What is the best initial evaluation and treatment approach for a patient presenting with a swollen knee, considering their age, medical history, and mechanism of injury?

X-Ray Imaging for Swollen Knee

Yes, obtain plain radiographs (minimum anteroposterior and lateral views) as the initial imaging study for a patient with a swollen knee, particularly if any Ottawa knee rule criteria are present or if there is concern for fracture, effusion, or structural abnormality. 1, 2

When to Order X-Rays

Apply the Ottawa knee rules to determine imaging necessity. Order radiographs if ANY of the following are present: 1, 2, 3

• Age ≥55 years
• Focal tenderness at the patella
• Focal tenderness at the fibular head
• Inability to bear weight for 4 steps (both immediately after injury and during examination)
• Inability to flex knee to 90 degrees

Override clinical decision rules and obtain radiographs regardless if the patient has: 1, 4

• Gross deformity
• Palpable mass
• Penetrating injury
• Prosthetic hardware
• Altered mental status (head injury, intoxication, dementia)
• Neuropathy (paraplegia, diabetes)
• History suggesting increased fracture risk

Required X-Ray Views

Minimum two views are mandatory: 1, 2, 5

• Anteroposterior (AP) view of the affected knee
• Lateral view with knee at 25-30 degrees flexion in lateral decubitus position (demonstrates joint effusion and patella in profile)

Additional views to consider based on clinical findings: 1, 2

• Cross-table lateral with horizontal beam - visualizes lipohemarthrosis (indicates intra-articular fracture)
• Patellofemoral (sunrise/Merchant) view - for anterior knee pain or suspected patellar fracture/subluxation
• Internal and external oblique views - for comprehensive fracture assessment

Clinical Context Matters

Acute Trauma Setting

If swelling follows trauma with focal tenderness or inability to bear weight, radiographs are the first-line imaging modality to identify fractures requiring immediate intervention. 1, 6

Chronic Knee Pain with Effusion

For chronic pain where initial radiographs show only joint effusion but pain persists, MRI without contrast becomes the next appropriate study to evaluate menisci, ligaments, cartilage, and bone marrow edema. 1 However, approximately 20% of patients inappropriately receive MRI without recent radiographs (within the prior year), making plain films essential first. 1

Pediatric Considerations (Age <18 years)

The Pittsburgh Decision Rule applies: obtain radiographs if the patient cannot take four weight-bearing steps in the emergency department. 1, 5 Three of 146 pediatric patients in validation studies had fractures despite negative bony tenderness, emphasizing the importance of weight-bearing assessment. 1

Critical Differential Diagnoses Requiring Imaging

Septic arthritis must be excluded if effusion is present with fever or systemic symptoms - this is an orthopedic emergency. 4 In children <5 years with C-reactive protein >2.0 mg/dL, there is >90% probability of septic arthritis. 4

Referred pain from hip or lumbar spine should be considered if knee radiographs are unremarkable, especially in patients with chronic symptoms. 1, 2

What Happens After Initial X-Rays

If Radiographs Show Fracture

Orthopedic consultation for fracture management. 4 CT may be needed for better characterization of complex fractures, particularly tibial plateau fractures where CT shows 100% sensitivity versus 83% for plain films. 1

If Radiographs Are Normal But Symptoms Persist

Conservative management with close follow-up is appropriate if the patient can bear weight. 2, 5 Consider MRI without contrast at 5-7 days if: 2, 5, 4

• Significant persistent joint effusion
• Inability to fully bear weight after initial treatment
• Mechanical symptoms suggesting meniscal injury (locking, catching)
• Joint instability suggesting ligamentous injury

If Radiographs Show Only Effusion

The physical examination becomes critical. For meniscal tears: joint line tenderness is 75% sensitive but only 27% specific, while McMurray test is 97% specific but only 52% sensitive. For anterior cruciate ligament tears, the Lachman test is more sensitive and specific than the drawer sign. 3

Common Pitfalls to Avoid

Do not skip radiographs and proceed directly to MRI - this wastes resources and may miss fractures or other bony pathology that changes management. 1, 6

Do not rely solely on ability to bear weight to exclude significant injury - occult fractures and soft tissue injuries can be present even when patients can walk. 2

Physician judgment should supersede clinical guidelines when appropriate, with the primary goal of identifying fractures requiring immediate intervention while avoiding unnecessary radiation exposure. 1, 2