In vitro investigation of stone ablation efficiency, char formation, spark generation, and damage mechanism produced by thulium fiber laser

Initial experience of thulium fiber laser in retrograde intrarenal surgery for ureteral and renal stones in Japan: surgical outcomes and safety assessment compared with holmium: yttrium-aluminum-garnet with MOSES technology

BACKGROUND

Thulium fiber laser (TFL) has been used for the treatment of ureteral and renal stones in Japan since October 2023. However, there are no reports on the initial results of TFL in Japan. This study aimed to assess the initial results of TFL in retrograde intrarenal surgery (RIRS) and to compare them with those of holmium: yttrium-aluminum-garnet (Ho: YAG) laser with MOSES technology.

METHODS

This retrospective single-center study compared perioperative results, complications, and the "stone-free" rate (defined as ≤ 2 mm fragments on computed tomography 1 month after RIRS) between the TFL (60 W) (group A, n = 48) and Ho: YAG laser with MOSES technology (120 W) (group B, n = 48) laser. The inclusion criteria were renal or ureteral stones ≤ 20 mm in diameter and those scheduled for single-stage RIRS.

RESULTS

The two groups had similar baseline patient characteristics. No significant differences were found in operative time (45 vs. 54 min, P = 0.10), laser time (15 vs. 10 min, P = 0.12), stone-free rate (97.9% vs. 95.8%, P = 1.00), ureteral injury (2.1% vs. 8.3%, P = 0.36), or postoperative fever (0% vs. 4.2%, P = 0.49) between groups A and B. However, significant differences were found in basketing time (7 vs. 21 min, P < 0.01) between groups A and B.

CONCLUSIONS

Our study showed that RIRS with TFL had similar results and no difference in complications compared to RIRS with Ho: YAG laser with MOSES technology. The TFL had a significantly shorter basketing time than the Ho: YAG laser with MOSES technology. Furthermore, future research is needed to determine suitable laser settings for the TFL.

Characterizing stone ablation with the thulium fiber laser: a single-pulse comparison of different pulse durations

Stone ablation using the thulium fiber laser (TFL) at different pulse duration/peak power or laser fiber-to-stone distances requires further research to understand effective and safe settings. In this study we characterized the vapor bubble anatomy and stone crater morphology at various settings and standoff distances. Optical profiles of 1 J short pulse (SP), 1 J long pulse (LP), 3 J SP, and 2.4 J LP of a TFL system (Fiberdust, Quanta) were assessed. We used high-speed imaging and segmentation to quantify the vapor bubble. Using Begostones, stone craters from a single pulse at each setting were compared at 0, 0.5, 1, 2 and 3 mm standoff distances. The temporal optical profile of TFL is rectangular in shape with a peak power of ~ 180W (LP) and ~ 470W (SP). Increasing the pulse energy did not increase the peak power. LP generated a channel-like bubble while SP formed multiple generations of a spherical bubble. Ablation volume with SP was greater than LP for all distances (p < 0.001) with up to 350% increased crater volume at contact. Ablation reduced as distance increased with both modes. There was no ablation at > 1 mm distance with LP. For SP, increasing the pulse energy had minimal impact on crater depth. The SP creates a bubble geometry that tends to collapse more quickly in comparison to LP. Peak power of TFL is a function of pulse duration, not pulse energy. LP results in weak ablation. When using SP, increasing the pulse energy increased ablation volume by increasing crater area rather than depth.

Simulation of the temperature distribution of kidney stones induced by thulium fiber laser and Ho: YAG laser lithotripsy

Simulation studies on temperature distribution in laser ablation help predict ablation rates, laser settings, and thermal damage. Despite the limited number of reported numerical studies on the temperature distribution of kidney fluid, there is no simulation study for kidney stone temperature distribution. We employ a numerical approach to study the kidney stone temperature distribution and predict ablation rates, which is an important parameter for clinical lithotripsy. The study looked at how the thulium fiber laser and the Ho:YAG laser differ in terms of temperature profile and ablation depth of kidney stones like calcium oxide monohydrate. The ablation depth increased from 152.7 µm to 489.7 µm when the TFL laser (operated at 10 Hz repetition rate and 1 ms pulse width) fluence increased from 764 J/cm2 to 1146 J/cm2. Correspondingly, the depth increased from 21 µm to 68 µm for the Ho: YAG laser operated at 3 Hz and 0.22 ms pulse width. We attribute this to an increase in temperature with laser energy. We further investigated the effect of pulse width on ablation depth by considering three different TFL pulse widths: 0.5 ms, 0.75 ms, and 1 ms. There was a decrease in ablation depths from 402.5 µm to 242.6 µm when the pulse width increased from 0.5 ms to 1 ms. Because of lower water absorption coefficients, the Ho:YAG laser (70 mJ/10 Hz) produced a smaller ablation depth and temperature profile than the thulium fiber laser (70 mJ/10 Hz). Experimental results from the literature validated the simulation. We found that the Ho:YAG laser worked better for ablation when it was set to 0.2 J/100 Hz for the Ho:YAG laser and 0.4 J/50 Hz for the TFL laser, which were clinical laser settings that we found in the literature. This indicates that, in addition to laser absorption by water, the laser parameters also significantly influence temperature distribution and ablation.

Exploring optimal settings for safe and effective thulium fibre laser lithotripsy in a kidney model

OBJECTIVES

To explore the optimal laser settings and treatment strategies for thulium fibre laser (TFL) lithotripsy, namely, those with the highest treatment efficiency, lowest thermal injury risk, and shortest procedure time.

MATERIALS AND METHODS

An in vitro kidney model was used to assess the efficacy of TFL lithotripsy in the upper calyx. Stone ablation experiments were performed on BegoStone phantoms at different combinations of pulse energy (EP ) and frequency (F) to determine the optimal settings. Temperature changes and thermal injury risks were monitored using embedded thermocouples. Experiments were also performed on calcium oxalate monohydrate (COM) stones to validate the optimal settings.

RESULTS

High EP /low F settings demonstrated superior treatment efficiency compared to low EP /high F settings using the same power. Specifically, 0.8 J/12 Hz was the optimal setting, resulting in a twofold increase in treatment efficiency, a 39% reduction in energy expenditure per unit of ablated stone mass, a 35% reduction in residual fragments, and a 36% reduction in total procedure time compared to the 0.2 J/50 Hz setting for COM stones. Thermal injury risk assessment indicated that 10 W power settings with high EP /low F combinations remained below the threshold for tissue injury, while higher power settings (>10 W) consistently exceeded the safety threshold.

CONCLUSIONS

Our findings suggest that high EP /low F settings, such as 0.8 J/12 Hz, are optimal for TFL lithotripsy in the treatment of COM stones. These settings demonstrated significantly improved treatment efficiency with reduced residual fragments compared to conventional settings while keeping the thermal dose below the injury threshold. This study highlights the importance of using the high EP /low F combination with low power settings, which maximizes treatment efficiency and minimizes potential thermal injury. Further studies are warranted to determine the optimal settings for TFL for treating kidney stones with different compositions.