Native joint infections

Infections of native joints generally occur in patients with predisposing factors such as trauma, underlying arthritis, immunosuppressive therapy, diabetes mellitus, malignancies, intravenous drug abuse, and other infections (e.g., endocarditis, skin infections, and urinary tract infections). Hematogenous spread of the organism through the highly vascular synovial space leads to an influx of polymorphonuclear leukocytes (PMLs) into the synovium and then to a release of enzymes that destroy the articular surface.

Diagnosis

Patients present with pain and limited motion of the joint. Fever may be mild, with only a few patients having a temperature higher than 39°C (102.2°F). Joint tenderness can be minimal to severe, but most patients have swelling as a result of joint effusions in response to the infection. Involvement of multiple joints is seen in 10% to 20% of cases, especially in viral arthritis and rheumatoid arthritis. Laboratory findings suggestive of septic arthritis include an elevated erythrocyte sedimentation rate and synovial fluid cell counts exceeding 50 000/mL, with more than 75% PMLs. In no individual case do any of these findings distinguish infected from inflammatory arthritis, such as rheumatoid or crystalline arthropathy, so the diagnosis is based on cultures of synovial fluid. On occasion, blood cultures may be positive. In patients with a chronic monoarticular process caused by mycobacterial or fungal organisms, synovial tissue cultures provide a better yield than synovial fluid cultures. Serum antibody tests provide the diagnosis of Lyme or viral arthritis. Polymerase chain reaction (PCR) assay of the joint fluid may yield the diagnosis in partially treated patients or in patients' infections caused by fastidious organisms such as Mycoplasma, Chlamydia, or Borrelia burgdorferi (Lyme disease). Plain radiographs are seldom of use diagnostically. Computed tomography (CT) and magnetic resonance imaging (MRI) provide more detail of the surrounding soft tissue and may reveal adjacent osteomyelitis. Radionuclear scans may be needed to visualize the sacroiliac joint; however, they are unable to distinguish septic arthritis from other inflammatory arthritis.