Characteristics

• Acute infection of a joint – usually monoarthritis, but may be at multiple sites.

• Usually bacterial; Staphylococcus being the most common infective group of organisms.

• Affects all ages (more common in young/old).

• Watch for immunodeficiency, e.g. steroids/chemotherapy, diabetes, HIV.

• Lower extremity (75%); over pressure points in diabetic foot.

• May originate from osteomyelitis or result in osteomyelitis.

Clinical features

• History of antecedent illness, recent surgery including dental work.

• Systemic illness – patients usually febrile and unwell.

• Hot, swollen, painful joint.

• Usually severely restricted movement and commonly held in a position as to reduce the pressure within the joint, e.g. in a septic knee, the patients hold the knee in approximately 10°–15° flexion to relax the posterior capsule.

• Raised WBC (neutrophils), CRP and ESR. May have positive blood cultures.

Radiological features

• Initial radiographs often normal.

• Soft-tissue swelling secondary to local hyperaemia and oedema.

• Joint distension ± subluxation of hip and humerus in children.

• Joint-space narrowing indicates rapid development of destruction of articular cartilage.

• Small erosions in articular cortex, loss of cortical outline and reactive bone sclerosis seen after about 8 to 10 days.

• Implants: lucency around the implant, subsidence and bone loss.

Management

• Patient resuscitation if shocked.

• Careful aseptic aspiration of the joint – send for urgent Gram stain, culture and sensitivity, and crystals.

• If patient unwell, start empirical antibiotics (now specimen taken), but if patient systemically stable await microscopy.

• If positive for organisms, then the joint is likely to need debridement (open or arthroscopic), but repeated aspiration and prolonged antibiotics have been used as a successful treatment of septic arthritis by rheumatologists for some time.

• With arthroplasty, debridement is unlikely to succeed unless the infection is ‘early’ – between 2 and 6 weeks after implantation. Otherwise a formal (one- or) two-stage revision will be needed.

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