Flexible ureteroscopy (fURS) with Holmium: YAG laser (Ho:YAG) is one of the standard procedures for renal urolithiasis as Ho:YAG has shown to efficiently fragment urinary stones of any composition. Several techniques of laser lithotripsy have been described although new laser parameters have been added to the Ho:YAG in recent years and studies on laser fiber handling have provided new recommendations to improve performance. Regarding fURS, treatment outcomes are associated with technical limitations of fURS when working in acute angle calices and Ho:YAG fiber breakages can be associated with costly repairs. The techniques and laser parameters for treating such difficult cases have not been widely described. The purpose of this thesis is to evaluate the non contact “pop-corn” (PC) technique with a wide range of Ho:YAG settings and fiber sizes as to evaluate the risk factors for fiber fracture while curved in systematic in-vitro assessments. The evaluations for the PC were done with 4 artificial stones in a Vacutaner. A fix ureteroscope was inserted trough a ureteral access sheath allowing constant irrigation flow. The laser was placed at 1 mm from the bottom of the Vacutaner within the stones. The combination of 0.5-1.5 Joules, 10-20 and 40Hz and long and short pulse were tested for 2 and 4 minutes. 273 and 365 μm laser fibers were used. All tests were repeated 3 times. The stones were weighed before and after the experiments to evaluate the setting efficiency. Significant predictors for a highly efficient technique were assessed. A total of 144 experiments were conducted. Stone groups had a consistent weight: mean of 0.23g. After the experiment median weight difference was 0.7 gr (0.01-0.243). Choosing a cumulative percentage of 50% of the stones volume reduction as the cut-off level the significant predictors for high efficiency PC technique were the use of long pulse (OR = 2.7, 95% CI 1.05 – 7.15), longer time (4 min) (OR = 11.4, 95% CI 3.88 – 33.29), small (273μm) laser fiber (OR = 0.23, 95% CI 0.08 – 0.70), and higher power (watt) (OR = 1.14, 95% CI 1.09 – 1.20). We concluded that higher energy (>1.5 J), longer pulse, higher frequency of (20Hz),longer time, and small laser fiber are predictors for high efficient PC technique. As for the curved fiber evaluations, 272m and 365m fibers were employed. Five different fiber curve diameters were tested: 9, 12, 15, 18 and 20mm. Fragmentation and dusting settings were used at 7.5W. The laser was activated for 5 minutes. Each experiment was repeated 10 times. We found that with dusting settings, fibers broke more frequently at a curved diameter of 9mm f or both 272m and 365m fibers (p=0,037 and 0,006 respectively). Using fragmentation setting, fibers broke more frequently at 12mm for 272m and 15mm for 365m (p=0,007 and 0,033 respectively). Short-pulse and high energy were significant risk factors of fiber fracture for the 365m fibers (p=0,02), but not for the 272m fibers (p=0,35). Frequency was not a risk factor for fiber rupture. Fiber diameters also seemed to be involved in the failure with a higher number of broken fibers for the 365m fibers but no significant differences were found comparing to the 272m fibers (p>0,05). We concluded that small fibers are more resistant than large ones as lower bending diameters (<9mm) are required to break small fibers. In acute angles the use small-diameter fibers, at a low energy and long-pulse (dusting setting) may reduce the likelihood of fiber rupture.