Diagnosis of Knee Inflammation
Begin with plain radiographs (anteroposterior and lateral views minimum) as the initial imaging modality for any patient presenting with knee inflammation, and proceed to MRI without IV contrast if radiographs are normal or show only effusion but symptoms persist. 1
Initial Clinical Assessment
Key Historical Features to Elicit
- Duration and onset: Acute inflammation (trauma, infection, crystal disease) versus chronic inflammation (osteoarthritis, inflammatory arthritis) 2, 3
- Age-specific considerations: Patellofemoral pain predominates in patients <40 years (25% lifetime prevalence), while osteoarthritis is most likely in patients ≥45 years with activity-related pain and <30 minutes of morning stiffness (95% sensitivity, 69% specificity) 3
- Mechanical symptoms: Popping, locking, catching, or giving way suggest meniscal tears (12% adult population prevalence) or loose bodies requiring further evaluation 4, 3
- Trauma history: Twisting injuries in patients <40 years suggest acute meniscal tears, while similar symptoms in patients ≥40 years indicate degenerative tears associated with osteoarthritis 3
Physical Examination Findings
- Temperature assessment: Use handheld infrared thermometry or hand palpation to detect temperature differences between knees, which indicates active inflammation 5
- Effusion detection: Palpate for joint effusion, which when present warrants consideration of aspiration for synovial fluid analysis 5, 1
- Joint line tenderness: 83% sensitivity and 83% specificity for meniscal tears 3
- McMurray test: Perform concurrent knee rotation (internal for lateral meniscus, external for medial meniscus) with extension (61% sensitivity, 84% specificity for meniscal tears) 3
- Valgus/varus stress testing: Assesses collateral ligament integrity 6
- Anterior knee pain during squat: 91% sensitivity and 50% specificity for patellofemoral pain 3
Diagnostic Imaging Algorithm
Step 1: Plain Radiographs (First-Line)
- Views required: Anteroposterior, lateral, and medial oblique projections; add sunrise/Merchant view for patellofemoral assessment and tunnel view if osteochondritis dissecans suspected 1, 4
- Weight-bearing preferred: Standing radiographs demonstrate malalignments better than non-weight-bearing films 5
- Bilateral imaging: Obtain bilateral views when possible for comparison, particularly in patients >70 years where unilateral structural abnormalities better correlate with symptomatic knees 5, 1
- Diagnostic codes: Use M25.461/M25.462 (effusion, right/left knee) combined with M25.561/M25.562 (pain, right/left knee) to justify imaging 1
Step 2: MRI Without IV Contrast (If Radiographs Normal or Non-Diagnostic)
- Indications: Normal radiographs with persistent symptoms, suspected meniscal tears, evaluation of medial patellofemoral ligament integrity, cartilage injury assessment, or identification of loose bodies 5, 1, 4
- Effusion-synovitis grading: MRI can grade effusion-synovitis (none/small versus medium/large), though synovial fluid white blood cell count shows limited correlation (71% sensitivity, 63% specificity at optimal cutpoint of 242 cells) 7
- Avoid premature MRI: Approximately 20% of chronic knee pain patients inappropriately receive MRI without recent radiographs—always obtain plain films first 1
Step 3: Ultrasound (Selective Use)
- Not for screening: Ultrasound is not useful as a comprehensive examination but can detect synovial pathology, effusions, cortical erosions, and meniscal extrusion 5
- Power Doppler utility: Demonstrates increased synovial blood flow associated with knee pain and can show reduced flow after therapeutic injections 5
- Aspiration guidance: Use ultrasound or fluoroscopy to guide joint aspiration when effusion is present and crystal disease or infection is suspected 5, 1
Step 4: Advanced Imaging (Rarely Indicated)
- CT without contrast: Consider for patellofemoral anatomy evaluation in chronic subluxation or to confirm prior osseous injury 5
- CT arthrography: May evaluate menisci, articular cartilage, and loose bodies (86-100% sensitivity and specificity) but rarely needed 5
- Nuclear imaging: Low specificity and decreased anatomic resolution compared to MRI; reserve for distinguishing bone versus soft-tissue pain origins when MRI contraindicated 5
Synovial Fluid Analysis
When to Aspirate
- Presence of effusion with concern for:
Interpretation Limitations
- White blood cell count: Median WBC of 335 (IQR 312) in medium/large effusion-synovitis versus 194 (IQR 272) in none/small effusion-synovitis, but limited diagnostic accuracy 7
- Correlation with MRI: Synovial fluid WBC shows suboptimal correlation with MRI effusion-synovitis grading 7
Critical Pitfalls to Avoid
- Never order MRI without recent radiographs first: This represents inappropriate utilization in approximately 20% of chronic knee pain cases 1
- Consider referred pain sources: If knee radiographs are unremarkable, evaluate hip and lumbar spine pathology with additional imaging and appropriate diagnostic codes 1
- Document laterality: Use specific right/left codes (M25.461/M25.462, M25.561/M25.562) rather than unspecified codes for improved reimbursement and clinical clarity 1
- Bilateral structural abnormalities: In patients >70 years, bilateral abnormalities on radiographs or MRI limit ability to discriminate painful from non-painful knees, even when using synovitis/effusion presence or Kellgren-Lawrence score ≥2 5
- Avoid bone scan for routine evaluation: Radionuclide imaging has low specificity and poor anatomic resolution compared to CT or MRI 5