13.2 Ureteroscopy (URS) & Retrograde Intrarenal Surgery (RIRS)

Ureteroscopy (URS) & Retrograde Intrarenal Surgery (RIRS)

Advances in miniaturized optics, active digital deflection, and laser lithotripsy have transformed ureteroscopy (URS) and retrograde intrarenal surgery (RIRS) into first-line interventions for ureteral and renal calculi. URS achieves exceptional stone-free rates across diverse clinical scenarios, including obese patients, patients with uncorrected coagulopathies, and calculi refractory to shock wave lithotripsy. Mastering scope biomechanics, intracorporeal lithotripsy physics, ureteral access sheath (UAS) hemodynamics, and complication avoidance is central to contemporary endourology practice.

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Semirigid URS vs. Flexible URS / RIRS

Selecting the appropriate endoscope depends on stone location, caliber of the ureter, and local pelvicalyceal architecture:

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|                                    URETEROSCOPY INSTRUMENTATION                                    |
|                                                                                                    |
|  1. SEMIRIGID URETEROSCOPY (URS)                                                                   |
|     - Tapered distal caliber: 6.0 to 9.5 Fr                                                        |
|     - Optical rod-lens or coherent fused silica fiber bundle                                       |
|     - Rigid shaft: Exceptional torque control, durability, and irrigation flow                     |
|     - Anatomical domain: Distal and mid-ureter; proximal access when anatomy permits               |
|                                                                                                    |
|  2. FLEXIBLE URETERORENOSCOPY (fURS / RIRS)                                                        |
|     - Distal outer diameter: 7.5 to 9.5 Fr                                                         |
|     - Active bidirectional deflection: up/down deflection, device-dependent                        |
|     - Digital CMOS chip-on-the-tip vs Fiberoptic image bundle                                      |
|     - Anatomical domain: Proximal (lumbar) ureter, renal pelvis, and intrarenal calyces            |
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1. Semirigid Ureteroscopy

• Design: Constructed with a rigid stainless steel outer sheath housing either a Hopkin rod-lens optical system or fused silica fiberoptic bundles, alongside a straight working channel (typically 3.0 to 4.5 Fr). Modern semirigid scopes feature a tapered distal tip (6.0 to 7.5 Fr) expanding to 8.5 to 9.5 Fr proximally.
• Clinical Role: Gold standard for distal and mid-ureteral calculi.
• Advantages: Outstanding column strength, superior rotational torque transmission (1:1 tactile feedback), high durability, and wide irrigation channels that maintain visibility even during minor bleeding.
• Limitations: Limited access in a curved or tortuous proximal ureter; forcing advancement risks perforation or avulsion. Flexible URS provides an alternative when safe semirigid access is not possible.

2. Flexible URS / RIRS

• Design: Miniaturized flexible shaft (outer caliber 7.5 to 9.5 Fr) with an active distal articulating segment capable of bidirectional active deflection of up to 270° in both upward and downward planes.
• Clinical Role: Standard of care for proximal ureteral calculi and intrarenal pelvicalyceal stones (RIRS), especially lower pole calyceal calculi.
• Optical Architecture:
  • Fiberoptic Scopes: Rely on coherent bundles of tens of thousands of individual quartz fiber cores. Prone to progressive breakage ("black dots" across the visual field), lower resolution, and image distortion when the scope is fully deflected.
  • Digital (Chip-on-the-Tip) Scopes: Feature a complementary metal-oxide-semiconductor (CMOS) sensor and integrated LED illumination at the extreme distal tip. Provides high-definition video output, good image resolution and illumination, with performance varying by device.
• Reusable vs. Single-Use (Disposable) Scopes:
  • Reusable Scopes: High initial capital cost, vulnerable to mechanical wear, channel puncture from laser fibers, and deflection loss over time; requires rigorous sterilization protocols.
  • Single-Use Scopes: Provide a new sterile instrument for each case and avoid reprocessing and repair requirements. Deflection and performance still depend on the device, accessories and mechanical loading. Particularly advantageous in high-risk lower pole anatomy where extreme deflection accelerates endoscope wear.

