What is the appropriate workup for a patient with a swollen right knee?

Workup for a Swollen Right Knee

The initial workup for a swollen right knee should begin with radiographs including anteroposterior (AP), lateral, sunrise/Merchant, and tunnel views, followed by appropriate advanced imaging based on clinical findings and radiographic results. 1

Initial Evaluation

Radiographic Assessment

• Standard radiographic views required:
  • Anteroposterior (AP) view
  • Lateral view (knee at 25-30 degrees flexion)
  • Sunrise/Merchant view (patellofemoral view)
  • Tunnel view
• A cross-table lateral view with horizontal beam may be added to visualize lipohemarthrosis in cases of suspected intra-articular fractures 1

When to Order Radiographs

Radiographs should be obtained when any of these findings are present:

• Focal tenderness
• Joint effusion
• Inability to bear weight
• Knee pain with swelling, locking, catching, popping, or giving way 1

Mandatory Radiographs (Regardless of Clinical Decision Rules)

Order radiographs immediately if the patient has:

• Gross deformity
• Palpable mass
• Penetrating injury
• Prosthetic hardware
• Unreliable clinical history due to multiple injuries
• Altered mental status (head injury, intoxication, dementia)
• Neuropathy (paraplegia, diabetes)
• History suggesting increased fracture risk 1

Advanced Imaging

MRI Without Contrast

Indicated as the next step when:

1. Initial radiographs are normal or show only joint effusion but pain persists 1
2. Radiographs demonstrate osteochondritis dissecans (OCD) 1
3. Concomitant knee pathology is suspected:
   • Meniscal pathology
   • Ligament injury (especially ACL)
   • Articular cartilage injury 1

CT Scan

Appropriate when:

1. Occult fracture is suspected after normal radiographs
2. Better characterization of tibial plateau fractures is needed
   • CT shows 100% sensitivity compared to 83% for radiographs in tibial plateau fractures 1
3. Patellofemoral anatomy evaluation is needed for suspected subluxation or maltracking 1

Joint Aspiration

Joint aspiration should be performed when:

1. Infection is suspected (fever, severe pain, warmth, erythema)
2. Crystal-induced arthropathy is suspected (gout, pseudogout)
3. There is a large, painful effusion requiring drainage 1, 2

Synovial Fluid Analysis

• Cell count and differential
• Crystal examination under polarized light
• Gram stain and culture
• Glucose and protein levels

Common Diagnostic Patterns

Age-Based Considerations

• Younger patients (<40 years):

  • Patellofemoral pain syndrome
  • Patellar tracking problems
  • Acute ligamentous injuries
  • Meniscal tears (often traumatic) 3, 4

• Middle-aged and older patients (>45 years):

  • Osteoarthritis
  • Degenerative meniscal tears
  • Crystal arthropathies 1, 4

Special Considerations

• Consider hip pathology if knee radiographs are unremarkable (referred pain) 1
• Consider inflammatory arthritis if there's significant morning stiffness
• Consider infection if there's fever, severe pain, or recent intra-articular injection 5
• Consider reactive arthritis in patients with recent infection (including COVID-19) 5

Pitfalls to Avoid

1. Premature MRI: Approximately 20% of patients with chronic knee pain have MRI without recent radiographs, which is often unnecessary 1

2. Missing septic arthritis: This is a medical emergency requiring prompt diagnosis and treatment; always consider infection in acute knee swelling, especially with fever or risk factors 5

3. Overlooking referred pain: Hip pathology can present as knee pain; consider hip imaging if knee evaluation is unremarkable 1

4. Misinterpreting incidental findings: Meniscal tears are common incidental findings in older patients, with the majority of people over 70 having asymptomatic tears 1

5. Inadequate radiographic views: Failure to obtain all four standard views may lead to missed diagnoses 1