Kidney stones are common in developed countries affecting around 10% of the population. Laser lithotripsy, predominantly via advanced ureteroscopy, has become a major technique for the minimally invasive surgical ablation of ureteral and kidney stones. The flashlamp-pumped solid-state Holmium:YAG laser has been the dominant technology in laser lithotripsy for over the past two decades. However, this mature technology has some fundamental limitations. Alternative technologies, such as Thulium Fiber lasers, Thulium:YAG lasers and Erbium:YAG lasers, have also been explored for laser lithotripsy.
What is laser lithotripsy?
Laser lithotripsy is a minimally invasive endoscopic technique (ureteroscopy) that uses a laser to fragment and remove stones from the urinary tract. Once the stone is localized in the bladder, ureter or kidney, an optical fiber is inserted into the working channel of the ureteroscope, and then the laser is activated to fragment the stone into smaller pieces. Stone fragmentation is primarily achieved by photothermal ablation. The direct absorption of the laser radiation leads to heat generation and subsequent melting and ablation of the stone. A secondary ablation mechanism results from the water absorption of the light. Water in pores rapidly vaporizes or expands creating a high localized pressure resulting also in ablation. In general, the surgeon removes the bigger pieces through the urethra with a small basket, and smaller pieces can be passed later with urination. However, the surgeon can proceed in a different way depending on the type of laser used and its parameters.
- Pulse energy:
The pulse energy available depends on the laser system used, but it can vary between 0.2 to 6.0 J, although typical settings during kidney stone ablation ranges from 0.2 to 2.0 J.
- Frequency or pulse rate
The typical frequency values range from 5 Hz to 80 Hz because of the technical constraints of the Holmium laser.
- Pulse duration
Typical pulse duration values for the Holmium lasers are between 150 and 500 µs.
- Laser wavelength
The main reason why Holmium:YAG lasers are currently the standard clinical lasers for lithotripsy is because its emission wavelength at 2120 nm. Light at this wavelength is strongly absorbed by the water contained within the stone pores and pockets leading to thermal expansion and vaporization of the water that results in enhanced ablation. In addition to this, Ho:YAG lasers has shown success ablation of a wide range of stone compositions.
The number of laser lithotripsy modes used for ablation of urinary stones have increased greatly in the recent years. However, these modes can be grouped into three main techniques:
In fragmentation, the ablation of the stone into multiple pieces is achieved using a high pulse energy at a low pulse rate. The pieces with higher diameter (> 2 mm) are retrieved using a basket. In dusting, the kidney stone is broken into small pieces with a diameter of less than 1 mm, in which active basket retrieval is not needed. Another technique is known as popcorning, where the fiber is held fix in a place and a high pulse energy is used to create turbulent flow and iteratively ablate the stones into small pieces. Table 1 summarizes the typical pulse energy and pulse rate values for each of the laser lithotripsy modes.
Laser lithotripsy mode Pulse energy (J) Pulse rate (Hz)
Dusting 0.2—0.5 50—80
Fragmentation 0.5—1.0 5—20
Popcorn ~1.5 20—40
Table 1. Most typical laser operation modes used with Holmium:YAG laser lithotripsy
What are the future trends of laser lithotripsy?
Solid-state Holmium:YAG (Ho:YAG) laser has become the most used laser for lithotripsy over the past decades. Its main benefits are its proven high success ratio in fragment many different stone types, and its relatively low cost for low power lasers. However, this technology has also its limitations and disadvantages. The most critical parameters are:
The emission wavelength of Ho:YAG lasers do not match exactly the water peak absorption around 2 µm. Thulium fiber lasers have closer emission wavelength to the water absorption peak, but Monocrom offers the possibility to tune the emission wavelength by design to perfectly match the water peak absorption, which has been proven to result in a more efficient stone ablation.
- Pulse rate
The maximum pulse rate or pulse frequency is typically in between the range from 20-80 Hz. This limits the possible treatment strategies especially in dusting mode operation. The latest Ho:YAG laser systems offer 100-120 Hz in repetition rate, showing how the trend is to continue increasing this value.
What Monocrom offers is a direct diode system that is electronically pulsed to a frequency that can exceed 1000 Hz. On top of this, our solder-free laser bar mounting technology (Clamping) is insensitive to the CTE mismatch between the semiconductor and the electrodes, allowing improved lifetime in hard pulsed applications.
- Average power
Similar trend is observed with the maximum average power with the latest high-power laser systems offering an output power of 120-140 W. Monocrom @FLEX laser achieves 105 W in average power, with the possibility to continue increasing this parameter in the next generation series.
- Wall-plug efficiency
The wall-plug efficiency of Ho:YAG lasers is rather low with values around 1-2%. The highest wall-plug efficiency has been shown by the thulium fiber laser, with maximum values being reported around 10%. A direct diode laser source from Monocrom can achieve a maximum wall-plug efficiency of about 5%, which is at least 4 times what can be achieved by a Ho:YAG laser.
Another important aspect to be considered relates to the fact that direct diode laser solutions can be effectively used in CW and pulsed operation. This benefit in combination with the ability to perfectly match the emission wavelength with low and high temperature water peak absorption makes it an ideal choice to use also in treatments that require soft tissue ablation and/or coagulation such as Benign Prostatic Hyperplasia (BPH). Below is a summary of the advantages and disadvantages of Ho:YAG laser sources and direct diode laser sources offered by Monocrom at 2 µm.
Advantages and disadvantages by laser type
- Proven to fragment all stone types
- Relatively low cost for low power version
- Does not match water peak absorption
- Limited to low pulse rates
- Very low wall-plug efficiency
- High cost of maintenance
- Advanced laser systems have a high
- Limited to use with fibers ≥200 µm
Direct diode laser
- Wavelength perfectly matches water absorption peak
- Operation in CW and pulse mode
- Pulse rate and duration is very flexible
- Wavelength can be tuned by design
- 4x higher wall plug efficiency in comparison to Ho:YAG (~5%)
- Very low cost of maintenance
- Clinical tests are not yet performed
- Limited to use with fibers ≥200
Written by Joan Montiel, Application Solution Manager at Monocrom