Ureteroscopy and laser lithotripsy is now the most common treatment modality in the United States for upper urinary tract stones. The sophistication of flexible ureteroscopy to treat renal and ureteral calculi has been possible because of advances in surgical laser technology. In particular, next-generation holmium:yttrium-aluminum-garnet (Ho:YAG) systems have been developed that provide the surgeon with a range of laser settings and parameters for stone fragmentation. The recent launch of the thulium fiber laser (TFL), which operates at a different wavelength to Ho:YAG, has renewed the enthusiasm for lasers.

Fiber-to-Stone Distance

Ho:YAG and TFL are most effective when the laser fiber is in contact with the stone. If the fiber tip-to-stone distance increases, it reduces fragmentation efficacy due to absorption of a portion of the laser energy by the intervening fluid resulting in vapor bubble formation.¹ Figure 1 demonstrates that with Ho:YAG when 1 J is applied with the fiber tip in contact with stone, the fragmentation volume is at its greatest. However, it is reduced by as much as 40% when the laser is activated with the fiber tip just 1 mm away from the stone surface. At 3 mm distance no fragmentation occurs. This effect is likely more pronounced for the TFL which has a wavelength closer to the absorption peak of water. Thulium laser energy is 4 times more absorbed in fluid than holmium laser energy.² The consequence of this is that at greater than 1 mm fiber-to-stone distances it may have no ablation effect on the stone, making it a truly contact laser.

Figure 1. Mean crater stone volume following single pulse of 1.0 J (short pulse mode) activated at different laser fiber-to-stone distances. Reproduced with permission.¹

Contact Laser Lithotripsy

Traditional fragmentation lithotripsy and modern day “dusting” are both considered contact techniques. With dusting, the focus has been on the production of fine fragments for spontaneous passage. The fiber tip is brushed along the stone’s surface in a painting, dancing or chipping motion.³ The surgeon constantly interrogates the stone, making sure not to stay in one area too long; otherwise fissures appear with large chunks breaking off. A typical setting for this would be 0.2–0.4 J and 30–70 Hz depending on the stone composition, size and location in the kidney.

Even with meticulous technique, laser fiber-to-stone contact is difficult to maintain. Using a light reflectance method in the lab to record laser fiber-to-stone distance, stone models in a simulated calyx were treated with Ho:YAG using a dusting technique at a setting of 0.3 J and 50 Hz.⁴ The percentage of pulses delivered when the laser fiber was >1 mm away from the stone was found to be 48%. Therefore, extrapolation of this finding to clinical practice suggests that a lot of pulses are fired when the laser and stone are too far apart to produce effective comminution. Pulse modulation which can split the pulse into 2 pulses increases stone ablation at 1 mm distance and is a helpful mode during ureteroscopy to improve fragmentation efficiency.¹

Noncontact Laser Lithotripsy

Despite the advantages of minimizing laser fiber-to-stone distance, there are specific scenarios where a noncontact approach is advantageous. No matter how skillful the surgeon when employing the dusting technique, the stone will eventually break into 2–4 mm sized fragments. Similarly with a fragmentation technique some material is too large for spontaneous passage, yet too small and numerous for targeted fragmentation. In both scenarios, the next step is to pulverize these fragments with noncontact laser lithotripsy, also known as “popcorning,” due to the chaotic and noisy movement of fragments. It is executed by activating the fiber tip a few millimeters away from the fragments. Using intermittent laser bursts, it results in a whirlpool-like effect that causes stone disintegration as the fragments move around and come in direct contact with the laser tip. Initially described by Chawla and colleagues, settings utilized were in the range of 1.0 J × 15–20 Hz.⁵

Figure 2. Optimal conditions for “Pop-dusting” non-contact laser lithotripsy.

With the advent of high-power, high-frequency lasers, additional laser settings became available to the urologist.⁶ In vitro studies were performed at the University of Michigan where model stones were treated in different-sized glass bulbs, simulating small and large calyces, at different pulse energy (0.5, 1.0 J) and frequency (20, 40, 80 Hz) settings while varying initial fiber tip-to-stone distance (0 and 2 mm).⁷ Improved submillimeter fragmentation outcomes occurred with higher pulse frequency and power settings, when performed in a smaller glass bulb, and with the laser fiber positioned closest to the stone surface. The term “pop-dusting” was introduced to differentiate these higher frequency techniques from the settings typically used for the popcorn technique. One must be mindful that high-power settings in the kidney can lead to heat generation and need to be mitigated with optimal irrigation conditions.⁸

Conclusions and Future Directions

Fragmentation efficiency is significantly affected by the laser fiber tip-to-stone working distance. The greatest volume of stone fragmentation occurs when the laser fiber is in contact with the stone and decreases with greater fiber tip-to-stone distance. When dusting a renal stone (low pulse energy/high frequency), we found that 48% of pulses are delivered in contact. Popcorning (high pulse energy/moderate frequency) is a noncontact technique used to pulverize the fragments for optimal stone passage. Pop-dusting (moderate pulse energy/high frequency) results in superior submillimeter fragmentation outcomes. The best results come from keeping the fiber tip as close to the stone as possible and performing it in a smaller space such as a calyx (vs renal pelvis, fig. 2).

Despite advanced Ho:YAG lasers that offer modulated pulses, there is still room for improvement. Future laser systems could have sensors on the laser fiber tip that gate the pulse, so they are only fired when in contact with the stone. Likewise, computer vision features on digital ureteroscopes could automatically identify the stone and its size,⁹ and then determine optimal fiber-to-stone distance and display it on the screen. Lastly, ureteroscopes with active suction can serve to pull the stone closer to the laser fiber tip, thereby improving the efficiency of contact laser lithotripsy even further.