Thulium versus holmium: Which is safer for the removal of entombed stents?

Abstract

Introduction:

Removal of entombed ureteral stents can be technically challenging, particularly if the stent were to fragment during removal. The purpose of this study was to compare the therapeutic suitability of the thulium fiber laser (TFL) and the holmium laser (HL) in the treatment of entombed stents.

Methods:

In this benchtop study, first, the time taken for each laser to transect the stent was recorded in 10 experiments/laser. Next, the force required to break the stent following 5 s of laser contact was measured in 15 randomized experiments/laser. Finally, seven experiments of simulated ureteroscopy on entombed stents were performed per laser. Lasers were operated at 0.8 J, 12 Hz with 270 µm fibers, and 6 Fr stents were utilized. Endpoints included time to release the stent, laser energy, and stent damage.

Results:

The stent transection time was shorter with the TFL compared to the HL (22.02 vs. 61.46 s; P < 0.001). After 5 s, the TFL transected the stent with lesser force compared to the HL (5.34 vs. 15.24 N; P = 0.004). Both required lesser force to break the stent compared to the baseline (33.8 N; P < 0.001). On simulated lithotripsy, the lithotripsy time (12.7 vs. 8.5 min; P = 0.11) and laser energy (4.7 vs. 2.7 kJ; P = 0.09) were similar between the TFL and HL. The mean stent damage score was higher when using the TFL compared to the HL (36.9 vs. 15.7; P = 0.017).

Conclusions:

The TFL resulted in faster stent transection, reduced breakage force, and greater stent damage. Urologists should be cautious when releasing entombed stents using the TFL as the laser may significantly weaken the stent, increasing the risk of fracture during removal.

INTRODUCTION

Since the first reported use of a stent in 1967, stents have become a vital tool utilized by urologists to improve patient care and outcomes.[1] Although they are crucial for renal drainage, stents can cause significant morbidity for patients if they become encrusted. Stent encrustation can lead to recurrent urinary tract infections, hematuria, obstruction, and renal failure.[1]

The risk factors for stent encrustation include the type of the stent material, patient-related factors, the presence of bacteria in the urine, and stent indwelling time. The stent indwelling time has been shown to be a significant risk factor for encrusted ureteral stents.[2,3] Studies looking at stent encrustation at different indwelling time intervals have shown an encrustation rate of 9%–26.8% at <6 weeks, a rate of 47.5%–56.9% at 6–12 weeks, and a rate of 75.9%–76.3% at >12 weeks.[4,5] When a retained stent becomes severely encrusted, making its removal difficult, it is called an entombed stent.

Laser lithotripsy may be used to remove an entombed stent.[2,6] While the holmium laser (HL) may still be considered as the gold standard for lithotripsy procedures, the introduction of the thulium fiber laser (TFL) and its widespread use challenges the HL as an alternative modality.[7] The preferred laser should be one that effectively treats the entombed stent while minimizing the damage to the stent and the ureter. The purpose of this study was to compare the therapeutic suitability of the HL and TFL for the treatment of entombed stents.

METHODS

The HL and TFL were compared utilizing a benchtop model. First, the stent transection time for each laser was compared. Second, the force required to break the stent after lasing for 5 s was measured. Finally, simulated lithotripsy of the entombed stents using both the lasers was performed. For all experiments, Olympus Gyrus 6 Fr stents were utilized, and both the lasers were operated at the settings routinely used clinically at our institution (0.8 J, 12 Hz with 270 µm laser fiber).[8] Both the HL and TFL were operated using a short pulse width. Randomization for all the sections of this project was done using a list randomizer to randomly determine the order of each experiment.

Stent transection

The stents were placed in a room-temperature saline bath and held straight using Kelly clamps at a fixed distance (2 cm). Enough tension was applied to keep the stent straight but not to pull the stent apart or aid in the transection. Ten experiments per laser were performed to transect the stent, and the transection time was recorded. The Mann–Whitney U test was used to compare the outcomes.

