In holmium laser lithotripsy of kidney and urinary stones, it has long been necessary to make direct contact between the laser fiber and the stone. However, contact is not always possible. Physicians may have difficulty reaching a stone that’s hiding in the lower pole or a difficult-to-reach calyx, beyond the scope’s capability to bend. Even when contact with the stone is achieved, some pulses of the laser will occur at a distance because of respiratory interference that causes kidney movement.
The latest generation of holmium lasers employs new parameters for varying pulse width and pulse modulation. These parameters allow the laser to ablate stones from a distance when contact is not possible or is not continuous because of movement.
With the advent of Moses technology for holmium lasers, with “Contact” and “Distance” pulse modulation modes (optimized for 0-1 mm and 1-2 mm fiber-to-stone distances, respectively), we began to wonder what ablation from distance looks like, compared to ablation at contact. We also questioned how efficient ablation was at various distances (for example, 1mm distance versus 2mm). In a study we presented this year at the World Congress of Endourology,1 we evaluated stone crater volume in a flat-plate stone model (BegoStone, Bego) using four modes of laser modulation at distances from 0mm to 3mm. The results led to some ideas for optimizing the procedure, as well as for future research.
Comparing Four Pulse Modulation Modes
In this study, we fired a single 1.0J pulse at the stone model in four modes (short pulse, long pulse, Moses Contact, and Moses Distance) using a 120W Ho:YAG laser (Moses P120, Lumenis) and a 230 μm core fiber (Moses 200 D/F/L, Lumenis). Consistent fiber-to-stone distances (0, 0.5, 1.0, 2.0, and 3 mm) were achieved with a 3D positioner. Using a freshly stripped and cleaved fiber each time, we performed five ablations for each mode and distance.
Measuring crater volume by reflectance microscopy, we found that volume for each mode and distance decreased as distance increased. For example, Moses Contact mode created the largest crater (0.125mm3) of all modes at contact, but volume dropped to 0.09mm3 at 0.5mm and 0.045mm3 at 1.0mm. As predicted, Moses Distance mode did ablate a greater volume than all other modes at distances of 1mm and 2mm (0.06mm3 and 0.0275mm3, respectively).
Figure 1: Mean stone crater volume produced when firing a single pulse at 1.0J at different laser fiber to stone distances utilizing short pulse (SP), long pulse (LP), Moses contact (MC), and Moses distance (MD) modes.
Using These Modes to Optimize Procedures
These results aligned with our expectations – when assessing single-pulse outcomes, Moses Contact mode did best at contact, and Moses Distance mode did best at distance. Still, the results shed some light on how physicians might get the greatest efficiency from the holmium laser during the lithotripsy procedure. Physicians can switch between Moses Contact and Distance modes for optimal results, but if that is impractical, Moses Distance mode’s efficacy at contact (0.115mm3), combined with its superior performance at distance, may make it a good choice for continuous use when treating renal stones. Certainly, with the stone repeatedly moving in relation to the laser fiber, distance mode guarantees maximum efficiency. However, one has to be cautious when using Distance mode to treat ureteral stones in order to avoid inadvertent ureteral injury from distance. More research is needed to define safe parameters.
In the future, we would like to delve deeper into how these holmium laser pulse modulation modes, used at different distances, affect stone dusting. It is possible that while the contact is the most efficient way to ablate stones, Moses Distance mode, used at specific distances, may produce better quality fragmentation for dusting. Additional study will help us determine how these modes can best support complete, efficient dusting.
Written by: Ali H. Aldoukhi, a Postdoctoral Research Fellow in Endourology at the University of Michigan, Ann Arbor, and Khurshid Ghani, MBChB, MS, FRCS, a Urologist and Associate Professor of Urology.