Under Direct Vision vs Under Non Direct Vision of Insertion of UAS in RIRS

Retrograde intrarenal surgery (RIRS) has been considered as the first-line choice for the management of \<20mm kidney stones. Insertion of a ureteral access sheath (UAS) before RIRS surgery is currently accepted as an effective method to improve the effectiveness of surgery, but can be accompanied by serious complications. In long-term clinical practice， the investigators has found that many ureteral injuries occur during UAS insertion of the ureter from ureteral orifice. And if we use rigid ureteroscopy to insert the UAS under direct vision, we can provide the placement success rate and reduce the incidence of complications. The investigator aims to perform a prospective and randomized controlled trial comparing the safety and efficacy of under direct vision and under non direct vision during the insertion of UAS.