Superpulsed Thulium Fiber Laser for Stone Dusting: In Search of a Perfect Ablation Regimen

Abstract

Objective:

To compare the efficacy of the standard and higher frequency regimens for superpulsed thulium fiber laser (SP TFL) retrograde intrarenal surgery (RIRS).

Materials and Methods:

A prospective study of patients with renal calculi of 10–30 mm was performed. For RIRS, we used the SP TFL (NTO IRE-Polus, Russia) and a 9.5F flexible ureteroscope with 270° deflection and 3.6F working channel. Retropulsion and visibility were assessed based on the surgeon's feedback using three-point Likert scales. The stone-free rate was assessed at 3 months with CT.

Results:

A total of 40 patients were included in the study with a mean age of 56 years, mean stone density of 880 ± 381 HU, mean stone size of 16.5 ± 6.8 mm, and median stone volume of 883 (interquartile range 606–1664) mm³. Both ablation efficacy and speed were higher in the 200-Hz mode (2.7 J/mm³ vs 3.8 J/mm³ and 5.5 mm³/second vs 8.0 mm³/second, respectively); moreover, the higher frequency correlated with increased ablation speed (r = −0.21, p = 0.019). However, both increased energy and frequency did not lead to increase of laser-on time or intraoperative complication rates.

Conclusions:

SP TFL is able to effectively disintegrate stones during RIRS with minimal complication rates. The use of higher frequency regimens showed higher efficacy and ablation speed and was not associated with increased complication rates.

Introduction

In the last two decades, lasers have become an integral part of urological surgery.¹ The flashlamp-pumped, solid-state holmium:yttrium–aluminum–garnet (Ho:YAG) laser has been the laser of choice for ureteroscopic lithotripsy for the past 20 years.² During the last decades, a number of alternatives such as FREDDY laser have been proposed,³ yet the Ho:YAG laser still holds the crown for the most efficient device for lithotripsy. The Moses effect introduced by a number of manufacturers was able to decrease the retropulsion rate, yet currently, researchers are not able to show a clear benefit over the conventional Ho:YAG laser in clinical settings.^4,5

Recently, a new player has been introduced for stone lithotripsy. The superpulsed thulium fiber laser (SP TFL) was able to show efficacy in in vitro settings with two to four times more efficient stone ablation.⁶ In a recent in vivo study by Andreeva et al., SP TFL was found to be superior to the Ho:YAG laser at the same pulse energy in terms of ablation efficiency during fragmentation (2-fold) and dusting (2.5-fold), with as much as four times lower stone retropulsion.⁷ Despite higher efficacy, it was proved that SP TFL did not lead to greater temperature increase and has a minimal water absorption distance, making it at least as safe as the Ho:YAG laser.^8,9

Previously, it was proven that the SP TFL is an effective tool for lithotripsy in percutaneous nephrolithotomy settings; however, the study was unable to show the dusting ability of TFL focusing on fragmentation regimens.¹⁰ Yet, one of the greatest advantages of the SP TFL is that it offers the most comprehensive and flexible range of parameters among laser lithotripters, with pulse frequencies as high as 2200 Hz, low to high pulse energies (0.005–6 J), short to long pulse durations (200 μs for 12 ms), peak power of 500 W, and average power of 50 W.¹¹ Such variation of parameters makes us think about what parameters are the most effective ones. Keller and Traxer supposed that high-frequency stone ablation, as high as 200 Hz, could lead to a three times increase in production of dust in comparison with the conventional Ho:YAG laser.¹¹ Despite its ability to work in several modes, they were not studied in a clinical setting. The aim of our study was to compare the efficacy of the standard and higher frequency regimens for SP TFL retrograde intrarenal surgery (RIRS).

Materials and Methods

A prospective study of patients with renal calculi who underwent RIRS in the period between February 2018 and July 2018 was performed. All patients were treated after obtaining informed consent and after IRB (Sechenov University, Russia) approval. Inclusion criteria were age 18 years and older and stone as large as 20 mm. Exclusion criteria included patients undergoing anticoagulant and antiplatelet therapy and need for secondary simultaneous surgical intervention (for benign prostatic hyperplasia, upper tract carcinoma, and urethral and ureteral strictures). Before inclusion in the study, all patients underwent contrast-enhanced CT of the urinary tract for anatomy and stone density assessment. All patients were prestented 4–7 days before the surgery for further use of ureteral sheaths (10–12F [Cook, USA] and 11–13F [Navigator HD; Boston Scientific, Marlborough, USA]).

