Abstract
Objective
To compare the efficacy and safety of thulium fiber laser (TFL) and holmium:yttrium-aluminum-garnet (Ho:YAG) laser for ureteric stone management with semi-rigid ureteroscopy.
Methods
In a prospective study from January 2020 to December 2021, we compared 40 patients in each group who underwent semi-rigid ureteroscopic lithotripsy with TFL and that with Ho:YAG laser. Stone volume, stone density, stone fragmentation rates, total lasing time, total operative time, endoscopic vision, retropulsion and stone free rates were analyzed in both groups and compared.
Results
Mean stone volume was comparable in the TFL group and the Ho:YAG laser group (282.45 [standard deviation, SD 139.79] mm3vs. 279.49 [SD 312.52] mm3; p=0.964). Mean stone density was also comparable in the TFL group and the Ho:YAG laser group (1135.30 [SD 317.04] Hounsfield unit vs. 1131.75 [SD 283.03] Hounsfield unit; p=0.959). The mean stone fragmentation rates calculated as stone volume divided by lasing time were 25.85 (SD 10.61) mm3/min and 21.37 (SD 14.13) mm3/min in the TFL group and the Ho:YAG laser group, respectively (p=0.113). The mean total lasing time (10.15 [SD] 4.69 min vs. 11.43 [SD 4.56] min; p=0.222), mean operative time (25.13 [SD 9.51] min vs. 25.54 [SD 10.32] min; p=0.866), and mean total hospital stay (2.62 [SD 0.77] days vs. 2.61 [SD 0.84] days; p=0.893) were comparable in the TFL group and in the Ho:YAG group. The vision was better and retropulsion was less in the TFL group. The stone-free rate at 1 month postoperatively was slightly better in the TFL group (100% vs. 90%; p=0.095).
Conclusion
TFL technology was associated with the comparable total surgical time, total lasing time, and stone fragmentation rate with Ho:YAG laser. However, TFL had better endoscopic vision, lesser stone retropulsion, and slightly better stone-free rates.
Keywords: Laser, Stone, Urolithiasis, Thulium, Holmium
1. Introduction
Endourological management of urolithiasis has rapidly developed over time. Almost every year, new working tools and technologies are being introduced into clinical practice so that existing treatments can be improved [1]. With these novel procedural techniques, there is complete clearance of stones from the urinary tract with less hospital stay, less need for auxiliary procedures, and fewer risks of complications [2].
Holmium:yttrium-aluminum-garnet (Ho:YAG) laser has revolutionized the management of urolithiasis over the last two decades with its ability to fragment all types of stones effectively [[1], [2], [3]]. However, the drawbacks of this laser are the size of the machines, low energy efficiency, and difficulty in focusing laser beams in small-size fibers.
Thulium fiber laser (TFL), on the other hand, can deliver high-power laser beams through small and more flexible fibers more effectively. TFL can emit a laser beam with a high frequency while sustaining a low pulse energy mode, which makes it ideal for optimizing the dusting of stones and reducing retropulsion [[4], [5], [6]].
Currently, there are very few studies in the literature comparing the efficacy and safety of TFL versus Ho:YAG laser in ureteric stone management. This prospective study aims to report our experience, comparing TFL versus Ho:YAG laser in the management of ureteric stones with semi-rigid ureteroscopy in terms of stone fragmentation rates, total lasing time, total operative time, vision while lasing, stone retropulsion, stone-free rates, and complications.
2. Patients and methods
This prospective study involving human participants was in accordance with the ethical standards of the institutional and national research committee and with the 1964 Helsinki Declaration and its later amendments. Institutional Ethics Committee gave the approval for the study (Muljibhai Patel Society for Research in Nephro-Urology Ethics Committee; No. EC/636/2020).
Patients who met the inclusion and exclusion criteria were enrolled in the study from January 2020 to December 2021. The inclusion criteria were patients aged over 18 years, with single ureteric stone on CT scan located in the mid or lower ureter (below the upper margin of sacro-iliac joint), with normal upper tract anatomy, that signed the informed consent, and that declared willingness for a one-month follow-up evaluation. We excluded patients who had significant medical co-morbidity contraindicating general anesthesia, consuming anti-platelet medications with deranged coagulation profile, with untreated culture-positive urinary tract infection, with multiple ureteric calculi, that were pregnant females, with presence of ureteric stricture, that had participated in any other clinical trial 30 days before and throughout the study, that were not able to give the informed consent, or that had other conditions contraindicating laser lithotripsy. All the patients enrolled were made to understand the procedure in detail and sign a written informed consent. At the initial appointment, we collected patients’ data like age, sex, blood counts, creatinine, urine culture, as well as basic stone parameters such as size, side, location, stone volume by scalene ellipsoid formula (length × breadth × thickness × π/6), and stone density, obtained from CT scan.
