Holmium:YAG percutaneous nephrolithotomy: the laser incident angle matters

Numerical Response Surfaces of Volume of Ablation and Retropulsion Amplitude by Settings of Ho:YAG Laser Lithotripter

Objectives

Although laser lithotripsy is now the preferred treatment option for urolithiasis due to shorter operation time and a better stone-free rate, the optimal laser settings for URS (ureteroscopic lithotripsy) for less operation time remain unclear. The aim of this study was to look for quantitative responses of calculus ablation and retropulsion by performing operator-independent experiments to determine the best fit versus the pulse energy, pulse width, and the number of pulses.

Methods

A lab-built Ho:YAG laser was used as the laser pulse source, with a pulse energy from 0.2 J up to 3.0 J and a pulse width of 150 μs up to 1000 μs. The retropulsion was monitored using a high-speed camera, and the laser-induced craters were evaluated with a 3-D digital microscope. The best fit to the experimental data is done by a design of experiment software.

Results

The numerical formulas for the response surfaces of ablation speed and retropulsion amplitude are generated.

Conclusions

The longer the pulse, the less the ablation or retropulsion, while the longer pulse makes the ablation decrease faster than the retropulsion. The best quadratic fit of the response surface for the volume of ablation varied nonlinearly with pulse duration and pulse number.

Lasers in percutaneous renal procedures

INTRODUCTION

Since the invention of lasers in 1960, they have been increasingly used in medicine. In this review paper, the types of lasers used in urology, in addition to their applications to percutaneous renal surgery will be reviewed. Specifically, use of lasers in the percutaneous management of renal stones, upper tract transitional cell carcinoma and stricture will be reviewed.

MATERIALS AND METHODS

Pubmed was searched for citations since 1966. The following terms were used: "lasers", "calculi", "endopyelotomy", and "transitional cell carcinoma".

RESULTS

Due to its minimal depth of penetration, holmium laser has proven to be safe and efficacious. It is currently the primary energy source for flexible instrumentation, and also has demonstrated efficacy in percutaneous lithotripsy (faster than ultrasonic lithotripsy and safer than electrohydraulic lithotripsy). Holmium laser been used for antegrade endopyelotomy and percutaneous resection of upper tract transitional cell carcinoma.

CONCLUSIONS

Holmium laser is safer than other lasers and has become the gold standard for laser lithotripsy for flexible instrumentation. It has been used successfully in the percutaneous management of renal stones, ureteropelvic junction obstruction, and upper tract transitional cell carcinoma. Holmium laser is an alternative energy source to conventional lithotripters and electrocautery for endopyelotomy and resection of upper tract transitional cell carcinoma.

[PCNL: technique, results and complications]

Recent technological changes of percutaneous nephrolithotomy are reviewed. Results and complications of the most recent publications are presented.

Side-Firing Germanium Oxide Optical Fibers for Use with Erbium:YAG Laser

BACKGROUND AND PURPOSE

During recent experimental studies, the erbium:YAG laser has been shown to be more efficient for lithotripsy and more precise for incision of soft urinary tissues than the conventional holmium:YAG laser. Mid-infrared optical fibers are being developed to allow endoscopic delivery of Er:YAG laser radiation. This paper describes the simple construction and characterization of a side-firing germanium oxide fiber for potential use with the Er:YAG laser in endourology.

MATERIALS AND METHODS

The 450-microm-core side-firing fibers were constructed from germanium oxide fibers by polishing the distal tip at a 45 degrees angle and placing a protective quartz cap over the tip. The Er:YAG laser radiation, with a wavelength of 2.94 microm, was transmitted through the fibers.

RESULTS AND CONCLUSION

The fiber transmission rate and damage threshold measured 48 +/- 4% and 149 +/- 37 mJ, respectively (n = 6 fibers). Sufficient pulse energies were transmitted through the side-firing fibers to produce contact tissue ablation.

In Vitro Assessment of Ultrasonic Lithotriptors

PURPOSE

Ultrasonic lithotriptors are commonly used to fragment and remove stones during percutaneous nephrolithotomy. To date a comparative assessment of current units has not been accomplished without potential operator bias. An objective testing environment is required for optimal appraisal of the efficiency of ultrasonic lithotriptors.