Lithotripsy was carried out using the SP TFL (NTO IRE-Polus, Russia) with a wavelength of 1.94 μm and peak/average power of 500/50 W. RIRS was preformed using flexible ureteroscopes: 7.5F. FLEX-X2 (Karl Storz GmbH, Germany) and LithoVue™ (Boston Scientific). Intraoperative irrigation was passive in all patients (falling water). At the end of the surgery, a ureteral stent (Double-J) was placed until the 10th postoperative day and a Foley catheter, 16–20F, was inserted. All procedures were performed under general anesthesia.

During surgery, the operative time (from instrument insertion to removal) and laser-on time (LOT, total time of laser ablation, measured by the laser apparatus) were assessed. Retropulsion was assessed based on the surgeon's feedback using a three-point Likert scale (from 0 to 2, where 0 is no retropulsion and 2 is high retropulsion, interfering with ablation), as well as intraoperative visibility (from 0 to 2, where 0 is clear visibility and 2 is poor visibility interfering in surgery). All interventions were performed by four attending surgeons. This number of participating surgeons allowed assessment of different opinions on the laser's performance to present reliable mean retropulsion and visibility scores. Nevertheless, all surgeries were recorded and attended by the researcher (R.K.) who aided in regimen selection and efficacy (visibility and retropulsion) estimation. If any discordance in Likert scaling was found, the record of the surgery was reviewed by the researcher and two surgeons taking part in the research, and scaling was done only after consent from all participants.

All patients included in the study were obliged to undergo a 3-month follow-up with contrast-enhanced CT to assess the stone-free rate and short-term complications (strictures). We have considered all patients with absence of stones larger than 2 mm on CT at 3 months as stone free. Postoperative complications were classified according to the Clavien–Dindo classification.¹²

To explore differences in stone ablation speed with different energy settings, we have calculated stone volume according to the ellipsoid volume formula (4/3*π*abc, where a, b, and c are radii of the stone). Parameters such as ablation speed (stone volume, mm³/LOT, seconds) and energy for ablation of 1 mm³ (total energy, J/stone volume, mm³) were calculated. In this particular subset of patients, we were planning to estimate the SP TFL dusting efficacy, therefore those patients who underwent fragmentation following lithoextraction were excluded. Patients with a stone volume less than 500 mm³ (stone less than 10 × 10 × 10 mm) were also excluded.

For statistical analysis, we used SPSS Statistics 23.0 (IBM, USA). Patient data are expressed as mean ± SD (range). Categorical variables were compared using Pearson's chi-square test. Noncategorical variables were compared with the Spearman test. A two-sided p-value of 0.05 was considered the threshold for statistical significance.

Results

A total of 40 patients were included in the study with a mean age of 56 years, mean stone density of 880 ± 381 HU, mean stone size of 16.5 ± 6.8 mm, and median stone volume of 883 (interquartile range [IQR] 606–1664) mm³ (Table 1). Mean surgery duration was 24.1 ± 10.9 minutes, median LOT was 240 (IQR 96–384) seconds, and median total energy for stone ablation was 5.6 (IQR 2.3–10.0) kJ. In the course of the study, only two regimens for dusting were used: −0.5 J × 30 Hz = 15 W and 0.15 J × 200 Hz = 30 W. Table 1 demonstrates their characteristics.

Table 1.

The Characteristics of the Most Used Lithotripsy Regimens

Mode	0.5 J × 30 Hz = 15 W	0.15 J × 200 Hz = 30 W	p
N (%)	15 (37.5)	25 (62.5)	—
Retropulsion, N (%)	3 (7.5)	1 (2.5)	0.13
Visibility, N (%)	1 (2.5)	2 (5)	0.688
Laser-on time, seconds, median (IQR)	219 (90–330)	372 (96–414)	0.169
Total energy for stone ablation, kJ, median (IQR)	5.6 (1.4–6.4)	6.6 (2.8–10.0)	0.619
Stone size, mm, mean ± SD	17.4 ± 5.8	15.0 ± 7.4	0.272
Stone density, HU, mean ± SD	857 ± 348	894 ± 407	0.773
Stone heterogeneity index	305	323	0.471
Ablation efficacy, J/mm³, median (IQR)	2.7 (1.8–9.8)	4.8 (2.6–11.3)	0.059
Energy consumption, J/second, mean ± SD	16.7 ± 4.1	28.9 ± 4.8	<0.001^*
Ablation speed, mm³/second, median (IQR)	5.5 (1.5–8.7)	8.5 (3.6–19.0)	0.014^*

p < 0.05 statistically significant.

IQR = interquartile range; SD = standard deviation.

