Ablation Efficiency of a Novel Thulium Fiber Laser: An In Vitro Study on Laser Setting and Fiber Usage

Abstract

Introduction:

To investigate the ablation efficiency of super-pulse thulium fiber laser (SPTFL) with different laser settings and fiber usage.

Materials and Methods:

SPTFL machine was attached with different fibers. Artificial stones were fixed in water, whereas laser fiber was driven on a platform for ablation. Pulse energy, frequency, fiber-moving speed, fiber-to-stone distance, and fiber size were adjusted in each trial. The cross-sectional area of craters on the lateral stone surface was measured for comparison of ablation rate, combined with fiber-moving speed.

Results:

There was a trend that the ablation rate increased as pulse energy or frequency increased. When pulse energy was set as 0.2 J and frequency was increased from 50 to 150 Hz, the cross-sectional area of the crater was enlarged from 0.21 to 0.37 mm² (p < 0.05); when the frequency was set as 100 Hz and pulse energy was increased from 0.1 to 0.3 J, the crater was enlarged from 0.10 to 0.45 mm² (p < 0.05). Furthermore, energy demonstrated greater impact on ablation rate and the crater was enlarged from 0.20 mm² in the 0.1 J × 300 Hz group to 0.44 mm² in the 0.3 J × 100 Hz group (p < 0.05). Then fiber was set at different moving speeds with the same laser setting; the ablation rate of 3 mm/second group was 3.64 times higher than 0.5 mm/second group (p < 0.05). Ablation diminished as fiber-to-stone distance grew. A 200 μm fiber produced thinner and deeper fissure than 272 and 550 μm fibers, and the ablation rate was the highest for the 200 μm fiber.

Conclusion:

Pulse energy is a more important factor in influencing ablation efficiency compared with frequency. Closer fiber-to-stone distance, faster fiber movement, and smaller fiber size increase ablation efficiency.

Introduction

Thulium fiber laser (TFL) is becoming a new player in the field of lithotripsy in urology.^1,2 A prominent advantage of fiber laser is the ability to transmit high-power energy through a small-sized fiber to increase energy density of irradiation.³ TFL belongs to the family of infrared laser as well as holmium:yttrium–aluminum–garnet (Ho: YAG) laser, which can be absorbed in water. TFL's wavelength of 1940 nm was closer to the peak of energy absorption than Ho: YAG laser in water, leading to a fourfold higher absorption coefficient.⁴

Two major mechanisms of laser lithotripsy are through photothermal and photomechanical effect.⁵ A number of studies suggest that TFL ablates stone predominantly by photothermal effect and microexplosive effect. Therefore, a higher absorption coefficient in water can be translated to promote greater ablation efficiency.^6
–8

Preclinical studies have shown TFL offers better performance on lithotripsy than Ho: YAG laser, particularly in the aspects of ablation speed, fragment size, and stone displacement.^9
–11 As super-pulse thulium fiber laser (SPTFL) is introduced into clinical practice in recent years, it has demonstrated efficacy and safety in the treatment of stone disease.^12,13 Optimal laser setting and fiber usage are still under investigation to achieve the full potential of SPTFL lithotripsy, and there is sparse data focusing on this area. In this study, we aim to investigate how laser setting and fiber usage affect ablation of SPTFL and provide evidence for clinical practice.

Materials and Methods

This study was approved by the Ethics Committee of Shanghai Changhai Hostpital. SPTFL machine (Raykeen Laser Technology Limited Corporation, Shanghai, China) with a peak power of 500 W was used in this study. A 550 μm core fiber was primarily attached, whereas 272 and 200 μm core fibers were also adopted in part of the experiment. Die-Stone (Heraeus Kulzer Dental Limited Corporation, Hanau, Germany) and water were blended (22 mL water/100 g powder) to make artificial stone cubes with 5 mm length of side. An electricity-driven 3D motorization platform (Shanghai Lianyi Instrument Factory of Optical Fiber and Laser, Shanghai, China) assembled with a fiber holder was utilized to move the fiber horizontally for stone ablation.

