JOURNAL OF ENDOUROLOGY
Volume 19, Number 8, October 2005
© Mary Ann Liebert, Inc.
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Effect of Holmium:YAG Laser Pulse Width on Lithotripsy
Retropulsion in Vitro
DAVID S. FINLEY, M.D.,1 JASEN PETERSEN,2 COROLLOS ABDELSHEHID, B.S.,1
MICHAEL AHLERING, B.S.,1 DAVID CHOU, M.D.,1 JAMES BORIN, M.D.,1 LOUIS EICHEL, M.D.,1
ELSPETH McDOUGALL, M.D.,1 and RALPH V. CLAYMAN, M.D.1
ABSTRACT
Background and Purpose: The effect of laser pulse width on calculus retropulsion during ureteroscopic
lithotripsy is poorly defined because of the limited availability of variable pulse-width lasers. We used an adjustable pulse-width Ho:YAG laser to test the effect of pulse width on in vitro phantom-stone retropulsion
and fragmentation efficiency.
Methods and Materials: An Odyssey 30 Ho:YAG laser (Convergent Laser Technologies, Oakland, CA) with
adjustable pulse width (350 or 700 sec) was used to treat spherical 10-mm plaster calculi in a model ureter
(N 40) and calix (N 16) utilizing 200- and 400-m fibers (10 Hz, 1.0 J). Calculi were placed in a waterfilled clear polymer tube, and laser energy was applied continuously in near contact until the stone had moved
8 cm. The time (seconds) and energy (joules) needed to cause the stone to traverse this distance was recorded.
Stones were also placed in a stainless-steel mesh calix model in which retropulsion was limited. Laser energy
was applied for 5 minutes at each pulse width. A laser-energy meter (Molectron Detector Inc, Portland OR)
was used to quantify fiber transmission efficiency after 1 minute of continuous lithotripsy for each fiber at
each pulse width.
Results: Retropulsion was greater for stones treated at 350 sec, indicated by a shorter time to traverse the
model ureter. For the 200-m fiber at 350 sec, the average time was 11.5 seconds v 20.3 seconds at 700 sec
(P 0.001). The average total energy delivered was 114.9 J at 350 sec v 199.8 J at 700 sec (P 0.001). For
the 400-m fiber at 350 sec, the average time was 5.8 seconds v 11.9 seconds at 700 sec (P 0.001). The
average total energy was 57.1 J at 350 sec v 127.3 J at 700 sec (P 0.001). In the caliceal model, at 350
and 700 sec with the 200- and 400-m fibers, mass loss was 34.9% and 33.4% (P 0.8) and 14.6% and
21.6% (P 0.04), respectively. The reduction in energy transmission at 350 sec and 700 sec with the 200m fiber after 60 seconds of continuous lasing was 8.82% v 9%, respectively (P 0.95). For the 400-m fiber,
the transmission loss was 18.4% at 350 sec v 4.4% at 700 sec (P 0.0002).
Conclusion: When treating ureteral calculi, retropulsion can be reduced by using a longer pulse width without compromising fragmentation efficiency. For caliceal calculi, the longer pulse width in combination with
a 400-m fiber provides more effective stone fragmentation.
INTRODUCTION
B
ECAUSE LASER APPLICATION to ureteroscopic
lithotripsy is relatively new, the optimal device and laser
parameters are still largely undefined. The holmium:YAG laser
has several advantageous characteristics: it can fragment or va-
1Department
2Convergent
porize virtually all types of stones with minimal collateral tissue damage. Its major drawback is uncontrolled retropulsion of
ureteral calculi up the urinary tract. Prior reports have noted
that retropulsion increases with both the optical fiber diameter
of the laser and the amount of energy the laser pulse delivers
to the stone.1 Although controlling the laser’s output may ap-
of Urology, University of California Irvine Medical Center, Orange, California.
Laser, Oakland, California.
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FINLEY ET AL.
pear straightforward, the physics of the laser–calculus interaction is complex and dependent on the laser’s physical parameters such as pulse width (i.e., duration), wavelength, and energy, as well as calculus characteristics.2 This study reports on
the efficacy of a new adjustable pulse-width laser in an in vitro
model of urinary calculi.
