Laser lithotripsy means breaking a stone in the kidney or ureter with a laser. A thin telescope is passed up the urinary tract to the stone, and a laser fibre, passed through it, fires at the stone under direct vision. The laser can break the stone into pieces, a process called fragmentation, or turn it into fine dust, a process called dusting. My talk is about the move from one to the other, and why dusting asks the surgeon to know the machine better than before.
In an earlier post, "Thulium Fibre Laser and Why I Argued Against Shock Wave Lithotripsy", I compared the three main stone treatments and introduced the thulium fibre laser. This post covers what that one did not: how a laser produces its light, how that light breaks a stone, how holmium and thulium machines differ, and the safety rules I think every user of these lasers should know.
How did the laser take over stone treatment?
Early in my talk, I show a chart redrawn from data by Doizi and colleagues, who examined 30 years of stone treatment at a single French academic institution. There, ureteroscopy (rigid and flexible together) caught up with shock wave lithotripsy in 2007 and became the main treatment, accounting for 76% of stone operations by 2014. In my view, the laser made flexible ureteroscopy practical, and that is why ureteroscopy displaced shock wave lithotripsy.
Before the laser, every method of breaking a stone in the ureter had a serious drawback. Electrohydraulic lithotripsy was very powerful, but it caused severe damage to the lining of the urinary tract and nearby organs, and it is no longer used. Ultrasound is slower than the laser, generates heat, and has a heavy handle. Today it is used mainly in PCNL (keyhole surgery through the back).
Pneumatic lithotripsy is still in use. It is cheaper and has good energy and frequency, but it works only through a rigid scope in the ureter and kidney. Its main problem is retropulsion, the way the stone gets pushed away each time it is hit. Retropulsion runs through the whole talk: it is the problem the laser eased, and the problem newer lasers attack in different ways.
How does a laser make its light?
Laser stands for light amplification by stimulated emission of radiation. Every laser has three parts. A pump source supplies the energy, and in a holmium laser that is a flashlamp. A gain medium converts energy into light, and in a holmium laser, that is a Holmium:YAG crystal rod. A resonant cavity, made of two mirrors, feeds the light back and forth to amplify it and shape the beam.
A holmium machine reaches higher power by adding more rods. That makes it bigger and heavier, lowers its energy efficiency, and requires special electrical installation for high-power units.
The thulium fibre laser (TFL) is built differently. Its gain medium is a long, thin fibre doped with thulium, pumped by diode lasers along its length. It is a different machine with a different architecture, which is why habits learned on holmium do not carry over to it. According to the manufacturer's figures, a TFL console is a fraction of the size and weight of a high-power holmium machine, and it runs from a standard socket.
Both names originate in Scandinavia. Holmium is named after Holmia, the Latin name for Stockholm, and thulium after Thule, the ancient name for Scandinavia. The Swedish chemist Per Teodor Cleve discovered thulium in 1879.
How does the light break a stone?
Both lasers work because water strongly absorbs their light. Holmium operates at about 2100 nm, and TFL at 1940 nm, which is closer to the water absorption peak. The ideal surgical laser operates in regions where water strongly absorbs, while the silica fibre can still carry the light.
The laser breaks stone in two ways. The first is heat, the photothermal effect: the stone absorbs laser energy directly and undergoes chemical breakdown through vapourisation, ablation, and fragmentation. The second is pressure, the photoacoustic effect: water within the stone's pores turns to vapour, the stone bursts from the inside, and cavitation bubbles form in the surrounding fluid.
Lasers with nearly the same wavelength can perform very different tasks. Standard holmium fragments stones, whereas modulated-pulse holmium dusts more effectively. A pulsed Thulium:YAG laser dusts well and ablates tissue, with the best coagulation among the group. The thulium fibre laser dusts. A continuous-wave Thulium:YAG laser works by heat alone and is a tissue laser; it is not for stones.
What decides dust or fragments?
A laser's power, in watts, is the energy of each pulse in joules multiplied by the frequency, the number of pulses per second. The same power can come from high energy at low frequency or from low energy at high frequency, and the two do different things. Higher energy breaks the stone into bigger pieces. Higher frequency produces finer dust and more heat. Pulse width, how long each pulse lasts, sets the mode as well: longer pulses push the stone less.
How do holmium and thulium lasers differ?
Holmium fires short, tall spikes of power. TFL fires long, low pulses that are almost flat, with a constant peak power. That pulse shape is why TFL dusts so finely and pushes the stone less. TFL can also run at far lower pulse energy and far higher frequency than holmium.
Moses technology on holmium machines addresses retropulsion differently by splitting each pulse into twin pulses. TFL does it with a long, low-power wave. I do not present one as beating the other. They suit different departments.
The advantages of TFL I list in my talk are fine dust, produced at very high frequency and very low energy; rapid dusting and a shorter procedure; and less retropulsion than with standard holmium. The dust brings its own problems, and companies do not include them in the brochure. Dust does not drain away. Hills of dust spoil the view. Very high frequency raises the temperature. Fragments that remain can be hidden in the dust.
What does this mean for patients?
In ureteroscopy, the telescope is inserted through the natural urinary tract, so the skin is not cut. The laser is a tool. How well and how safely it works depends on how the surgeon sets and handles it.
More power is not better. Companies keep selling machines with ever-increasing power and very high frequency; power in a small space, like the kidney, can do more damage than expected. Which treatment suits a particular stone is a decision for each patient and their urologist, after an assessment.
What should urologists using these lasers know?
Few urologists read the manual. I advise learning each machine from the manufacturer's product specialist, starting with the basics, then applying them to the machine you actually have. With TFL, never start at maximum power or high frequency; start low and adjust gradually, according to need.
In my own practice, I stopped using ureteral access sheaths because of complications. I now perform all RIRS without a sheath. Traxer and Thomas described ureteral wall injuries caused by inserting an access sheath during RIRS: in 359 patients, 46.5% had a ureteral wall injury, and 13.3% had a severe one. In the same study, placing a stent before the operation reduced the risk of severe injury by about sevenfold.