Stone cubes were fixed and immersed in plexiglass filled with saline. Fiber tip was adjusted to vertically contact the stone surface or close to it at designated distances. Before laser firing, fiber tip was placed beside the stone cube. Fiber was cleaved and checked before each trial and moved across the stone surface during ablation. In this way, regular fissure on the upward stone surface and representative crater profile on the lateral stone surface could be produced.

Pulse energy was set as 0.1, 0.2, and 0.3 J in various combinations with frequency of 50, 100, 150 Hz, and even 300 Hz. Generally, fiber-moving speed was set as 3 mm/second except for trials studying on different fiber-moving speeds, at which fissures ablated on the stone were clean and regular for each laser setting. Pulse energy, frequency, fiber-moving speed, fiber-to-stone distance, and fiber size were adjusted, respectively, to explore their influence on ablation.

Each stone was proceeded with three laser settings on the same surface, and each trial was repeated three times. Ablated stone samples were delivered to be photographed under optical microscope (1:10 magnification ratio). To study the morphology of crater profiles, the maximum depth, opening width, and cross-sectional area of each crater on the stone surface were then analyzed by ImageJ software package (National Institutes of Health, Bethesda). As ablation rate could be estimated by cross-sectional area multiplied by fiber-moving speed, it was substituted by cross-sectional area, except for trials when fiber-moving speed was changed. Student's t-test was conducted using SPSS v.21 (IBM Corporation, New York), and p < 0.05 was considered statistically significant. Figures in this article were made by GraphPad Prism 5 (GraphPad Software, Inc., San Diego).

Results

Frequency influence on ablation efficiency

Pulse energy was set as fixed 0.2 J, and frequency was increased from 50, 100, to 150 Hz. Highly regular fissures were formed on the stone. The maximum crater depth was about 0.41 mm in the 50 Hz group, and the crater got deeper as frequency became quicker. The craters represented by cross-sectional area enlarged as frequency was increased, from 0.21 mm² in 50 Hz group, 0.27 mm² in 100 Hz group, to 0.37 mm² in 150 Hz group (Fig. 1).

FIG. 1.

SPTFL lithotripsy with different frequencies in combination with a constant pulse energy. (A) Craters on the lateral stone surface, from left to right were of 0.2 J × 50 Hz, 0.2 J × 100 Hz, and 0.2 J × 150 Hz, respectively (the scale bar equals 1 mm). (B) The maximum crater depth and width-changing trend caused by frequency. (C) The cross-sectional area changing trend caused by frequency (*p < 0.05). SPTFL = super-pulse thulium fiber laser. Color images are available online.

Pulse energy influence on ablation efficiency

Frequency was set as fixed 100 Hz, and pulse energy was increased from 0.1, 0.2, to 0.3 J. The maximum crater depth was about 0.25 mm in the 0.1 J group, and the crater got much deeper as energy became higher, 0.55 mm in the 0.2 J group and 0.82 mm in the 0.3 J group. When pulse energy was increased to three times higher, the cross-sectional area became even more than three times larger, from 0.10 mm² in the 0.1 J group to 0.45 mm² in the 0.3 J group (Fig. 2).

FIG. 2.

SPTFL lithotripsy with different pulse energy in combination with a constant frequency. (A) Craters on the lateral stone surface, from left to right were of 0.1 J × 100 Hz, 0.2 J × 100 Hz, and 0.3 J × 100 Hz, respectively (the scale bar equals 1 mm). (B) The maximum crater depth and width-changing trend caused by pulse energy. (C) The cross-sectional area changing trend caused by pulse energy (*p < 0.05). Color images are available online.

Comparison of influence on ablation between pulse energy and frequency

Output power was set as fixed 30 W, with different pulse energy and frequency combinations. Crater depth was about 0.41 mm in the 0.1 J × 300 Hz group, and the crater got much deeper as energy became larger, to 0.72 mm in the 0.2 J × 150 Hz group and 0.81 mm in the 0.3 J × 100 Hz group. The crater was enlarged as pulse energy was increased, despite decreased frequency and constant output power setting, from 0.20 mm² in the 0.1 J × 300 Hz group, 0.39 mm² in the 0.2 J × 150 Hz group to 0.44 mm² in the 0.3 J × 100 Hz group (Fig. 3).