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MATERIALS AND METHODS
The Odyssey 30 holmium:YAG laser (Convergent Laser
Technologies, Oakland, CA) is a solid-state laser with a wavelength of 2100 nm adapted for intracorporeal lithotripsy. It possesses a novel feature that allows pulse width variation at two
discrete settings: 350 and 700 sec; 200-, 400-, 600, and 1000m optical fibers can be used.
We irradiated ceramic phantom calculi in an in vitro model
designed to mimic ureteral stones and calix-entrapped stones.
Similar size and weight (9 mm and 550 50 mg) spherical ceramic phantom calculi were soaked in deionized water for 20
minutes after Greenstein et al.3 A 200- or 400-m optical laser
fiber was fed through a 3.6F working channel of a 70-cm, 7.5
F flexible ureteroscope (Karl Storz, Culver City, CA) and extended 5 mm beyond the ureteroscope tip. To mimic a ureteral
stone, a phantom calculus was placed inside a 16-cm clear polymer tube, (inner diameter 12 mm), open on each end, and inscribed with distance markings (Fig. 1A) after White and
coworkers.4 The tube was secured to the base of a polymer tub
and filled with deionized water (Fig. 1B).
Thirty trials each were conducted at a pulse width of 350
and 700 sec with both the 200- and the 400-m fiber, respectively, for a total of 120 trials. A pulse frequency of 10 Hz
and pulse energy of 1.0 J was utilized. For each trial, a stone
phantom was placed in the tube at a starting point marked as
zero. Laser energy was then applied continuously to the center
of the stone at a normal incident angle. The optics of the uretero-
A
B
C
D
FIG. 1. Experimental set-up. (A) Laser and in vitro ureter with flexible ureteroscope tip inserted adjacent to phantom calculus.
(B) Magnified view of polymer in vitro ureter with ureteroscope tip adjacent to phantom calculus. (C) In vitro calix with phantom calculus. (D) Phantom calculi after 5 minutes of lasing at 700- and 350-sec pulse widths utilizing 400-m fiber.
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PULSE WIDTH AND RETROPULSION
to recut the laser fiber tip to ensure maximal transmission efficiency. A laser-energy meter (Molectron Detector, Portland,
OR) was used to quantify transmission efficiency after 1
minute of continuous lithotripsy at each pulse width and fiber
diameter. Four stones were treated at each pulse width using
both the 200-m and the 400-m fibers, for a total of 16 trials. After each trial, the stone was dried and weighed to calculate the percent mass lost.
Data were analyzed using Student’s t-test with Microsoft Excel V. 1997 on a Dell PC. Statistical significance was reported
at P 0.05.
RESULTS
scope were not utilized; instead, delivery of laser energy onto
the stone was controlled by direct observation from outside the
plastic tubing. After each pulse, the stone was displaced distally (retropulsed), and the ureteroscope was advanced until the
stone traversed a total of 8 cm—the finish line. The total time
(seconds) and energy (joules) from start to finish were recorded
for each trial. Each stone was used for 10 trials. Air bubbles
were flushed between trials. Only one investigator applied the
laser energy in all trials.
In a second experiment, designed to model an intracaliceal
stone, retropulsion was limited to only a few millimeters by
placing the stone in a stainless-steel mesh basket 12 mm in diameter with a pore size of 2 to 2.2 mm (Fig. 1C), with the fiber
tip positioned from above at approximately 1 mm from the
stone surface at a normal incident angle. After measurement
of the dry weights of the artificial stones, they were soaked in
deionized water for 20 minutes. A stone was then placed in
the basket and submerged in water. Laser energy (10 Hz, 1.0
J) was applied for 5 minutes at a pulse width of 350 or 700
sec. Each lithotripsy trial was interrupted every 60 seconds
In our ureter model, retropulsion was greater for phantoms
treated at a pulse width of 350 sec than at 700 sec (P 0.001), indicated by a shorter time to traverse the 8-cm distance
and by a lower total energy discharge (Fig. 2). As expected,
retropulsion was also found to increase as fiber diameter increased. For the 200-m fiber at a pulse width of 350 sec, the
average traverse time was 11.5 seconds ( 3.4 seconds) v
20.3 seconds ( 4.3 seconds) at 700 sec (P 0.001) (Fig.
