Illustration of the course of a needle and catheter during arthrocentesis. An axial radiograph of a right shoulder in supine position is shown. The abducted position in this radiograph is used for better illustration only. During the aspirations in this study the shoulder was adducted. The green area represents the fluid in the joint, the orange line represents a rigid steel needle and the yellow line represents a flexible catheter which is advanced into the posterior recess.