FIG. 3.

SPTFL lithotripsy with the same output power in different combinations of pulse energy and frequency. (A) Craters on the lateral stone surface, from left to right were of 0.1 J × 300 Hz, 0.2 J × 150 Hz, and 0.3 J × 100 Hz, respectively (the scale bar equals 1 mm). (B) The maximum crater and width-changing trend caused by different combinations of pulse energy and frequency. (C) The cross-sectional area changing trend caused by different combinations of pulse energy and frequency (*p < 0.05). Color images are available online.

Considering the same speed of fiber moving in each laser setting, all of the above results showed that although ablation efficiency could be enhanced by increasing pulse energy or frequency, pulse energy would cause a greater change on ablation efficiency than frequency given a constant output power.

Influence of fiber-moving speed on ablation

Fiber tip was set at different moving speeds with the same laser setting of 0.2 J × 100 Hz. The crater was diminished as fiber moved faster, as indicated by cross-sectional area, from 0.33 mm² in the 0.5 mm/second group, 0.26 mm² in the 1 mm/second group, to 0.20 mm² in the 3 mm/second group. However, considering different speeds of fibers moving, the ablation rate ( = cross-sectional area × fiber-moving speed) in the 3 mm/second group and 1 mm/second group was 3.64 times and 1.58 times higher than that in the 0.5 mm/second group, respectively. These results suggested that faster fiber movement on the stone led to increased ablation efficiency (Fig. 4).

FIG. 4.

SPTFL lithotripsy with different speed of fiber movement on condition of the laser setting. (A) Craters on the lateral stone surface, from left to right were of 0.5, 1, and 3 mm/second, respectively (the scale bar equals 1 mm). (B) The maximum crater depth and width-changing trend caused by fiber movement. (C) The cross-sectional area changing trend caused by fiber movement, whereas it should be noted that ablation rate was decided by cross-sectional area × fiber-moving speed (*p < 0.05). Color images are available online.

Influence of fiber-to-stone distance on ablation

Fiber-to-stone distance was set as 0, 0.2, 0.4, 0.6, 0.8, and 1.0 mm, respectively, in arithmetic progression. As the distance increased, craters were transformed from sharp and deep profiles to blunt and shallow ones. Figure 5B showed a negative linear relationship between crater depth and fiber-to-stone distance. The cross-sectional area appeared to be largely decreased when the distance reached 0.6 mm, and there was nearly no ablation when the distance reached 1.0 mm. These results suggested that ablation in a contact mode was the most efficient, and ablation efficiency decreased as fiber-to-stone distance widened (Fig. 5).

FIG. 5.

SPTFL lithotripsy with different fiber-to-stone distance on condition of the same laser setting. (A) Craters on the lateral stone surface, from left to right were of 0, 0.2, 0.4, 0.6, 0.8, and 1.0 mm, respectively (the scale bar equals 1 mm). (B) The maximum crater depth and width-changing trend caused by fiber-to-stone distance. (C) The cross-sectional area changing trend caused by fiber-to-stone distance (*p < 0.05, and some were omitted). Color images are available online.

Influence of fiber size on ablation

Fibers of different sizes were used with the same fiber-moving speed (3 mm/second) and laser setting (0.2 J × 100 Hz). Crater depth surpassed opening width for 272 and 200 μm fibers, whereas the crater produced by 550 μm fiber had a different crater profile. The cross-sectional area was slightly larger in the 200 μm fiber group (0.36 mm²) than the 272 μm fiber group (0.31 mm²) and the 550 μm fiber group (0.27 mm²). These results suggested that smaller fibers might be more efficient in stone ablation than larger fibers (Fig. 6).

FIG. 6.

SPTFL lithotripsy with different fiber sizes on condition of the same fiber-moving speed (3 mm/second) and laser setting (0.2 J × 100 Hz). (A) Craters on the lateral stone surface, from left to right were of 550, 272, and 200 μm fibers, respectively (the scale bar equals 1 mm). (B) The maximum crater depth and width-changing trend caused by fiber size. (C) The cross-sectional area changing trend caused by fiber size (*p < 0.05). Color images are available online.

