Septic Arthritis

Orthopedics / Infectious Disease

OverviewWhoPresentationMechanismWorkupManagementPivots

Illness script · Orthopedics / Infectious Disease

Septic Arthritis

Bacterial infection of a synovial joint causing acute inflammation; a surgical emergency requiring urgent drainage to prevent joint destruction.

This illness script for Septic Arthritis covers predisposing factors, classic presentation, mechanism, workup, management, and the clinical pivots that separate it from look-alikes—written for USMLE Step 1 and clerkship reasoning.

Updated Jul 19, 2026All scripts

01

Predisposing factors

  • Most common organism: S. aureus (all ages); N. gonorrhoeae in sexually active adults <45
  • IV drug use, immunocompromise (DM, HIV, RA, steroids) increase risk
  • Prosthetic joints, prior joint surgery, or intra-articular injections are major risk factors
  • Hematogenous seeding most common route; contiguous spread or direct inoculation also occur
  • Children: S. aureus most common; neonates also group B Strep; Kingella kingae in <2 yo

02

Presentation

  • Classic triad: acute monoarthritis + fever + joint effusion (warm, erythematous, exquisitely tender)
  • Knee most commonly affected (~50%); hip second most common
  • Severely restricted ROM in ALL planes — passive motion acutely painful (key exam finding)
  • Gonococcal: migratory polyarthralgia → monoarthritis + tenosynovitis + pustular skin lesions
  • Patients appear ill; fever >38.5°C supports infectious etiology over crystal arthropathy

03

Pathophysiology

  • Bacteria seed synovium via bacteremia → rapid leukocyte influx into joint space
  • Proteolytic enzymes and cytokines from PMNs directly degrade cartilage within hours–days
  • Elevated intra-articular pressure → vascular compromise → avascular necrosis if untreated
  • Non-gonococcal septic arthritis causes faster, more destructive joint damage than gonococcal

04

Diagnostics

  • JOINT ASPIRATION is both diagnostic and therapeutic — do before antibiotics if possible
  • Synovial WBC >50,000/μL with >75% PMNs highly suggestive; >100,000/μL essentially diagnostic
  • Synovial fluid Gram stain (50–75% sensitive) + culture (gold standard, ~90% sensitive)
  • Blood cultures positive in ~50% — always obtain before antibiotics
  • ESR/CRP elevated but nonspecific; X-ray early: only soft-tissue swelling; MRI best for hip/deep joints

05

Management

  • Urgent surgical washout (arthrotomy or arthroscopy) + IV antibiotics — do NOT delay
  • Empiric IV antibiotics: vancomycin (covers MRSA) ± gram-negative coverage (ceftriaxone) based on risk
  • Gonococcal septic arthritis: IV ceftriaxone; may transition to oral after clinical improvement
  • Prosthetic joint infection: requires hardware removal in most cases for cure
  • Duration: 2–4 weeks IV/oral antibiotics total; do NOT use oral steroids acutely

06

Clinical pivots

How to separate this script from the look-alikes that show up on exams and on the wards.

  • Crystal Arthropathy (Gout/Pseudogout)

    Synovial fluid crystals on polarized microscopy; WBC rarely >100k; can coexist — always culture

  • Reactive Arthritis

    Follows GI/GU infection by 1–4 weeks; sterile joint fluid; no organisms on culture

  • Transient Synovitis (pediatric)

    Afebrile or low-grade fever; WBC <50k; Kocher criteria help; treat conservatively vs. OR for septic hip

  • Gonococcal Arthritis

    Migratory polyarthralgia + tenosynovitis + skin lesions; lower synovial WBC (~20–50k); cervical/urethral swab positive

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Educational use only. This illness script is a study framework, not medical advice. Confirm decisions with current guidelines and your clinical supervisors.