Septic arthritis
Background
- Bacterial infection of a joint space — a true orthopedic emergency
- Rapid cartilage destruction occurs within hours if untreated[1]
- Staphylococcus aureus is the most common pathogen in adults (~50%)
- Neisseria gonorrhoeae is the most common cause in sexually active young adults
- Knee is the most commonly affected joint (~50%)
- Mortality: 5-15% overall; higher in elderly and prosthetic joints
Risk Factors
- Pre-existing joint disease (rheumatoid arthritis, osteoarthritis)
- Prosthetic joint
- Recent joint surgery or injection
- IV drug use
- Immunosuppression (diabetes, HIV, steroids)
- Skin infection or bacteremia
- Advanced age
Clinical Features
- Acute monoarticular joint pain, swelling, warmth, erythema
- Pain with both active and passive range of motion (distinguishes from periarticular pathology)
- Effusion
- Fever (present in ~60%, absence does not exclude)
- In gonococcal arthritis: migratory polyarthralgias, tenosynovitis, dermatitis (pustular skin lesions), may involve multiple joints
- Prosthetic joint infection: may have subtle presentation with chronic pain and loosening
Differential Diagnosis
- Gout / Pseudogout (crystal arthropathy)
- Reactive arthritis
- Rheumatoid arthritis flare
- Hemarthrosis
- Lyme disease (Lyme arthritis)
- Viral arthritis
- Osteomyelitis with joint extension
- Periarticular abscess or bursitis
Evaluation
- Arthrocentesis — must be performed in any suspected septic joint[2]
- Send for: cell count with differential, Gram stain, culture, crystal analysis
- WBC >50,000/mm³ with >90% PMNs strongly suggests infection
- WBC >100,000/mm³ is virtually diagnostic
- Lower counts do not exclude — partially treated or early infection may have lower counts
- Gram stain positive in ~50% of non-gonococcal cases
- Labs: CBC, ESR, CRP, blood cultures (positive in ~50%), lactate
- If gonococcal suspected: GC/CT NAAT (urine, cervix/urethra, pharynx, rectum)
- Imaging:
- X-ray: evaluate for effusion, osteomyelitis, fracture
- Ultrasound: guide arthrocentesis, confirm effusion
- MRI if concerned for adjacent osteomyelitis
Management
- Empiric IV antibiotics after arthrocentesis (do NOT delay if aspiration will be delayed):
- Vancomycin 15-20 mg/kg IV (covers MRSA) PLUS
- Ceftriaxone 2 g IV (covers gram-negatives and gonococcus)
- If prosthetic joint: add Vancomycin + Cefepime or Meropenem
- Orthopedic consultation for:
- Joint washout/irrigation (arthroscopic or open)
- Prosthetic joint infections require urgent surgical intervention
- Gonococcal arthritis: Ceftriaxone 1 g IV/IM daily + treat for chlamydia
- Pain management: IV analgesics, joint immobilization, ice
Disposition
- Admit all confirmed or suspected septic arthritis
- Orthopedic surgery consultation for joint washout
- Gonococcal arthritis: may be managed with close outpatient follow-up after initial IV antibiotics in select cases