7. Knee arthrogram, lateral projection

An arthrogram is an X-ray study of a joint obtained after the injection of sterile, water-soluble iodine-containing liquid into the joint cavity. For a knee arthrogram air is injected as well. The radiopaque (and radiolucent) contrast provide(s) more anatomic information than does the plain film alone. In most cases these studies have been supplanted by MRI (see Image #8 in this unit), which can yield more information about the joint contents and surrounding structures without the risks associated with an arthrogram (infection, allergic reaction to the contrast agent, etc).

However, this image is still useful in illustrating radiologic anatomy. It demonstrates that the suprapatellar bursa communicates with the knee joint, since the bursa becomes distended by air and its surface is seen lined by radiopaque contrast after these substances are injected into the joint. Thus when the joint abnormally accumulates fluid (effusion) in some pathologic conditions, fluid will also collect in the suprapatellar bursa, distending it and increasing the radiographic density of this region on the plain lateral radiograph.