Force required to break stent

Similarly, the stents were placed in a room-temperature saline bath and held at a fixed distance (2 cm) with two Kelly clamps using just enough tension to keep the stents straight. After 5 s of lasing (either TFL or HL), the force required to break the stent was measured using force transducers (MARK 10 Force Guage Model M5-2 [Copiague, NY] and Asani Mini Crane Scale [Seattle, WA]). The baseline force to break the stents was measured using stents that had not been previously lased. Fifteen randomized experiments per laser were recorded and compared to the baseline force required to break the stent. The Mann–Whitney U test was used to compare the outcomes.

Simulated lithotripsy of entombed stents

A plastic mold (10 mm in diameter) and BegoStone (Bego USA, RI) were used to simulate an entombed stent. The BegoStone was prepared using a 15:6 ratio of BegoStone to water (calcium oxalate monohydrate consistency). After placing the stent in the center of the mold, the BegoStone preparation was poured around the stent for a length of 15 mm and allowed to dry. The completed model consisted of a stent encased in a cylindrical stone (10 mm in diameter and 15 mm long) inside a plastic mold simulating an entombed stent. The stones were rehydrated 24 h prior to the experiment.

The entombed stents were placed in room-temperature saline, and laser lithotripsy was performed in a prospective randomized order. Seven experiments were conducted for each laser with endpoints of lithotripsy time to release the stent, laser energy, and objective scoring of the stent damage. For the objective scoring of stent damage, two blinded urologists (who were not involved in the benchtop portion of the project) used a novel semi-quantitative scale (length of damage [mm] × severity [1–5]) to calculate the damage score. Evaluation of severity was determined using the following criteria: Grade 1 = little to no damage, Grade 2 = damage penetrating < ¼ of stent diameter, Grade 3 = damage penetrating ¼–-½ of stent diameter, Grade 4 = damage penetrating ½–-¾ of stent diameter, and Grade 5 = damage penetrating >¾ of stent diameter. The reviewers’ scores were combined and averaged to determine the mean stent damage score for each laser. The Mann–Whitney U and Chi-square tests were used to compare outcomes.

The authors confirm the availability of, and access to, all the original data reported in this study.

RESULTS

Stent transection

The time taken to completely transect the stent with the TFL was significantly shorter than with the HL (22.02 vs. 61.46 s; P < 0.001) [Figure 1].

Figure 1.

Mean stent transection time of thulium fiber laser and holmium laser

Force required to break stent

The force required to break the stent after 5 s of lasing was significantly lower with the TFL compared to the HL (5.34 vs. 15.24 N; P = 0.004) [Figure 2]. Both required a lower force to break the stent when compared to the baseline (33.8 N; P < 0.001).

Figure 2.

Mean force to break stent after 5 s of lasing with either thulium fiber laser or holmium laser

Simulated lithotripsy of encrusted stents

On simulated lithotripsy of the entombed stents, lithotripsy time (12.7 vs. 8.5 min; P = 0.11) and laser energy (4.7 vs. 2.7 kJ; P = 0.09) were similar between TFL and HL. The mean stent damage score was significantly higher when using the TFL compared to the HL (36.9 vs. 15.7; P = 0.017) [Figures 3 and 4].

Figure 3.

Mean stent damage score after simulated lithotripsy with thulium fiber laser or holmium laser

Figure 4.

(a) Thulium fiber laser damage to stent during simulated laser lithotripsy. (b) Holmium laser damage to stent during simulated laser lithotripsy

DISCUSSION

Both the HL and TFL have been used to treat entombed stents.[1,9,10] However, to the best of our knowledge, no studies comparing the effects of the TFL and HL on stent integrity have been previously reported. A known risk of using laser lithotripsy in the removal of entombed ureteral stents is the transection or fragmentation of the stent, which may make subsequent complete removal more challenging. Transection of the stent may require percutaneous nephrolithotomy to remove the remaining stent fragment. In addition, any fragments left behind from the damaged stent may be lithogenic, increasing the chances of stone recurrence. Damage to the ureter is also a known risk. Therefore, the use of a laser that minimizes the risk of damage during removal of the entombed stent is preferred.

In this study, when comparing the direct effects of the laser on the stent, the TFL resulted in a quicker stent transection time and weakened the stent at a faster rate than the HL. In addition, after 5 s of lasing, the stents in the TFL arm required less force to break than those in the HL arm. Even if the stent was not transected, the use of the TFL while releasing an encrusted stent increased the chances of its breakage during manual removal. When the performance of the laser was measured during simulated lithotripsy of entombed stents, there was no significant difference in the lasing time or laser energy required to release the stent. However, the use of the TFL resulted in significantly higher damage to the stent than the HL. The increased damage further increases the risk of stent transection, stent fragmentation, and breakage. This mirrors our clinical experience, where we have observed greater stent damage and fracture when treating entombed stents with the TFL.

