the most difficult portion of the procedure is creating the tract between the kidney and the flank skin. most of the time this is achieved by advancing a needle from the flank skin into the kidney, known as the 'antegrade' technique. a 'retrograde' technique has recently been updated wherein a thin wire is passed from inside the kidney to outside the flank with the aid of a flexible ureteroscope. this technique may reduce radiation exposure for patient and surgeon.