MATERIALS AND METHODS

An in vitro test system was devised to evaluate the ability of ultrasonic lithotriptors to core through artificial stones. The system consisted of an irrigation sheath (Cook Urological, Spencer, Indiana) through which ultrasonic probes were placed. Ultrasonic hand pieces and probes were secured in an upright position. An Ultracal-30 (U.S. Gypsum, Chicago, Illinois) stone cylinder (mean length 12.8 +/- 0.6 mm, mean diameter 7.6 +/- 0.07 mm) was centered on the probe tip. A weight (62.7 gm) was placed atop the stone to provide a constant force. We evaluated the Olympus LUS-1 and LUS-2 (Olympus, Melville, New York), Circon-ACMI USL-2000 (Circon-ACMI, Southborough, Massachusetts), Karl Storz Calcuson (Karl Storz, Culver City, California) and Richard Wolf model 2271.004 (Richard Wolf, Vernon Hills, Illinois). All probes had outer diameters of 3.4 mm except for the Circon-ACMI unit (3.8 mm). Using 100% power settings times for complete stone penetration were assessed for all units. Differences in mean stone penetration times were compared using ANOVA.

RESULTS

The Olympus LUS-2 had the fastest mean stone penetration time (28.8 +/- 2.7 seconds). This value was used to normalize the data into efficiency ratios, where other unit times were expressed as multiples of the LUS-2 time: Olympus LUS-2 (1.0 +/- 0.1) equals Circon-ACMI USL-2000 (1.1 +/- 0.3) greater than Karl Storz Calcuson (1.4 +/- 0.3) greater than Olympus LUS-1 (2.1 +/- 0.5) greater than Richard Wolf (3.6 +/- 0.8). Efficiencies of the LUS-2 and USL-2000 units were essentially equivalent, with all others significantly less efficient (p <0.05).

CONCLUSIONS

This new in vitro testing model provides an objective, reproducible method for evaluating the efficiency of intracorporeal lithotriptors. Of the units tested the Olympus LUS-2 and Circon-ACMI USL-2000 were the most efficient.

Stone retropulsion during holmium:YAG lithotripsy

PURPOSE

We modeled retropulsion during holmium:YAG lithotripsy on the conservation of momentum, whereby the force of ejected fragment debris off of the calculous surface should equal the force of retropulsion displacing the stone. We tested the hypothesis that retropulsion occurs as a result of ejected stone debris.

MATERIALS AND METHODS

Uniform calculous phantoms were irradiated with holmium:YAG energy in air and in water. Optical fiber diameter and pulse energy were varied. Motion of the phantom was monitored with high speed video imaging. Laser induced crater volume and geometry were characterized by optical coherence tomography. To determine the direction of plume laser burn paper was irradiated at various incident angles.

RESULTS

Retropulsion was greater for phantoms irradiated in air versus water. Retropulsion increased as fiber diameter increased and as pulse energy increased (p <0.001). Crater volumes increased as pulse energy increased (p <0.05) and generally increased as fiber diameter increased. Crater geometry was wide and shallow for larger fibers, and narrow and deeper for smaller fibers. The ejected plume propagated in the direction normal to the burn paper surface regardless of the laser incident angle.

CONCLUSIONS

Retropulsion increases as pulse energy and optical fiber diameter increase. Vector analysis of the ejected plume and crater geometry explains increased retropulsion using larger optical fibers. Holmium:YAG lithotripsy should be performed with small optical fibers to limit retropulsion.

Free electron laser lithotripsy: threshold radiant exposures

PURPOSE

To determine the threshold radiant exposures (J/cm2) needed for ablation or fragmentation as a function of infrared wavelengths on various urinary calculi and to determine if there is a relation between these thresholds and lithotripsy efficiencies with respect to optical absorption coefficients.

MATERIALS AND METHODS

Human calculi composed of uric acid, calcium oxalate monohydrate (COM), cystine, or magnesium ammonium phosphate hexahydrate (MAPH) were used. The calculi were irradiated in air with the free electron laser (FEL) at six wavelengths: 2.12, 2.5, 2.94, 3.13, 5, and 6.45 microm.

RESULTS

Threshold radiant exposures increased as optical absorption decreased. At the near-infrared wave-lengths with low optical absorption, the thresholds were >1.5 J/cm2. The thresholds decreased below 0.5 J/cm2 for regions of high absorption for all the calculus types. Thresholds within the high-absorption regions were statistically different from those in the low-absorption regions, with P values much less than 0.05.

CONCLUSIONS

Optical absorption coefficients or threshold radiant exposures can be used to predict lithotripsy efficiencies. For low ablation thresholds, smaller radiant exposures were required to achieve breakdown temperatures or to exceed the dynamic tensile strength of the material. Therefore, more energy is available for fragmentation, resulting in higher lithotripsy efficiencies.

Holmium:YAG lithotripsy in children

PURPOSE

We determined the safety and efficacy of holmium:YAG lithotripsy in children.