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Laser Lithotripsy Physics: Holmium:YAG vs. Thulium Fiber Laser

Intracorporeal lithotripsy has evolved from pneumatic and electrohydraulic probes to high-precision laser systems. Two distinct laser technologies dominate contemporary endourology:

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|                                   LASER LITHOTRIPSY BIOPHYSICS                                   |
|                                                                                                  |
| HOLMIUM:YAG (Ho:YAG)                           | THULIUM FIBER LASER (TFL)                       |
| Wavelength: 2,120 nm                           | Wavelength: 1,940 nm                            |
| Flashlamp-pumped solid crystal                 | Diode-pumped thulium-doped fiber                |
| Water absorption: ~30 cm^-1                    | Water absorption: ~120 cm^-1                    |
| Optical depth: ~0.4 mm                         | Optical depth: ~0.1 mm                          |
| Conventional or modulated pulses               | Super-pulsed emission                           |
| Frequency: platform-dependent                  | Some platforms: up to 2,400 Hz                  |
| Example low energy: 0.2 J                      | Example low energy: 0.025 J                     |
| Mainly photothermal ablation                   | Mainly photothermal ablation                    |
| Retropulsion varies with settings              | Low retropulsion at low pulse energy            |
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1. Holmium:YAG Laser (Ho:YAG)

• Biophysics: Solid-state laser utilizing a crystal of yttrium-aluminum-garnet doped with chromium, thulium, and holmium ions, pulsed via a flashlamp. Its wavelength of 2,120 nm matches an absorption peak in liquid water (water absorption coefficient $\mu_a \approx 30\ \text{cm}^{-1}$; tissue penetration depth $\approx 0.4$ mm).
• Ablation Mechanism: Strong water absorption creates a vapor channel through which energy reaches the stone. Comminution is predominantly photothermal ablation and decomposition; bubble dynamics can also contribute. The vapor-channel phenomenon underlies the Moses effect, while commercial pulse modulation deliberately shapes energy delivery through that channel.
• Settings & Modes:
  • Fragmentation Mode (Basketing): High pulse energy (0.8 to 2.0 J), low frequency (5 to 10 Hz). Cleaves calculi rapidly into large chunks for extraction with nitinol baskets. Downside: marked stone retropulsion pushing fragments into inaccessible calyces.
  • Dusting Mode: Low pulse energy (0.2 to 0.5 J), high frequency (15 to 40+ Hz). Vaporizes the calculus surface into fine dust particles (< 1 mm) that pass spontaneously in urine. Retropulsion is minimized.
  • Pulse Modulation (Moses Technology): Modulates the pulse shape by delivering a small introductory energy pulse to separate water and generate the vapor bubble, immediately followed by the main therapeutic energy pulse. This can reduce retropulsion and improve energy delivery; effects depend on the settings and system.

2. Thulium Fiber Laser (TFL)

• Biophysics: Uses a long, hair-thin silica glass fiber core doped with thulium ions, continuously pumped by laser diodes and electronically modulated to release super-pulsed square waves. Its wavelength of 1,940 nm lies near a strong absorption peak of liquid water.
• Superior Water Absorption: The water absorption coefficient of TFL ($\mu_a \approx 120\ \text{cm}^{-1}$) is four times higher than that of Ho:YAG. The optical penetration depth is only 0.1 mm (one-fourth of Ho:YAG).
• Clinical Advantages:
  • Ultra-Dusting Capability: Operates at extremely low pulse energies (down to 0.025 J) and exceptionally high frequencies (up to 2,000 to 2,400 Hz), comminuting stones into an impalpable microscopic powder ("snowstorm" effect).
  • Negligible Retropulsion: Low pulse-energy settings generally produce little retropulsion; higher settings can still move the stone.
  • Miniaturized Fiber Cores: Can operate through small fibers, including 150 µm or 200 µm designs; inspect fiber condition and use compatible settings. Smaller fibers preserve endoscope deflection in acute lower pole calyces and permit higher irrigation flow.
  • Compact Footprint: Operates on standard wall electricity (110–230 V) with air cooling, avoiding the bulky water chillers and high noise of Ho:YAG consoles.

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Ureteral Access Sheaths (UAS): Physiology vs. Risks

A ureteral access sheath (UAS) consists of a hydrophilic-coated outer sleeve and an inner dilator (standard calibers: 9.5/11.5 Fr to 12/14 Fr) deployed over a stiff safety guidewire under fluoroscopic visualization into the upper ureter.

