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Updated: Nov 29 2022

Septic Arthritis - Adult

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  • summary
    • Septic Arthritis is the inflammation of the joints secondary to an infectious etiology, most commonly affecting the knee, hip, and shoulder. 
    • Diagnosis is made with an aspiration of joint fluid with a WBC count > 50,000 being considered diagnostic for septic arthritis. Lower counts may still indicate infection in the presence of positive gram stains or cultures results. 
    • Treatment is usually urgent surgical irrigation and debridement followed by culture directed IV antibiotics.
  • Epidemiology
    • Anatomic location
      • most commonly affected joints in descending order
        • knee (~ 50% of cases) >
        • hip >
        • shoulder >
        • elbow >
        • ankle >
        • sternoclavicular joint
          • found in IV drug users
          • pseudomonas aeruginosa was most common pathogen in 1980's
          • staphylococcus aureus is now the most common pathogen in all patients, including IV drug users
          • advanced imaging (CT/MRI) should be obtained preoperatively to rule out retrosternal abscess or chest wall phlegmon 
    • Risk factors
      • age > 80 years
      • medical conditions
        • diabetes
        • rheumatoid arthritis
        • cirrhosis
        • HIV
      • history of crystal arthropathy
      • endocarditis or recent bacteremia
      • IV drug user
      • recent joint surgery
  • Etiology
    • Pathophysiology
      • pathoanatomy
        • 3 etiologies of bacterial seeding of joint
          • bacteremia
          • direct inoculation
            • from trauma or surgery
          • contiguous spread
            • from adjacent osteomyelitis
      • cellular biology
        • septic arthritis causes irreversible cartilage destruction in an involved joint
          • cartilage injury can occur by 8 hours
        • caused by release of proteolytic enzymes from inflammatory cells (PMNs)
      • microbiology
        • most common pathogens is staphylococcus aureus (accounts for >50% of cases)
        • see Classification below
    • Associated conditions
      • prosthetic implant infection
  • Classification
    • By organism
      • staphylococcus species
        • staphylococcus aureus
          • most common and accounts for >50% of cases
        • MRSA
        • staphylococcus epidermis
      • neisseria gonorrhea
        • account for ~20% of cases
        • most common organism in otherwise healthy sexually active adolescents and young adults
        • manifests as a bacteremic infection
          • arthritis-dermatitis syndrome in ~60% of cases
          • localized septic arthritis in ~40% cases
      • gram-negative bacilli
        • account for 10-20% of cases
        • pathogens include
          • E coli, proteus
          • klebsiella
          • enterobacter
        • risk factors
          • neonates
          • IV drug users
          • elderly
          • immunocompromised patients with diabetes
      • streptococcus
        • streptococcus pyogenes (Group A)
          • most common
        • Group B streptococcus (e.g., agalactiae)
          • predilection for infants, elderly and diabetic patients
      • propionibacterium acnes
        • associated with shoulder surgery
      • salmonella or streptococcus pneumoniae
        • seen in patients with sickle cell disease
      • bartonella henselae
        • seen in patients with HIV
      • pseudomonas aeruginosa
        • seen in patients with history of IV drug abuse
      • pasteurella multocida
        • seen in patients after dog or cat bite
      • eikenella corrodens
        • seen in patients after human bite
      • fungal/candida
        • found in immunocompromised host
  • Presentation
    • Symptoms
      • pain in affected joint
      • fevers (only present in 60% of cases)
      • may appear toxic
    • Physical exam
      • inspection
        • erythema
        • effusion
        • extremity tends to be in position of maximum joint volume
          • hip would be in FABER position (flexed, abducted, externally rotated)
      • palpation
        • warmth
        • tender
      • motion
        • inability to bear weight
        • inability to tolerate PROM
  • Imaging
    • Radiographs
      • recommended views
        • AP and lateral of the joint in question
      • findings
        • may show joint space widening or effusion
        • periarticular osteopenia
    • Ultrasound
      • indications
        • may help in confirming joint effusion in large joint such as hip
          • can be used in guiding aspirations
    • MRI
      • indications
        • detects joint effusion, and may detect adjacent bone involvement such as osteomyelitis
  • Studies
    • Serum labs
      • WBC >10K cells/mL with left shift
      • ESR >30 mm/hr
        • ESR is often elevated but may be normal early in process
          • rises within 2 days of infection and can rise 3-5 days after initiation of appropriate antibiotics, and returns to normal 3-4 weeks
      • CRP >1 mg/dL
        • most helpful
        • best way to judge efficacy of treatment, as CRP rises within few hours of infection, and may normalize within 1 week of treatment
    • Joint fluid aspirate
      • gold standard for treatment and allows directed antibiotic treatment
      • should be analyzed for
        • cell count with differential
        • gram stain
        • culture
        • glucose level
        • crystal analysis
          • septic arthritis occurs concurrently with gout or pseudogout in < 5% of cases
      • characteristic findings
        • joint fluid appears cloudy or purulent
        • cell count with WBC > 50,000 is considered diagnostic for septic arthritis, however lower counts may still indicate infection
          • antibiotics administered within 24 hours of arthrocentesis can lower synovial WBC count and lead to false negative results 
        • gram stains only identifies infective organism 1/3 of time
        • glucose less than 60% of serum level
        • negative "string" sign
          • septic synovial fluid has low viscosity compared to normal synovial fluid (high viscosity)
    • Saline load test
      • utilized to determine if wound near a joint communicates with the joint
      • for the knee
        • 155 mL of saline is needed to reach 95% sensitivity
        • 175 mL of saline is needed to reach 99% sensitivity
  • Differential
    • Crystal arthropathy
      • gout
      • pseudogout
    • Cellulitis
    • Bursitis
      • prepatellar bursitis
  • Treatment
    • Operative
      • IV abx, operative irrigation and drainage of the joint
        • indications
          • considered an orthopaedic surgical emergency
        • IV antibiotic therapy
          • initiate empiric therapy prior to definitive cultures based on patient age and or risk factors
            • young, healthy adults
              • staphylococcus aureus and neisseria gonorrhea
            • immunocompromised patients
              • staphylococcus aureus and pseudomonas aeruginosa
          • transition to organism-specific antibiotic therapy based once obtain culture sensitivities
        • outcomes
          • treatment can be monitored by following serum WBC, ESR, and CRP levels during treatment
    • Nonoperative
      •  gonococcal septic arthritis can be treated with antibiotics and aspiration
        • typical antibiotic therapy includes ceftriaxone or fluoroquinolones
        • high resistance pattern to penicillin and tetracyclines
  • Technique
    • Operative irrigation and drainage of the joint
      • approach
        • can be performed open or arthroscopically (depending on joint)
      • irrigation
        • remove all purulent fluid and irrigate joint
      • debridement
        • synovectomy can be performed as needed
      • cultures
        • obtain joint fluid and tissue for culture
  • Complications
    • Arthritis
    • Fibrous ankylosis
    • Osteomyelitis
  • Prognosis
    • Delayed diagnosis can lead to profound, extensive cartilage damage within 8 hours
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