Reprinted from www.antimicrobe.org

Septic Arthritis - Causes

               Septic arthritis is a medical and surgical emergency that can lead to rapid destruction of the joint and irreversible loss of function. It is caused by invasion of the microorganisms into the synovial space. Bacteria are the most common cause of joint of infection although other pathogens such as viruses, fungi and mycobacteria may also be responsible.

               Bacteria enter the joint through several mechanisms including direct inoculation, penetrating trauma, surgical incision including arthroscopy, spread from a contiguous focus of soft tissue infection or hematogenous seeding from a distant focus. Hematogenous seeding is the most common route. Negative prognostic factors include older age, delay in onset of treatment, pre-existing arthritis, immunosuppressive therapy, resistant organisms and multiple joint involvements. Septic arthritis results in loss of function in 25% to 50% of patients despite appropriate therapy. Table 1 lists the microbiology of bacterial arthritis in relationship to patient’s age.

Table 1  Microbiology of Bacterial Septic Arthritis Related to Age of Patient.

Organism	Children (6 mo. to 5 yrs.)	Young adult	Adult	Elderly
Staphylococcus aureus	10% - 20%	15% - 20%	60% - 70%	45% - 65%
Streptococci	5% - 10%	1% - 5%	15% - 20%	10% - 15%
Gram-negative bacteria	1% - 5%	Rare	10% - 15%	15% - 35%
Haemophilus influenzae	30% - 50%	1% - 5%	1% - 5%	Rare
Neisseria gonorrhoeae	1% - 5%	60% - 80%	1% - 5%	Rare

Adapted from Gorbach SL et al (Eds), Infectious Diseases, 2^nd Edition, 1998

RISK FACTORS

               Degenerative joint disease, rheumatoid arthritis, and corticosteroid therapy are the most common predisposing conditions. Total joint arthroplasties are susceptible to intraoperative or hematogenous seeding which can result in subsequent prosthetic infection. While patients infected with HIV demonstrate a higher prevalence of septic arthritis or osteomyelitis than does the general population this may be due to intravenous drug abuse and multiple transfusions in this patient population.

NON GONOCOCCAL BACTERIAL SEPTIC ARTHRITIS

               In adults non gonococcal acute septic arthritis occurs with an incidence of 0.034% to 0.13%. One to two percent of patients with total joint arthroplasty may develop septic complications. 90% of cases are mono-articular with knee being involved in 50% of these cases. Staphylococcus aureus and streptococci remain the predominant organism although certain other unusual organisms may be seen in specific clinical settings (Table 2). In addition, certain microorganisms may be associated with reactive arthritis (Table 3). Several unusual organisms such as Kingella spp., A. actinomycetemcomitans, M. catarrhalis and P. multocida have been implicated in osteoarticular infections.

Table 2.   Septic Arthritis Complicating a Clinical Setting

Table 3.  Microorganisms Associated with Reactive Arthritis

PATHOPHYSIOLOGY

               Bacterial invasion of the highly vascular synovium results in bacterial trapping and multiplication in the subsynovium. The host responds with acute inflammatory reaction and phagocytosis by the polymorphonuclear leucocytes. Toxins and enzymes are released by bacterial and stimulated T-cells leading to destruction of the articular cartilage. Revascularization, synovial proliferation and granulation tissue develop. Synovial fluid white blood cell count can exceed over 50,000/mm3 and increased intracavitary of pressure from accumulation of the purulent fluid resulting in necrosis of the synovium and cartilage. Pro-inflammatory cytokines such as interluken-1 and tumor necrosis factor α also contribute to joint injury.