MATERIALS AND METHODS

We retrospectively reviewed the records of all holmium:YAG lithotripsy done in patients 17 years old or younger. Demographic, preoperative, intraoperative and postoperative data were collected.

RESULTS

A total of 9 boys and 10 girls (26 stones) with a mean age of 11 years (range 1 to 17) were treated with holmium:YAG lithotripsy, which was chosen as initial therapy in 10 (53%). Retrograde ureteroscopy was performed in 15 patients to treat 13 ureteral and 6 renal calculi, and percutaneous nephrolithotripsy was done in 4 to treat 3 ureteral and 4 renal calculi. A complete stone-free outcome after 1 procedure was achieved in 16 children (84%) and 3 patients were rendered stone-free after 2 procedures. No patient had an intraoperative injury. Followup ranged from 0.5 to 12 months (mean 3). Followup imaging has shown no evidence of stricture or hydronephrosis.

CONCLUSIONS

Holmium:YAG lithotripsy is safe and effective in children. It is a reasonable option for failed shock wave lithotripsy, or in children with a known durile stone composition or contraindications to shock wave lithotripsy.

Holmium:YAG lithotripsy for large renal and bladder calculi: strategies for efficient lithotripsy

Holmium:YAG lithotripsy is effective for all stone compositions, and high success rates may be expected. Large renal and bladder calculi may be treated effectively with Ho:YAG lasertripsy. Using angled optical fibers and increasing power settings may be particularly useful to increase lithotripsy speed.

Holmium:YAG laser lithotripsy: A dominant photothermal ablative mechanism with chemical decomposition of urinary calculi

BACKGROUND AND OBJECTIVE

Evidence is presented that the fragmentation process of long-pulse Holmium:YAG (Ho:YAG) lithotripsy is governed by photothermal decomposition of the calculi rather than photomechanical or photoacoustical mechanisms as is widely thought. The clinical Ho:YAG laser lithotriptor (2.12 microm, 250 micros) operates in the free-running mode, producing pulse durations much longer than the time required for a sound wave to propagate beyond the optical penetration depth of this wavelength in water. Hence, it is unlikely that shock waves are produced during bubble formation. In addition, the vapor bubble induced by this laser is not spherical. Thus the magnitude of the pressure wave produced at cavitation collapse does not contribute significantly to lithotripsy.

STUDY DESIGN/MATERIALS AND METHODS

A fast-flash photography setup was used to capture the dynamics of urinary calculus fragmentation at various delay times following the onset of the Ho:YAG laser pulse. These images were concurrently correlated with pressure measurements obtained with a piezoelectric polyvinylidene-fluoride needle-hydrophone. Stone mass-loss measurements for ablation of urinary calculi (1) in air (dehydrated and hydrated) and in water, and (2) at pre-cooled and at room temperatures were compared. Chemical and composition analyses were performed on the ablation products of several types of Ho:YAG laser irradiated urinary calculi, including calcium oxalate monohydrate (COM), calcium hydrogen phosphate dihydrate (CHPD), magnesium ammonium phosphate hexahydrate (MAPH), cystine, and uric acid calculi.

RESULTS

When the optical fiber was placed perpendicularly in contact with the surface of the target, fast-flash photography provided visual evidence that ablation occurred approximately 50 micros after the initiation of the Ho:YAG laser pulse (250-350 micros duration; 375-400 mJ per pulse), long before the collapse of the cavitation bubble. The measured peak acoustical pressure upon cavitation collapse was negligible (< 2 bars), indicating that photomechanical forces were not responsible for the observed fragmentation process. When the fiber was placed in parallel to the calculus surface, the pressure peaks occurring at the collapse of the cavitation were on the order of 20 bars, but no fragmentation occurred. Regardless of fiber orientation, no shock waves were recorded at the beginning of bubble formation. Ablation of COM calculi (a total of 150 J; 0.5 J per pulse at an 8-Hz repetition rate) revealed different Ho:YAG efficiencies for dehydrated calculus, hydrated calculus, and submerged calculus. COM and cystine calculi, pre-cooled at -80 degrees C and then placed in water, yielded lower mass-loss during ablation (20 J, 1.0 J per pulse) compared to the mass-loss of calculi at room temperature. Chemical analyses of the ablated calculi revealed products resulting from thermal decomposition. Calcium carbonate was found in samples composed of COM calculi; calcium pyrophosphate was found in CHPD samples; free sulfur and cysteine were discovered in samples composed of cystine samples; and cyanide was found in samples of uric acid calculi.

CONCLUSION

These experimental results provide convincing evidence that long-pulse Ho:YAG laser lithotripsy causes chemical decomposition of urinary calculi as a consequence of a dominant photothermal mechanism.

