All Holmium Laser Fibers are the Same, Right? Pt.4 -- Fiber Design Evolution April 20 2016
Part 4: Fiber Design Evolution -- the early days as reflected in today's products
First and foremost, for differentiation among holmium laser fibers, is the issue of fiber dimensions. Frankly, I am astonished at the lack of clarity or standardization in identifying fiber sizes within this industry. Almost all “200 micron” fibers are actually much larger than 200 microns; not as egregiously large as described in some of the urological literature, where total fiber diameters are reported as opposed to the fiber core diameters used by manufacturers, but definitely misleading to the point where incredulity dominates. I’ll have more on fiber sizes in a later part: identifying the two dimensions that actually matter to urologists.
Of “200 micron” fiber offerings, only a true 200 micron core fiber, like ProFlex LLF200, permits full deflection of your ureteroscope where false 200s reduce the maximum deflection available by as much as 20%.
Most holmium fibers are pretty much the same under certain metrics. Large core fibers, for example, all work on SMA lasers just fine, at least on first blush. ProFlex™ LLF works a bit better by filling less of the fiber numerical aperture (NA) and shaping this lower divergence into as collimated a working beam as possible at the working tip, but these differences don’t produce a huge gap in functionality except in extreme cases (huge, hard stones or challenging access or in reusable configurations where our fibers retain performance much longer than others do). I’d question the wisdom of using a lesser fiber where ProFlex is a cost effective alternative and you never know when you’ll need the ProFlex™ advantage.
A good example of the differences between a generic holmium fiber and a superior fiber is found in the SureFlex™ 365 when it was manufactured at InnovaQuartz Inc. prior to 2007 (features now embodied in the ProFlex LLF 365). Around the turn of the millennium it was assumed all larger core holmium fibers were the same -- that only fibers smaller than 365 could be distinguished in performance. Then some aggressive mobilizers found that IQ’s SureFlex 365 could be used dozens of times; one reported 99 cases. (I’ll have more to say on the risks associated with reusable fibers, validation and why so-called “unlimited reuse” fibers should be called “guaranteed failure and liability” in a later segment. For now, just respect the label’s limits.)
Then came the AUA meeting in San Antonio wherein Dr. Bodo Knudsen’s (et al.) podium presentation titled “Performance and Safety of Holmium:YAG Laser Optical Fibers” (published later in the Journal of Endourology, Volume 19, Number 9, November 2005) identified the SureFlex 365 as the only fiber of any size, made by anyone, that the researchers simply could not make fail, under any conditions, including a 1 cm bend diameter under the full 80 watts of power: “The IQinc 400 was the only fiber that did not fracture during multiple trials with a bend diameter of 1 cm, and the finding approached statistical significance (P = 0.06)”. Our new ProFlex LLF 365 is even better -- I’ll deep dive into the reasons why in a future installment but it can be boiled down to the fact that we figured out why SureFlex 365 was so bulletproof and applied that knowledge to produce the new product line (and two US patents).
The differences between others’ fibers that are smaller than 365 micron core boils down to which failure mode is dominant -- connector failures or burn through failures – as this is the realm where the laser focus can be larger than the fiber’s aperture (core diameter): where technological savvy is truly tasked.
FIGURE 1: Standard SMA Termination (buffer-stripped fiber glued into the SMA)
At the inception of laser lithotripsy, most fibers were terminated as depicted in Figure 1; this simple termination is still in use for larger core fibers from other manufacturers (365 micron, 550 micron and 910 micron). Figure 1 depicts a 273 micron core fiber termination for continuity with Figure 2 and modern issues.
At relatively low average laser powers, pulse energy density can be high enough to vaporize the glue and polymer cladding around the fiber -- and even the stainless steel of the SMA ferrule -- as depicted in Figure 2. Historically, connector failure problems arose even for larger core fibers as laser powers were increased. The most frequent consequence of large core fiber failures was laser lens damage, thus taking the laser out of service while the costly repairs were performed. (Even minor overfill, misalignment or leaking causes problems by heating the connector enough for the fiber adhesive to expand and creep onto the fiber face, setting off thermal runaway that results in catastrophic failure.)