Sample size for each group was calculated using Power and Sample Size Calculation Program Version 3.1.6 [7]. The power of the study was kept at 0.80 and the level of significance (α) was accepted at 0.05 (95% confidence interval). The patients were then randomized into two study groups using the online research randomizer tool (https://www.randomizer.org/). The first group of 40 patients was managed with TFL by IPG Photonics Pvt. Ltd., Karnataka, India, and the second group of 40 patients was managed with Ho:YAG laser by Lumenis Pulse® (Boston Scientific corporation, Hemel Hempstead, UK).
2.1. Technique of procedure
After giving general anesthesia, patients were placed in lithotomy position. After antiseptic preparation with povidone-iodine, a standard cystoscope with a working channel was introduced into the bladder. The corresponding ureteral orifice was cannulated with a sterile 0.035-inch (1 inch=2.54 cm) straight glide wire and under fluoroscopic guidance, it was then passed into the pelvicalyceal system. A sterile 6 Fr ureteric catheter was threaded on the glide wire until the tip reached the lower border of the ureteric calculus.
The ureter was dilated with serial Teflon dilators (Meditech Devices Pvt. Ltd., Ahmedabad, India) as per our institutional preference. Ureteroscopic lithotripsy was performed using 4.5 Fr, 6 Fr, 7 Fr, or 8 Fr tapered semi-rigid ureteroscopes (Richard Wolf GmbH [Knittlingen, Germany] or Karl Storz SE & Co [Tuttlingen, Germany]) (Fig. 1). The stones were fragmented using TFL or Ho:YAG laser according to randomization. Laser pulse energy in both groups ranged from 0.8 kJ to 1 kJ with frequencies ranging from 10 Hz to 12 Hz. Pulse width was kept constant. The intraoperative endoscopic vision was observed while using both lasers and the surgeon was asked to mark it on a Likert scale. Forceps were used in some cases to remove fragments. Intraoperatively various parameters noted were total lasing time, total laser energy used, vision during lasing, stone retropulsion, and total operative time (from cystoscopy to end of the procedure). The procedure was terminated once stone clearance was confirmed under fluoroscopy and ureteroscopy.
Figure 1.
Trolley showing instruments and components for semi-rigid ureteroscopy.
An indwelling double-J stent was placed after the procedure in all the patients and the stent was removed after 4 weeks. Per-urethral catheter was placed and was removed on the first postoperative day.
The primary endpoints of the study were the successful completion of the intended surgical procedure and the ability to effectively treat the stone. The secondary endpoints were procedure- or device-related adverse events or complications. Patients in both groups were analyzed for stone fragmentation rates, stone-free rates, intraoperative complications, and the need for auxiliary procedures and retreatment.
All patients underwent ultrasound examination and plain X-ray on the first postoperative day and low dose non-contrast CT scan at a one-month follow-up to assess stone-free rates. Routine blood investigations were repeated on the first postoperative day. Complete stone clearance was defined as the non-visualization of stones on ultrasound, X-rays, or CT scans.
2.2. Assessment of vision while lasing
The operating surgeon was asked to assess the endoscopic vision during lasing and mark it on a Likert scale from 1 to 5 corresponding to from blurred vision to excellent vision.
Stone retropulsion was also assessed with a Likert scale from 0 to 2 where 0 represented no retropulsion, 1 represented mild retropulsion but allowed easy lasing, and 2 represented severe retropulsion which made lasing difficult.
2.3. Statistical analysis
After data collection, data entry was done in a Microsoft Excel sheet. Data were analyzed using Statistical Package for the Social Sciences software (version 25.0, IBM Corp, Armonk, NY, USA). The qualitative data were presented as number and percentage, while quantitative data were presented as mean (standard deviation [SD]) or median (range), depending on the normal or skewed distribution of data. The normal distribution of quantitative data was assessed by an independent sample t-test. A comparison of qualitative variables between the groups was done using the Chi-square test. Comparative analysis of parameters between pre- and post-operative variables was done using the paired sample t-test. A p-value of less than 0.05 was considered statistically significant.
3. Results
There were 40 patients included in each group (TFL group and Ho:YAG laser group). The demographic data of the patients are depicted in Table 1. Both the groups had comparable patient characteristics.
