What is the appropriate assessment and management for a patient presenting with sudden knee pain and swelling not associated with injury?

Assessment of Sudden Knee Pain and Swelling Without Injury

For a patient presenting with sudden atraumatic knee pain and swelling, immediately obtain plain radiographs (AP, lateral, Merchant/sunrise, and tunnel views) and perform urgent arthrocentesis if infection is suspected based on fever, systemic symptoms, or inability to bear weight. 1, 2

Immediate Clinical Priorities

Rule out septic arthritis first – this is an orthopedic emergency that can cause permanent joint destruction if treatment is delayed. 3, 4

Critical Red Flags for Infection

• Fever or systemic symptoms strongly suggest septic arthritis requiring urgent intervention 2, 3
• Inability to bear weight (cannot take 4 steps) indicates serious pathology 2, 5
• Recent bacteremia or skin infection increases infection risk 2, 3
• Age >80 years, diabetes, rheumatoid arthritis, immunosuppression, or prosthetic joint are major risk factors for septic arthritis 3

Essential Physical Examination Findings

• Document weight-bearing status by having the patient take at least 4 steps 1
• Assess range of motion – specifically whether the knee flexes to 90 degrees 1, 2
• Palpate for joint effusion – presence indicates inflammatory process or structural damage 1, 2
• Inspect for erythema, warmth, and swelling – these suggest infection or inflammation 3, 4
• Check for focal bony tenderness at the patella, fibular head, and tibial plateau 5

Diagnostic Algorithm

Step 1: Obtain Plain Radiographs Immediately

All patients with atraumatic knee swelling require plain radiographs as first-line imaging, including AP, lateral, Merchant/sunrise, and tunnel views. 6, 1, 2 This rules out fractures, prosthetic complications, chronic degenerative changes, and provides baseline assessment. 2

Step 2: Perform Arthrocentesis Based on Clinical Suspicion

Urgent arthrocentesis is indicated when:

• Suspected acute infection (fever, systemic symptoms, inability to bear weight) 2, 3
• Atraumatic effusion of unknown etiology 2
• Age <5 years with CRP >2.0 mg/dL (>90% probability of septic arthritis in children) 5

Send synovial fluid for:

• Cell count with differential – WBC >50,000/μL suggests septic arthritis 3
• Gram stain and culture – identifies causative organism 3
• Crystal analysis – rules out gout or pseudogout 4, 7

Step 3: Obtain Laboratory Studies

Order ESR and CRP – the combination provides best sensitivity and specificity for infection, especially in prosthetic joints. 2 Elevated inflammatory markers support infectious or inflammatory etiology. 3

Blood cultures if fever present or acute symptom onset suggests bacteremia. 2

Management Based on Etiology

If Septic Arthritis Suspected or Confirmed

• Initiate empiric IV antibiotics immediately after obtaining synovial fluid 3
• Staphylococcus aureus is the most common pathogen – tailor antibiotics based on Gram stain and local resistance patterns 3
• Orthopedic surgery consultation for operative drainage vs. serial aspirations 4
• Oral antibiotics are non-inferior to IV in most cases after initial treatment 3
• Total antibiotic duration: 2-6 weeks depending on organism and clinical response 3

If Inflammatory Arthritis (Crystal Disease, Reactive Arthritis)

• Intra-articular corticosteroid injection for pain flares with effusion 1
• NSAIDs with gastroprotection if gastrointestinal risk factors exist 1
• Consider less common causes like Clostridium difficile-induced reactive arthritis, especially post-COVID-19 or in HLA-B27 positive patients 8

If Structural Pathology Suspected (Normal Initial Workup)

• MRI without contrast at 5-7 days if symptoms persist despite conservative treatment, mechanical symptoms develop (locking, catching), or joint instability present 1, 2
• MRI evaluates meniscal tears, ligamentous injury, cartilage lesions, and bone marrow edema 6, 1

Conservative Management for Non-Urgent Cases

• Acetaminophen up to 4g/day as first-line oral analgesic 1
• Exercise therapy with progressive loading individualized to patient tolerance 1
• Patient education about pain management and condition prognosis 1
• Topical NSAIDs or capsaicin as second-line if acetaminophen ineffective 1

Critical Pitfalls to Avoid

• Never skip radiographs even when effusion is clinically obvious – fractures and chronic changes must be ruled out 2
• Do not delay aspiration if infection suspected – septic arthritis causes irreversible joint damage within hours to days 2, 3
• Avoid intra-articular corticosteroid injection before ruling out infection – this can worsen septic arthritis 8
• Do not routinely order MRI, bone scan, or PET for initial diagnosis – these should not replace standard workup of radiographs and aspiration 2
• Consider reactive arthritis mimics especially in the COVID-19 era or with recent diarrheal illness – these can present identically to septic arthritis 8, 4

When to Refer to Orthopedics

• Immediate referral: Confirmed or suspected septic arthritis 3, 4
• Urgent referral: Radiographic fracture or prosthetic complication 2
• Elective referral: Persistent symptoms after 6+ weeks of adequate conservative treatment with concerning MRI findings 1