ProFlex LLF -- Advanced Technology for Holmium Laser Lithotripsy June 11 2015
InnovaQuartz developed the first quartz ferrule reinforced surgical fiber termination 20-years ago; we called it the Black Hole™ because it could capture and absorb even highly errant laser focus energy without damaging the laser or the connector. The holmium laser fiber product line came to be known as SureFlex™ for its unprecedented high tolerance to tight bending under power.
InnovaQuartz was acquired by Laserscope in 2006 and then became part of American Medical Systems a month later when Laserscope was acquired. In 2012, the scientist who had designed and perfected SureFlex and AccuFlex™ (and founder of InnovaQuartz, Stephen Griffin) had an “ah ha” moment while working on a laser design: a realization that he had not considered focal angle overfill in the SureFlex design.
There are two categories of laser focus energy that overfill holmium laser fibers and render them susceptible to burning through in bending: spatial, where the beam is off center or too large for the fiber core, and angular, where the beam may well be smaller than the core and perfectly aligned, but edge of the focal cone imparts the fiber at angles that are higher than the fiber was designed to accept. SureFlex fibers are robustly designed to be impervious to spatial overfill, but no accommodation had been made for angular overfill, even though angular overfill rapidly renders a fiber sensitive to burn through.
Testing of a new holmium fiber design concept, code named “ASPIRE”, began in 2013, shortly after InnovaQuartz was reconstituted. ASPIRE = Angular Separation of Populations for Introduction or Rejection of Energy. A patent application was drafted in 2014 and the High Power Connector design, branded Pulsar™ HPC, was integrated with high performance optical fiber, tested and validated to produce the newest holmium fiber for lithotripsy and litholysis: ProFlex™ LLF.
The first production run of ProFlex LLF is going into sterilization this week and will be available in four sizes – a true 200 micron core in LLF200, one made of fiber that others misrepresent as 200 micron in LLF273, a 365 micron in LLF365 and a 550 micron in LLF550. A large core, 910 micron LLF910 will follow next month and a 150 micron fiber is in development.
The Science Behind Pulsar HPC Explained (in every day English)
Holmium fibers burn through when the angles of light within the fiber get to be too high to be reflected and guided down the fiber core. Holmium laser fibers can only contain light up to about 13 degrees off the fiber’s axis (as measured in air). Accordingly, holmium lasers are designed to focus light into the fiber at angles up about 10 degrees, so on first blush there should be no problem with light leaking from the fiber.
Small holmium fibers are also asked to bend through some rather tight and tortuous pathways in reaching their target. Bending the fiber causes low angle light to reflect at higher angles than they would in loose bends or within a straight fiber. The tighter the bend, the more the higher the angle the light becomes.
Angles of Reflected Light Increase in Tight Bending
Worse yet, holmium laser beam quality degrades as the laser heats up. The laser rod suffers thermal distortion much like the heat waves that cause mirages above the pavement on a hot summer day. The diameter and position of focal spot changes with each pulse, particularly when surgery is prolonged. This instability is often called ‘beam bloom’ because the most obvious consequence is a larger focal spot that won’t fit into a small fiber. The inverse also occurs -- the focal spot is reduced in diameter -- and this is actually worse because, rather than blowing the laser connector, the fiber burns through in the ureteroscope.
Why is this? The produced beam of a laser -- the beam existing the laser rod before it is focused -- normally diverges a bit so that the diameter of the beam at the lens is larger than the diameter at the end of the laser rod. When the rod heats up, the divergence changes and the diameter of the beam at the focusing lens increases and decreases.The exaggerated drawings below tell the story best.
Large Beam = Larger Angle and Smaller Spot
Normal Beam = Proper Angle and Spot
Small Beam = Small Angle and Large Spot
The Pulsar HPC is designed to correct both of these extremes in aberrant beams. Within the Pulsar HPC, the fiber is hermetically fused into a quartz sleeve much like SureFlex and AccuFlex, which makes the input face impervious to spatial overfill by larger than expected focus spots and misalignment. The primary difference in the Pulsar HPC is that an annular lens is machined onto the fused surface: a single surface lens with opposing curvatures. A concave lens in the center of the fused face shifts a normal focus, or the center of blooming focus, to lower than normal angles within the fiber, essentially collimating the light in the fiber.