These findings may be explained by the intrinsic differences between the TFL and HL. One difference is that the absorption coefficient of the TFL in water (129.2 cm⁻¹) is more than four-fold higher than that of the HL laser (31.8 cm⁻¹). In water, these coefficients translate to optical penetration depths of 0.314 mm for HL and 0.077 mm for TFL. For each respective optical penetration depth traveled by each laser, the energy of the laser beam passing through water is reduced by a factor of e (2.78182). For example, at a 1 mm distance, the HL will be at 4% of its initial intensity, while the TFL will be at a nearly undetectable 0.00024% of its initial intensity. Due to the logarithmic nature of radiation absorption, at this distance, the TFL’s energy is absorbed approximately 16,000 times more than the HL.[11] These factors contribute to TFL’s higher ablation efficiency.[12] Due to the increased ablation efficiency of the TFL, when the laser comes in contact with a stent, there is an increased risk of transecting the stent or causing damage that leads to fragmentation or breakage. Another difference is that the TFL has been demonstrated to generate significantly more heat when used in simulated lithotripsy in the proximal ureter.[13,14] This issue becomes even more relevant when the irrigation rate decreases, causing large increases in the temperature even at lower power settings.[15] Increased temperatures combined with the increased cutting efficiency likely resulted in the greater observed damage to the stent.

There is controversial evidence regarding the safety of the TFL, with some studies demonstrating its safety, while others have not found this to be true.[7,16,17] Adverse events of ureteral damage have been reported utilizing the TFL, such as thermal damage leading to kidney failure and ureteral stenosis.[18,19] In a 2-year prospective study (involving 478 patients) comparing the use of HL (n = 240) and TFL (n = 238) in laser lithotripsy, TFL demonstrated a significantly higher incidence of ureteric strictures (TFL vs. HL, 4.62% vs. 1.67%, respectively).[17] In addition, recalls have been issued for software upgrades to mitigate thermal injuries from the use of the TFL.[20,21] When using the TFL, higher irrigation rates may be used to reduce the risk of thermal injury. However, higher irrigation flow rate may result in higher intrarenal pressure and is associated with an increased risk of tissue injury, fluid extravasation, systemic inflammatory response syndrome, and sepsis.[22,23,24,25] The higher amount of heat produced by the TFL remains a cause for concern while removing entombed stents. In clinical practice, studies have demonstrated similar ureteroscopy times, stone-free rates, length of hospital stay, and complication rates between the two treatment modalities.[26,27] In addition, the HL has retropulsion, which assists in removal of the stone from the stent, while the TFL does not have this additional benefit.

This study does have limitations. This was a benchtop study, which does not fully replicate all the aspects of laser lithotripsy for an entombed stent in clinical practice. The volume of stone, type of stone, width of the ureter, and the length of lasing time do not perfectly mirror what may be commonly encountered in clinical practice. Moreover, a single type of stent was used in this study. Other types of stents may interact with each laser differently. In addition, this simulation allowed the removal of the entombed stent in a single operation, in clinical practice, the removal of encrusted ureteral stents may require more complicated procedures or multiple procedures.[2] However, using this model allowed for the standardization of the parameters, which allowed precise comparison between the two lasers.

To summarize, although the TFL has been shown to have similar outcomes to the HL in many previous studies, our benchtop study demonstrates an increased risk of stent breakage when treating entombed stents with the TFL. If using the TFL for an entombed stent, caution is advised as there may be a greater risk of stent damage leading to an increased risk of stent fracture during removal.

CONCLUSIONS

In this study, the TFL resulted in greater stent damage compared to the HL when lasing entombed stents. The TFL had a lower stent transection time and required lower force to break the stent. Urologists should be cautious when releasing entombed stents using the TFL as the damage conferred by the laser may significantly weaken the stent, increasing the risk of fracture during removal.

Conflicts of interest

There are no conflicts of interest.

Funding Statement

Nil.