Paediatric Imaging

Septic arthritis

bacterial joint infection. The most common pathogen is Staphylococcus aureus but Haemophilus influenzae and Group B streptococcus are also relatively common in toddlers. Joint damage results from enzymes produced by the bacteria and leukocytes. Secondary vascular damage may occur from thrombosis or direct compression of vessels.

The clinical presentation is usually acute with rapid onset of joint pain progressing to a febrile systemic illness. On examination the joint is swollen and tender, the range of movement is very limited and painful, and white blood count (WBC), C-reactive protein (CRP) and erythrocyte sedimentation rate (ESR) are elevated. The exception is the neonate in whom clinical presentation may be subtle, the neonate typically presents with loss of spontaneous movement of an extremity or posturing of the joint at rest. Typically neonates do not appear systemically unwell, fever is often absent and ESR normal. There may be a preceding history of mild trauma to the extremity or simultaneous illness or infection for example septic arthritis may follow chicken-pox. Dislocation of the hip joint may occur, secondary to the effusion and infection, so-called Tom Smith's arthritis (Fig.1) (Fig.2).

Conventional radiographs performed early in the course of the disease are often unremarkable except for soft tissue swelling. Widening of the joint space may develop with time. With time radiographs may demonstrate osteopenia due to disuse or local hyperaemia and may show a joint effusion. Bone scintigraphy allows earlier diagnosis showing focal, abnormal increased uptake around the joint. Ultrasound shows the joint effusion. Blood cultures may be helpful but joint aspiration confirms the diagnosis. The joint fluid will be cloudy with leukocytosis and polymorphocytosis. Culture may confirm the pathogen but cultures can be negative in 20 - 30%. Treatment is with effective drainage and parenteral antibiotics.

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Fig.1

Radiograph (a) and CT scan (b) in a 2-week-old infant. The right hip is dislocated and there is swelling of the buttock, with faint calcification within it. There is a metaphyseal defect in the proximal femur. CT scan shows the extensive abscess around the hip joint, with calcified debris within it.
Septic arthritis, Fig.1 (a)
Septic arthritis, Fig.1 (b)
Septic arthritis, Fig.2