marked widening of the medial joint space is noted in the right hip as compared to the left hip (arrows)
The main finding here is marked widening of the "teardrop" distance of the right hip when compared to that of the left hip. This distance is defined as the distance between the medial margin of the ossification center of the femoral head and the medial subchondral margin of the articular "teardrop" of the acetabulum (see the drawing below). The "teardrop" is formed by the medial acetabular wall and the quadrilateral surface, as seen on an AP radiograph. The teardrop distance approximates the medial joint space of the hip.
In a normal child this space should be fairly symmetrical between the right and left side. Indeed, if these measurements differ by as much as 1 mm, one should consider it to be abnormal. The differential diagnosis of such widening includes:
While some of the entities listed above are not terribly alarming, a septic joint is a true surgical emergency. Even with rapid diagnosis and intensive medical and surgical treatment, a bacterial infection of a joint can completely destroy the articular cartilage of that joint in as little as 24 hours. Therefore, when one sees a symptomatic patient with an asymmetric teardrop space, one should treat it as if it were a septic joint until proven otherwise, because the consequences of missing a septic hip are so devastating.
How should one then proceed in the workup of such a patient? The main thing is that they should undergo an aspiration arthrogram as soon as possible. It is critical to quickly prove either that there is an infection or that infection is unlikely. In my opinion, one should always inject a small amount of radiographic contrast into the joint just to prove that one has indeed reached the joint space. This is especially important if one cannot aspirate fluid from the joint. In this event, one should inject some nonbacteriostatic saline or contrast to lavage the joint and then aspirate that back from the joint and send it to the lab for cultures and Gram stain. The most common organism grown will be staphylococcus aureus.
As a resident, I was taught that one should operate at a very high index of suspicion for septic arthritis, because of its potentially devastating consequences. I was taught that if one had to tap 9 negative joints for every positive joint aspirate found, one was operating at an appropriate index of suspicion. How has this worked out in practice? Well, in my career as a musculoskeletal radiologist, I have been involved with 4 cases of a widened teardrop distance. They all underwent an aspiration arthrogram and 3 of them grew staphylococcus aureus from the aspirate. Your mileage may vary, but my limited experience has so far made a believer out of me.
One interesting phenomenon that I have found in these cases is a general hesitation on the part of orthopedists and other clinicians to tap the joint of a child, particularly an infant. This has evolved several times into the unlikely scenario of an aggressive radiologist pushing a reluctant surgeon into a procedure (the aspiration arthrogram). The problem seems to be that, other than pediatric radiologists, not that many people get an opportunity to tap tiny little hips, and they feel nervous about it. This is, of course, no excuse. Since these little patients can usually give us little or no history, they usually need an aspiration arthrogram even more than an adult does. The prosecution rests.