1. Physiological and Technical Advantages

• Intrarenal Pressure (IRP) Decompression: Normal renal pelvic pressure is low, commonly described as 0–20 cmH2O. Pressurized irrigation can raise it substantially. Elevated pressure promotes pyelorenal backflow, with pyelotubular reflux often described around 27–41 cmH2O and pyelovenous backflow around 41–68 cmH2O; pressure duration also matters. Backflow can carry bacteria and endotoxins into tissue and circulation. A UAS improves outflow and usually lowers pressure, but does not guarantee a safe value. Use moderate inflow, unobstructed outflow and, where available, suction or pressure monitoring.
• Continuous Irrigation Outflow: Flushes out blood and dust, maintaining crystal-clear optical visualization.
• Scope Preservation & Operative Speed: Allows rapid, repeated endoscope re-entries for fragment extraction without subjecting the ureteral orifice or intramural ureter to repetitive shear trauma.

2. Mechanical Risks & PULS Grading

Forceful UAS insertion into a small-caliber or non-pre-stented ureter can cause significant shear trauma, mucosal stripping, wall ischemia, and perforation. The severity of ureteral injury is standardized using the Post-Ureteroscopic Lesion Scale (PULS):

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|                               POST-URETEROSCOPIC LESION SCALE (PULS)                               |
|                                                                                                    |
|  Grade 0: No visible mucosal injury                                                                |
|  Grade 1: Superficial mucosal lesion and/or significant edema or hematoma                          |
|  Grade 2: Submucosal lesion without perforation                                                    |
|  Grade 3: Perforation with less than 50% partial transection                                       |
|  Grade 4: More than 50% but less than complete partial transection                                 |
|  Grade 5: Complete ureteral transection                                                            |
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• Ureteral Stricture Risk: Ureteral trauma, particularly perforation, increases later stricture risk. Grade 2 denotes submucosal injury; grades 3–5 describe perforation or transection. Document injury and plan drainage and follow-up accordingly.
• Clinical Rule: If significant mechanical resistance is encountered during UAS advancement, forceful dilation must never be attempted. The sheath must be withdrawn, and the surgeon should either perform primary URS without a sheath or place a double-J stent for 1 to 2 weeks of passive ureteral pre-stenting before returning.

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Evidence-Based Indications for Post-URS Stenting

The decision to place an indwelling double-J ureteral stent at the conclusion of URS must balance therapeutic benefits against known stent-related morbidity (dysuria, frequency, urgency, flank pain on voiding, and encrustation).

1. Uncomplicated URS (Stentless Approach)

In accordance with EAU guidelines, routine post-operative stenting is NOT necessary in patients who meet all of the following criteria:

• Complete calculus extraction/clearance without residual fragments.
• Uncomplicated procedure without ureteral injury requiring drainage.
• No anatomical narrowing or pre-existing ureteral stricture.
• Normal contralateral functioning kidney.
• No pregnancy, infection, renal impairment or other factor that makes postoperative obstruction particularly hazardous.

2. Increased-Risk Cases Requiring Drainage

Insert a stent when complication risk is increased or drainage is uncertain. Duration depends on the injury and clinical course; EAU specifies no single ideal duration. Important situations include:

• Ureteral Mucosal Trauma / Perforation: Stent submucosal injury or perforation to support healing and drainage; major transection may require reconstruction. Stenting reduces obstruction and leakage risk but does not guarantee freedom from stricture.
• Solitary Functioning Kidney: Postoperative obstruction in a solitary kidney can cause acute renal failure, so secure drainage is particularly important.
• Significant Residual Stone Burden / Dust: Stenting maintains luminal patency, preventing fragment impaction and steinstrasse.
• Extensive Balloon or Serial Dilation: Dilation induces localized edema that can occlude the ureter.
• Impacted Calculi: Long-standing impacted stones induce chronic mucosal inflammation, ureteral polyps, and muscular thinning.
• Pregnancy: Pregnancy increases the need to ensure reliable postoperative drainage.
• Suspected or Confirmed Sepsis / Bacteriuria: Ensure drainage and treat infection. If infection is recognized during stone surgery, stop definitive lithotripsy and manage the infected system; a stent alone is not complete sepsis treatment.

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Comparative Reference Tables

Laser Lithotripsy Physics Comparison

Ureterorenoscopy Equipment Selection