, ligamentous injuries, and occult fractures 1
• Administer IV contrast if evaluating for soft tissue abscesses or to delineate extent of infection 1
• A negative MRI excludes osteomyelitis 1

CT with IV contrast is an alternative if MRI is contraindicated or unavailable 1:

• CT allows evaluation of soft tissue compartments and can differentiate cellulitis, myositis, abscess, and septic arthritis 1
• CT is well suited for detecting gas in tissues, foreign bodies, and guiding aspiration or surgical debridement 1

If Vascular Compromise Is Suspected:

CTA of the lower extremity with IV contrast should be obtained urgently if there are signs of acute limb ischemia (absent pulses, pallor, paresthesias, paralysis, or pain out of proportion) 1:

• CTA is less invasive than conventional angiography and has similarly high accuracy for detecting vascular injuries 1
• Vascular injuries require prompt surgical intervention to preserve the limb 1

Ultrasound Considerations

Ultrasound may be useful as a bedside adjunct but should not replace radiographs as the initial study 1:

• US can detect: joint effusion, soft tissue abscesses, periosteal thickening, and guide aspiration 1
• US is limited by inability to detect bone abnormalities, metal artifact, and operator dependence 1

Critical Pitfalls to Avoid

• Never skip radiographs, even when clinical findings strongly suggest infection or soft tissue pathology, as radiographs are essential to exclude fractures, tumors, and chronic bone changes 1, 2, 3
• Do not delay arthrocentesis if septic arthritis is suspected; this is an orthopedic emergency requiring urgent drainage and antibiotics 2, 3
• Do not assume normal radiographs exclude serious pathology: early osteomyelitis, soft tissue abscesses, and internal derangement require MRI for detection 1
• Assess vascular status immediately: spreading leg swelling with pain may indicate compartment syndrome, deep vein thrombosis, or acute limb ischemia, all of which require urgent intervention 1
• Consider deep vein thrombosis in the differential diagnosis of unilateral leg swelling; if suspected, obtain Doppler ultrasound of the lower extremity 1

Algorithm Summary

1. Obtain plain radiographs (AP and lateral views of the knee) 1, 2
2. Perform arthrocentesis if infection suspected (fever, warmth, erythema, systemic symptoms) 1, 2
3. Send labs: ESR, CRP, blood cultures if febrile 2
4. If radiographs show fracture: obtain CT without IV contrast for further characterization 1
5. If radiographs normal or show only effusion/swelling: obtain MRI without IV contrast (or with IV contrast if abscess suspected) 1, 2
6. If vascular compromise suspected: obtain CTA lower extremity with IV contrast urgently 1
7. If DVT suspected: obtain Doppler ultrasound of lower extremity 1