The 1990s saw a boom in laser TURP (Transurethral Resection of the Prostate) using mostly Nd:YAG lasers. Nd:YAGs showed promise for lowering the classical side effects of standard TURP, where a hot wire loop is used to melt/carve away strips of hyperplastic tissue; their CW (continuous wave) output produced smoother surfaces than holmium lasers do, they did not strongly interact with the sterile irrigation fluid and were less damaging to fibers than holmium lasers. Laserscope’s ADD-Stat™ fiber -- the progenitor to the Model 2090 used in the GreenLight™ revolution of 2000 -- was a fairly popular device that was used with the 800 series laser: a 100 watt Nd:YAG laser that could be switched to 40 watts of 532nm output using a KTP frequency doubling crystal.
We previously discussed the ADD-Stat fiber, as designed by Russell Pon, but I’d like to refresh your memory because it was an important advancement. Pon recognized that the backscatter -- and indeed most of all scatter in generic side fire fibers -- were due to what I’ve coined “Snell Reflections”: reflections at and above the critical angles encountered near the edges of the cylindrical optical fiber output (see figure, immediately below).
The figures above and below comes from one of my patents and represent my interpretation of Pon’s original drawings, as provided in Part 2. If you are sure that you understand the issues addressed by Pon, feel free to skip this next paragraph.
In the figure, the central ray A exists as At (where “t” is transmission) without refraction and minimal Fresnel reflection (not shown). Ray B is closer to the edge of the fiber and escapes as a refracted ray Bt and suffers a stronger Fresnel (f) reflection Bf that is also refracted on escape as Bft, with its own Fresnel reflection Bff, ad infinitum. Moving closer to the edge of the fiber, ray C encounters the glass:air interface at an angle that is at or larger than the critical angle so it is totally reflected (r) within the fiber as Cr, then again as Cr’, until it encounters the flat plane of the TIR bevel again, but from the opposite direction, escaping Ct and suffering Fresnel reflection Cf in the opposite direction that refracts upon escape as Cft and suffering a Fresnel reflection Cff that gets captured by total reflection again, ad infinitum. (I had to stop somewhere and, admittedly, the third order Fresnel reflections are quite small – but they do add up to produce a very strange output profile.)
The ray closest to the edge of the fiber D is also totally internally reflected as Dr, the Dr’ and escapes through the flat bevel as Dt while suffering a Fresnel reflection Df and that’s as far as I’ll go with that. You should be getting the picture clearly by now: it’s an absolute mess inside a generic, bevel-tipped side fire fiber. Pon’s fix was to displace the glass:air interface farther from the core such that only very extreme edge rays would see critical angles after reflection from the TIR bevel. Pon’s fix simplifies the figure immensely:
Other than the use of a 1.4 CCDR fiber within the Add-Stat™, there is nothing substantially different about the Laserscope device over a simple, generic side fiber. The TIR bevel is mechanically ground and polished directly upon the fiber and the bevel-tip is protected by a fused silica cap that is glued directly to the fiber hard cladding.
Another fairly successful device of this era was an ‘adaptation’ of an optical design I’d done for Laserscope in 1990: specifically the LDD-Stat™ as described in US Pat. No. 5,257,991. The LDD-Stat was intended for laser discectomy and it was my very first laser surgical fiber design. Laserscope did not see fit to name me as an inventor on that patent, being an employee of a contractor. This kind of thing has happened so many times since that I’ve become accustomed to it, but this was my first job out of graduate school and I was surprised by the slight. There was little that I could do about it but I got a bit of satisfaction; I feed Laserscope an incorrect rationale for a unique feature of the design and they used that explanation within the patent. I know this was petty, but the ego has its hungers.
I’m covering the LDD-Stat design because it was later incorporated into a Heraeus Surgical BPH fiber offering -- the first fiber where the protective quartz cap was fused to the fiber. Back in 1989 when I was designing this fiber, the fiber construction that was most commonly used in laser surgical devices was far different from that we use today. Instead of a low refractive index polymer coating and Tefzel® or Nylon® over the glass cladding, all that was present was a thin coating (0.015 mm to 0.020 mm) of polyimide. I had a rudimentary (manual motion) CO2 laser system but had not become terrifically proficient with using it at that time. The mechanically polished bevel tips were also far from perfect which resulted in substantial axial leakage through the TIR bevel, and this was the basis of my trivial revenge.
When I was asked by the Laserscope why I’d used a beveled spacer in the fiber cap, I claimed that it was there to redirect axial leakage, but Snell’s law says that’s not possible; total internal reflection can only occur at a boundary from high refractive index to lower refractive index, not vice versa. There are some Fresnel reflections of axial leakage, but the bulk continues axially, and although this leakage is refracted in the generally correct direction, the angle of emission if too low to be of surgical relevance. Laserscope’s specification for the position of the output bevel was quite tight, owing to the fact that this optical assembly was to be housed in a steel test tube (or ported cannula), with a fairly small exit hole for the laser emission. The actual reason for the plug was for locating the fiber at the proper position, axially, within the cap.
