Teaching Case

A six-year-old boy presented with a painful hip. Inflammatory markers were raised.

A six-year-old boy first presented in July 1997 with pain in the right hip and limping. He was treated with arthrotomy and lavage of the hip for septic arthritis. The organism grown was a coagulase negative Staphylococcus. He developed a Brodie’s abscess, and was put on long term antibiotics, and regular follow-up. He subsequently developed secondary osteoarthritis (OA) of right hip.

The importance of acute septic arthritis was first recognised by Smith in 1874.the mortality is not as high as in the pre-antibiotic era. However, the sequelae are seen to this day. The single most important factor in determining outcome is the delay in instituting treatment. Most authors agree that neonates and infants are more likely to have a poor outcome. Other factors that are of importance include infecting organism, associated osteomyelitis and adequacy of treatment. Acute septic arthritis is an orthopaedic emergency and requires immediate recognition and effective treatment. This will prevent growth anomalies and limitation of motion. It is sometimes difficult to establish diagnosis especially in new-borns. Often the only symptom is a child not using a limb. The symptoms can be mistaken for rheumatoid or other arthritides. The laboratory data may remain normal, and the initial radiographs difficult to interpret or unhelpful. WBC and temperature are normal in one-third cases. Hence, puncture with aspiration and examination of the joint fluid (microscopy, culture and WBC) is mandatory whenever infection is suspected. The causative organism in septic arthritis is usually Staph. aureus and H influenzae. The initial x-rays do not always predict outcome. A normal radiograph does not give any idea of the capital epiphysis, growth plate or the triradiate cartilage. Translucent zones do not always mean that growth plate or capital epiphysis is destroyed. Widening of the joint space is a warning sign of increased joint pressure. Ignore at your peril! MRI is helpful in documenting complications such as fistulas, abscesses, or osteitis. MRI is also useful when the diagnosis is uncertain on other investigations. Inflamed synovium enhances after gadolinium. When the margin of the synovium is irregular and the adjacent soft tissue also enhances, infective arthritis should be considered. Associated osteomyelitis is seen as an area of low T1W and high T2W signal within the bone marrow and the subchondral bone. The inflamed tissue shows a high STIR signal. Cortical destruction may not be easily appreciable on MRI. Mr will however show the marrow oedema when the results of CT and scintigraphy are equivocal. However false positive MR diagnosis may be made in case there is non-specific bone marrow oedema. Infected and non-infected synovial effusion is indistinguishable on MR imaging. Initial antibiotic therapy must be affective against penicillin-resistant gram-positive cocci. The duration of therapy is empirical.