Subsequent discoveries in the field of light transmission over the next several decades led to the field of fiberoptics. Documentation of these advancements was undertaken by Curtis and colleagues who incorporated fiber-optic technology into medical instruments, thus allowing image conduction [4]. Curtis and Hirschowitz first built fiberoptic scopes for use in gastroenterology. They were the first to arrange coherent bundles of fibers while protecting adjacent fibers from image mixing [5]. Their work sparked the interest of others in the potential of flexible endoscopy for use in separate fields. To understand how flexible ureteroscopes were devised, one must first understand the mechanics of fiberoptic imaging. The light from the object of interest travels in a glass fiber surrounded by a cladding with a lower refractory index . Through the phenomenon of total internal reflection , this light is transmitted over the length of the ureteroscope with minimal degradation (Fig. 3.2). The fiberoptic strands are clad with a material of a lower index of refraction, thus preventing leakage of light. A standard fiberoptic bundle will contain several hundred thousand individual fibers, each of which has a diameter of approximately 10 μm. Each fiber in a given coherent bundle essentially accepts one pixel of information about the specific image. That information is then transmitted to the other end of the bundle to allow for visualization of that image. Since the fibers have an identical orientation at the ends of each bundle, the exact image is transmitted to the eyepiece. The simple eyepiece then presents a magnified view to the eye [6]. The image obtained by fiberoptic bundles is not a single image but a composite matrix of each fiber within the bundle, giving it the classic “honeycomb” appearance. The fibers are only fixed to each other at the ends; thus, much of the length of the endoscope is able to be flexed allowing significant maneuverability and the capacity to navigate tortuous ureters and previously difficult to access calyces. It is this flexibility that was revolutionary and a significant advance over rigid ureteroscopes, thus allowing access to the entire urinary system.

The performance of flexible ureteroscopy was first reported by Marshall with the use of their 9 French endoscope first during an open ureterostomy and subsequently during a transurethral procedure by his colleagues [7]. In 1968, Takagi and Asi reported on their work in developing flexible fiberoptic access to the upper tract of the urinary system [8]. They reported their successful attempts at visualizing the renal collecting system including pelvis and papillae utilizing an 8 French fiberoptic endoscope and evaluated its performance in both cadavers and patients. The Takagi group was the first to present their findings using still photographs of the renal papillae with their newly developed pyelo-ureteroscope [8]. Takayasu subsequently was the first to show video evidence of renal pelvis visualization with the ureteroscope in 1970 [9].

This group did much of the earliest work of flexible ureteroscope development , devising strategies for addressing various limitations. They also recognized difficulty in inserting the ureteroscope from the bladder into the ureter and initially addressed this by employing the cystoscope sheath before later developing a flexible polytetrafluoroethylene introducer sheath [10]. This difficulty was largely due to the struggle to successfully manipulate the endoscope tip. Passing the flexible ureteroscope through a catheter served to provide axial rigidity and transmitted torque along the length of the endoscope. This technique enabled the ability to maneuver the tip in the chosen direction. This experience led to the realization that a flexible tip was required for a successful endoscope.

In another breakthrough, Takayasu et al. realized that the lack of irrigation was problematic, and their first attempt at irrigation utilized the 12 Fr sheath as a means of ureteral luminal distension and better visualization. Continued innovation led to their group being the first to introduce a channel for irrigation and working instruments [9].

Ureteroscopes initially utilized optical lenses arranged sequentially in series for imaging (Fig. 3.3), resulting in large external diameters. Additionally, with even minor flexion, these lenses would become improperly aligned with subsequent instrument failure [11], thus strictly relegating them to use in rigid endoscopes. The advantage of a fiberoptic bundle lens was that even upon circumferential deflection, it would continue to clearly illuminate the field. The early fiberoptic lenses were fashioned from equally spaced packed quartz bundles which produced a completely flexible optical imager [6]. The progress of fiberoptic imaging allowed for the introduction of the flexible ureteroscope which eventually revolutionized the way urologists approached ureteral and renal stones. However, widespread adoption of flexible ureteroscopy at that time was deterred by its limitation as a strictly diagnostic procedure. Additionally, urologists were initially hesitant to embrace flexible ureteroscopes because of the very valid concern that scope diameters were prohibitively large, which introduced the significant risk of ureteral damage .

