Continuous-wave and quasi-continuous wave thulium-doped all-fiber laser: implementation on kidney stone fragmentations

Evaluating the learning curve for flexible ureteroscopic thulium fiber laser lithotripsy using CUSUM analysis

OBJECTIVES

Effective training methods and learning curve (LC) assessment are crucial for more difficult endoscopic procedures. The present study sought to assess the LC of flexible ureteroscopes (fURS) for thulium fiber laser (TFL) lithotripsy and determine the effectiveness of using a porcine kidney model in training modality, to inform future training.

METHODS

Postgraduate medical students without experience in ureteroscopy were recruited, and surgical data were collected from 10 consecutive flexible ureteroscopic lithotripsy (fURL) procedures performed on our homemade porcine kidney training model. Cumulative sum (CUSUM) analysis and pooled mean CUSUM curves were applied to determine the LC turning points (TPs), and pre-LC and post-LC data were compared with that of an experienced attending physician.

RESULTS

Data from 110 surgeries were included in the analysis. The mean number of surgical units to overcome the LC for the duration of fURS TFL lithotripsy was 4. The operation time, number of tissue damage, and number of central visual shifts were significantly lower in students after the TP than before. No statistically significant difference in operation time was found between the students after the LC (817.50 [703.75-964.75]) and the surgeon group (732.50 [51.00-822.25]).

CONCLUSION

Overcoming the LC of fURS TFL lithotripsy requires four surgeries, and the developed ex vivo porcine kidney is a conveniently accessible and effective clinical training model.

What to expect from the novel pulsed thulium:YAG laser? A systematic review of endourological applications

INTRODUCTION

Several preclinical studies about a novel pulsed-thulium:yttrium-aluminum-garnet (p-Tm:YAG) device have been published, demonstrating its possible clinical relevance.

METHODS

We systematically reviewed the reality and expectations for this new p-Tm:YAG technology. A PubMed, Scopus and Embase search were performed. All relevant studies and data identified in the bibliographic search were selected, categorized, and summarized.

RESULTS

Tm:YAG is a solid state diode-pumped laser that emits at a wavelength of 2013 nm, in the infrared spectrum. Despite being close to the Ho:YAG emission wavelength (2120 nm), Tm:YAG is much closer to the water absorption peak and has higher absorption coefficient in liquid water. At present, there very few evaluations of the commercially available p-Tm:YAG devices. There is a lack of information on how the technical aspects, functionality and pulse mechanism can be maximized for clinical utility. Available preclinical studies suggest that p-Tm:YAG laser may potentially increase the ablated stone weight as compared to Ho:YAG under specific condition and similar laser parameters, showing lower retropulsion as well. Regarding laser safety, a preclinical study observed similar absolute temperature and cumulative equivalent minutes at 43° C as compared to Ho:YAG. Finally, laser-associated soft-tissue damage was assessed at histological level, showing similar extent of alterations due to coagulation and necrosis when compared with the other clinically relevant lasers.

CONCLUSIONS

The p-Tm:YAG appears to be a potential alternative to the Ho:YAG and TFL according to these preliminary laboratory data. Due to its novelty, further studies are needed to broaden our understanding of its functioning and clinical applicability.

Optimizing the performance of a monolithic Tm3+, Ho3+-codoped fiber laser by FBG reflected wavelength and fiber gain matching

We present the optimization of a 2.1-µm continuous wave monolithic single-oscillator laser by adapting the Fiber Bragg Grating (FBG) reflected wavelength to the maximum gain wavelength of the Tm³⁺, Ho³⁺-codoped fiber. Our study examines the power and spectral evolution of the all-fiber laser and demonstrates that matching these two parameters improves the overall performance of the source.

