Numerous risk factors exist for septic arthritis (Table 12.2), with skin infection being the most significant. Following arthroscopic surgery, septic arthritis is relatively uncommon with a reported incidence of 0.14 % [5], though infection following anterior cruciate ligament reconstruction is more common with 0.14–2.6 % of procedures affected [6]. The risk of joint infection following intra-articular corticosteroid injection is low with a reported incidence of 0.04 % [2]. Systemic conditions that are associated with the condition include diabetes mellitus, chronic renal failure, malignancy and alcohol abuse. Diabetic patients are at increased risk of septic arthritis due to immune system suppression, propensity to skin infections and ulceration, and the potential requirement for haemodialysis [7]. In a study from the Netherlands, increased age, particularly over 80 years, was associated with an increased risk of septic arthritis [5]. Conditions that lead to joint injury, in particular rheumatoid arthritis and to a lesser extent osteoarthritis, also confer an increased risk of joint infection [2]. The use of anti-tumour necrosis factor alpha (TNFα) therapies is reported to double the risk of septic arthritis in patients with rheumatoid disease [8]. Intravenous drug abusers are at greater risk than the general population of developing polymicrobial septic arthritis, potentially with atypical pathogens including fungi.

The aetiological agents for septic arthritis vary with age (Table 12.3), but in all age and risk groups, the principal causative organism is Staphylococcus aureus followed by other Gram-positive organisms, particularly streptococci [2]. S. aureus has a strong predilection for joints due to the presence of multiple microbial surface components that facilitate binding to joint extracellular matrix. Some strains of S. aureus (the Panton-Valentine Leukocidin complex) are producers of cytotoxins and can survive within neutrophils; these can be responsible for severe joint infections in otherwise healthy individuals [9]. The finding of methicillin resistant strains of S. aureus (MRSA) causing septic arthritis has been a particularly worrying trend over the past decade [2].