Over the next two decades, advances in rigid ureteroscope technology allowing for the fragmentation of ureteral and even renal pelvis calculi in some cases emphasized the need for ureteroscopes that could navigate the renal collecting system [12]. Changes to the original flexible ureteroscope began in the 1980s first with a passively deflectable instrument which was only able to access limited regions of the intrarenal collecting system. In 1983, Bagley and colleagues reported their experience with a new pyeloscope designed with a flexible tip that was able to be deflected 160° in one direction and 90° in the opposite direction all in the same plane [13]. This ureteroscope was also of a small enough diameter that it was able to be placed through the sheath of a rigid telescope on removing the actual telescope. This strategy of fitting the flexible ureteroscope through the rigid sheath mitigated two problems previously encountered with flexible ureteroscopy: difficulty in maneuvering the flexible ureteroscope in the renal pelvis and obtaining sufficient irrigation for visualization. The Lyon group later attempted to address the problem of ureteral access by using both a rigid and a flexible endoscope. This practice of using complimentary ureteroscopes, a semirigid ureteroscope for access to the distal ureter, and more proximal endoscopy with an actively deflectable flexible scope [13] has proven useful.

Urologists quickly recognized the need to visualize the entire collecting system which would require a steerable scope with a small deflectable tip [14]. Bagley and Rittenberg performed investigations to determine the tip deflection required for total visualization of the renal collecting system [15]. On evaluating patient radiographs, they found that the average angle between the major axis and the lower pole infundibulum was 140° and a maximum of 175°. Hence, they proposed a ureteroscope that would deflect 175°. However, on clinical use, the endoscope manufactured to these parameters often could not access the lower pole since deflection was frequently limited by instruments in the working channel and/or by variations of the collecting system itself. As such, active deflection angles continued to increase until they reached 270°, now considered the industry standard [16].

Also by the late 1980s, miniaturization and tighter packing of fiberoptic fibers produced smaller bundles, resulting in smaller-diameter flexible ureteroscopes. Eventually, several different flexible ureteroscope designs offered in sizes ranging from 8.1 to 10.8 Fr were introduced, all of which could be inserted over a wire and used to provide panoramic visualization of the intrarenal architecture without the need for a stabilizing sheath [17].

While access to portions of the lower pole could still be challenging, visualization was often able to be achieved using active (primary) and passive (secondary) deflection of the ureteroscope tip. Primary deflection up and down is controlled by the lever, while secondary deflection is a further degree of deflection that can be accomplished with an already deflected ureteroscope tip. This was achieved by introducing a more bendable segment approximately 6 cm proximal to the tip which allowed for passive endoscope buckling when the ureteroscope was maximally deflected. Secondary deflection proved particularly helpful in not only gaining access to the lower pole calyces but in allowing for the treatment or retrieval of calculi located there [18]. It has since become a standard component of flexible ureteroscope design.

Innovations in ureteroscope technology were inextricably connected to the development of working instruments. In fact, one might argue that the crucial phenomenon that made the use of ureteroscopes feasible was the development of smaller, functional instruments. In many cases the introduction of new or improved working instruments spurred the development of endoscopes to more appropriately take advantage of the advances allowed by the new or updated ancillary equipment. Many of the improvements in ureteroscope design paralleled advances in intracorporeal lithotripsy . In particular, the potential opportunity presented by the development of ever smaller electrohydraulic lithotripsy probes of 2.5, 1.9, and 1.7 French [19] encouraged the design of increasingly smaller ureteroscopes.

The introduction of the 3.6 French endoscope working channel facilitated placement of an ancillary instrument while still allowing the passage of irrigant. This design allowed the use of a wide assortment of working instruments, including guide wires, baskets and other stone retrieval implements , laser fibers , and electrohydraulic lithotripter probes . Most currently available flexible ureteroscopes continue to have a single 3.6 Fr working channel. One notable exception is the Wolf Cobra™ dual-channel ureteroscope with two 3.3 Fr channels [20]. This allows for simultaneous ancillary instrument use or for increased irrigation flow rate. However, the compromise for having two channels is a 9 Fr sheath diameter.