Table 1.
Demographic patient data in both TFL group and Ho:YAG laser group.
| Characteristic | TFL | Ho:YAG laser | p-Value |
|---|---|---|---|
| Agea, year | 44.93±14.11 | 47.72±12.88 | 0.411 |
| Sex, n (%) | 0.742 | ||
| Male | 25 (62.5) | 32 (80.0) | |
| Female | 15 (37.5) | 8 (20.0) | |
| Side, n (%) | 0.820 | ||
| Right | 18 (45.0) | 16 (40.0) | |
| Left | 22 (55.0) | 24 (60.0) | |
| Stone location, n (%) | 0.687 | ||
| Mid ureter | 24 (60.0) | 26 (65.0) | |
| Lower ureter | 16 (40.0) | 14 (35.0) | |
| Volume of stonea, mm3 | 282.45±139.79 | 279.49±312.52 | 0.964 |
| Stone densitya, HU | 1135.30±317.04 | 1131.75±283.03 | 0.959 |
| Stone fragmentation ratea, mm3/min | 25.85±10.61 | 21.37±14.13 | 0.113 |
| Total laser energy consumeda, kJ | 6.73±2.75 | 7.06±3.53 | 0.674 |
HU, Hounsfield unit; TFL, thulium fiber laser; Ho:YAG, holmium:yttrium-aluminum-garnet.
Values are presented as mean±standard deviation.
Various stone characteristics like mean stone volume, mean stone density, stone laterality, and stone location have also been depicted in Table 1. The mean total operative time was 25.13 (SD 9.51) min in the TFL group and 25.54 (SD 10.32) min in the Ho:YAG laser group, which was comparable (p=0.866). The mean total lasing time was also similar in the TFL group and Ho:YAG group (10.15 [SD 4.69] min vs. 11.43 [SD 4.56] min; p=0.222).
The mean stone fragmentation rates were comparable in the TFL group and the Ho:YAG laser group (25.85 [10.61] mm3/min vs. 21.37 [SD 14.13] mm3/min, p=0.113). The mean total energy dissipated in the fragmentation of stones was similar in the TFL group and the Ho:YAG laser group (6.73 [SD 2.75] kJ vs. 7.06 [SD 3.53] kJ; p=0.674) (Table 1).
The deterioration of endoscopic vision was studied while lasing the stones with both the lasers and excellent endoscopic view was observed in 75.0% of cases of the TFL group versus 52.5% cases of the Ho:YAG laser group whereas severe deterioration of the vision (hazy or blurred vision) was observed in 2.5% cases in the TFL group versus 7.5% cases in the Ho:YAG laser group (Table 2).
Table 2.
Comparison of deterioration of vision during lasing in both groups during ureterolithotripsy.
| Group | Likert scale for grading vision, n (%) |
||||
|---|---|---|---|---|---|
| 1 (blurred) | 2 (hazy) | 3 (grainy) | 4 (sharp) | 5 (excellent) | |
| Ho:YAG laser | 1 (2.5) | 2 (5.0) | 10 (25.0) | 6 (15.0) | 21 (52.5) |
| TFL | 0 | 1 (2.5) | 5 (12.5) | 4 (10.0) | 30 (75.0) |
TFL, thulium fiber laser; Ho:YAG, holmium:yttrium-aluminum-garnet.
The stone retropulsion was defined as the dislocation of stone from its original location during process of lasing. Retropulsion was also assessed on a Likert scale; it was observed that there was no retropulsion in 37 (92.5%) cases in the TFL group and 25 (62.5%) cases in the Ho:YAG laser group; mild retropulsion was seen in 3 (7.5%) cases in the TFL group and 10 (25.0%) cases in the Ho:YAG laser group; and severe retropulsion was seen in 5 (12.5%) cases only in the Ho:YAG laser group. Severe retropulsion in few cases (n=5) in the Ho:YAG laser group led to stone migration to kidney (Table 3).
Table 3.
Comparison of degree of retropulsion of stone while lasing in both groups.
| Group | Likert scale for grading stone retropulsion, n (%) |
||
|---|---|---|---|
| 0 (no retropulsion) | 1 (mild retropulsion but allowed easy lasing) | 2 (severe retropulsion which made lasing difficult) | |
| Ho:YAG laser | 25 (62.5) | 10 (25.0) | 5 (12.5) |
| TFL | 37 (92.5) | 3 (7.5) | 0 |
TFL, thulium fiber laser.