2). For the 400-m fiber at a pulse width of 350 sec, the average time was 5.8 seconds ( 1.1 seconds) v 11.9 seconds
( 3.2 seconds) at 700 sec (Fig. 2). The total energy at 350
and 700 sec for the 200- and 400-m fibers was 114.9 J ( 33.2 J) and 199.8 J ( 40.6 J) and 57.1 J ( 10.0 J) and
127.3 J ( 39.2 J), respectively. The loss of stone mass accured in all the ureteral stone trials was negligible.
By contrast, in our caliceal model, there was significant loss
of stone mass. The percent loss after 5 minutes of continuous
lithotripsy at 350 and 700 sec with the 200-m fiber (10 Hz,
1.0 J) was 34.9% and 33.4%, respectively (P 0.8) (Fig. 3).
With the 400-m fiber, the percent loss of stone mass was
14.6% at 350 sec and 21.6% at 700 sec (P 0.04) (Figs.
1D; 3). The drop in laser transmission efficiency at 350 sec
and 700 sec with the 200-m fiber after 60 seconds of continuous lasing was 8.82% v 9%, respectively (P 0.95) (Fig.
4). For the 400-m fiber, the efficiency loss was 18.4% at 350
sec v 4.4% at 700 sec (P 0.0002) (Fig. 4).
FIG. 3. Fragmentation in caliceal-stone model. Percent stone
fragmentation at 350 and 700 sec after 5 minutes of continuous lasing utilizing 200- and 400-m fibers. (Bars ; *P 0.05).
FIG. 4. Percent loss of laser energy transmission at 350 and
700 sec after 60 seconds of continuous lasing utilizing 200and 400-m fibers. (Bars ; *P 0.05).
FIG. 2. Retropulsion in ureteral-stone model. Average total
time (seconds) to traverse 8-cm distance at 350- and 700-sec
pulse widths for 200- and 400-m fibers. (Bars ; *P 0.05).
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FINLEY ET AL.
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DISCUSSION
Laser light has the unique dual-function ability to both fragment urinary calculi and ablate or coagulate soft tissue. The
laser achieves this by two fundamental effects: (1) photoacoustic—the generation of a shockwave that causes localized
mechanical rupture of the stone—and (2) photothermal—irradiation and heating of the calculus, causing chemical decomposition or vaporization.5 Whether a particular laser exerts a
predominantly acoustic or thermal effect is determined principally by its pulse duration (sec) and energy (joules).1 The
longer the pulse duration, the more photothermal the effect. Because the Ho:YAG laser produces a relatively long pulse, its
effect is predominantly photothermal, resulting in intense direct irradiation and resultant thermal decomposition of the calculus with little acoustic tissue damage.
Stone irradiation is accelerated by the rapid heating and vaporization of water at the calculus surface. Vaporized water generates a stream of microbubbles that channel the laser’s energy
to the stone surface by parting the ambient fluid (“the Moses
effect”).6 In addition, heating of interstitial water within the
stone contributes to stone dissolution by generating an internal
shockwave that helps fracture the stone’s crystalline structure.5
A major drawback of laser lithotripsy is stone retropulsion.
This distal motion necessitates repositioning of the laser tip
and extends the operative time. Retropulsion during Ho:YAG
lithotripsy is known to increase in proportion to both the size
of the laser transmission fiber and the total pulse energy output.1 Narrow fiber lasers produce a deeper crater in which laser
energy is confined to stone dissolution and not undesirable stone
motion. However, changing fiber size will not by itself control
retropulsion. This motion also results from the sum of other distinct forces created by the laser’s pulse energy during calculus
fragmentation such as vapor-bubble expansion and collapse.1
Photothermal decomposition produces a plume consisting of vapor bubbles and diminutive stone fragments that pushes the residual calculus away from the laser tip, similar to jet propulsion. This surface plume combines with an internal shockwave
generated by sudden thermal expansion of the stone’s interstitial water.7 Their net force causes retropulsion.
Control of laser pulse energy has been hindered until recently
by a lack of variable pulse-width lasers (VPW). The advent of
VPW technology could allow the urologist to control stone
retropulsion while retaining effective fragmentation capability.
The Ho:YAG laser’s pulse width is usually fixed from 250 to 350
sec. Recently, Yoshida and colleagues8 used a VPW Ho:YAG
laser (Sphinx Ho 40, Heraeus Corp) to fragment artificial stones.