No correlation between stone density and LOT was found (r = 0.049, p = 0.56). To justify this conclusion, we assessed the stone heterogeneity index (SHI), yet no correlation was found (r = 0.052, p = 0.745). Higher energy (r = 0.145, p = 0.078) showed no effect on LOT or surgery duration and high frequency was even able to cut the LOT (r = −0.241, p = 0.003). Importantly, we did observe higher energy consumption for single stone ablation in the 200-Hz mode. Both the ablation efficacy and speed were higher in the 200-Hz mode (2.7 J/mm³ vs 3.8 J/mm³ and 5.5 mm³/second vs 8.0 mm³/second, respectively); moreover, the higher frequency correlated with increased ablation speed (r = −0.21, p = 0.019). No difference in total energy for single stone ablation was found (p = 0.619) (Table 2). We did observe that a higher SHI led to increased speed of stone ablation (r = 0.348, p = 0.026).

Table 2.

Pearson and Spearman Correlations

	Regimen	Total energy for stone ablation	Stone density	Stone heterogeneity index	Stone volume
Laser-on time
Correlation coefficient	0.145	0.908	−0.049	0.052	−0.405
P	0.078	0.000^*	0.557	0.745	0.010^*
Complication rate (Clavien–Dindo I–II)
Correlation coefficient	−0.041	0.068	−0.049	0.101	−0.121
P	0.620	0.412	0.561	0.208	0.462
Ablation efficacy (J/mm³)
Correlation coefficient	−0.042	0.64	−0.249	−0.270	−0.401
P	0.079	0.001^*	0.126	0.088	0.011^*
Energy consumption (J/second)
Correlation coefficient	−0.802	0.150	−0.093	−0.064	0.116
P	0.001^*	0.361	0.572	0.689	0.481
Ablation speed (mm³/second)
Correlation coefficient	−0.213	0.439	−0.352	0.348^*	−0.406
P	0.019^*	0.005^*	0.028^*	0.026	0.010^*

Pearson or Spearman (where appropriate) correlation coefficients (r and r ²) for the relationship of laser and stone parameters with intraoperative features. Significant correlation are in bold.

p < 0.05 indicates statistical significance.

No association of laser parameters or visibility and retropulsion with complications (Clavien–Dindo I–II) was found (Table 3). Retropulsion interfered with surgery only in 3 (7.5%) patients (0.5 J × 30 Hz). Poor visibility was reported in 2 (5%) cases in a 200-Hz regimen. A total of 92.5% of patients (37 of 40 men) showed a stone-free rate at 3 months. The postoperative complication rate was relatively low, with only Clavien I–II complications. No complications related to using TFL were found. Three months after surgery, CT with contrast enhancement showed no strictures or stenosis in the upper urinary tract (Table 3).

Table 3.

Complications

Complications	Total (n = 40)
Grade I
Fever, n (%)	2 (5)
Transient creatinine elevation, n (%)	1 (2.5)
Grade II
Urinary tract infection, n (%)	1 (2.5)

Discussion

The SP TFL has been proven to be an efficient tool for stone lithotripsy. As expected, the high-frequency regimens were associated with increased dust formation and higher lithotripsy efficacy. Although the frequency of 200 Hz leads to worse visibility, almost no retropulsion was found in this setting. On the contrary, higher power was associated with increased retropulsion. However, worsening visibility (in a limited number of cases) and increased retropulsion do not affect outcomes of the surgery.

The efficacy of SP TFL in different regimens has been shown previously, it was found to be effective with settings of 0.1–0.2 J/15–30 W for dusting of kidney stones, 0.2–0.5 J/10–15 W for dusting and fragmentation of ureteral stones, and 2–5 J/30–50 W for fragmentation of bladder stones.⁹ In the current article, we focused on the most interesting regimens—those used for dusting during RIRS. The second most often used regimen was a standard 0.5 J × 30 Hz = 15 W. A low pulse frequency of 30 Hz allows maintaining good visibility, even in the limited space of the renal calix. Moreover, the energy of 0.5 J allows for fast dusting with minimal retropulsion.

One of the most intriguing additions of SP TFL is the ability to work at a high frequency (as high as 2000 Hz). However, in clinical work, the regimen of 0.15 J × 200 Hz = 30 W proved to be efficient for dusting. Among the main advantages are safety, the absence of retropulsion, and the ability of fine dusting into small fragments. Indeed, we observed worse visibility in some patients; however, it was more due to the snowstorm effect, rather than due to microbleeding, as previously proposed.¹³ It was shown that higher frequencies may shorten operative time by as much as 20%; we found a similar result in our study where higher frequencies were associated with shorter LOT.¹⁴ The higher frequency regimen was characterized with higher ablation efficacy and speed (p = 0.019). This finding shows that a higher frequency may not only allow for smaller fragments or lesser retropulsion but it also eventually increases efficacy.

