What is the initial management for a patient presenting with infectious knee bursitis?

Management of Infectious Knee Bursitis

For infectious knee bursitis, immediately aspirate the bursa for diagnostic confirmation, then initiate empiric oral antibiotics targeting Staphylococcus aureus (including MRSA coverage if risk factors present) while awaiting culture results, reserving hospitalization and IV antibiotics for patients with systemic toxicity, immunocompromise, or failure of outpatient management. 1, 2

Initial Diagnostic Approach

Bursal aspiration is mandatory when infection is suspected to distinguish septic from non-septic bursitis, as this fundamentally changes management from conservative measures to antimicrobial therapy 1, 2. The distinction is critical because the primary treatment of cellulitis or bursitis inflammation is antimicrobial therapy, whereas purulent collections require drainage 3.

Aspiration Technique and Analysis

• Perform bursal aspiration and send fluid for: Gram stain, bacterial culture (aerobic and anaerobic), cell count with differential, glucose measurement, and crystal analysis 1, 2
• Staphylococcus aureus causes >80% of septic bursitis cases, with other gram-positive organisms accounting for most remaining cases 2
• Gram-negative organisms, including Pseudomonas aeruginosa, can occur particularly in immunocompromised patients or those with atypical presentations 4

Clinical Features Suggesting Infection

• Localized erythema, warmth, swelling, and tenderness over the bursa (prepatellar or infrapatellar most commonly affected in the knee) 1, 5
• Fever and systemic symptoms may be present but are not required for diagnosis 1
• History of preceding trauma or chronic microtrauma (e.g., repetitive kneeling) increases risk 1, 2

Empiric Antibiotic Selection

Start empiric antibiotics immediately after aspiration in suspected septic bursitis while awaiting culture results, as delays increase morbidity 6.

Outpatient Oral Therapy (for non-acutely ill patients)

• First-line empiric coverage should target S. aureus, including MRSA if risk factors present (recent hospitalization, known MRSA colonization, failed prior therapy, or high local prevalence) 3
• For MRSA coverage: trimethoprim-sulfamethoxazole, doxycycline, or minocycline 1
• For methicillin-susceptible S. aureus (MSSA): cephalexin, dicloxacillin, or other first-generation cephalosporins 3

Inpatient IV Therapy Indications

Hospitalize and initiate IV antibiotics for patients with: 1, 2

• Fulminant local infection with extensive surrounding cellulitis
• Evidence of systemic toxicity (fever, tachycardia, hypotension, confusion)
• Immunocompromised state (diabetes, chronic kidney disease, immunosuppressive medications)
• Failed outpatient oral antibiotic therapy

IV antibiotic options: 3

• MSSA: nafcillin 1.5-2g IV q4-6h or cefazolin 1-2g IV q8h
• MRSA: vancomycin 15mg/kg IV q12h or daptomycin 6mg/kg IV q24h

Duration of Antibiotic Therapy

• Mild infections: 1-2 weeks of antibiotics typically suffices, though some require 3-4 weeks total 3
• Moderate to severe infections: 2-4 weeks of therapy depending on clinical response, extent of surrounding soft tissue involvement, and patient comorbidities 3
• Transition from IV to oral antibiotics once clinical improvement is evident (typically 48-96 hours), using highly bioavailable oral agents 3

Adjunctive Management

Bursal Drainage

• Perform repeated aspiration as needed for symptomatic relief and to remove purulent material 1, 2
• Serial aspirations may be required every 2-3 days until fluid reaccumulation ceases 2

Conservative Measures

• Rest, ice, elevation, and immobilization of the affected joint 1, 5, 7
• Avoid corticosteroid injection in suspected or confirmed septic bursitis, as this can worsen infection 1, 5

Surgical Intervention

Reserve surgical drainage or bursectomy for: 2, 5

• Failure to respond to IV antibiotics plus repeated aspiration after 4-7 days
• Recurrent septic bursitis despite appropriate medical management
• Presence of loculated fluid collections not amenable to needle aspiration

Monitoring and Follow-Up

• Reassess within 48-72 hours to confirm clinical improvement (decreased erythema, warmth, swelling, pain) 6
• If no improvement by 72-96 hours, consider treatment failure: obtain repeat aspiration for culture, reassess antibiotic choice, and consider hospitalization for IV therapy 6
• Monitor inflammatory markers (ESR, CRP) if obtained initially—these should trend downward with effective treatment 6

Critical Pitfalls to Avoid

• Do not inject corticosteroids into a potentially infected bursa, as this dramatically worsens outcomes and can lead to systemic sepsis 1, 5
• Do not delay aspiration in favor of empiric antibiotics alone—culture data is essential for targeted therapy and to rule out resistant organisms 1, 2
• Do not assume absence of fever excludes infection—chronic or indolent infections may present with local findings only 1
• Do not use aspiration for chronic microtraumatic (non-infectious) bursitis due to risk of introducing infection 1