3D positioning system to popcorn the stones via a 120W holmium laser (P120, Lumenis). A setting of 0.5J x 80Hz was used for all experiments, and conducted for a total duration of 3 minutes done in 30 second intervals. Fiber to stone distance differences were studied with the fiber placed 0, 1-2 and 3-4 mm away from the stone surface; 1-2 mm was the default for all other experiments. Flow rates studied were 1L irrigation bag at 60 cm (7-8 mL/min), 100 cm (14-15mL/min), and 100 cm þ 150 mmHg pressure (38-40 mL/min); 14-15mL/min was the default for all other experiments. Custom designed glass models were used to assess calyceal sizes (11, 15 and 19 mm). A 365 mm fiber (Flexiva 365) was compared to a 200 mm fiber. The primary outcome was percentage of fragments <1 mm.RESULTS: Using the 200 mm laser fiber led to better submillimeter fragmentation results than when using the 365 mm fiber (68.5% vs 50.3%, p <0.05). Similarly, the 11 mm calyceal model and shorter fiber to stone distance (0 mm) had significantly better fragmentation outcomes (p <0.05) (Figure 1). A higher flow rate of 38-40 mL/min led to larger fragments when popcorning (p <0.05).CONCLUSIONS: A smaller laser fiber diameter, reduced stone to fiber distance, and smaller calyceal model sizes improved submillimeter fragmentation results when popcorning in an in vitro model. In clinical practice, displacing the stone into a smaller sized calyx, and avoiding high pressure flow rates may improve the efficiency of the technique and lead to a better dusting technique.