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Introduction
  • Epidemiology
    • body 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
    • 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
  • 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 topic
  • Prognosis
    • delayed diagnosis can lead to profound, extensive cartilage damage within hours
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 with left shift
    • ESR >30
      • 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 >5
      • 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
        • prosthetic joint with WBC >1,100 is considered septic
      • gram stains only identifies infective organism 1/3 of time
      • glucose less than 60% of serum level
  • 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 
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
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
 

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