a retrograde pyelogram is done to locate the stone in the kidney. with a small 1 centimeter incision in the loin, the percutaneous nephrolithotomy (pcn) needle is passed into the pelvis of the kidney. the position of the needle is confirmed by fluoroscopy. a guide wire is passed through the needle into the pelvis. the needle is then withdrawn with the guide wire still inside the pelvis. over the guide wire the dilators are passed and a working sheath is introduced.