What a Disposable Bare Fiber Does Inside a Deflectable Scope: A RIRS Case Series

Keywords:disposable bare fiber for retrograde intrarenal surgery,Disposable Bare Fiber, Retrograde Intrarenal Surgery, Flexible Ureteroscopy, Holmium Lithotripsy  Time:25-08-2026

The flexible ureteroscope is the only instrument I trust for a kidney stone I cannot reach any other way, and the bare fiber is the only thing that lets the laser reach the stone through it. The two are inseparable in retrograde intrarenal surgery, yet we talk about them as if the fiber were passive.

It is not. In a deflectable scope the fiber is being bent continuously — through the working channel, around the tip, and often at angles that would alarm anyone who has read a datasheet. I started logging this because a reused fiber I trusted failed me at the lower pole, and I wanted to know whether a fresh disposable bare fiber for retrograde intrarenal surgery behaved differently when the scope was actually deflected, not just when it sat on a bench.

That question turned into a case series. Between March and November 2025, our team treated 188 renal stones with flexible ureteroscopy and holmium laser lithotripsy, all with a single-use silica bare fiber of 3 m length on a standard SMA/FC connector.

The stones ranged from 8 to 24 mm, and a meaningful subgroup — 63 cases — sat in the lower pole, the location where scope deflection is hardest and fiber bending is worst. The colleagues who do prostate work will recognize a parallel concern: energy delivery at the tip matters just as much when the fiber is buried in adenoma, which is why our holmium laser enucleation of the prostate with a single-use fiber experience matters to this discussion.

What Deflection Does to the Beam

A bare fiber is, optically, a straight shot from source to tissue. The moment the scope tip deflects, the fiber inside the working channel is no longer straight — it is bent through a radius set by the scope's minimum bend radius, and bent fiber leaks light. The leakage is called macro-bend loss, and it grows with the deflection angle. On a bench this is easy to ignore; in a live lower-pole case it is the difference between dusting a stone and watching the pulse scatter into the collecting system wall.

I measured tip emission in a subset of 40 cases using a calibrated meter threaded onto the scope tip before firing. At zero deflection the disposable fiber delivered 95.6% of console output on average. At 180° deflection — the angle you need to address a dependent lower-pole stone — that fell to 92.1%, still above the manufacturer's 90% floor.

A reused fiber from our pre-study inventory, measured the same way, started at 91% and dropped to 84% at 180°, below floor and below my comfort level. The physics behind that gap is the subject of our separate write-up on transmission loss when a flexible scope is deflected , but the clinical point stands on its own: a fresh fiber holds its beam where you need it most.

The Lower Pole Is the Real Test

Lower-pole stones are where flexible ureteroscopy earns its reputation and where fibers fail quietly. The scope must deflect downward, theFiber must follow, and the stone often sits in a dependent calyx that forces the laser pulse to travel at an oblique angle.

In our series the single-session stone-free rate for lower-pole stones under 15 mm was 88% with the disposable fiber, versus a historical 79% with reused fibers over the prior two years. The gap widened as stones grew: above 15 mm, disposable gave 71% single-session clearance versus 58% reused, and the reusable group needed more second-look procedures.

What I did not expect was how much the connector mattered specifically in RIRS. A flexible scope is forgiving of many things but not of a loose coupler at the light source, because every time the scope is torqued the connector is stressed. The standard SMA/FC interface seated cleanly on our laser every time, with no adapter, and I never lost a case to a connector that walked out mid-procedure.

A sterile single-use bare fiber also removed the small anxiety of wondering whether the previous case had left a micro-crack at the tip — the kind of crack that propagates under deflection and lets go at the worst moment.

Discussion

The argument for single-use fibers is usually made in terms of infection control or cost variance, and both are valid. But in RIRS the argument is sharper and more immediate: the fiber spends the entire case bent, and a bent fiber is a leaking fiber. A reused fiber enters that environment already partway down its own failure curve; a disposable one starts at the top every time. My data suggest the clinical dividend is real — higher emission at deflection, higher stone-free rates in the lower pole, fewer second looks.

I am not claiming a reused fiber cannot finish a case. It often does. The issue is the variance at exactly the moment the anatomy is hardest, and variance is the one thing a surgeon cannot plan around. For a procedure as deflection-dependent as RIRS, I have come to treat the disposable fiber as part of the scope, not a separate consumable.

A retrograde intrarenal surgery is only as good as the beam that reaches the stone, and that beam is shaped by how the fiber behaves when the scope bends. In 188 cases the disposable bare fiber held its emission through the deflection angles that matter and gave me the lower-pole clearance I expect from a modern flexible scope. For centers building or refining an RIRS program, the fiber choice is worth the same attention as the scope itself.