In the last few decades, the treatment of urinary stones has witnessed tremendous advances with the development of several innovations in this ever-changing field. The evolution in laser technology has made retrograde intrarenal surgery (RIRS) with laser lithotripsy an attractive minimally invasive option. Holmium:yttrium-aluminum-garnet (Ho:YAG) laser was first described more than 20 years ago, and it has become the gold standard for endoscopic laser lithotripsy.¹ However, a new laser technology: the thulium fiber laser (TFL), has gained increased attention owing to its properties in both preclinical and clinical settings.^2–4 The most recent literature shows multiple advantages of TFL over Ho:YAG—does this mean we should abandon Ho:YAG altogether?

Physical Characteristics

Owing to the complex physical characteristics of the Ho:YAG laser, a large water-cooling system is necessary, resulting in a bulky, heavy, and noisy machine that requires specific high-power outlets that need specialized electrical installation in operating rooms.⁴ The TFL console has a highly efficient design that is lighter, smaller, less energy-consuming, and uses a standard electrical outlet available in any operating room.

Wavelength

The emitted laser beam of the TFL has a wavelength of 1,940 nm that is 4 to 5 times more absorbed by water than Ho:YAG. This may explain the higher ablation efficiency of TFL when applied to all stone types in bench testing.⁵ With this wavelength, the TFL is also more absorbed in hemoglobin, meaning better hemostasis. Because of this, it may have advantages in ablating upper tract urothelial tumors.

Energy and Frequency

“Dusting” is an attractive lithotripsy technique to create tiny stone particles <1 mm. TFL produces pulses with a lower peak power of 500 W (maximum) compared to >2,000 W for Ho:YAG, and is also capable of generating very small pulse energy (minimum 0.025 J) at longer pulse widths compared to Ho:YAG. The TFL can fire at a high frequency of over 2,000 Hz compared to 120 Hz of most Ho:YAG systems. All these characteristics lend themselves more to a dusting technique using TFL compared to Ho:YAG.⁶ TFL has shown a four- to fivefold higher dusting rate compared to Ho:YAG and produced twice as much dust compared to the newer pulse modulation technologies (Moses^TM, Lumenis).⁷

Dusting vs Fragmentation

Stone retropulsion threshold is up to 4 times higher with the TFL compared to Ho:YAG at equal pulse energies because of the lower and prolonged peak power with a longer pulse duration of the TFL.⁸ However, retropulsion is not always a bad thing. In some challenging lithotripsy cases, retropulsion is often desirable. For instance, for stones in a calyceal diverticulum, impacted in the ureter, or a sharply angled calyx, retropulsion may help to dislodge the stones to facilitate access. Consequently, a complete absence of retropulsion may not be a favorable feature in every lithotripsy procedure. We have been using Ho:YAG in these challenging situations in conjunction with TFL. We also use Ho:YAG when we want to fragment the stone into pieces more amenable for basket removal. One of the disadvantages to TFL is its inability to fragment into pieces larger than dust (eg for basket removal). Along with the “dust storm” that comes with TFL lasering is decreased visualization, which may lead to potential workarounds including increasing the irrigation pressure and its potential consequences. It remains to be seen if the stone-free rate is better with TFL given that this dust may take time to clear and, depending on when the postoperative imaging is done, may continue to show dust fragments remaining. Furthermore, ultrasound imaging may amplify these sizes. The clinical outcomes of patients undergoing TFL lithotripsy still need to be elucidated, while the results of Ho:YAG are well described over its 30 years of clinical use.

Smaller Fibers

Given the uniform and focused laser beam generated by TFL, smaller fiber lasers (150 μm) can be used safely compared to Ho:YAG generators that accept only fibers with a core diameter of ≥200 μm.⁷ The smaller fibers may lead to improved irrigation flow, better instrument deflection, more flexibility, and less burnback when compared to Ho:YAG laser fibers.⁸ As a result, TFL offers opportunities for the miniaturization of endoscopic instruments.

Safety

Regarding the safety of TFL technology, several in vitro studies and prospective patient cohorts have shown that the TFL is safe to use.⁹ However, a recent controversial issue that has been heatedly discussed is the local temperature rise caused by TFL. Nevertheless, many published papers have shown a similar temperature change when similar energy and frequency settings were used by both TFL and Ho:YAG.¹⁰ The current recommendations are to keep the power around 10 W in the ureter and not higher than necessary for stone ablation when operating in the kidney. Maintaining constant irrigation flow with cooled irrigation fluid, use of a ureteral access sheath, and pausing intermittently between laser bursts are thought to negate any temperature rises from any laser.

Conclusion

In our opinion, despite the evidence that TFL may be a superior “duster” compared to Ho:YAG, there is little evidence to say that one laser is superior to another. There is a lack of clinical studies comparing the 2 modalities in real-world situations to suggest one laser over another. The lower peak power and longer pulse width make TFL an ideal “dusting” laser. The higher peak power and retropulsion of Ho:YAG still has a role during ureteroscopic lithotripsy—particularly for those surgeons performing fragmentation and basket removal. Randomized clinical trials may not tell us more than we already know. In reality, there is a need and utility for both lasers in clinical practice depending on the clinical situation and surgical technique.