Septic arthritis
Formal Definition
A surgical/medical emergency involving bacterial infection of a joint space, most commonly Staphylococcus aureus, presenting with acute onset of severe joint pain, exquisite tenderness, swelling, warmth, erythema, marked reluctance to move the joint, and often fever; diagnosis requires arthrocentesis with synovial fluid analysis showing WBC >50,000 cells/mm³ (often >100,000), neutrophil predominance, and positive Gram stain/culture; treatment requires empiric IV antibiotics and urgent joint drainage (arthroscopy, serial aspiration, or open washout) to prevent irreversible cartilage destruction within hours to days.
How It's Used on the Ward
"Septic joint" — the joint is infected; this is a true emergency because bacterial enzymes destroy cartilage in hours; the patient refuses to move the joint at all (compared to crystal arthritis where they will reluctantly move through pain); get a tap, start empiric antibiotics, and call ortho for drainage.
Example
""68-year-old man with diabetes presents with 36h of severe left knee pain, swollen, hot, refusing to bear weight. T 38.6°C. Exam: knee erythematous, effusion visible, warm to touch, cannot tolerate any passive motion. Arthrocentesis: 60 mL of purulent-appearing fluid, WBC 85,000 with 95% PMNs, Gram-positive cocci in clusters. Diagnosis: septic arthritis of left knee, likely Staph aureus. Empiric vancomycin + cefepime started, urgent orthopedics consult for operative irrigation and debridement. Blood cultures also positive — bacteremia source.""
Clinical Context
Risk factors: advanced age, IV drug use (think Staph aureus, gram-negatives), immunocompromise, recent joint surgery or intra-articular injection, gout or pseudogout (damaged joint more vulnerable), prosthesis (PJI — prosthetic joint infection requires different management, often 2-stage revision). Most common pathogen: S. aureus (~50% across all populations). By population: S. aureus across all (MSK, IVDU), Neisseria gonorrhoeae in sexually active young adults (migratory polyarthralgia, tenosynovitis, dermatitis — "arthritis-dermatitis syndrome"), gram-negatives in IVDU, immunocompromised. Differential WBC in synovial fluid: <2000 = non-inflammatory (OA); 2000-7500 = inflammatory (RA, crystal); >50,000 = septic (overlap with crystal possible). Crystals do NOT exclude septic arthritis — co-occurrence possible. Empiric antibiotics: vancomycin + ceftriaxone (covers Staph aureus incl. MRSA + gonococcus in young adults), vancomycin + cefepime for gram-negative coverage (elderly, IVDU). Treatment window: arthrocentesis + antibiotics + joint drainage within hours — delays >24h significantly increase permanent joint dysfunction risk.