Msk Dermatology · Year 2 · from Msk Dermatology

Case 1: Septic Arthritis of the Knee

Patient Presentation

Demographics: 58-year-old man

Chief Complaint: "My right knee is extremely swollen and painful, and I've had fevers."

History of Present Illness: The patient presents with 3 days of progressive right knee pain and swelling. He initially thought he had tweaked his knee while walking but noted rapid worsening with development of fever and chills. He now cannot bear weight on the leg. He has difficulty moving the knee in any direction due to severe pain. He denies any penetrating trauma but reports he had a skin boil on his leg drained about 2 weeks ago.

Past Medical History:

• Type 2 diabetes mellitus (poorly controlled, HbA1c 9.2%)
• Rheumatoid arthritis (on methotrexate)
• Hypertension

Medications:

• Methotrexate 15 mg weekly
• Folic acid 1 mg daily
• Metformin 1000 mg twice daily
• Glipizide 10 mg twice daily
• Lisinopril 20 mg daily

Social History:

• Retired truck driver
• Former smoker
• Occasional alcohol

Physical Examination

• Vital Signs: BP 108/68 mmHg, HR 110 bpm, Temp 39.2C, RR 20
• General: Ill-appearing, diaphoretic
• Right Knee:
• Large tense effusion
• Marked erythema and warmth
• Exquisite tenderness - patient guards against any movement
• Held in 20-30 degrees of flexion
• Unable to actively extend or flex
• Skin: Healing wound on right lower leg (site of prior furuncle drainage)
• Other joints: Chronic MCP changes from RA, no acute inflammation

Workup and Results

Laboratory Studies:

Synovial Fluid Analysis:

Imaging:

• Knee X-ray: Large joint effusion, soft tissue swelling, no fracture or erosions

Clinical Image

Clinical photograph and diagram demonstrating a large knee joint effusion with visible swelling and loss of normal joint contour, characteristic presentation of septic arthritis. Source: Wikimedia Commons, CC BY-SA 3.0.

Diagnosis

Septic Arthritis of the Right Knee - Presumed Staphylococcus aureus

Diagnostic Features:

• Acute monoarticular arthritis with systemic signs
• Synovial WBC >50,000/mm3 with >90% PMNs
• Gram-positive cocci on Gram stain (presumed S. aureus)
• Risk factors: diabetes, immunosuppression (methotrexate), prior skin infection

Treatment Plan

Emergent Management:

1. Joint drainage: Orthopedic surgery consulted for arthroscopic lavage vs. daily needle aspirations
2. Hold methotrexate until infection cleared
3. IV antibiotics:

• Vancomycin 1.5g IV loading, then 1g IV q12h (MRSA coverage given community onset, prior skin infection)
• Adjust based on culture and sensitivity

1. Blood cultures x 2 sets drawn before antibiotics

Supportive Care:

1. IV fluid resuscitation
2. Insulin sliding scale for glucose control
3. Pain management
4. DVT prophylaxis

Culture Results (48 hours):

• Synovial fluid and blood cultures: Methicillin-sensitive Staphylococcus aureus (MSSA)
• De-escalate to nafcillin 2g IV q4h (or cefazolin 2g IV q8h)

Duration:

• Minimum 4 weeks IV antibiotics for native joint septic arthritis
• Consider oral step-down to ciprofloxacin + rifampin or high-dose oral beta-lactam if organism susceptible and good clinical response

Follow-up:

• Repeat aspiration to confirm sterilization
• Monitor inflammatory markers (CRP, ESR) for response
• Physical therapy once infection controlled
• Discuss restarting RA therapy (wait 2-4 weeks post-antibiotics)

Teaching Points

1. Septic arthritis is a medical emergency: Untreated leads to rapid cartilage destruction within days; prompt diagnosis and drainage are critical
2. Synovial WBC >50,000 with >90% PMN: Strongly suggests septic arthritis; Gram stain is positive in only 50-75% of cases
3. Staphylococcus aureus: Most common cause across all age groups; always suspect MRSA in community-acquired cases
4. Risk factors: Diabetes, immunosuppression, pre-existing joint disease (RA, OA), skin infections, and prosthetic joints increase susceptibility
5. Drainage is essential: Antibiotics alone insufficient; surgical or repeated needle drainage required

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