Introduction. For rational planning of laser lithotripsy tactics, determination of pulse settings and laser modes, considering patient’s condition, stone size and composition as well as for more rationale planning of operation theatre loading and the use of high-technology equipment the knowledge of the time required for complete stone fragmentation is needed. It is still assessed subjectively base on the urologist’s own experience.
Aim. To determine the specific value of urinary stone’s mass decrease in order to evaluate exact time for complete fragmentation during holmium laser lithotripsy.
Materials and methods. Fragmentation time values were derived from in vitro studies in which lithotripsy using holmium laser was compared to other lithotriptors’ types, as well as our own results of time and energy level measurement during urinary stones fragmentation in vivo were used.
Results. As baseline data for estimating of the urinary stones destruction using holmium laser lithotripsy we suggest to use not stone volume but its mass which combine in some extent the volume and density. To take into account energy level for stone fragmentation it is suggested to introduce new value, the specific decrease of the stone mass per unit of laser energy (mg/J). It is established that such a value is a constant for a given laser type and fiber size. This value is calculated using stone dimensions, its density in HU and the total laser energy delivered during complete fragmentation. Based on experimental data it is estimated that specific decrease of the stone mass per unit of holmium laser energy during fragmentation in vitro of BegoStone is 0.442±0,083 mg/J (p=0.05). This value, measured in real conditions in vivo during urinary stones lithotripsy using holmium laser Triple («Medoptotech») with 600 μm fiber is 0.401±0.12 mg/J (р=0.05).
Conclusion. The use of specific decrease of the stone mass per unit of the laser energy during lithotripsy allows to assess complete fragmentation time of the urinary stones, based on their dimensions and density in HU. The absolute error in estimating of stone fragmentation time does not exceed several minutes, which is quite acceptable for calculating the total time of using high-technologic equipment and operative time.

Keywords: urinary stones, lithotripsy, holmium laser, specific decrease of stone mass, fragmentation rate