Holmium: YAG lithotripsy: photothermal mechanism

OBJECTIVE

A series of experiments were conducted to test the hypothesis that the mechanism of holmium:YAG lithotripsy is photothermal.

METHODS AND RESULTS

To show that holmium:YAG lithotripsy requires direct absorption of optical energy, stone loss was compared for 150 J Ho:YAG lithotripsy of calcium oxalate monohydrate (COM) stones for hydrated stones irradiated in water (17+/-3 mg) and hydrated stones irradiated in air (25+/-9 mg) v dehydrated stones irradiated in air (40+/-12 mg) (P < 0.001). To show that Ho:YAG lithotripsy occurs prior to vapor bubble collapse, the dynamics of lithotripsy in water and vapor bubble formation were documented with video flash photography. Holmium:YAG lithotripsy began at 60 microsec, prior to vapor bubble collapse. To show that Ho:YAG lithotripsy is fundamentally related to stone temperature, cystine, and COM mass loss was compared for stones initially at room temperature (approximately 23 degrees C) v frozen stones ablated within 2 minutes after removal from the freezer. Cystine and COM mass losses were greater for stones starting at room temperature than cold (P < or = 0.05). To show that Ho:YAG lithotripsy involves a thermochemical reaction, composition analysis was done before and after lithotripsy. Postlithotripsy, COM yielded calcium carbonate; cystine yielded cysteine and free sulfur; calcium hydrogen phosphate dihydrate yielded calcium pyrophosphate; magnesium ammonium phosphate yielded ammonium carbonate and magnesium carbonate; and uric acid yielded cyanide. To show that Ho:YAG lithotripsy does not create significant shockwaves, pressure transients were measured during lithotripsy using needle hydrophones. Peak pressures were <2 bars.

CONCLUSION

The primary mechanism of Ho:YAG lithotripsy is photothermal. There are no significant photoacoustic effects.

Holmium:YAG lithotripsy: photothermal mechanism converts uric acid calculi to cyanide

PURPOSE

Holmium:YAG lithotripsy fragments stones through a photothermal mechanism. Uric acid when heated is known to be converted into cyanide. We test the hypothesis that holmium: YAG lithotripsy of uric acid calculi produces cyanide.

MATERIALS AND METHODS

Human calculi of known uric acid composition were irradiated with holmium:YAG energy in water. Stones received a total holmium:YAG energy of 0 (control), 0.1, 0.25, 0.5, 0.75, 1.0 or 1.25 kJ. The water in which lithotripsy was performed was analyzed for cyanide concentration. A graph was constructed to relate holmium:YAG energy to cyanide production.

RESULTS

Holmium:YAG lithotripsy of uric acid calculi in vitro produced cyanide consistently. Cyanide production correlated with total holmium:YAG energy (p <0.001).

CONCLUSIONS

Holmium:YAG lithotripsy of uric acid calculi risks production of cyanide. This study raises significant safety issues.

Holmium:YAG lithotripsy efficiency varies with energy density

PURPOSE

We test the hypothesis that holmium:YAG lithotripsy efficiency varies with optical fiber size and energy settings (energy density).

MATERIALS AND METHODS

The 272, 365, 550 and 940 microm. optical fibers delivered 1 kJ. total holmium:YAG energy to calcium oxalate monohydrate calculi at energy output/pulse of 0.2 to 1.5 J. Stone mass loss was measured for each fiber energy setting. Stone crater width was characterized for single energy pulses. Fiber energy outputs were compared before and after lithotripsy.

RESULTS

Stone mass loss correlated inversely with optical fiber diameter (p <0.05). Stone loss correlated with energy/pulse for the 365, 550 and 940 microm. fibers (p <0.001). The 272 and 365 microm. fibers yielded equivalent stone loss at 0.2 and 0.5 J. per pulse. At energies of 1.0 J. per pulse or greater the 272 microm. optical fiber was prone to damage, and yielded reduced energy output and stone loss compared to the 365 microm. fiber (p <0.01). Stone crater width for single pulse energies correlated with energy settings for all fibers (p <0.001).

CONCLUSIONS

Lithotripsy efficiency with the holmium:YAG laser depends on pulse energy output and diameter of the optical delivery fiber, implying that lithotripsy efficiency correlates with energy density. The 365 microm. fiber is indicated for most lithotripsy applications. The 272 microm. fiber is susceptible to damage and inefficient energy transmission at energies of 1.0 J. per pulse or greater. The 272 microm. fiber is indicated at energies of less than 1.0 J. per pulse for small caliber ureteroscopes or when maximal flexible ureteroscope deflection is required.