The high-frequency regimens of SP TFL have been shown to be highly effective in in vitro settings, previously shown by Andreeva and colleagues; however, the efficacy of high-frequency regimens was not shown. Fried mentioned that such a regimen may have potential advantages for dusting and described the laser's ability to work in this setting as the one of the biggest SP TFL advantages.⁶ This was also mentioned by Kronenberg and Traxer, who described that SP TFL is able to produce four times more dust with a high-frequency regimen than the Ho:YAG laser with Moses technology.¹⁵ The main drawbacks of high-frequency settings are the possibility of microbleeding due to fragment bouncing inside the pelvis/calix and worsening visibility.¹³ It should be mentioned that despite a correlation between frequency and quality of vision, poor visibility (1 on the Likert scale) was found only in two patients (5%). It is a clear indication that higher frequency is linked to worse visibility, but will eventually affect only a minor percent of cases. High-frequency regimens of the SP TFL have been shown to produce much less retropulsion than the Ho:YAG laser, sometimes even no retropulsion at all when low pulse energies and frequencies under 150 Hz are used.¹⁶

A recent article from Panthier and colleagues dealt with the assessment of the required energy for ablation of 1 mm³ of stone during Ho:YAG laser lithotripsy.¹⁷ The authors observed that if calcium oxalate monohydrate stones required an average of 24 J for ablation of 1 mm³, the amount of energy for uric acid and cystine stones was much less than 2.5 and 7.6 J, respectively.¹⁷ Unfortunately, stone composition was not assessed routinely in our study. However, data on the required energy per 1 mm³ of stone ablation are encouraging with 2.5–4.8 J, on average, in all stone types and densities. This could be another confirmation of SP TFL being superior in terms of stone ablation over conventional Ho:YAG laser devices. However, a direct head-to-head comparison is necessary. For more precise estimation of stone dusting capabilities of SP TFL, we assessed the SHI, which had been introduced by Lee and colleagues.¹⁸ The authors proved that a higher SHI is associated with an increased success rate in extracorporeal shockwave lithotripsy and could be used as a more precise characteristic of stone density. We did not find any connections between the SHI and LOT, yet we observed that a higher SHI is associated with increased speed of stone ablation (mm³/second). Therefore, we believe that this important issue should be investigated in further studies.

The choice of mode depends on visibility and localization of the stone. All regimens were highly safe and effective. None of the complications found were associated with the use of a specific regimen. Over the entire period of the intraoperative study, no case of significant bleeding or pronounced mucosal damage has been reported. Minor bleeding that was found in some of our patients was more likely due to rupture of the forniceal veins (after pressure increase) and not by the laser. Hemorrhage in these cases was insignificant. It should be mentioned that prestenting is a general practice in our hospital. In this particular series, prestenting was done to enable access to the kidney.

Limitations

The main limitation of the study was the absence of a control group. However, in the current study, our main aim was to describe possible and most efficient regimens of RIRS with SP TFL. Moreover, it was already proven that SP TFL is an efficient tool for lithotripsy both in vivo and in vitro. In addition, it would hardly be a comparison of similar machines due to the ability of SP TFL to work in two to twenty times higher frequencies than the Ho:YAG laser. Another limitation was the use of Likert scales for visibility and retropulsion; however, the fact that the researcher participated in all surgeries and discussed the final mark of each scale with surgeons allowed us to exclude inter-rater variability and present data with minimal bias. One of the most important limitations of the study is the higher power of the 200-Hz regimen (30 W) compared with the standard regimen (15 W). However, the total energy for stone ablation does not differ between the regimens (0.619).

Conclusions

Footnotes

Ethical Approval

The study was approved by the institutional review board of Sechenov University (Moscow, Russia).

Informed Consent

Informed consent was obtained from all participants included in the study.

Author Disclosure Statement

No competing financial interests exist.

Funding Information

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Abbreviations Used

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Superpulsed Thulium Fiber Laser for Stone Dusting: In Search of a Perfect Ablation Regimen—A Prospective Single-Center Study

Abstract

Objective:

Materials and Methods:

Results:

Conclusions:

Introduction

Materials and Methods

Results

Discussion

Limitations

Conclusions

Footnotes

Ethical Approval

Informed Consent

Author Disclosure Statement

Funding Information

Abbreviations Used

References