Snapshot A 22-year-old man presents to his primary physician's office for worsening knee pain. He reports that his symptoms began approximately 1 week ago and has worsened to the point of not being able to bend the knee. He states to being sexually active with multiple partners and inconsistently using condoms. On physical exam, the left knee is erythematous, swollen, and tender to palpation. There is decreased range of motion on the affected joint. Laboratory testing demonstrates an elevated erythrocyte sedimentation rate and C-reactive protein. An athrocentesis is obtained and demonstrates a white blood cell count of 55,000 cells/μL and gram-negative organisms. Empiric antibiotics are administered and orthopedic surgery is consulted. Introduction Clinical definition infection of a joint leading to arthritis that can be caused by bacteria fungi mycobacteria Epidemiology risk factors increasing age diabetes mellitus rheumatoid arthritis prosthetic joints intravenous drug abuse skin infection alcohol use sexual activity Etiology Staphylococcus aureus most common cause streptococci second most common cause of nongonococcal arthritis Neisseria gonorrhoeae suspect in healthy and sexually active young patient Pseudomonas aeruginosa typically seen in healthcare-associated infections Borrelia burgdorferi typically seen in patients in endemic areas Mycobacterium tuberculosis Pathogenesis infectious agent can be spread to the joint via the blood (hematogenously) direct inoculation contiguously from the adjacent skin (e.g., cellulitis and cutaneous ulcers) once the infectious agent arrives to the synovial membrane an inflammatory response ensues Prognosis dependent on a number of comorbid conditions (e.g., age and immunosuppresion) ranges 10-15% Presentation Symptoms fevers may be present joint pain Physical exam in most cases 1 joint is affected a joint that is warm erythematous swollen restricted to movement Imaging Radiography indication can be used to get a baseline assessment of joint damage Studies Labs ↑ erythrocyte sedimentation rate and C-reactive protein Arthrocentesis the best initial test for the evaluation for septic arthritis Synovial fluid culture the most accurate test and must be performed before antibiotic administration findings fluid is usually purulent Gram stain may be positive 30-50% sensitive white blood cell count > 50,000 cells/μL Differential Gout Pseudogout Inflammatory arthritis (e.g., rheumatoid arthritis) Viral arthritis (e.g., hepatitis B and C and parvovirus B19) Treatment Medical intravenous empiric antibiotics indication treatment of choice after athrocentesis is performed to target the most likely organism antibiotics then becomes tailored to the specific organism when susceptibility results return drugs vancomycin used when Gram stain of synovial fluid shows gram-positive cocci third-generation cephalosporin used when Gram stain of synovial fluid shows gram-negative bacilli Operative joint drainage indication used along with antibiotics to treat septic arthritis septic arthritis represents a closed abscess modalities needle aspiration arthroscopic drainage arthrotomy Complication Osteomyelitis Sepsis