High-peak-power pump-modulated quasi-CW fiber laser

Quasi-continuous wave (CW) laser output with high peak power and high energy is preferred in some industrial applications. Due to the non-linear effects and transverse mode instability, such high-peak-power laser output is difficult to achieve via monolithic fiber lasers in CW mode. For diode-pumped monolithic fiber lasers, by applying overshoot pulse modulation to the pumping diodes, we obtain a pulse laser output with a peak power much higher than that in CW mode. In this paper, it has been theoretically studied that stable pulses with the width of µs level can be generated with quasi-CW operation without distortion according to our simulation. We also experimentally demonstrate a bi-directional pumped quasi-CW monolithic fiber laser operating in CW and pulse modes. In quasi-CW mode, by applying overshoot pulse modulation to the diodes, with a frequency of 1 kHz and a pulse width of 100 µs, the peak power of the output laser reached 9713 W with an average power of 898 W and M² of 2.4 and 2.3 in the two orthogonal directions, respectively. To the best of our knowledge, this is the very first quasi-CW fiber laser of a 10-kW level with the M² level of 2.

Thulium fiber laser in urology: physics made simple

PURPOSE OF REVIEW

In this narrative review, we will focus on a novel thulium fiber laser's physical properties in terms of its clinical applicability.

RECENT FINDINGS

TFL has successfully moved forward from the preclinical trials into clinical practice and now is being widely used in clinics around the world. The available data suggest that the device effectively operates in soft tissues - benign prostate hyperplasia (BPH) and bladder tumors, as well as in lithotripsy. Also, the first promising results were obtained from laparoscopic surgery showing its possible applicability in the management of renal cell carcinoma. The constructional changes in fiber laser's design, lead to alteration of laser-tissue interactions, which resulted in clinical advantages of the device. Yet, the exact mechanism often is considered complex for understanding. With this work, we are aiming to build a bridge between biophysics and clinical practice and give a simple explanation of how the devices is working and why the knowledge of it is important for a clinician.

SUMMARY

The more effective wavelength (closer to the water absorption peak), favorable beam profile, different modes of action allowing to decrease carbonization on one hand and retropulsion on the other, all this makes TFL an evolution in urologic surgery. Further trials investigating on the possible pros and cons of the device are awaited.

New Lasers for Stone Treatment

Laser technology has been a breakthrough in urology. The new era in endocorporeal laser lithotripsy has recently begun in mid-2020, where promising technologies tested in vitro have reached their approval for clinical use and, in that way, have made it possible to confirm their safety and advantages in the real world, for the patient and for the urologist.

Usefulness of Thulium-Doped Fiber Laser and Diode Laser in Zero Ischemia Kidney Surgery-Comparative Study in Pig Model

Background: The aim of this study was to evaluate the usefulness of a thulium-doped fiber laser and a diode laser in zero ischemia kidney surgery, by carrying out a comparative study in a pig model. Material and methods: Research was carried out on 12 pigs weighing 30 kg each. A thulium-doped fiber laser (TDFL) and a diode laser (DL) operating at wavelengths of 1940 and 1470 nm, respectively, were used. The cut sites were assessed both macroscopically and microscopically. The zone of thermal damage visible in the histopathological preparations was divided into superficial and total areas. Results: During partial nephrectomy, moderate to minimal bleeding was observed, which did not require additional hemostatic measures. All animals survived the procedure. On day 0, the total thermal damage depth was 837.8 µm for the TDFL and 1175.0 µm for the DL. On day 7, the depths were 1556.2 and 2301.7 µm, respectively. On day 14, the overall thermal damage depth for the DL was the greatest (6800 µm). The width of the superficial zone was significantly reduced on days 7 and 14 after TDFL application. Conclusion: Both lasers are suitable for partial wedge nephrectomy without ischemia in pigs. The TDFL produced similar or better hemostasis than the DL, with a smaller zone of thermal damage and, therefore, seems more suitable for application in human medicine.

Diffraction limited 195-W continuous wave laser emission at 2.09 µm from a Tm3+, Ho3+-codoped single-oscillator monolithic fiber laser

We present a bi-directionally 793-nm diode-pumped Tm³⁺, Ho³⁺-codoped silica polarization maintaining double-clad all-fiber laser based on a single-oscillator architecture emitting 195 W at 2.09 µm in continuous wave mode of operation, with a beam quality near the diffraction limit (M² = 1.08). The power scaling of the laser is only pump-power-limited in the range of the total available pump power (540 W).

