Literature Sharing | Intracorporeal Laser Lithotripsy
【Introduction】
Over the past few decades, with the continuous advancement of medical technology, surgical treatment for urinary calculi has made significant breakthroughs. Currently, ureteroscopy (URS) is a precise and minimally invasive diagnostic and therapeutic technique that can comprehensively assess the urinary tract drainage system and effectively treat calculi through in-body lithotripsy. The application of laser technology in the development of lithotripsy fibers has completely revolutionized the clinical model of in-body lithotripsy, significantly improving the treatment outcome and reducing surgical trauma.
Source: Papatsoris AG, Skolarikos A, Buchholz N. Intracorporeal laser lithotripsy. Arab J Urol. 2012 Sep;10(3):301-6. doi: 10.1016/j.aju.2012.02.006. Epub 2012 Mar 26. PMID: 26558041; PMCID: PMC4442925.
【Laser Lithotripsy Variables】
The efficient fragmentation of stones of diverse compositions and with minimal collateral tissue damage is primarily contingent on laser variables (wavelength, pulse duration and pulse energy) and the physical properties of the stones (optical, mechanical and chemical) . The pulse duration governs the dominant mechanism in stone fragmentation, which is either photothermal or photoacoustic/photomechanical. Lasers with long pulse durations (i.e. >10 μs) induce a temperature rise in the laser-affected zone, with minimal acoustic waves . Stone material is removed by means of vaporisation, melting, mechanical stress and/or chemical decomposition. Short-pulsed laser ablation (i.e.<10 μs), produces shock waves, and the resultant mechanical energy fragments the stones .
【Ho:YAG Laser Efficacy】
Currently, holmium laser lithotripsy has become the gold standard for both rigid and flexible ureteroscopic lithotripsy. Researchers have found that at any given pulse energy, shorter pulse durations produce more stone rebounds compared to longer pulses. Regardless of the pulse duration, higher pulse energy and larger fibers result in larger ablation volumes and rebounds (P < 0.05). For shorter pulse durations, bubble expansion is faster and the amplitude of the rupture pressure wave is greater (P < 0.05). Therefore, when the duration of the holmium:yttrium aluminum garnet laser pulse increases, the rebounds decrease, but the lithotripsy effect remains comparable.
Even for patients with kidney stones larger than 2 centimeters, flexible ureteroscopy (URS) combined with holmium laser lithotripsy is an ideal choice for some patients. These patients include those who do not agree to percutaneous nephrolithotomy (PCNL), those who are undergoing anticoagulant therapy that should not be discontinued, morbidly obese patients, patients with an isolated kidney, or those with chronic renal insufficiency. Finally, the treatment of calcified kidney stones is extremely challenging. Although PCNL is the standard treatment for large calcified kidney stones, flexible ureteroscopy combined with laser lithotripsy can also be used for treatment. Laser lithotripsy is usually combined with the ureteral sheath to facilitate the flushing of mucoid matrix stones.
【Non-Ho:YAG Laser Systems】
Compared with the simple YAG laser, the Ho:YAG laser exhibits rapid absorption in water (3 millimeters) and minimal tissue penetration (0.4 millimeters), which reduces thermal damage and enhances safety.
New types of lasers, such as erbium:YAG lasers, are currently in the research and development stage. They are more effective than holmium lasers and have lower toxicity. Preliminary experiments with erbium:YAG lasers have shown that they have higher stone-fragmentation efficiency, and better precision in ablation and cutting compared to holmium:YAG lasers. However, the lack of suitable optical fibers currently limits their application. In particular, when the wavelength of the erbium:YAG laser is 2.94 μm, its high-temperature water absorption coefficient is approximately 30 times higher than that of the holmium laser with a wavelength of 2.12 μm. This increases the stone-fragmentation efficiency by two to three times. However, the erbium laser wavelength cannot be transmitted through the existing standard quartz fibers; special infrared fibers are required, which are usually less flexible, more expensive, and have poorer biocompatibility than quartz fibers.
【Technical and Clinical Implications】
The pressure waves generated by holmium laser lithotripsy are less than those produced by other stone-breaking methods, but there is still a certain recoil phenomenon. The duration of the laser pulse affects the generation of shock waves and the movement of the stones. A longer pulse width results in less movement of the stones after a single impact, but more energy transfer during repeated impacts. Clinically, this may reduce the frequent and cumbersome adjustments of optical fibers and improve the efficiency of stone fragmentation. Another prospective multicenter study evaluated a series of reusable holmium laser fibers. The study showed that reusable laser fibers are more cost-effective than disposable fibers. Additionally, the study found that fibers with a core diameter of 365 μm have a higher reusability rate than those with a core diameter of 270 μm.
Regarding the routine use of ureteral stents, a prospective randomized trial compared ureteroscopic lithotripsy without stent placement after holmium laser lithotripsy with ureteroscopic lithotripsy with stent placement. A total of 110 patients underwent uncomplicated ureteroscopic laser lithotripsy. After the operation, the patients were randomly divided into the stent non-placement group and the stent placement group (55 cases in each group). The stents were routinely placed for 3 weeks. The researchers indicated that uncomplicated ureteroscopic laser lithotripsy can be safely performed without stent placement. Patients without stent placement had shorter operation time, lower incidence of pain and hematuria.
【Conclusions】
The field of laser lithotripsy is developing in two different directions: one is to improve the existing Ho:YAG laser platform, and the other is to develop new laser platforms. The most significant improvement in Ho:YAG laser lithotripsy may come from optical fibers with better transmission performance. In fact, developing more flexible, more economical, and more durable new optical fibers is the challenge of the future.
