Septic arthritis can present at any age but is most common in infants and teenagers. It is extremely important to exclude septic arthritis when confronted with a painful hip because failure to properly diagnose this entity can result in joint destruction. In the pediatric population, septic arthritis most commonly occurs from infectious extension from the adjacent metaphysis. Bacterial organisms (Staphylococcus aureus > Group A Streptococci) are the most common etiologic agents of septic arthritis. Most cases are monocular and involve large joints (hip > knee).

Radiographic findings of septic arthritis include asymmetric widening of the hip joint spaces by > 2mm (the distance is measured between the teardrop of the acetabulum and the medial cortex of the femoral metaphysis. Other findings include displacement or obliteration of the fat pads surrounding the hip (obturator internus, iliopsoas and gluteus). Unfortunately, these findings are not sensitive for the presence of a joint effusion. A normal appearing plain film by no means excludes the diagnosis of septic arthritis.

Toxic Synovitis is a diagnosis of exclusion. It occurs most commonly in children under the age of 10 who present with no limping and no pain on palpation. These patients have a positive joint effusion that is negative for organisms on aspiration and that resolves with rest. The effusion is likely due to a viral infection.