Discussion

In this study, crater depth, opening width, and cross-sectional area were measured to analyze the morphology of stone crater produced by SPTFL. A previous study on Ho: YAG laser suggested that crater shape was affected by the laser beam profile. The crater was deepest under the fiber center because energy density was greatest along this line.¹⁴ Craters of SPTFL appeared to be bullethead-shaped in our experiment, probably because the spatial beam profile was Gaussian distribution, even more uniform, and symmetrical than Ho: YAG laser.⁴ Fissures produced during ablation was observed to be highly regular and consistent, hence ablation rate could be represented by cross-sectional area due to the constant fiber-moving speed in most settings, except for those on condition of different fiber-moving speeds.

TFL with high peak power or SPTFL improved lithotripsy efficiency vastly and made its debut in retrograde intrarenal surgery and percutaneous nephrolithotomy.^12,13 As peak power has been set as default, pulse energy and frequency are commonly adjusted parameters during the surgery. Our results showed both energy and frequency affected ablation efficiency. Specifically, pulse energy had a greater influence on ablation efficiency than frequency given a constant output power. In studies by Hardy and colleagues and Andreeva and colleagues,^9,10 SPTFL demonstrated higher ablation rate with increased energy or frequency, however, different combinations of energy and frequency under a constant output power was not discussed in their studies. Panthier and colleagues applied SPTFL in vitro ablation experiment using three different modes: “fine dusting” (0.15 J/100 Hz), “dusting” (0.5 J/30 Hz), and “fragmentation” (1 J/15 Hz). Three-dimensional scanning on fissures showed that “fragmentation” mode had faster ablation rates than “fine dusting” or “dusting,”¹¹ supporting our findings in this study.

Nonetheless, stone displacement caused by higher energy can be a concern in some circumstances, in contrast with stones fixed in this study. Eugenio and colleagues studied the effect of laser pulse shape on stone retropulsion, showing bigger retropulsion was induced by higher energy with lower frequency of SPTFL.¹⁵ In addition, the size of stone fragment could vary in different combinations with frequency and energy, which was beyond the scope of this study. As a result, optimal laser setting should be a process taking ablation rate, stone displacement, and fragment size into consideration at least. A one-size-fits-all solution of laser setting for SPTFL surgery may not be the optimal approach to the problem.

For Ho: YAG laser lithotripsy, “painting” on the stone to produce a “dusting” effect has been proposed as a useful technique by urologists.¹⁶ By setting different moving speeds of fiber, the 0.5 mm/second fiber was nearly motionless during lithotripsy, while the 3 mm/second fiber created a “painting” effect on stone. Our results demonstrated that faster “painting” might lead to more efficient ablation. This phenomenon could be attributed to bubble characteristics of TFL. Hardy and colleagues found pear-shaped bubbles were formed during TFL firing, and predicted effective working distance of TFL based on the maximum bubble length.¹⁷

Actually, TFL and Ho: YAG laser share a similar underlying mechanism of bubble formation, and the vapor tunnel for energy transmission is referred to as the Moses effect.¹⁸ Once the crater depth surpassed bubble length, no ablation would occur despite continuous laser transmitting from a motionless fiber tip. However, for a “painting” fiber, there was less chance for crater depth surpassing bubble length and thus ablation continued as long as fiber moved on stone surface. This explained why crater depth and cross-sectional area were larger for motionless fiber than moving fiber, whereas ablation rate of motionless fiber was lower. Another possible reason was that larger portion of laser energy was expensed on Moses effect to create energy tunnel for lithotripsy as crater depth increased, whereas “painting” on stone could minimize energy expense on Moses effect and maximize energy expense on ablation.

