How Lasers Ablate Stones: In Vitro Study of Laser Lithotripsy (Ho:YAG and Tm-Fiber Lasers) in Different Environments

Abstract

Introduction:

There are two main mechanisms of stone ablation with long-pulsed infrared lasers: photothermal and photomechanical. Which of them is primary in stone destruction is still a matter of discussion. Water holds importance in both mechanisms but plays a major role in the latter. We sought to identify the prevailing mechanism of stone ablation by evaluating the stone mass loss after lithotripsy in different media.

Materials and Methods:

We tested a holmium:yttrium–aluminum–garnet (Ho:YAG) laser (100 W; Lumenis), a thulium-fiber laser U1 (TFL U1) (120 W; NTO IRE-Polus, Russia), and a SuperPulse thulium-fiber laser U2 (TFL U2) (500 W; NTO IRE-Polus). A single set of laser parameters (15 W = 0.5 J × 30 Hz) was used. Contact lithotripsy was performed in phantoms (BegoStones) in different settings: (a) hydrated phantoms in water, (b) hydrated phantoms in air, (c) dehydrated phantoms in water, and (d) dehydrated phantoms in air. Laser ablation was performed with total energy of 0.3 kJ. Phantom mass loss was defined as the difference between the initial phantom mass and the final phantom mass of the ablated phantoms.

Results:

All lasers demonstrated effective ablation in hydrated phantoms ablated in water; no visual differences between the lasers were detected. The ablation of dehydrated phantoms in air was also effective with visible vapor during ablation and condensation on the cuvette wall. Dehydrated phantoms in water and in air show minimal to no ablation accompanied with formation of white crust on phantom surface. Among laser types, TFL U2 had the highest phantom mass loss in all groups except for dehydrated phantoms ablated in air.

Conclusions:

Our results suggest that both photothermal and thermomechanical ablation mechanisms (explosive vaporization) occur in parallel during laser lithotripsy. In Ho:YAG and TFL U2 stone ablation explosive vaporization prevails, whereas in TFL U1 ablation photothermal mechanism appears to predominate.

Introduction

Lasers have been the subject of intense research in urology for the past several decades. The holmium:yttrium–aluminum–garnet (Ho:YAG) laser is one of the most commonly used lasers for lithotripsy, demonstrating high safety and efficacy.¹ Active research has led to the emergence of several number of new laser lithotripters. Based on encouraging in vitro studies, the novel SuperPulse thulium fiber laser (500 W) now seems to be a suitable tool for lithotripsy in routine clinical practice.^2,3

Despite wide adoption of lasers in clinical practice, the exact mechanism of laser stone ablation is still poorly understood.⁴ Two primary mechanisms of stone ablation have been described: photothermal (chemical decomposition of stone components heated with laser irradiation or heated by boiling and vaporizing water) and thermomechanical (vaporization of water inside stone pores leading to stones bursting from the inside).⁵ Water media play a major role in thermomechanical ablation (explosive vaporization).⁶ Use of waterless settings for laser lithotripsy could shed light on predominant mechanism of stone destruction in different types of lasers. Herein, we sought to identify the prevailing mechanism of stone ablation by evaluating the stone mass loss after lithotripsy in water vs air media.

Materials and Methods

We compared three lasers with 550 and 600 μm bare-ended fibers: a Ho:YAG laser (wavelength of 2.1 μm; Lumenis), a thulium-fiber laser U1 (TFL U1) (wavelength of 1.9 μm, 120 W; NTO IRE-Polus), and a SuperPulse thulium-fiber laser U2 (TFL U2) (wavelength of 1.9 μm, 500 W; NTO IRE-Polus). A single set of laser parameters (15 W = 0.5 J × 30 Hz) was used.

For ablation, 5 × 5 × 5 mm artificial square stones (phantoms) were used. A total of 60 BegoStones (Bego GmbH, Bremen, Germany) were used in the experiment to achieve uniformity. The phantoms were weighted prior the study (with scales DEMCOM DL-123 [precision −0.001 g]). Half of them (30) were stored in air for at least 1 day at 22°C (dry = dehydrated phantoms). The remaining 30 were stored in water at 22°C for at least 24 hours before the experiments (hydrated phantoms).

The water ablation setup consisted of an outer (20 × 20 × 40 mm) and an inner (10 × 10 × 40 mm) quartz cuvette (Fig. 1). The bottom of the inner cuvette had several orifices 1 mm in diameter. The water for irrigation was pumped in by two pumps at the top and at the bottom of the cuvette. The phantoms were placed in the inner cuvette, and the laser fiber was inserted from the top during laser exposure. The air ablation was performed using the same setup and approach, but without water in the cuvettes.

FIG. 1.

The experimental setup.

Contact lithotripsy was performed in four groups, with 15 artificial stones per group (five with each of lasers): (a) hydrated phantoms in the water medium, (b) hydrated phantoms in the air medium, (c) dehydrated phantoms in the water medium, and (d) dehydrated phantom in the air medium. Ablation of stones in air lasted for 20 seconds in total and were stopped after first 10 seconds for 5 seconds to let the fiber cool down and limit smoke formation. Fiber was cleaved after each ablation to exclude potential effect of fiber tip damage on experiment.

The fiber was fixed in a fiber holder and moved along the surface by the urologist (M.T.) to make sure that ablation was performed in contact mode. Each phantom was ablated with total energy of 0.3 kJ. The fiber was moved along the surface without drilling to avoid fragmentation of the phantoms.

After all experiments were performed, the phantoms were stored in air at 22°C for at least 24 hours. The phantoms were then weighed, and the phantom mass loss (mg) for each of the four groups and three lasers was calculated. Mass loss was defined as the difference between the initial mass and the final mass of the ablated phantoms.

For statistical analysis, the SPSS 23 software package (IBM Corp, Armonk, NY) was used. To determine statistical differences between the independent parameters of the groups the one-way analysis of variance (ANOVA) test was used. Achieved data were expressed as mean ± standard deviation. A p-value of 0.05 was considered to indicate statistical significance.

Results

The mass loss data are presented in Table 1. There was marked difference between four groups in mass loss and visible stone formation changes (Fig. 2):

FIG. 2.

Stone ablation in different settings: (a) hydrated phantoms in water, (b) hydrated phantoms in air, (c) dehydrated phantoms in water, and (d) dehydrated phantoms in air.

Table 1.

Phantom Mass Loss (mg, Mean ± Standard Deviation) for Ho:YAG, TFL U1, and TFL U2 (0.5 J/Pulse at 15 Hz)

Mass loss, mg	Hydrated phantoms		p	Dehydrated phantoms		p
Mass loss, mg	In water (n = 15)	In air (n = 15)	p	In water (n = 15)	In air (n = 15)	p
Ho:YAG	35.8 ± 7.5	31.4 ± 2.2	0.245	20.4 ± 0.9	7.5 ± 0.7	<0.001
TFL U1	38.8 ± 4.3	27.8 ± 5.0	0.004	11.3 ± 1.4	13.5 ± 4.9	0.115
TFL U2	61.3 ± 7.8	53.7 ± 12.7	0.008	38.8 ± 5.5	3.0 ± 0.1	<0.001
p-Value (Ho:YAG vs TFL U1)	0.461	0.180		<0.001	0.005
p-Value (Ho:YAG vs TFL U2)	<0.001	<0.001		<0.001	<0.001
p-Value (TFL U1 vs TFL U2)	<0.001	<0.001		<0.001	<0.001

Ho:YAG = holmium:yttrium–aluminum–garnet; TFL U1 = thulium-fiber laser U1; TFL U2 = SuperPulse thulium-fiber laser U2.

(a) Hydrated phantoms in water: effective ablation, no visual differences between the lasers were detected.

(b) Hydrated phantoms in air: effective ablation, visible vapor during ablation and condensation on the cuvette wall was observed during ablation.

(c) Dehydrated phantoms in water: minimal ablation, an easily removable white crust (debris) on the phantom surface formed within the first few seconds of ablation.

(d) Dehydrated phantoms in air: minimal to no ablation, an easily removable white crust (debris) on the phantom surface, no visible indentation (zone of stone ablation).

The biggest mass loss after a Ho:YAG ablation was found in the hydrated phantoms in the water medium (35.8 ± 7.5 mg), whereas the smallest mass loss was seen in the dehydrated phantoms in air (7.5 ± 0.7 mg).

TFL U1 had a similar ablation efficacy as Ho:YAG for hydrated phantoms in both water and air (38.8 ± 4.3 and 27.8 ± 5.0 mg, respectively). For dehydrated phantoms ablated in air, TFL U1 demonstrated the best mass loss among the three lasers (13.5 ± 4.9 mg). Mass loss of dehydrated phantoms in water was less with TFL U1, however (11.3 ± 1.4 mg).

TFL U2 was characterized by the highest phantom mass loss in hydrated phantoms in water and air (61.3 ± 7.8 mg and 53.7 ± 12.7 mg, respectively). Visually TFL U2 demonstrated the best efficacy for stone ablation with the deepest hollow under the fiber tip. For dehydrated phantoms in water, the most significant changes were seen in the TFL U2 group (38.8 ± 5.5 mg), whereas dehydrated phantoms in air demonstrated a minimal mass loss among all the phantom groups and settings (3.0 ± 0.1 mg). Retropulsion occurred only with Ho:YAG laser both in air and water.

Discussion

Stone fragmentation results from the absorption of laser energy (e.g., photons).⁶ Depending on the amount of energy absorbed by the water vs the stone, laser stone ablation is a compromise between a photothermal ablation and a photomechanical destruction. A photothermal ablation mechanism involves direct laser energy absorption by the stone (or in some cases, indirect, when stone material is heated with boiling/vaporizing water), leading to heat generation causing chemical decomposition and ablation.^7,8

In contrast, there are several studies that support the opposite primary mechanism of stone ablation by mechanical effects because of cavitation collapse for ultrashort and short pulse durations of the bubble formed at the tip of laser fiber while it is emitting.⁹ However, several authors proved that lasers operating at >2 microseconds do not create cavitation that generate significant shockwaves, such that at pulse durations longer than exist for ultrashort or short pulse lasers, conditions do not exist for shockwave-induced fragmentation.^6,8

In our study, white crust of the phantom was found mostly using the TFL U1, which could be because of chemical decomposition of the phantom material. Ablation of the stone with it was accompanied by formation of the crust on the stone surface (Fig. 2c, d), which was larger in air setups than in water setups. On the contrary, using Ho:YAG and TFL U2 lasers we found that the ablation of the hydrated phantoms in water resulted in the largest mass loss, whereas ablation of dehydrated phantoms in air provided the least mass loss.

Vassar and colleagues suggested that the primary photothermal mechanism of Ho:YAG lithotripsy involves direct absorption of laser energy by a stone.¹ In that study, lithotripsy was most efficient for dry stones and occurred before the collapse of a vapor bubble. Further research in the field allowed authors to suggest that in water medium more energy was necessary to vaporize the water layer between the fiber tip and the surface of the phantom, aside from interstitial water heating.⁵ Authors reported that the laser irradiation exposure increases the temperature above a critical threshold causing stone chemical breakdown and plume ejection.⁵

We found a similar mechanism in our in vitro experiment in the TFL U1 laser group. Among the three lasers, this group had the highest mass loss in dehydrated phantoms in air (13.5 ± 4.9 mg), whereas mass loss of dehydrated phantoms in water was less (11.3 ± 1.4 mg). This may indicate that the prevailing mechanism of stone ablation with a TFL U1 laser is photothermal.

Contrary to the results of Vassar and colleagues,¹ we observed the best Ho:YAG ablation in hydrated phantoms in water compared with dehydrated phantoms in air. The differences between our results and Vassar and others might be explained by the lower peak power of the Ho:YAG laser used in their study compared with ours. Similar results were found in our study for the TFL U2 laser, supporting the fact that in Ho:YAG and TFL U2 lasers, another mechanism of stone ablation appears predominant. Yet it bears emphasis to repeat that cavitation-induced shockwave is not a significant contributor to fragmentation with any of the longer pulse duration lasers tested, so mechanical or acoustical stress was not a factor in lithotripsy we observed.

Possible explanation for this is an explosive vaporization mechanism (thermomechanical) reported by Beghuin and associates.¹⁰ Authors reported that Ho:YAG may break stone through explosive vaporization of interstitial water: during laser lithotripsy stone surface undergoes chemical decomposition, whereas remaining energy vaporizes stone organic constituents and interstitial water, leading to increase of pressure and ejection of stone fragments.¹⁰ Support of this theory was demonstrated by Hardy and colleagues in an experimental setup with Ho:YAG and TFL lasers. Authors proved that irradiation can vaporize the water trapped in pores and fissures near the surface of hard calculi resulting in a high local pressure.¹¹ This produces a mechanical stress wave inside the stone, which may be sufficient for ablation and removal of weakened material from the exposed stone wall.^9,12 Thermomechanical mechanism (explosive vaporization) is usually considered a part of photothermal ablation. However, it is important to stress that this mechanism played a major role in stone ablation with Ho:YAG and TFL.

We found results supporting this theory: while the hydrated phantoms in water were ablated well, dehydrated phantoms in water were ablated during the first seconds of laser exposure (while the water absorbed in the stone surface was vaporized), after which ablation stopped (water was not able to infiltrate the stone pores anymore) and a superficial crust of chemically destroyed stone compounds was formed. This crust prevented further stone ablation. Therefore, we suppose that the water in the cracks and pores could play a major role in stone ablation. Without it, chemically destroyed stone compounds cannot be removed from the surface.

It is well known that different lasers' parameters can lead to different prevailing mechanism of stone ablation. Wavelength is usually mentioned as the major parameter shaping laser–tissue interactions.^4,13 TFL operates at 1.9 μm in contrast to Ho:YAG operating at 2.1 μm; this allows TFL for three to four times better energy absorption in water.^14
–16

However, TFL U1 and U2 shared the wavelength, yet, the effects of the lasers differ significantly. Such difference in ablation rates for TFL U1 and TFL U2 meant that it is not only wavelength of the laser, which is influencing stone ablation. We suppose that our findings can be associated with laser peak power: 120 W (peak) in TFL U1 in contrast to 500 W (peak) in TFL U2 and 2–10 kW (peak) of Ho:YAG. To estimate them in experimental regimen (15 W = 0.5 J × 30 Hz) we used InGaAs photodiode (Hamamatsu G12182-003K; Hamamatsu Photonics, Japan) with spectral response range from 0.9 to 2.1 μm. Use of this photodetector did not allow us to estimate exact values of each laser peak; however, dramatic difference between Ho:YAG and TFL peaks is seen easily (Fig. 3). Higher impulse peak power allows for instantaneous heating of stone with its chemical decomposition and explosive vaporization of water trapped inside of it. Together these two mechanisms allow for fast ablation.¹⁷

FIG. 3.

Peak profiles of Ho:YAG and TFL lasers (InGaAs PIN photodiode, Hamamatsu G12182-003K; Hamamatsu Photonics, Japan). Ho:YAG = holmium:yttrium–aluminum–garnet; TFL = thulium-fiber laser.

However, it should be mentioned that a higher peak power is always better. In some cases its increase may not contribute to better ablation but lead to energy loss.¹⁷ On the contrary, less peak power leads to effective heating of the stone compounds, yet water in pores of the stone is not vaporized at a rate necessary for momentary stone destruction. Therefore, we suppose that better TFL energy absorption in water allows the laser to partially compensate for the lack of peak power compared with Ho:YAG. It can be seen in dry phantoms in air where Ho:YAG showed better ablation than TFL U2.

Therefore, the role of water absorption is indisputable. Also, a prolonged peak of TFL allows for uniform heating of the stone. However, high peak power (500 W) speeds up the process leading to rapid vaporization of water and thermomechanical damage allowing for at least equal (or even greater) ablation rate (Fig. 3). Moreover, low efficacy of 100 W TFL hints at the fact that the higher peak power plays the leading role.

Limitations

Among possible limitations of the study was the fact that we did not use a Ho:YAG laser with Moses effect. However, we feel this was unnecessary, since we sought to identify the exact mechanism of stone ablation rather than compare the efficacy of different lasers. The second limitation was the use of phantoms (BegoStone). The benefit was the ability to experiment with stones of the same density, size, and configuration. Therefore, we could change environments and hydration of stones while keeping all other factors constant. We supposed that use of BegoStones may translate into increased reproducibility of the experiment and make it more uniform. However, such differences in results with some previous studies could be because of use of phantoms. We plan to use natural stones in future studies, since different components can affect the ablation process. The third limitation involved BegoStones immediately absorbing water through cracks and pores from the water, which would have influenced the results of dehydrated phantoms in water. It should also be mentioned that the so-called dry phantoms in our study had not been desiccated and some thermomechanical damage (explosive vaporization) might have occurred. However, we believe that the fact that we compared mass loss of the phantoms between the groups (and found extensive differences) and that all of phantoms were stored in similar conditions makes prior desiccation of phantoms unnecessary. Moreover, after freeze-drying of the phantoms (during results checkup) we observed maximum of only 2–3 mg decrease in stone mass. White crust (debris) on the surface of air-ablated phantoms may be led to laser irradiation shielding as a potential factor that made these cohorts less robust in terms of ablation. However, we believe that debris removal is a part of explosive vaporization mechanism; therefore, it is not a limitation solely. Another limitation is the absence of a hands-free fragmentation setup, as previously described by Bader and colleagues,¹⁸ which allows elimination of bias and technical variation. However, because of extensive differences in amounts of stone ablation between groups, we felt that it is unnecessary. Yet, all experimental setups in our research were performed by the same urologist and in the same manner to minimize variability between experiments.

Conclusion

We believe that ablation mechanisms (photothermal and thermomechanical) occur in parallel during laser lithotripsy. Initial stage of stone ablation involves photothermal decomposition, whereas later thermomechanical breakdown. Our data suggest that Ho:YAG and TFL U2 predominantly involve a thermomechanical stone ablation mechanism (explosive vaporization), whereas TFL U1 involves a mostly photothermal mechanism. TFL U2 was characterized by the highest stone mass loss in all groups except dehydrated stones ablated in air.

Footnotes

Author Disclosure Statement

No competing financial interests exist.

Funding Information

No funding was received.

Abbreviations Used

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