As mentioned above, the base fiber was 400 μm core, 1.1 CCDR, silica/silica fiber with a polyimide buffer. The step on the cap served to reduce the wall thickness of the cap at the point where the hermetic seal was to be formed between the cap inner wall and the fiber’s glass cladding. The vent holes proved necessary to allow water vapor and combustion gases from burning polyimide to escape. Polyimide coatings on fibers are typically incompletely “cured”, meaning the imide rings had not all closed. The ring closure of amide to imide is a dehydration reaction so one mole of water is evolved for each mole of imide formed and the polyimide would also burn a bit where it was closest to the seal. Evolving gasses prevented full seal formation.
Heraeus Surgical’ version of this LDD-Stat™ fiber for BPH was larger by 50%. It was based upon 600 μm core fiber with polyimide coating but with the addition of a nylon jacket. It used the same stepped cap and hermetic seal, although the beveled plug was omitted. For the BPH side fire, the step on the cap allowed for overlapping heat shrink tubing to stiffen and reinforce the cap to fiber junction, but as you’ve likely surmised, this fiber had several shortcomings: ~28% backscatter, ~5% axial leakage and a tendency to break off at the step in the cap OD. Even still, it was a fairly popular fiber at the time, selling at ~$800.
The most successful fiber of the period was likely one made by Trimdyne: Urolase™. Although Trimedyne did (and still does) make side fire fibers based design upon TIR, Urolase was not. It was essentially a tiny, semi-parabolic metal mirror that was crimped onto the end of the fiber (see below).
The image above is taken from one of the Trimedyne patents, e.g. US Pat. No. 5,437,660, but I’ve colorized it for clarity. Note first that the rays depicted are generally collimated, a misleading feature of most patent drawings for side fire fibers. Still, you’ll get the gist of the function: the fiber core is light blue, the fiber cladding is dark blue, the base metal of the crimped on reflector of brass is yellow and the gold plating is an orange like color. R marks the supposed output rays.
While hundreds of thousands of these fibers sold, they really weren’t very good. True, they were fairly robust being absent a glass capsule, but the cup-like void for the parabolic reflector was readily contaminated by tissue and the reflector surface was damaged quickly. These fibers typically got hot, so hot in fact that a couple of Trimedyne engineers formed a new company called Energy Life Systems to exploit the heat, making metal capped side fire fibers that purposefully got hot. The ELS fiber design never really went anywhere but foreshadowed the use of metal caps over glass.
The other thing about Urolase that was different from the norm at the time was its diameter; it was huge in comparison to the other fibers offered (save one). The metal cup diameter was on the order of 4 mm diameter. That it was among the most widely used fibers of the era spoke to the fact that the mantra of many other companies was probably wrong, that side fire fibers HAD to be kept to 6 French channel compatibility. Sure, 6 Fr. is nice in permitting use in just about every scope that could be used for BPH, but it was definitely not necessary for success and when one is forced to stay under 1.8 mm there is precious little room to do build substantive optics for turning the light at right angles.
Coherent Medical also broke with the 6 Fr. tradition and used a very thick quartz capsule with an outer diameter on the order of 3.5 mm. That side fire fiber lasted longer than any other of the time, but it did not sell well for reasons I do not know. The sales success of side fire fiber models WAS known to me because I made most of the quartz caps (for the major players) but by 1996 there were almost two dozen minor players as well, in a total market that could support two or three of them, at best.
It is my thesis that proliferation of mediocre side fire fibers killed laser BPH in the mid-1990s. Prices for most side fire fibers were too high because nobody won sufficient market share to exploit the economies of scale. One of the poorest performing fibers won the lion’s share of the market because it was both early to the marketplace and distributed by the 900 lb. gorilla of the time. The ADD-Stat™ was a good fiber design, but it also cost too much due to its reliance upon 1.4 CCDR fiber.
About ½ of the side fire market was held by the two dozen “me too” fibers and many of these were simple bevel tipped fibers with the quartz capsule glued on. There were a few innovations that briefly won favor, but opportunists’ reacted quickly to adopt similar enhancements. For example, Heraeus Surgical introduced a heat shrink orientation marker (above), featuring an dashed orange line on the output side of the fiber and green line on the back side: the idea being that if you could see the green line it was safe to fire the laser (fiber output facing away from the scope optics and generally pointed at tissue). Within a couple of years just about everyone was using a similar strategy, patent or no patent.