Preoperatively two patients in the TFL group underwent percutaneous nephrostomy and three patients underwent double-J stent placement whereas, in the Ho:YAG laser group, two patients underwent percutaneous nephrostomy and two patients underwent double-J stent placement for diversion. The mean hemoglobin drop was 0.52 [SD 0.41] g/dL in the TFL group and 0.64 [SD 0.46] g/dL in the Ho:YAG laser group (p=0.229).
There were a total of seven patients with Clavien–Dindo Grade II complications in the form of urinary tract infection, two patients in the TFL group, and five patients in the Ho:YAG laser group.
Mean hospital stays were comparable in both groups, 2.62 [SD 0.77] days in the TFL group and 2.61 [SD 0.84] days in the Ho:YAG laser group (p=0.893). Stone-free rates at Day 1 were 92.5% (n=37) in the TFL group and 82.5% (n=33) in the Ho:YAG laser group. However, at the end of 1 month, the stone-free rates were 100% (n=40) in the TFL group and 90% (n=36) in the Ho:YAG laser group (p=0.095). These residual stones in Ho:YAG group were managed with repeat ureteroscopic intervention (flexible ureteroscopy).
4. Discussion
Laser lithotripsy, via advanced ureteroscopy, is now a major technique for the minimally invasive surgical removal of ureteral stones, decreasing the operative time, surgical risk, and hospital stay [8].
Ho:YAG laser has the ability to fragment all types of stones effectively [3,9]. High-power Ho:YAG lasers after being introduced into clinical practice have made fragmentation and fine dusting faster and therefore, more efficacious. Although Ho:YAG laser lithotripsy is capable of high energy, it has limitations in attaining high frequency. Newer high-power Ho:YAG machines can attain maximum energy of up to 6 J and frequency of only 100 Hz and can reach a total power of up to 140 W. In addition, there are higher retropulsion rates with Ho:YAG laser [10].
A significant drawback of Ho:YAG laser for stone management is pronounced deterioration of the endoscopic view while the laser is emitting, which is due to the so-called “snowstorm” effect [11]. This prolongs the operating and lasing time, which causes increased risks of leaving residual clinically significant fragments, and may also lead to ureteral perforation. Moreover, to overcome this problem, an increase in irrigation flow is mandatory, which sometimes can lead to an increased risk of inflammatory complications [12]. In the present study, we assessed the deterioration of endoscopic vision while lasing by asking the operating surgeon to grade the endoscopic vision on a Likert scale of 1–5. It was observed that the TFL group had excellent endoscopic vision in 75.0% of cases versus 52.5% of cases in the Ho:YAG laser group, whereas in 7.5% of cases in the Ho:YAG laser group, there was a severe deterioration of vision. This observation is well supported in literature [5,13].
The limitations of Ho:YAG laser are overcome by the new TFL, which has the capability to work at a very high frequency with low energy levels. TFL can deliver high-power ablative energy through small, more flexible fibers, potentially down to 50 μm core diameter, thus occupying less space in the instrument's working channel while delivering a high-power output, which allows better irrigation as well as larger deflection angles [4]. The rationale for using the TFL in lithotripsy is based on the major emission line of the TFL (1940 nm) closely matching a strong water absorption peak near 1940 nm [14]. It is hypothesized that water is a primary absorber facilitating the conversion of laser energy into mechanical and thermal energy during laser lithotripsy [15]. This shows why the stone ablation threshold is lower in thulium laser as compared to Ho:YAG laser for equivalent laser parameters [16]. TFL was cleared for clinical use in 2017 after large series of preclinical studies showing faster stone fragmentation with lower retropulsion [3,4,9,12,17].
In this study, we compared the clinical efficacy and safety profile of TFL and Ho:YAG laser for semi-rigid ureterolithotripsy. All the patients were successfully treated with both lasers. There was no statistically significant difference between the two groups concerning demographic data, postoperative hemoglobin drop, and pre- and post-operative creatinine. The stone lithotripsy with TFL had a comparable stone fragmentation rate to the Ho:YAG laser technique with similar total lasing and operative time. The stone-free rate was comparatively higher in the TFL group.
Martov et al. [13] fixed the laser parameters at 1 J pulse energy, 10 Hz repetition rate with both TFL as well as Ho:YAG laser in their study, but in our study, the pulse energy in both groups ranged from 0.8 kJ to 1 kJ with frequency ranging from 10 Hz to 12 Hz. The mean total energy dissipated in the fragmentation of stones was comparable in both the groups; it was 6.73 (SD 2.75) kJ in the TFL group and 7.06 (SD 3.53) kJ in the holmium group (p=0.674).
