Fig. 1

A transverse ultrasound section of the mid supra-patellar pouch area of the right knee during the very early (a) and later (b) phase of injection, showing a positive air arthrosonogram. There is an emerging area of intra-articular air bubbles (arrows), initially faintly (a) and then more clearly visible (b), with posterior acoustic shadowing (*). Varying transducer pressure demonstrated the echogenic layer to be displaceable and display buoyancy, consistent with intra-articular placement of the injection. SPP denotes suprapatellar pouch