These underlying mechanisms are also responsible for findings inferred from the experiment with different fiber-to-stone distances. In contact ablation mode, it is the most efficient way for laser energy deposition on stone. In noncontact mode, Moses effect plays a key role. More energy is expensed on Moses effect rather than ablation as fiber-to-stone distance increases. When fiber-to-stone distance increases beyond the maximum bubble length, no energy can be transmitted to stone. Studies on Ho: YAG laser showed no ablation occurred when fiber-to-stone distance reached 3 mm, irrespective of pulse modes (short pulse, long pulse, and Moses mode).¹⁹ As the TFL bubble dimension was smaller than Ho: YAG, the effective working distance for TFL was expected to be smaller than Ho: YAG.¹⁷

According to our study, ablation stallout occurred when distance reached 1 mm, consistent with a previous study. Although SPTFL working distance was smaller than Ho: YAG, stone displacement in SPTFL lithotripsy was reported to be smaller than Ho: YAG.⁹ Therefore, it was more likely to keep fiber tip close to stone to increase ablation efficiency for SPTFL than Ho: YAG, which offset the shorter working distance.

Panthier and colleagues compared difference created by ablation rates between 150 and 272 μm fiber for SPTFL lithotripsy, and found that 150 μm fiber had lower ablation rates than 272 μm fiber in most laser settings. Additionally, thinner and deeper fissure for 150 μm fiber was also noticed.¹¹ Blackmon and colleagues found the ablation rates of the 50 μm fiber were comparable to 100 μm fiber, suggesting that a smaller fiber would not compromise lithotripsy efficiency.²⁰

It is noteworthy that both TFL and Ho: YAG laser ablate stone predominantly by photothermal effect. Lithotripsy efficiency for Ho: YAG increases when fiber diameter decrease in equivalent power setting, implying ablation efficiency is positively correlated with energy density.²¹ Our results showed that the 200 μm fiber produced thinner and deeper fissure than 550 μm fiber, and the ablation rate was higher for smaller fiber, which might be a benefit for ureteroscopy lithotripsy.²² Anyway, fiber sizes, laser settings, and stone composition were different in this study from previous studies.

There are some limitations in this study. First, we used cross-sectional area of fissures to represent ablation rate, however, this method was more cost effective when compared with 3D scanning. Furthermore, craters on the lateral stone surface provided direct visualization of the ablation by laser. Second, fiber condition might be changing due to burnback effect, although it could be argued that this closely mimics actual surgical condition than using a brand-new fiber for each trial. We chose the 550 μm fiber and made comparisons using data yielded from consecutive trials to minimize the effect of this error. Fine condition of all tested fibers was reflected by craters' shape resembling Gaussian distribution.¹⁴

Third, artificial stones cannot represent various kinds of stone compositions in reaction to SPTFL ablation. However, artificial stones bear homogeneity that human stones lack for reliable comparison. Crater characteristics were verified on human stone in a preliminary study to compensate the shortage (Supplementary Fig. S1). Fourth, laser parameters were restrained to “dusting” rather than “fragmenting” mode of SPTFL. Our preliminary study found “fragmenting” mode made quantitative comparison difficult, which was the reason why “dusting” mode was used. This study attempts to identify the optimal settings that maximize lithotripsy efficiency. Further study is needed to test if our conclusion can be applied for all laser settings.

Footnotes

Author Contributions

X.G. contributed to study design, supervision, and revision of article; M.L. contributed to study design, experiment accomplishment, and composition of article; Y.P. and Z.W. contributed to results analysis and revision of article; L.L., S.M., and Z.F., contributed to proof-reading and revision of the article; figure editing was accomplished by H.D. and R.L.

Acknowledgments

Chun Ki Chan revised the language of article. Hui Zuo provided technique support.

Author Disclosure Statement

SPTFL products used in this study were provided by Raykeen Laser Technology Limited Corporation.

Funding Information

This work was supported by the National Natural Science Foundation of China (81900644; 81800624).

Supplementary Material

Supplementary Figure S1

Abbreviations Used

References

Kronenberg

, Traxer

. The laser of the future: Reality and expectations about the new thulium fiber laser—A systematic review. Transl Androl Urol, 2019; 8(S4):S398–S417.

Fried

. Thulium fiber laser lithotripsy: An in vitro analysis of stone fragmentation using a modulated 110-watt Thulium fiber laser at 1.94 microm. Lasers Surg Med, 2005; 37:53–58.