A successful strategy for stone treatment is the use of a small diameter ureteroscope along with an effective small diameter lithotrite. This has largely been achieved with the combination of flexible ureteroscopes with laser fibers. The first documented use of lasers was in 1966 as a result of work by Bush and coworkers who used a bovine renal model to evaluate the effect of a focused argon laser beam on renal calculi [21]. However, this and several subsequent lasers were inadequate or inappropriate for kidney calculus fragmentation [22, 23]. The development of pulsed lasers, where the energy is emitted in pulses of a given duration instead of in a continuous mode, was a major breakthrough for laser lithotripsy [24]. This type of laser allows for precision in control of the laser beam while minimizing lateral heat conduction and subsequent overheating of nearby tissue. The holmium: yttrium-aluminum-garnet (YAG) lase r, first championed for use in urology by Johnson and colleagues in the early 1990s [25, 26], has proven quite versatile and rapidly became the laser fiber of use for stone fragmentation [16]. As the laser becomes more accepted in ureteroscopy for its ability to fragment any stone with minimal tissue damage, ureteroscopes were devised to work in concert with this working instrument. Depending on fiber size, lasers can be advanced to the stone through flexible ureteroscopes with little loss of deflection capability of the scope [27].

Ureteroscope tip design has also undergone enhancements over the years. The earliest flexible ureteroscopes had flush “tin-can” tips. However, several subsequent scope iterations were introduced with beveled tips. The rationale for this engineering change was that these tips would better facilitate insertion of the scope into the ureteral orifice, thus decreasing injury. Most flexible ureteroscopes have a 0° angle of view.

There were several attempts in the 1990s to decrease ureteroscope diameter from approximately 10 Fr [28]. One of the more impactful offerings was from the Storz company in 1994; they produced an optical quality small diameter fiberoptic bundle. The miniaturization of the fiberoptic cable facilitated a substantial decrease in the size of the resulting ureteroscope’s outer diameter. Along with a corresponding increase in tip deflection, these innovations facilitated enhanced ureteral intubation and scope maneuverability throughout the upper tract. The collective work of several instrument makers that has since effected a decrease in ureteroscope diameter from 10 Fr to what is now 7.5 Fr has enabled the surgeon to gain routine endoscopic access to the intrarenal calyceal system often without a need for ureteral dilation, thus noticeably increasing treatment efficacy.

Digital Ureteroscopes

The next major breakthrough in imaging technology after the development of fiberoptics was the introduction of digital imaging. Compared to fiberoptic cables, digital sensors are composed of millions of photodiodes, which convert photons into electric current that is subsequently converted into voltage then amplified and changed into a digital form (Table 3.2).

Table 3.2

Comparison of technology behind digital and fiberoptic ureteroscopes

	CCD and CMOS chip (digital)	Fiberoptic bundle
Parameters
Initial reception of field of vision	“Chip on the tip”: CCD or CMOS chip positioned on distal tip of the ureteroscope with LED	Objective lens at distal end of the ureteroscope with a light diode receives the reflected light rays and focuses them onto a fiberoptic bundle
Reception of initial image	Convert photons of light to a series of electrons	Reflected light rays focused onto a fiberoptic bundle
Transmission of image through the scope	Wires within ureteroscope transmit the flow of electrons to the image processor	Light received from the objective lens and transmitted through the fiberoptic bundle
Reception of final image and display	Image processor receives the electric signal and converts it into an image for real-time display	Camera located at the proximal end of the scope receives the light rays from the fiberoptic bundle and displays the image on-screen

The first digital ureteroscopes utilized the charge-coupled device (CCD) chip technology which employed two miniature light-emitting diodes (LEDs) as the light source. They are positioned adjacent to the distal lens (Table 3.3), allowing images to be transmitted from a digital sensor on the ureteroscope tip to a proximal point via a single wire. This was advantageous because it offered greater image clarity with the capability for digital magnification in a system that was also able to autofocus. The digital image appears on a standard monitor with a clear image that no longer included the honeycomb effect associated with the image produced by fiberoptic quartz bundles [30]. The improvement in image quality has implications not only for the treatment of calculi but also for the diagnosis and possible treatment of upper tract urothelial carcinoma as well as multiple other pathologies. An early comparative study revealed parity of the digital and fiberoptic ureteroscopes [31], while later studies have reported superior visibility with digital ureteroscopes [32].

Table 3.3

Comparison of digital and fiberoptic ureteroscopes

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