Comment on: Majdalany SE,Levin BA,Ghani KR. The Efficiency of Moses Technology Holmium Laser for Treating Renal Stones During Flexible Ureteroscopy: Relationship Between Stone Volume,Time,and Energy. J Endourol 2021 Dec;35(S3):S14–S21.

Dear Editor,

In their clinical study, Majdalany et al. ¹ analyzed the efficiency of intracorporeal lithotripsy using Holmium:YAG (Ho:YAG) laser with Moses technology (MT), using a reporting methodology proposed by our group in 2020. ² The authors found that Ho:YAG with MT achieved a fragmentation speed of 0.9 mm³/s using 38.2 J/mm³ of stone volume, and 47% laser activity. Stone volume was correlated to total lasing time. The authors should be commended for reporting their results using this methodology, which allows comparison with existing data. Our reply has the intent to clarify and highlight some key points.

This study does not involve MT lithotripsy as a stand-alone procedure, because of the use of high-frequency and high-power (HP) settings. This means that it is difficult to determine whether the results presented are influenced either by MT or by HP settings themselves. Comparison with previously available series sheds some light in this regard. A recent meta-analysis of low power (LP) and HP Ho:YAG lithotripsy ³ showed that HP lasers could only reduce operative time by 50% despite a theoretical fivefold increase in repetition rate, and this reduction was even less relevant when differences in stone volume were taking into account.

Similarly, the results of Majdalany and colleagues show that despite a slightly higher lithotripsy speed (0.9 vs 0.7 mm³/s), HP lasers with MT laser doubled J/mm³ values (38 J/mm³ vs 19 J/mm³) and reduced laser activity (47% vs 84%) compared with standard LP laser lithotripsy. ² What does this mean for daily clinical practice? First, it means that HP lasers require twice as much energy as LP devices when ablating two identical stones, which means that this additional amount of energy is absorbed by the water and increases the temperature of the surrounding medium. ^2,4

To mitigate the temperature increase, it is possible to fire the laser intermittently, which explains both the decrease in laser activity and the increase in operative time, thus reducing the benefits of high frequency. ³ Another explanation for the decrease in laser activity is the loss of visibility because of the high frequencies. Another point to consider is the role of stone density. We have already shown that the stone density is not linearly related to the J/mm³ values, since they increase at density values >1000 HU. This means that soft stone has little impact, whereas hard stones decrease the lithotripsy efficiency, as demonstrated in previous studies. ^4,5

In conclusion, we agree with the authors that J/mm³, mm³/s, and laser activity must be reported to describe the efficiency of laser lithotripsy, and we urge researchers to adopt such a methodology to promote the debate regarding this topic.