Example of high-powered laser
Lumenis (Yokneam, Israel) has developed a new technology for their Lumenis Pulse™ 120H laser dubbed the “Moses effect .” The laser pulse is modulated to create a vapor channel between the tip of the fiber and the target stone or tissue. The Olympus EMPOWER H65 system employs a “stabilization mode” to provide a similar vapor channel effect. Pulse modulation is designed to improve energy transmission to the target stone or tissue and thereby improve fragmentation, reduce retropulsion, and ideally shorten procedural times . Future Ho:YAG laser designs will likely further work to optimize pulse modulation.
The (1) silica glass core of the fibers used with the Ho:YAG laser is the laser light transmitting portion. Ideally laser energy should travel efficiently through the core through a process termed total internal reflection. The core is surrounded by the cladding. The (2) cladding may be made of similar material to the core but has a lower index of refraction, which is important for total internal reflection to occur at the boundary of the core and cladding. The (3) jacket, or outer coating, encases the core and cladding and functions to protect the delicate glass components of the fiber. The jackets are often colored which aids in visualizing the fiber both endoscopically and outside the patient.
Laser Fiber Size
The diameter of a laser fiber affects how that fiber might perform. For example, larger fibers may be less flexible and limit irrigation flow. Often urologists will request a “200 μm” laser fiber to use for their procedures not necessarily understanding that the fibers are not 200 μm in overall diameter. In fact, there are few commercially available fibers that can be used for laser lithotripsy that are 200 μm in diameter. The true diameter of most fibers is significantly greater as the diameter must take into account the combination of the fiber’s core, cladding, and jacket. For example, the Cook (Spencer, IN) HLF-S200 fiber is marketed as having a 200 μm diameter, but it is the fiber core that measures 200 μm and the true diameter of the fiber, when taking the core, cladding, and jacket into account, is approximately 374 μm. Another even more confusing example is the Boston Scientific (Marlborough, MA) Flexiva 200 fiber. While the name of the fiber implies it is 200 μm, the core is about 240 μm and the true diameter of the fiber is 443 μm, so in fact no part of the fiber is 200 μm .
Preferred laser fiber core diameter
Ball-tip fiber preferred to preserve inner lining of flexible ureteroscope
Ball-tip fiber preferred to preserve inner lining of flexible ureteroscope
Semirigid ureteroscopes (4.5–6F)
Semirigid ureteroscopes (>6F)
The beam profile of the Ho:YAG lasers couples best with core sizes of greater than 200 μm and ideally larger than about 240 μm. Smaller core sizes risk launching the laser energy into the cladding which can damage or destroy the fiber. Prior bench testing of fibers has demonstrated that fibers with core sizes <240 μm were more prone to failure . For flexible ureteroscopy with intracorporeal lithotripsy, choosing a fiber with a core size of 240–270 μm offers a fair trade-off between durability and size.
Fiber Performance: Flexibility
The flexibility of a laser fiber is an important performance component for fibers used during ureteroscopy, especially for stones located in the lower pole. The flexibility of a fiber is affected by both the diameter of the fiber and the components used to construct the fiber. The deflection of a ureteroscope can be limited if a stiffer laser fiber is used, potentially limiting access to lower pole stones in certain situations. When a selection of fibers with 240–270 μm core diameters were evaluated for flexibility, approximately 30–60° of baseline deflection was lost when inserted into a Stryker (Kalamazoo, MI) U-500 flexible ureteroscope that has 275° of baseline deflection. Fibers with a slightly smaller core size of 200 μm had slightly less deflection loss, averaging 20–30° of deflection loss in the U-500 . Therefore if maximal deflection is needed to reach a stone, then a 200 μm core fiber may be the best option to reach the target. Stiffer, less flexible fibers have the potential to put added strain on the deflection mechanism of a delicate flexible ureteroscope which could lead to premature failure of the device.