Advances in laser technology and fibre-optic delivery systems in lithotripsy

The flashlamp-pumped, solid-state holmium:yttrium-aluminium-garnet (YAG) laser has been the laser of choice for use in ureteroscopic lithotripsy for the past 20 years. However, although the holmium laser works well on all stone compositions and is cost-effective, this technology still has several fundamental limitations. Newer laser technologies, including the frequency-doubled, double-pulse YAG (FREDDY), erbium:YAG, femtosecond, and thulium fibre lasers, have all been explored as potential alternatives to the holmium:YAG laser for lithotripsy. Each of these laser technologies is associated with technical advantages and disadvantages, and the search continues for the next generation of laser lithotripsy systems that can provide rapid, safe, and efficient stone ablation. New fibre-optic approaches for safer and more efficient delivery of the laser energy inside the urinary tract include the use of smaller-core fibres and fibres that are tapered, spherical, detachable or hollow steel, or have muzzle brake distal fibre-optic tips. These specialty fibres might provide advantages, including improved flexibility for maximal ureteroscope deflection, reduced cross section for increased saline irrigation rates through the working channel of the ureteroscope, reduced stone retropulsion for improved stone ablation efficiency, and reduced fibre degradation and burnback for longer fibre life.

Ex vivo testing of air-cooled CW/modulated 30 W thulium fiber laser for lithotripsy

A diode-pumped, air-cooled, all-fiber, quasi-continuous-wave thulium laser at an operating wavelength of 1.94 μm has been designed to study the performance of the laser parameter on the rate of fragmentation and its dependence on stone composition, fragmented particle size, as well as the retropulsion effect. The optimized laser cavity with an active fiber core/cladding diameter of 10/130 μm under a counter-propagating pump provides a stable laser power of 30 W at a slope efficiency of 50% and wall plug efficiency of 17%. The rate of fragmentation along with the retropulsion effect has been studied with human calcium oxalate monohydrate (COM) urinary stones (N=36) of different composition by using the designed laser and 200-μm-core low OH silica delivery fiber. The thulium fiber laser setting of 2.7 J pulse energy at the pulse rate of 10 Hz, pulse width of 90 ms, and peak power of 30 W is successful in breaking human COM stones in a controlled manner at a fragmentation rate of 0.8±0.4  mg/s, with almost uniform fragments of particle size less than 1.6 mm. During the stone fragmentation, the stone displacement (retropulsion effect) is less than 15 mm, even for the fragmented stone mass of 15±5  mg.

High power holmium:YAG versus thulium fiber laser treatment of kidney stones in dusting mode: ablation rate and fragment size studies

OBJECTIVES

The experimental Thulium fiber laser (TFL) is currently being studied as a potential alternative to the gold standard Holmium:YAG laser for lithotripsy. Recent advances in both Holmium and TFL technology allow operation at similar laser parameters for direct comparison. The use of a "dusting" mode with low pulse energy (0.2-0.4 J) and high pulse rate (50-80 Hz) settings, is gaining popularity in lithotripsy due to the desire to produce smaller residual stone fragments during ablation, capable of being spontaneously passed through the urinary tract.

METHODS

In this study, Holmium and TFL were directly compared for 'dusting' using three laser groups, G1: 0.2 J/50 Hz/10 W; G2: 0.2 J/80 Hz/16 W; and G3: 0.4 J/80 Hz/32 W. Holmium laser pulse durations ranged from 200 to 350 μs, while TFL pulse durations ranged from 500 to 1,000 μs, due to technical limitations for both laser systems. An experimental setup consisting of 1 × 1 cm cuvette with 1 mm sieve was used with continuous laser operation time limited to ≤5 minutes. Calcium oxalate monohydrate stone samples with a sample size of n = 5 were used for each group, with average initial stone mass ranging from 216 to 297 mg among groups.

