OBJECTIVES Laser lithotripsy, often used during ureteronephroscopy (URNS), requires the Ho:YAG optical fiber transmit energy via total internal reflection (TIR). In critical lower pole deflections, energy may refract into the cladding causing fiber failure and scope damage. New optical fiber technology aims to have increased tolerance for high degrees of flexion. We compared two brands of laser fibers with sub-300 micron cores (Sureflex, Boston Flexiva) to determine failure rates and scope repair costs. METHODS A retrospective cohort study comparing these two fibers for patients at a single academic institution who underwent flexible URNS with laser lithotripsy was performed from September 2013 to October 2015. Preoperative imaging was evaluated for stone burden and location. Intraoperative variables were collected, including energy use, lower pole lasering, laser fiber malfunction, and scope damage. The primary outcome was scope damage caused by laser fiber malfunction. Secondary outcome was scope repair costs. Fisher's exact test and two tailed t-tests were used. RESULTS Of 223 subjects, 143 met inclusion criteria, and 8 had laser fiber failure. All failures occurred with the Sureflex fiber (8 of 63, 13%) vs the Boston Flexiva fiber (0 of 80, 0%) (P