There were “me too” non-TIR fibers like Xintec’s (above) and fibers that were just bent (below) to redirect the energy, akin to Biolitec’s Twister™ of today. Other innovations involved improved fiber control, infrared feedback (akin to AMS’ FiberLife™) and such, but the bulk of fibers remained simple bevel tipped designs and before long every fiber looked like the next and most worked poorly. The driving force for the market became cost it became a race to the bottom for quality and function, with a couple of notable exceptions.
First, a word or three on bent fibers: they don't work. OK, that's a bit harsh. How about they don't work the way that you think they will? You can't bend a multimode, step index and large core fiber that sharply, even if it has been done with heat, without consequences. It leaks from the bend. That' kinda the point for the ELS fiber, where the metal plug 14 is supposed to get hot, but in the ESP and Biolitec products this leakage is no less a problem than generic side fire fibers. Sure, the designs are simplified versus basic TIR-based designs, so they are probably better than THAT old technology, but it's 2017! Good, I got that off my chest.
Laser Peripherals had started out selling laser eyewear and had been quite successful and an early entrant able to command extraordinary profit margins. LP also sold generic laser fibers for several surgical specializations and while their fiber prices were consistently lower than most others’ their market was primarily focused on laser rental companies that reused the fibers. Around 1995 LP introduced the ScatterFree™ side fire fiber (below, US Pat. No. 5,537,499) but it did not gain traction.
As you can see from the Xintec/Trimedyne and ESP/ELS/Biolitec similarities, there was considerable parallel development going on in the field: everyone trying to gain the upper hand and lawsuits were flying about freely. The ScatterFree fiber was the first of a subcategory of TIR-based fibers that have earned the moniker “fused fibers” because the problematic cylindrical exit surface of the fiber is fused 125 to the inner wall of the protective capsule and eliminating the reflection problems; first to market, anyway – but it was not the first fused fiber, even though the LP patent was filed first.
Until a couple of years ago, the US was a ‘first to invent’ country with respect to priority of inventorship, or who gets the patent. This is one reason why we scientists were taught to keep meticulous lab notebooks and have a colleague periodically read and sign the pages, indicating understanding of the contents. The US is now a ‘first to file’ country, meaning whomever is first to the patent office with an idea gets the patent, regardless of who actually invented something first. I filed my patent application for MaxLight™ (below) after LP had filed for ScatterFree, but I’d disclosed MaxLight to my attorney (in a letter dated earlier) and signed lab notebooks predating that, dates well before LP’s patent was filed (and Laserscope/Pon, in fact). MaxLight never made it to the marketplace, however, at least as a medical device.
MaxLight had a flaw that made it too risky for use in surgery, at least in my opinion, so we decided not to sell it, preferring to wait until we had a better solution. The LP ScatterFree has the same flaw, as does any fused fiber, including Lumenis’ Duotome™ SideLite™. You see, fusion requires very high temperatures be applied to melt the fused silica surfaces together -- almost 1800°C (3270°F) – but the plastics in the fiber can’t exceed about 200°C just a couple of millimeters away or else they’ll melt or even burn. The only way to accomplish this is to heat the glass cap very rapidly and in a very confined area, keeping the rest of the capsule cool. This abuse would shatter the capsule instantly, like a hot marble dropped in ice water, if it were not for the very low thermal expansion of fused quartz glasses.
Fused quartz does not shatter, at least not right away. But the stresses produced by the spot fusion do not go away: they are locked into the glass’ structure, predisposing it to shattering later, when stressed in surgery. In other words, caps of fused fibers are prone to snapping off at the fusion junction. This is at least partially why DuoTome has a metal cap around the glass cap. But we’ll get to DuoTome, in its modern configuration, later.
Knowing what I now know about patents, MaxLight should have been filed as multiple patents because it was, in fact, multiple inventions rolled into one. But it was just my second patent application. I didn’t know anything about patent construction or prosecution back then. Heck, I didn’t even know what kind of attorney to use; the disclosure I mentioned above was sent to my father’s copyright attorney, and he died shortly thereafter. It then passed to a powerful, New York patent firm, but they were so intimidating that I just accepted what they said and did without question or comment. Rest assured that I do not make such mistakes any longer.
Even with its flaws, MaxLight solved problems with side fire fibers well beyond the backscatter issues, but this chapter of the blog series is already running long, so we’ll discuss MaxLight as compared to ScatterFree, and maybe a couple of other designs of the era, next time.
Thanks for reading,
Stephen Griffin
@doctorsilica
#LDD85 #LDDduet #LDDtrio #QuadraLase #MaxLight
© InnovaQuartz 2017
The trademarks used above are the property of the companies, or their successors, mentioned with the trademarks. MaxLight is a trademark of InnovaQuartz for side fire fibers used in spectroscopy.