Pulsar Annular Lens Input
The ridge of quartz surrounding the negative lens has a convex curvature to shift light away from the fiber core: light at too large a diameter to enter the fiber or at too high of an angle to correct with the concave lens. Again, the drawings must be exaggerated because the actual dimensions are exceedingly small.
By lowering the maximum angle of light within the fiber before a tight bend is encountered, the light is not raised in angle to as high a level as in standard fibers, making ProFlex less susceptible to burning through. The annular lens of the Pulsar HPC also ensures that light in the laser focus cannot enter the fiber at dangerous angles.
Smaller SureFlex fibers have been known to burn through at the back of the connector on occassion and the same proved true for ProFlex in the earliest prototypes. InnovaQuartz determined that cause was that there was sometimes too much light missing the fiber core for SureFlex' tiny, internal beam absorber to handle. With potentially more energy being denied entry to the fiber in the ProFlex design, IQ had to find a solution to this problem.
Our answer is the installation of a highly reflective groove that is cut into the quartz ferrule wall within the center portion of the connector. Normally, energy that misses the fiber is carried by the surrounding quartz sleeve to the back of the connector. The helical groove in the Pulsar termination catches bits of this energy and reflects it into the wall of the connector so that the energy is dissipated along the length of the connector, rather than all in one place.
Uniform Dissipation of Errant Laser Energy in the Pulsar HPC Termination of ProFlex
The bottom line is that ProFlex has to be bent more than other fibers in order to start leaking, but what happens when leaking does begin? In all double clad fibers used for holmium lasers, when light begins to escape the fiber core, it is partially trapped by the secondary cladding made of organic polymer, but only partially; the plastic cladding is not as transparent to holmium energy as the primary glass cladding. This is much better than having no secondary cladding, but not all secondary cladding is equal. Most silica fiber is made to perform over a broad range of wavelengths so optimizing the secondary cladding for just one wavelength is just not considered. Some holmium fiber producers simply do not understand fiber optic design, or perhaps they focus too much on cost savings and choose inappropriate materials like silicone for the polymer cladding. Better fibers are made using fluoropolymers, also known as hard cladding (as opposed to soft silicone), but the optical and mechanical properties of these hard polymer coatings vary considerably and provide varying levels of secondary containment at holmium wavelengths.
Silica Core, Silica Cladding, Hard Cladding, Tefzel
At InnovaQuartz, we used out draw tower to study dozens of potential candidates and found a polymer that was both more transparent to holmium wavelengths and contained higher angles than any other and that’s what we use, exclusively. This polymer cladding is a bit tricky to apply because it is about as thin as water before curing, but the low viscosity also serves to penetrate much smaller glass defects than the syrupy coatings used elsewhere, so the ProFlex fiber is also about twice as strong as competing holmium fibers.
Most small holmium fibers also damage the forceps channel of ureteroscopes, even when passed though without deflecting the scope. They scratch the plastic tube walls and collect Teflon curlicues that may partially block irrigation flow or contaminate the fiber tip. Worse yet, some tips snap off in the forceps channel and damage the rest of the fiber as it passes through. These ‘cleaved’ or 'ground and polished' tips are imperfect in other ways as well, contributing to scattering in the fiber output that reduces efficiency in coupling to the target tissue.
InnovaQuartz produced the first laser flat-polished fibers to address these problems a decade ago, in SureFlex and AccuFlex. The tips of ProFlex 200 and ProFlex 273 are branded as “Smooth Passage” because they pass forceps channels without causing or suffering any damage, even if the scope is fully deflected. Larger ProFlex fibers also pass forceps channels without damage, but cannot pass through a fully deflected scope, nor can a scope fully deflect while containing the larger fibers, preloaded. Flexible scopes can fully deflect with the ProFlex 200 preloaded and can deflect to about 95% of unloaded deflection with a ProFlex 273 preloaded.
In summary, InnovaQuartz has again set the standard for performance in holmium laser fibers. Rest assured, we’ve never been comfortable in laurel beds…