Blackmon et al. [18] demonstrated that for equivalent laser energy and frequency settings, the TFL produced 2.5 to three times higher ablation rates than Ho:YAG laser in dusting mode and two times higher in fragmentation mode. A systematic review of TFL by Kronenberg et al. [19] summarized that the ablation efficiency of TFL was two times faster for fragmentation and four times faster for dusting mode as compared to Ho:YAG laser. Hardy et al. [20] also demonstrated 1.5, 4.3, and 7.3 times faster lithotripsy in favor of the TFL at 150 Hz, 300 Hz, and 500 Hz frequency respectively. In our study, when we compared the fragmentation mode of TFL with Ho:YAG laser in semi-rigid ureteroscopy, we observed that the mean stone fragmentation rate was comparable in both groups (25.85 [SD 10.61] mm3/min in the TFL group vs. 21.37 [SD 14.13] mm3/min in Ho:YAG laser group). Post-ureterolithotripsy stenting was done in almost all the patients (97.5%) in the thulium group and 92.5% cases in the Ho:YAG group, which was comparable.
Another parameter, stone retropulsion, had already been investigated in vitro. In our study, mild retropulsion was seen in 7.5% (n=3) cases in the TFL group and 25.0% (n=10) cases in the Ho:YAG group, and significant retropulsion was seen in 12.5% (n=5) cases in the Ho:YAG group and none in the TFL group. This observation is well supported in literature [13]. The possible explanation for lower retropulsion is the four times smaller size bubbles produced by TFL and ten times lower local pressure than those of the Ho:YAG laser [6,17].
Literature demonstrated statistically the shorter hospitalization time and lower complication rate in TFL due to shorter operation and laser time with better endoscopic vision [13,21]. However, in our study, the mean hospital stay was comparable in both TFL and Ho:YAG laser groups (p=0.893).
The most common postoperative complications seen in the present study were Clavien–Dindo Grade II complications in the form of urinary tract infection (two patients in the TFL group and five patients in the Ho:YAG laser group) and we did not come across any cases of Grades III, IV, or V complications in the present study. All these patients were treated with culture-sensitive antibiotics. A similar rate of complications was observed by Mahajan and Mahajan [21]. Another prospective study showed an unexplained exacerbation of pyelonephritis associated with TFL lithotripsy [5].
The most important parameter is the stone-free rates. In literature, investigators have used ultrasonography and kidney-ureter-bladder X-ray to assess stone clearance [21]. The sensitivity of ultrasonography and kidney-ureter-bladder X-ray is comparatively less for assessing residual ureteric stones. One of the advantages of our study is the estimation of the stone-free rate with low dose non-contrast CT scan at a one-month follow-up. The one-month follow-up showed no residual stones in the TFL group and 10% (n=4) cases of residual stones in the Ho:YAG group. These stones were managed with repeat ureteroscopic intervention (flexible ureteroscopy).
This study is among very few prospective studies which have compared the effectiveness of TFL versus Ho:YAG laser in semi-rigid ureteroscopy procedure. We used almost the same laser settings for both the lasers and observed better stone-free rates with TFL than with the Ho:YAG laser, probably due to the production of smaller stone fragments with TFL, which effectively facilitates drainage of all the stone particles.
Our study also had limitations, as only one set of laser parameters was studied. Dusting mode was not studied. We have studied only a limited range of laser settings. This is a single tertiary care center study. We need to conduct a multicenter randomized clinical study, ideally in a double-blinded fashion. As we have used different sized ureteroscopes, there might be bias in regard to better visibility with broader working channels, which needs further studies.
5. Conclusion
TFL technology was associated with the comparable total surgical time, total lasing time, and stone fragmentation rate with Ho:YAG laser. However, TFL had better endoscopic vision, lesser stone retropulsion, and slightly better stone-free rates.
Author contributions
Study concept and design: Ankit Gupta, Arvind P. Ganpule.
Data acquisition: Ankit Gupta, Ankush Puri.
Data analysis: Ankit Gupta.
Drafting of manuscript: Ankit Gupta, Arvind P. Ganpule.
Critical revision of manuscript: Abhishek G. Singh, Arvind P. Ganpule, Ravindra B. Sabnis, Mahesh R. Desai.
Conflicts of interest
The authors declare no conflict of interest.
Footnotes
Peer review under responsibility of Tongji University.
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