They used four pulse widths: 150, 300, 600, and 800 sec, noting that the shortest pulse produced the highest peak power and
shockwave energy. They provided no data on retropulsion.
We used an Odyssey 30 Ho:YAG laser, which allows discrete pulse-width settings of 350 or 700 sec. We wanted to
determine the optimal pulse width for stone fragmentation in
an in vitro model mimicking two clinical settings: (1) intraureteral stones, in which there is little or no restriction on stone
retropulsion, and (2) intracaliceal stones, in which retropulsion
is limited to a few millimeters. We used a variable pulse-width
Ho:YAG laser with two fiber widths (200 and 400 m) as well
as the two pulse widths (350 and 700 sec).
In our model, retropulsion was nearly 50% less at a pulse width
of 700 sec than at 350 sec, irrespective of fiber width. The
laser’s peak power is equal to its energy divided by its pulse width.
Because of this inverse relation of pulse width to peak laser power,
retropulsion should be less at a long pulse width, as we observed.
By contrast, in the entrapped-stone model, we found fragmentation efficiency was similar for either pulse width using a
200-m fiber (see Fig. 3). For the 400-m fiber, we found a
small (7%) but significant increase in fragmentation efficiency
at the longer pulse width (Fig. 3). For this larger fiber, tip damage was greater at 350 sec, resulting in an 18.4% reduction
in energy transmission compared with a 4.4% loss at 700 sec
(Fig. 4). The increased power and retropulsion at a short pulse
width, although limited by the calix model, may be significant
enough to cause more severe fiber-tip damage than is seen at
700 sec. As expected, for the 200-m fiber, retropulsion was
minimal, regardless of the pulse width, resulting in equivalent
tip damage and no difference in fragmentation efficiency.
CONCLUSIONS
Variable pulse-width lasers allow the operator to control
retropulsion. When treating ureteral calculi, retropulsion can be
reduced by using a longer pulse width without compromising
fragmentation efficiency. For caliceal calculi, a longer pulse
width, in combination with a 400-m fiber, provides slightly
more effective stone fragmentation, possibly because of less
fiber-tip damage.
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AJ. Stone retropulsion during holmium:YAG lithotripsy. J Urol
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3. Greenstein A, Matzkin H. Does the rate of extracorporeal shock wave
delivery affect stone fragmentation? Urology 1999;54:430–432.
4. White MD, Moran ME, Calvano CJ, Borhan-Manesh A, Mehlhaff
BA. Evaluation of retropulsion caused by holmium:YAG laser with
various power settings and fibers. J Endourol 1998;12:183–186.
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and photomechanical. Surg Clin North Am. 1992;72:531–556.
6. Jansen ED, Asshauer T, Frenz M, et al. Effect of pulse duration on
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7. Vassar GJ, Chan KF, Teichman J, et al. Holmium:YAG lithotripsy:
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Address reprint requests to:
Ralph V. Clayman, M.D.
Dept. of Urology
University of California, Irvine Medical Center
101 The City Drive, Building 55, Room 304, Rt 81
Orange, CA 92868
E-mail: rclayman@uci.edu
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This article has been cited by:
1. Pawan Kumar Gupta . 2007. Is the Holmium:YAG Laser the Best Intracorporeal Lithotripter for the Ureter? A 3-year Retrospective
Study. Journal of Endourology 21:3, 305-309. [Abstract] [PDF] [PDF Plus]
2. Andrew J. Marks, Joel M. H. Teichman. 2007. Lasers in clinical urology: state of the art and new horizons. World Journal of
Urology 25:3, 227. [CrossRef]
3. Sean Pierre, Glenn M. Preminger. 2007. Holmium laser for stone management. World Journal of Urology 25:3, 235. [CrossRef]
4. 2006. LiteratureWatch. Journal of Endourology 20:5, 362-368. [Citation] [PDF] [PDF Plus]
5. Hyun Wook Kang, Ho Lee, Joel M.H. Teichman, Junghwan Oh, Jihoon Kim, Ashley J. Welch. 2006. Dependence of calculus
retropulsion on pulse duration during HO: YAG laser lithotripsy. Lasers in Surgery and Medicine 38:8, 762. [CrossRef]