Fried

, Irby

. Advances in laser technology and fibre-optic delivery systems in lithotripsy. Nat Rev Urol, 2018; 15:563–573.

Fried

. Recent advances in infrared laser lithotripsy [Invited]. Biomed Opt Express, 2018; 9:4552–4568.

Chan

, Pfefer

, Teichman

, Welch

. A perspective on laser lithotripsy: The fragmentation processes. J Endourol, 2001; 15:257–273.

Taratkin

, Laukhtina

, Singla

, et al. How lasers ablate stones: In vitro study of laser lithotripsy (Ho:YAG and Tm-fiber lasers) in different environments. J Endourol, 2020 [Online ahead of print]; DOI: 10.1089/end.2019.0441.

Keller

, De Coninck

, Doizi

, Daudon

, Traxer

. Thulium fiber laser: Ready to dust all urinary stone composition types?. World J Urol, 2020; DOI: 10.1007/s00345-020-03217-9.

Chan

, Kang

, Irby

, Hardy

, Fried

. Scanning electron microscopy of real and artificial kidney stones before and after Thulium fiber laser ablation in air and water. Ther Diagn Urol, 2018; DOI: 10.1117/12.2285069.

Andreeva

, Vinarov

, Yaroslavsky

, et al. Preclinical comparison of superpulse thulium fiber laser and a holmium:YAG laser for lithotripsy. World J Urol, 2019; 38:497–503.

10.

Hardy

, Vinnichenko

, Fried

. High power holmium:YAG versus thulium fiber laser treatment of kidney stones in dusting mode: Ablation rate and fragment size studies. Lasers Surg Med, 2019; 51:522–530.

11.

Panthier

, Doizi

, Lapouge

, et al. Comparison of the ablation rates, fissures and fragments produced with 150μm and 272μm laser fibers with superpulsed thulium fiber laser: An in vitro study. World J Urol, 2020 [Online ahead of print]; DOI: 10.1007/s00345-020-03186-z.

12.

Enikeev

, Taratkin

, Klimov

, et al. Thulium-fiber laser for lithotripsy: First clinical experience in percutaneous nephrolithotomy. World J Urol, 2020; 38:3069–3074.

13.

Enikeev

, Taratkin

, Klimov

, et al. Superpulsed thulium fiber laser for stone dusting: In search of a perfect ablation regimen-a prospective single-center study. J Endourol, 2020; 34:1175–1179.

14.

Lee

, Ryan

, Teichman

, et al. Effect of lithotripsy on holmium:YAG optical beam profile. J Endourol, 2003; 17:63–67.

15.

Ventimiglia

, Doizi

, Kovalenko

, Andreeva

, Traxer

. Effect of temporal pulse shape on urinary stone phantom retropulsion rate and ablation efficiency using holmium:YAG and super-pulse thulium fibre lasers. BJU Int, 2020; 126:159–167.

16.

Giusti

, Proietti

, Villa

, et al. Current standard technique for modern flexible ureteroscopy: Tips and tricks. Eur Urol, 2016; 70:188–194.

17.

Hardy

, Kennedy

, Wilson

, Irby

, Fried

. Analysis of thulium fiber laser induced bubble dynamics for ablation of kidney stones. J Biophotonics, 2017; 10:1240–1249.

18.

Ventimiglia

, Traxer

. What is Moses effect: A historical perspective. J Endourol, 2019; 33:353–357.

19.

Aldoukhi

, Roberts

, Hall

, Ghani

. Watch your distance: The role of laser fiber working distance on fragmentation when altering pulse width or modulation. J Endourol, 2019; 33:120–126.

20.

Blackmon

, Hutchens

, Hardy

, Wilson

, Irby

, Fried

. Thulium fiber laser ablation of kidney stones using a 50-μm-core silica optical fiber. Opt Eng, 2014; 54:011004.

21.

Vassar

, Teichman

, Glickman

. Holmium:YAG lithotripsy efficiency varies with energy density. J Urol, 1998; 160:471–476.

22.

Scott

, Cilip

, Fried

. Thulium fiber laser ablation of urinary stones through small-core optical fibers. IEEE J Sel Top Quantum Electron, 2009; 15:435–440.

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