Sacrificial blast shields were added to most lasers between the connector face and the laser focusing lens to reduce the cost and downtime associated with what were, at the time, rather routine connector failures. Replacing a sacrificial blast shield with an available spare does not require technical skill in most cases and the costs are typically an order of magnitude lower than replacing a focusing lens. Connector failures that occur at higher pulse energies are generally more dramatic, expelling chunks of stainless steel and glass at high velocity. Some thicker blast shields are able to protect the laser lens against even catastrophic failures, but not all blast shields are thick nor are thick blast shields always thick enough.
The first trend in laser connector design involved preserving the blast shield or focusing lens. Strategically, the location of the damage was sequentially displaced away from the laser, into the connector body and then (unwittingly) towards the physician and patient. This spared the laser but, in hindsight, it was not the best idea. As it turned out, just about all of the new designs had unforeseen consequences, i.e. burn through failures became more common.
The most common strategy for producing high power connectors in holmium laser lithotripsy dates to the 1990s and is commonly referred to as a “air well termination” or just “well connector” (Figure 3). The idea is simple because it is based upon an extremely simplistic view of why fibers fail; if the damage results from the intense laser focus hitting adhesive and metal, why not just remove these materials from the laser focal plane? Problem solved?
The solution looks elegantly simple, and logical, and it does work for minor misalignment issues in some cases, but it also introduces new sources of misalignment, particularly for smaller core fibers (Figure 4). The initially problem arose because standard SMA connectors provided just about 2 mm of the precise bore for locating the fiber in the center of the connector ferrule (originally from ~A to the input face in the connector depicted in the figure). By counterboring from the input face to form the air well, most of this precision bore length is lost and the foreshortened alignment bore allows the fiber to tilt significantly, or become canted.
Even where the laser focus is small enough to couple completely with the fiber aperture, as depicted in Figure 4, a couple of degrees of fiber canting results in gross misalignment. Worse, canted fibers always preload high angle modes, predisposing the fiber to distal burn through. The potential problems with air well terminations do not end with misalignment. Thin, highly flexible fibers are impossible to polish in situ within an air well; there is no support near the face of the fiber being polished so it chatters and chips. Polishing the fiber prior too installation offers a better result, but polished fiber faces are easily damaged by installing them in the SMA’s metallic ferrule bores and the faces must be perfectly aligned with the ends of the ferrule least the fiber aperture fall before or after the focal waist, where the beam is bigger.
Fiber raw material used to manufacture laser fibers are made with a protective polymer cladding, also known as a second cladding, between the glass cladding over the fiber core and the protective jacketing. (I’ll delve deeper into the fiber raw material construction in a later installment – the rationale for double cladding and the unintended consequences of having two claddings, in the segment discussing fiber diameters.) Manufacturers must leave the polymer cladding on the fiber to protect it while it’s being “bare” polished, and for protection during installation within the SMA; while the adhesive and metal are removed from the focal plane, the cladding coating remains.
Do you think this polymer burns when hit by laser energy? Of course it does. It’s a tiny amount of burnt plastic -- about a dozen nanograms – so does it matter? Over time polymer “smoke” builds up on the blast shield. If the blast shield is not routinely replaced and is allowed to accumulate enough burnt cladding debris, it will fail (and can take out the laser focus lens or the fiber you’re using when it goes).
Air well terminations also fail to go far enough. Figure 5 shows a burn in the bottom of a commercially available air well fiber: a 273 micron core (called a “200 micron” bare fiber by the manufacturer). This burn occurred on half of the fibers we tested from this manufacturer and all of the burns were observed after less than 150 pulses at 0.8 joules per pulse (15 pps, 12 watts average power, less than 10 seconds lase). We opted not to test these fibers further since we were low on blast shields.
The shape and position of the burn mark in the air well (the dark crescent at the bottom of the well in Figure 5) is caused by laser energy missing an eccentric (or canted) fiber within the laser focus. As shown in Figure 6, where the holes in the beam profiles at right are where the light entered the fiber aperture, the remaining overfill energy is crescent shaped where it is highest in intensity.
Lastly, it is very difficult to clean the face of a fiber within an air well termination; attempting to do so easily breaks the fiber and at least pollutes the air well with cleaning cloth fibers. Air wells readily collect detritus, especially when used, and a thin film of sticky polymer cladding residue makes cleaning next to impossible. The air well is a wholly inappropriate design for reusable fibers.
Next time in “All Holmium Laser Fibers are the Same, Right?” Part 5: Beyond Air Well Terminations.