What are Holmium Laser Fiber Sizes, Really? November 21 2016

Section 502 of the Federal Food, Drug and Cosmetic Act (FFDCA) says that a device is misbranded if its labeling is false or misleading in any particular. Maybe the FDA doesn’t read Urology?

I’m referring to a paper that appeared in the journal Urology a couple of years ago*, questioning the veracity of holmium laser fiber manufacturers in describing the diameters of their fiber devices. The conclusion was that nobody told the truth. That would actually be a good case against holmium fibers being misbranded, in that, if all fibers are uniformly mislabeled, it is not misleading. While the cynic in me appreciates that conclusion -- and indeed it has some foundation -- the Urology paper’s conclusion incorrect: at least some fibers are named and labeled correctly.

InnovaQuartz has been diligent in naming, and describing, our holmium laser fibers accurately for more than 25 years. We’ve also not been shy about calling out others for their convenient lack of precision in naming their fibers. And it does matter. The stiffness of a fiber is directly proportional to the diameter, as is their occlusion to irrigation flow. 

For anyone interested in where the oddball sizes like 365 μm and 550 μm came from, I’ll put a bit of history at the end of this blog to feed your curiosity. For the rest of you, the take home message is that optical fiber is named for the core diameter in the fiber optics industry, regardless of the thickness or number of other layers. There are four layers in holmium laser fibers. Te core is the fused silica rod that carries the laser energy. The cladding surrounds the core and is also fused silica, but doped with fluorine to alter the refractive index. Over the cladding is a thin and transparent plastic coating that protects the glass underneath and acts as a fail-safe secondary cladding. The blue plastic that surrounds that is a tough fluoropolymer jacket. (It’s green for Boston Scientific’s Flexiva.)

If you use a micrometer (gauge) to measure the glass diameter of a fiber you will be measuring the cladding diameter, not the core. The cladding dimension is typically 1.1-fold or 1.2-fold the core diameter, so a 273 μm core fiber will measure as 300 μm glass diameter. If the transparent plastic coating is still intact the fiber will measure about 30 μm larger as these coatings vary between 10 μm and 20 μm thick depending on the fiber size (thinner on smaller fibers). The blue jacket varies from about 60 μm thick up to 200 μm thick, also increasing with fiber size.  The figure below is to scale, showing the various layers making up a holmium laser fiber. The blue jacket stripped back 4 mm and the transparent plastic coating stripped back about 0.5 mm.

Actual Holmium Fiber Dimensions

You cannot physically measure the core dimension. It can be measured optically but it takes a special microscope to do so precisely. You can measure it by inference and with fairly good certainty if you know that any glass diameter under 300 is probably 1.2 CCDR and anything over 300 is probably 1.1 CCDR, where CCDR is the Cladding to Core Diameter Ratio. Measure 405 μm and it’s a 365 μm core, for example (405 divided by 1.1 = 368 ≈ “365”, keeping in mind that there is a variation in the actual dimensions of such things, that micrometers aren’t perfect and that there may be remnants of the thin plastic coating, or dust on the glass surface).

Time for a shameless plug espousing the benefits of InnovaQuartz’ ProFlex™ fibers: Not only are ProFlex fibers the most efficient and reliable fibers, they are also the only ones that are properly named for their physical size, throughout the product line. ProFlex 200 has a 200 μm core…ProFlex 273 has a 273 μm core, etc.  Others call their 273 μm and 242 μm fibers, “200 micron”, which is not only incorrect, but bad nomenclature. (The “micron” was discarded by international standards authorities 30-years ago, in favor of the more confusing term, “micrometer”, the same as the tool.)  

I suppose I can see Boston Scientific’s marketing rationale for calling the Flexiva™ a 200 micron fiber -- after all “everybody else is doing it” and “we’ll suffer a disadvantage if we call it a 242” -- but they didn’t mislead customers at the beginning; why start now? When InnovaQuartz made the fibers that brought Boston Scientific to laser URS -- AccuFlex™ -- all five sizes were properly named.  I would be remiss if I did not point out that the Boston 242 μm core Flexiva™ and AccuMax™ are almost exactly the same diameter as our ProFlex™ 273 -- and others “200 micron fibers” -- and just as stiff as a result.

If you truly need flexibility, use a truly 200 micrometer fiber: ProFlex 200.

Now for that bit of history; first, why the odd ball sizes?  When the ‘special fiber coatings’ some companies claim they invented were first developed, their purpose was to replace the expensive, fluorine doped-silica glass cladding to make bargain priced fiber optics: so-called “hard polymer clad” fibers, now called simply “hard clad” or “HCS”. The ‘hard’ notion came from the fact that the plastic coatings were thin and rigid compared to the soft silicones that preceded it.

Die coating systems and manufacturing parameters were developed for making the standard fiber sizes of 200, 300, 400, 600, 800 and 1000, meaning the glass core strands were these diameters. A few years later saw the introduction of the holmium laser for lithotripsy and the old fiber constructions just did not work. The solution turned out to be using the new hard polymer cladding as a coating, but upon the expensive and already glass clad AFS fiber (“All Fused Silica” or, as explained above, fused silica core that is clad with fluorine-doped silica glass). Rather than develop new die coating systems and production parameters, the new fiber construction was simpler to make at the same glass diameter as had been the HCS fiber. A 600 μm doped silica outer diameter rod yields a 550 μm core, and so on.

This simple solution blew apart with 200 μm fiber due to optical limitations that I’ll not delve into here, but because InnovaQuartz made the first true 200 μm fiber for flexible laser URS anyway, we sized it to permit full scope deflection rather than to fit into existing fiber manufacturing equipment or size convention. At least we named it correctly… Thanks for reading.

@doctordsilica

#proflexfibers

Flexiva™, Accumax™ and AccuFlex™ are trademarks of Boston Scientific.

ProFlex™ is a trademark of InnovaQuartz.

© 2016 InnovaQuartz LLC

* December 2014