Flexible ureteroscopes are miniaturized instruments with a tiny working channel of 3.6 Fr. It is quite consensual to use small 200–270 μm fibers for many reasons. First, these fibers are the most flexible and are able to reach all parts of the collecting system including lower pole of the kidney. Furthermore, they allow ensuring adequate irrigation during the whole procedure. However, their small diameter permits transmitting only low power levels up to 15 W to the stone.

Before determining optimal laser settings, a clear understanding of the interaction between laser fiber and stone is required. There are three distinct effects that happen when firing on a stone: fragmentation, stone retropulsion, and fiber tip degradation [4, 5, 9]. It has been shown, that higher energy levels produce more ablation but also more retropulsion [3]. Retropulsion occurs mainly on smaller stones that have low kinetic inertia. Retropulsion significantly reduces the efficacy of the fragmentation process, because it increases the distance of the tip of the laser fiber and the stone at the time of subsequent shots. It has also been shown, that with the same energy settings (Joule) small fibers produce the same ablation volume as bigger ones, but the crater is deeper and narrower [3]. However, bigger fibers provoke more retropulsion. On the other hand, there is more fiber tip degradation on smaller fibers [4, 5, 7].

Considering these observations, four fragmentation strategies have been proposed during a flexible ureteroscopy [2]: