Eugenio Ventimiglia1,2,3, Olivier Traxer1,2
1. Sorbonne Université, Service d’Urologie, AP-HP, Hôpital Tenon, F-75020 Paris, France
2. Sorbonne Université, GRC n°20, Groupe de Recherche Clinique sur la Lithiase Urinaire, Hôpital Tenon, F-75020 Paris, France
3. Division of Experimental Oncology/Unit of Urology; URI; IRCCS Ospedale San Raffaele, Milan, Italy
The term laser is an acronym which stands for “light amplification by stimulated emission of radiation”. Laser emission is therefore a light emission whose energy is used, in the case of intracorporeal lithotripsy, for targeting and ablating the stone. Several laser sources were proposed and evaluated during the last decades in the endourological field [1]. Since its first urological application in 1992 [2], Holmium – Yttrium Aluminum Garnet (Ho:YAG) laser has become the main player among lasers currently used for lithotripsy, due to its effectiveness, versatility, and safety profile [3]. Ho:YAG laser, thanks to a holmium doped YAG crystal, emits light at a wavelength of 2100nm, in the infra-red spectrum.
When using Ho:YAG lasers in the clinical daily practice, urologists have the possibility of setting three main parameters: energy (measured in Joules, J), frequency (pulses per seconds or Hertz, Hz), and pulse duration (microseconds). Laser power is defined by the product of energy and frequency (measure in Watts, J*Hz=W). The values for energy frequency and frequency can be decided discretely by the urologist. Energy values will determine the “strength” of the single laser pulse, and are considered low in the 0.2-0.5J range and high in the 1-2J range. The frequency determines the speed at which the laser pulse are conveyed by the laser fiber, and is considered as low in the 1-5Hz range and high in the 15-80Hz. Pulse duration is related to the single pulse (therefore, it will not affect power), and is represented by the time stint during which a single laser pulse is emitted. Range for pulse duration is 200 vs 800 microseconds and usually classified as “long” or “short” by laser producers, being not precisely adjustable when using urological lasers. At this regard, a strong consent on the proper definition of either long or short pulse duration is lacking.
The proper setting of laser parameters is fundamental during lithotripsy in order to obtain the desired effect. The right combination of energy, frequency and pulse duration determines whether dusting, fragmentation, or popcorning will be achieved (Figure 1)
Figure 1. Laser parameter specification according to the desired effect
In order to dust a stone it is indicated to use low energy (~5Hz), high frequency (15-20Hz), and long pulse (800 microseconds), with a total power of 7.5-10 W. Conversely, high energy (1.5-2Hz), low frequency (5Hz), and short pulse (200 microseconds) are required for fragmentation. In order to achieve the so called popcorn effect, high energy (1-1,5J), high energy (15-20Hz), and long pulse (600 microseconds) should be used. Latest lasers devices come with pre-defined laser settings, i.e. it is possible to get suggestions for the most appropriate parameters according to each specific desired effect (dusting-fragmentation-popcorning) and to the stone composition.
Generally speaking, low power (20-30Hz) laser emitting devices are sufficient for stone treatment and especially for dust production. The advantage of using high power (up to 120W) devices lies in the opportunity of ideally performing fastest procedures due to the possibility of using high frequencies.
Besides laser settings, the choice of a proper laser fiber is a fundamental step when performing lithotripsy. Laser fibers are available with different diameters, usually ranging from 200 up until 550 µm. Considering lithotripsy during flexible ureteroscopy (fURS), small calipered laser fiber (i.e. 200-273 µm) are usually the most suitable ones, since they were described to have same efficiency, more flexibility, more irrigation, and less retropulsion compared to larger ones [4]. Fiber diameter directly determines the energy density, i.e. the energy delivered in a single pulse per mm2 of fiber surface; using the same amount of energy, a fiber with a smaller diameter will deliver an higher energy density thanks to its smaller surface (Figure 2).
Figure 2. Energy density according to the laser fiber section
A. Using a 1J single pulse, different energy densities are obtained according to the fiber density. A 273 micron laser fiber (area of the section 0,058mm2) will have a density of 1J/0,058mm2 = 17 J/mm2, whereas a smaller (150 micron fiber) will have 3.3x energy density (56 J/mm2).
B. Lower energy levels are requested in order to reach the same energy density level when using smaller fibers
Practical advices for better using laser fibers include its cleavage. It was shown [5] that cleaving the fiber tip may restore its effectiveness, although only for a limited time; it is therefore advisable to cut the laser fiber every 10 KJ or every 10-15 mins, especially in case of stressful conditions for the fiber itself such as the use of high energy, short pulse duration, and hard stones. Fiber tips can be safely cut with metallic scissor without reducing lithotripsy efficacy. Further advantages of fiber tip cleavage include preserving the scopes from damage during laser fiber insertion and better identification of the laser tip during lithotripsy [6]. In order to properly use the laser fiber without increasing risk of damaging the scope during pulse emission, a good general rule is to visualize the fiber tip at 1/4 of the screen diameter (Figure 3);
Figure 3. Laser fiber tip position in relation to the tip of the flexible ureteroscope
The laser fiber tip (blue, at 9 o’clock position) is visually located at the first quarter of the screen starting from the left hand side. As a thumb rule, the screen can be dived vertically on the midline, further subdividing the left sector again on the vertical midline in order to obtain an indicator of the proper position of the laser fiber. With such a setting, the distance between laser fiber and ureteroscope tip is 3mm.
such a distance corresponds to a 3mm distance from the scope, safe enough for avoiding damages related to the backburn effect.
Despite the superiority of Ho:YAG laser, still considered the gold standard of endoscopic laser lithotripsy [3], the need for a more efficient laser lithotripsy prompted research towards the development of new laser sources. This is the case for thulium fiber laser (TFL), a laser whose functioning is based on a 10 to 30m fiber with 10µm core doped by active thulium ions and pumped by diode laser. TFL emission wavelength is at 1940nm, more closely matching major water absorption peak in tissue and calculi compared to Ho:YAG. Although ideal parameters for TFL lithotripsy have not been determined yet, pre-clinical laboratory studies gave interesting insights. TFL is capable of working at very high frequencies (up to 2000Hz), very low energy levels (50mJ), and using smaller fibers (minimum 50µm): all in all, these features can explain the higher dusting efficiency (both in terms of speed and dust production) observed during laboratory based lithotripsy. Future laboratory and clinical studies will tell whether TFL will be an effective new player in laser intracorporeal lithotripsy.
References
[1]Talati JJ, Tiselius HG, David MA, Ye Z, Abbas F, Ather H, et al. Urolithiasis: Basic science and clinical practice. Urolithiasis Basic Sci Clin Pract 2012:1–982. doi:10.1007/978-1-4471-4387-1.
[2]Johnson DE, Cromeens DM, Price RE. Use of the holmium:YAG laser in urology. Lasers Surg Med 1992;12:353–63.
[3]Türk C, Neisius A, Petrik A, Seitz C, Skolarikos A, Thomas K, et al. EAU Guidelines on Urolithiasis 2018.
[4]Kronenberg P, Traxer O. In vitro fragmentation efficiency of holmium: yttrium-aluminum-garnet (YAG) laser lithotripsy–a comprehensive study encompassing different frequencies, pulse energies, total power levels and laser fibre diameters. BJU Int. 2014 Aug;114(2):261-7.
[5]Haddad M. et al. Impact of laser fiber tip cleavage on power output for ureteroscopy and stone treatment. World J Urol. 2017 Nov;35(11):1765-1770.
[6]Talso M, Emiliani E, Haddad M, et al. Laser Fiber and Flexible Ureterorenoscopy: The Safety Distance Concept. J Endourol. 2016 Dec;30(12):1269-1274.