What is the initial approach for a 7-year-old male presenting with knee pain on extension and effusion?

Initial Approach for a 7-Year-Old Male with Knee Pain on Extension and Effusion

Obtain plain radiographs (anteroposterior and lateral views) as the initial imaging study, and assess for septic arthritis versus traumatic injury based on clinical presentation, with urgent aspiration if infection is suspected. 1, 2

Immediate Clinical Assessment

Key Historical Features to Elicit

• Trauma history: Determine if there was a fall, twisting injury, or significant mechanism (motor vehicle accident) 1, 2
• Fever and systemic symptoms: Presence suggests septic arthritis, which is an orthopedic emergency requiring immediate intervention 1, 3
• Weight-bearing ability: Inability to take four weight-bearing steps is a critical Ottawa rule criterion 1, 2
• Time course: Acute onset (<48 hours) versus subacute presentation 4

Critical Physical Examination Findings

• Focal bony tenderness: Palpate the patella, fibular head, and tibial plateau for point tenderness 1, 2
• Ability to flex knee to 90 degrees: Inability is an Ottawa rule criterion requiring radiographs 1, 2
• Gross deformity or palpable mass: These findings mandate immediate radiographs regardless of other criteria 1
• Joint effusion assessment: Compare with contralateral knee to quantify severity 2, 5

Initial Imaging Strategy

Radiographic Evaluation

Obtain a minimum of two views (anteroposterior and lateral at 25-30 degrees flexion) if any Ottawa rule criteria are present, which includes focal tenderness, effusion, or inability to bear weight 1, 2. The lateral view allows evaluation for joint effusion and lipohemarthrosis (suggesting intra-articular fracture) 1.

Additional views to consider:

• Cross-table lateral with horizontal beam for lipohemarthrosis visualization 1
• Patellofemoral view if patellar injury is suspected 1
• Internal and external oblique views for comprehensive fracture assessment 1

When to Bypass Ottawa Rules

Obtain radiographs immediately regardless of Ottawa criteria if:

• Gross deformity is present 1
• Penetrating injury or prosthetic hardware exists 1
• Altered mental status or unreliable examination due to multiple injuries 1
• Clinical suspicion for high-energy trauma 1

Differentiating Septic Arthritis from Traumatic/Aseptic Effusion

High-Risk Features for Septic Arthritis

Age <5 years and C-reactive protein >2.0 mg/dL together predict >90% probability of septic arthritis in a child with painful swollen knee 3. This is a critical distinction because septic arthritis requires urgent surgical drainage to prevent joint destruction 1, 6.

Additional red flags for infection:

• Fever (though absence does not exclude septic arthritis) 1, 4
• Refusal to bear weight or move the joint 1, 3
• Bacteremia or recent infection elsewhere 1

Aspiration Indications

Perform urgent ultrasound-guided joint aspiration if:

• Clinical suspicion for septic arthritis based on age <5 years and elevated CRP 3
• Atraumatic effusion of unknown etiology to establish diagnosis 1, 7
• Fever with joint effusion, even with negative radiographs 1, 3

Aspiration improves diagnostic accuracy substantially: Adding synovial fluid analysis with percent polymorphonuclear cells (%PMN) to clinical criteria raises diagnostic certainty to 96% for septic arthritis 3. Aspiration is highly accurate for diagnosing septic arthritis and can prevent unnecessary arthrotomy 1.

Aspiration Technique and Analysis

• Use ultrasound guidance for improved accuracy and safety 1
• Send fluid for: Cell count with differential, Gram stain, culture, and crystal analysis 1, 5
• Synovial fluid WBC >50,000 with >75% PMNs strongly suggests septic arthritis 3

Management Based on Initial Findings

If Radiographs Show Fracture

Proceed with orthopedic consultation for fracture management 1. Consider MRI if tibial plateau fracture is identified to evaluate for additional occult fractures and meniscal/ligamentous injuries 1.

If Radiographs Normal but Trauma History Present

Conservative management with close follow-up is appropriate if the child can bear weight 8. The ability to bear weight after trauma with negative radiographs rules out most fractures requiring surgical intervention 8.

Consider MRI at 5-7 days if:

• Significant persistent effusion 8
• Inability to fully bear weight 8
• Mechanical symptoms suggesting meniscal injury 8
• Joint instability suggesting ligamentous injury 8

If Atraumatic Effusion

Aspiration is strongly recommended to establish diagnosis and differentiate between infectious, inflammatory, and malignant etiologies 1, 4, 7. In atraumatic effusions, aspiration aids in earlier diagnosis establishment 7.

Common Pitfalls to Avoid

• Do not skip radiographs before ordering MRI: Approximately 20% of patients with chronic knee pain receive MRI without recent radiographs, which is inappropriate 2
• Do not rely solely on fever absence: Septic arthritis can present without fever, especially in younger children 4, 3
• Do not delay aspiration in suspected septic arthritis: Early diagnosis and treatment prevent severe joint destruction and functional limitation 1, 6
• Recognize aspiration provides only temporary relief in traumatic effusions: Re-accumulation occurs within one week, so aspiration should be performed primarily for diagnostic purposes in trauma cases 7
• Physician judgment supersedes clinical guidelines: In any scenario where clinical suspicion is high, proceed with appropriate imaging or aspiration regardless of decision rules 1

Advanced Imaging Considerations

MRI is not routinely used as initial imaging for acute knee trauma but becomes appropriate when radiographs are normal and pain persists, or when soft tissue injury evaluation is needed 2, 8. MRI has 82-100% sensitivity and 75-96% specificity for osteomyelitis in children, making it valuable if infection extends beyond the joint 1.

CT may be considered if radiographically occult fractures are suspected, as it shows 100% sensitivity for tibial plateau fractures compared to 83% for radiographs 2.