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 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