Simulating manual manipulation of small optical fibers within flexible ureteroscopes for potential application in thulium fiber laser lithotripsy

Thulium fiber laser (TFL) lithotripsy has recently been introduced in the clinic. Previous TFL laboratory studies demonstrated successful high-power delivery through ultra-small (50-150-µm-core) optical fibers. This preliminary study simulates the forces on fibers during insertion into an ureteroscope and determines the mechanical feasibility of ultra-small fibers in a clinical setting. Simulations were conducted for commercially avalaible silica fiber sizes (core/cladding): 50/70, 72/108, 100/140, 150/165, 150/180, and 200/240 μm. Solidworks software intregrating Euler’s buckling equation was used to calculate fiber buckling thresholds as a function of typical manual forces (0.3- 2.0 N) applied near the proximal end of a standard ureteroscope. Forces on fibers being inserted were modeled, assuming support from saline flow and resistance by the working channel wall. Simulation results were categorized based on force values previously reported in the literature, with smaller forces ( 1.6-2.0 N) at risk of damaging the working channel. Fiber sizes were simulated with two different types of holdings on each end to find a range of possible values that most closely simulate clinical behavior. The smallest usable standard flat tip fiber was found to be 150/190-μm (core/cladding), assuming a cladding thickness of ten times the laser wavelength of 1.94 μm (or extra 40 μm OD) to prevent leakage of evanescent waves through the core/cladding interface. The smallest usable ball tip fiber was found to be 110/150 μm. Numerical simulations predicted that optical fibers for TFL lithotripsy should be larger than 110/150 μm to provide effective manual manipulation within flexible ureteroscopes.