For bacterial infection, Pseudomonas aeruginosa has been found to infect joints, for example in children who have sustained a puncture wound. This bacteria also causes endocarditis.^[1]

In newborns, the most common pathogen is Group B Streptococci. In children older than 2 or 3, about 50% are due to Staphylococcus aureus. Haemophilus influenzae type B used to be a major cause before the use of the vaccine. In sexually active individuals, a major cause is Neisseria gonorrhoeae.

Indications

Septic arthritis should be suspected when one joint (monoarthritis) is affected and the patient is febrile. In seeding arthritis, several joints can be affected simultaneously; this is especially the case when the infection is caused by staphylococcus or gonococcus bacteria.

Diagnosis is by aspiration (giving a turbid, non-viscous fluid), Gram stain and culture of fluid from the joint, as well as tell-tale signs in laboratory testing (such as a highly elevated neutrophils (approx. 90%), ESR or CRP).

Treatment

Therapy is usually with intravenous antibiotics, analgesia and washout/aspiration of the joint to dryness.

Radiologic Findings

Traditionally, the diagnosis of septic arthritis was based on clinical assessment and prompt arthrocentesis. However, the clinical picture may be obscured by multiple confounding factors and a paucity of specific findings especially for the deep joints, ie. the hip or shoulder. Imaging can be used to confirm the diagnosis of septic arthritis and more importantly, imaging findings suggestive of septic arthritis can direct the clinician to a diagnosis that may not have been considered.

Plain film findings of septic arthritis include: joint effusion, soft tissue swelling, periarticular osteoporosis, loss of joint space, marginal and central erosions and bone ankylosis. CT is more sensitive than plain films for the detection of early bone destruction and effusion.

The role of MRI in the diagnosis of septic arthritis has been increasing in recent years in an effort to detect this entity earlier. Findings are usually evident within 24 hours following the onset of infection and include: synovial enhancement, perisynovial edema and joint effusion. Signal abnormalities in the bone marrow can indicate a concomitant osteomyelitis. The sensitivity and specificity of MRI for the detection of septic arthritis has been reported to be 100% and 77% respectively.