RESULTS

Holmium laser ablation rates were lower than for TFL at all three settings (G1: 0.3 ± 0.2 vs. 0.8 ± 0.2; G2: 0.6 ± 0.1 vs. 1.0 ± 0.4; G3: 0.7 ± 0.2 vs. 1.3 ± 0.9 mg/s). The TFL also produced a greater percentage by mass of stone dust (fragments <0.5 mm) than Holmium laser. For all three settings combined, one out of 15 (7%) stones treated with Holmium laser were completely fragmented in ≤5 minutes compared to nine out of 15 (60%) stones treated with TFL.

CONCLUSIONS

These preliminary studies demonstrate that the TFL is a promising alternative laser for lithotripsy when operated in dusting mode, producing higher stone ablation rates and smaller stone fragments than the Holmium laser. Clinical studies are warranted. Lasers Surg. Med. 51:522-530, 2019. © 2019 Wiley Periodicals, Inc.

Recent advances in infrared laser lithotripsy [Invited]

The flashlamp-pumped, solid-state, pulsed, mid-infrared, holmium:YAG laser (λ = 2120 nm) has been the clinical gold standard laser for lithotripsy for over the past two decades. However, while the holmium laser is the dominant laser technology in ureteroscopy because it efficiently ablates all urinary stone types, this mature laser technology has several fundamental limitations. Alternative, mid-IR laser technologies, including a thulium fiber laser (λ = 1908 and 1940 nm), a thulium:YAG laser (λ = 2010 nm), and an erbium:YAG laser (λ = 2940 nm) have also been explored for lithotripsy. The capabilities and limitations of these mid-IR lasers are reviewed in the context of the quest for an ideal laser lithotripsy system capable of providing both rapid and safe ablation of urinary stones.

Advances in Lasers for the Treatment of Stones-a Systematic Review

PURPOSE OF REVIEW

Laser lithotripsy is increasingly used worldwide and is a continuously evolving field with new and extensive research being published every year.

RECENT FINDINGS

Variable pulse length Ho:YAG lithotripters allow new lithotripsy parameters to be manipulated, and there is an effort to integrate new technologies into lithotripters. Pulsed thulium lasers seem to be a viable alternative to holmium lasers. The performance of similar laser fibers varies from manufacturer to manufacturer. Special laser fibers and "cleaving only" fiber tip preparation can be beneficial for the lithotripsy procedure. Different laser settings and the surgical technique employed can have significant impact on the success of laser lithotripsy. When safely done, complications of laser lithotripsy are rare and concern the endoscopic nature of procedure, not the technology itself, making laser lithotripsy one of the safest tools in urology. Laser lithotripsy has had several new developments and more insight has been gained in recent years with many more advances expected in the future.

Fiber optic muzzle brake tip for reducing fiber burnback and stone retropulsion during thulium fiber laser lithotripsy

The experimental thulium fiber laser (TFL) is being explored as an alternative to the current clinical gold standard Holmium:YAG laser for lithotripsy. The near single-mode TFL beam allows coupling of higher power into smaller optical fibers than the multimode Holmium laser beam profile, without proximal fiber tip degradation. A smaller fiber is desirable because it provides more space in the ureteroscope working channel for increased saline irrigation rates and allows maximum ureteroscope deflection. However, distal fiber tip burnback increases as fiber diameter decreases. Previous studies utilizing hollow steel sheaths around recessed distal fiber tips reduced fiber burnback but increased stone retropulsion. A “fiber muzzle brake” was tested for reducing both fiber burnback and stone retropulsion by manipulating vapor bubble expansion. TFL lithotripsy studies were performed at 1908 nm, 35 mJ, 500 ?? ? s , and 300 Hz using a 100 - ? m -core fiber. The optimal stainless steel muzzle brake tip tested consisted of a 1-cm-long, 560 - ? m -outer-diameter, 360 - ? m -inner-diameter tube with a 275 - ? m -diameter through hole located 250 ?? ? m from the distal end. The fiber tip was recessed a distance of 500 ?? ? m . Stone phantom retropulsion, fiber tip burnback, and calcium oxalate stone ablation studies were performed ex vivo. Small stones with a mass of 40 ± 4 ?? mg and 4-mm-diameter were ablated over a 1.5-mm sieve in 25 ± 4 ?? s