Septic arthritis and osteomyelitis are the most common of the sinister causes of a limp in a child. In the hip it is often difficult to differentiate this from a transient synovitis but this is extremely important as catastrophic avascular necrosis of the femoral head can occur with sepsis unless surgical decompression is performed urgently.
The clinical appearances of both are similar. Clinical evaluation is most important and the child with the septic joint is usually recognizable to the experienced clinician. The child is usually unwilling to move around the bed and will hold the leg completely still. Kocher’s criteria suggest four parameters that can be used to predict the likelihood of septic arthritis of the hip (pyrexia, elevated ESR > 40mm/hr, increased WBCC > 12,000/mm3, inability to weight bear).
The number of positive parameters correlates with the probability of septic arthritis – Zero parameters: 0.2%; One parameter: 3.0%; Two parameters: 40.0 %; Three parameters: 93.1%; Four parameters 99.6% (4). CRP and ultrasound are now also very helpful but the presence or absence of an effusion is not diagnostic. If there is any suspicion, a paediatric orthopaedic consultation should be requested. Failure to treat septic arthritis effectively within 1 – 3 days leads to a poor outcome but it is best to institute treatment within hours where appropriate.
This involves surgical drainage of the joint and intravenous antibiotics for a prolonged period. Osteomyelitis is becoming less common but similarly should be considered. The white cell count, CRP and ESR are usually elevated and imaging with MRI or USS can display the infection. Radiographs are often normal in the early stage and again treatment with resuscitation and intravenous antibiotics should be undertaken.