Fiber Performance: Durability
Durability refers to the resistance of the fiber to fracture with bending. Typically the fibers do not fail with bending alone but rather fail when the laser is activated with the fiber in a deflected position. The concept is that with bending, there can be a loss of total internal reflection of the laser energy within the fiber core, and when the energy leaks into the cladding and especially the jacket, the fiber will fail due to thermal damage . Increasing both the pulse energy setting of the laser and the tightness of the fiber bend increases the risk of fiber failure . Should this occur during a clinical case, it could result in catastrophic damage to the flexible ureteroscope secondary to damage from the laser energy. The broken piece of the fiber could fall into the kidney and require extraction, which may be technically difficult. Moving and displacing stones from the lower pole to an easier to access location such as the renal pelvis or upper pole is also a prudent strategy to reduce the risk of fiber failure. This decreases the strain on the deflection mechanism of the flexible ureteroscope and may increase the stone-free rates after the procedure [11, 12].
The fiber tip is often degraded with burnback. Preparation of the fiber tip consists of stripping the terminal portion of the jacket and then cleaving several mm off the end of the core. Specialized tools such as laser fiber strippers and ceramic scissors exist for that purpose. In one in vitro study, the coated fibers (compared to stripped) regardless of how they were cut (metal or ceramic scissors) yielded better lithotripsy performance. The authors hypothesized that stripping the fibers may damage the cladding layer .
Single-Use Versus Reusable Fibers
There is currently a wide range of commercially available laser fibers , with both single use and reusable variants available. Historically, reusable fibers were more costly to purchase, but with repeated use the cost is amortized over the life of the fiber and reusable fibers can be more cost-effective than single-use variants . In general terms, performance between single-use and reusable fibers has been similar, although there have been examples where the reusable version from a manufacturer outperformed their single-use version [9, 10]. In recent years, a shift has begun to occur with some laser fiber manufacturers focusing on high-cost, single-use fibers. An example of this is the Boston Scientific Flexiva and Flexiva TracTip 200 fiber line. The fibers are single use only and reusable variants are not available. These fibers sit at the high end of the price spectrum for Ho:YAG fibers but have been shown to have excellent performance characteristics .
Use of Laser Fibers in Ureteroscopy
The primary advantage of the Ho:YAG laser in ureteroscopy is that it can be used to fragment stones of any composition. The procedure is performed by carefully advancing the laser fiber in through the working channel of the ureteroscope. The tip of the fiber should always be visualized. The tip of the fiber should be in contact with the stone for efficient fragmentation. Failure to see the tip of the fiber may indicate that the fiber is inside the working channel of the ureteroscope and, if activated, can cause catastrophic damage to the ureteroscope. Furthermore, the cavitation bubble collapse created by the individual pulses may be of sufficient magnitude to damage the ureteroscope. In one study, when the laser fiber was advanced out to a point that it occupied a quarter of the video screen, the bubble generated by laser activation never touched the flexible ureteroscope tip, thus preserving the scope from damage. The authors dubbed this the “safety-distance concept” .
Outcomes of Ho:YAG Laser Lithotripsy
Preliminary experience using the Ho:YAG laser to fragment stones highlighted a few important points . First, the laser proved effective at treating stones, with a stone-free rate of 92% in 21 patients. Second, the laser was versatile in treating stones located throughout the entire urinary tract and proved particularly helpful in treating calyceal stones away from the nephrostomy tract in percutaneous nephrolithotomy when used in flexible nephroscopy. Third, the laser had the ability to treat stones of all compositions, including cystine stones and calcium oxalate monohydrate stones, which had failed previously attempted treatment modalities. In addition, the laser proved to be safe, with one case of ureteral perforation when the device was used under fluoroscopic control rather than direct visualization.
Larger series with over 500 patients validated the initial findings and had similar conclusions highlighting the Ho:YAG laser’s efficacy at treating stones, with stone-free rates >90% and very low rates of complications including ureteral perforation and stricture formation (<1%) [19, 20].
As experience with Ho:YAG laser lithotripsy grew, studies began to investigate its use with more challenging patient populations. Excellent outcomes were reported in various clinical situations, including patients with morbid obesity , bleeding diatheses [22, 23], anomalous kidneys , and the gravid  and pediatric  patient population. The combination of small caliber highly deflectable ureteroscopes coupled with Ho:YAG laser allowed surgeons to perform procedures that may have historically required more invasive means such as percutaneous or even open approaches [27, 28].