A laser probe is a laser probe, right? Well, like most things, not exactly.
Katalyst Surgical employs as CEO one of the most prolific inventors in the history of Retina Surgery. Founder and former employee of Advanced Surgical Products, Inc. and Synergetics, Inc., Gregg Scheller is an engineer with more than 85 patents and patent applications pending. During the span of his 30-year Ophthalmic career, he has frequently been involved in fiberoptics and he has invented or participated in the invention of many of the commonly used devices in Retina Surgery including the Disposable Endoilluminator, the first multifunction laser probe, the directional laser probe, the first lighted surgical instruments, the first asymmetrical forceps, the first diamond membrane scrapers, the first high-brightness illuminator and the first chandelier illuminators as well many others. He has worked with many of the famous names in Ophthalmic Surgery including Dr’s Awh, Chang, Chow, DeJuan, Ducournau, Dunn, Hida, Holekamp, Lewis, Morris, Opremcak, Peyman, Rice, Tano, Thomas, and Witherspoon.
Gregg developed the first multifunction aspirating laser probe with Dr. Stanley Chang. He developed the first directional laser probe with Dr. Eugene DeJuan. He developed the first Illuminated Laser Probe as well. The latest development is the “Steerable™” Laser Probe.
The anatomy of a simple straight laser probe is simple. About 5 parts. A connector, a fiber optic fiber, a sheath covering the fiber, a handle, and a stainless steel tubing tip. How much difference can there be?
Let’s start with the connector. This is an off-the-shelf item for many manufacturers, with suppliers supplying the components to a known specification. These are VERY precisely dimensioned devices, however. When you consider that most ophthalmic lasers produce a focal spot of around 50 microns in diameter. Most Laser Probes feature an optic of around 200 microns in diameter. The connector’s job is therefore to center the optic on the focal spot precisely enough that it can capture all of the light and transmit it down the fiber. Easier said than done.
The 200 micron core and the 50 micron spot only allow a mismatch of 75 microns per side. 75 Microns translates to about .006 inch or about the width of two human hairs. The manufacturing tolerances include those of the laser centered on the connector center, the inside diameter of the female connector on the unit, the outside diameter of the male connector on the probe and the centering of the fiber in the male portion of the connector. Any variance beyond this tolerance range will result in less power at the tip.
Fibers can be constructed of a variety of materials. Most Laser-transmitting fibers use silica glass as the core transmitting material. This is then surrounded by a plastic or glass “cladding” that has an Index of Refraction that “holds” the light inside of the core material. Then, it is usually surrounded by a protective layer, sometimes called the “buffer”. The fiber buffer material varies widely from the Polyimide (Kevlar) material used ONLY in Katalyst products currently, to commonly, Tefzel (derivative of Teflon), to even Silicone elastomer. Katalyst has done extensive testing on these materials and, especially where the fiber is used in a dynamic (flexing) product, the additional strength of the polyimide buffer offers reliability advantages by protecting the fiber better. Yes, this material is more expensive, but the added reliability on the OR is the goal.
There are also different fiber core diameter sizes. Depending on the manufacturer and, among other things, the amount of money that they are willing/ able to spend on the connector as described above, the diameter of the fiber in some products goes up to 300 microns. This allows a less precise laser, or lower tolerance (cheaper) parts in the connector to be used to save cost. This usually results in a larger spot size at the Retinal surface. Most precision products utilize 200 micron core fiber. A 50% increase in diameter up to 300 microns translates to a treated area that is 2.25X the size of the 200 micron core. This is a tradeoff that most surgeons are not willing to live with.
The choice of cladding material determines, to a large extent, the Numerical Aperture (NA) of the fiber. This NA describes the rate of beam divergence as it exits the tip of the fiber. As you can envision, this also as a determining effect on the spot size at the Retina as well as determining how far from the Retina the tip of the probe is held. Katalyst products feature a low-beam divergence NA to allow the surgeon a smaller, more precise, spot and to also allow the tip to be further away from the Retina.
Spot integrity at the Retina can also be a product of the way the fiber is optically finished on BOTH ends of the fiber. Some manufacturers finish fibers through a process called “cleaving”. This process involves stripping the protective fiber buffer off of the fiber assembly, stretching the fiber tight, and then nicking the fiber, usually with a diamond or sapphire blade. The fiber then “cleaves” at this point with the objective being a flat, optically-clean surface. In practicality, however, the fiber almost always cleaves in a manner which creates optical defects. These defects come in many forms. Most common is a surface that is not optically flat. This often looks like a potato chip when magnified. The effect of this is that the output pattern usually has “hot spots” in it, so it is not homogenous. Another common defect is a chip at the edge. Under high magnification, one can often see where the blade contacted the fiber at the edge. In practical terms, this results in a “stray ray”, which is laser power outside of the spot. This is perhaps clinically of a larger concern than the “hot spots” previously described, because now there is the potential for treatment beyond the intended region and yes, potentially into the macula. This concern should be compounded by the fact that the aiming beam is so weak that the surgeon will likely never see it there, but the treatment beam is blocked by filters in the microscope. These “stray rays” are readily visible in a dark room when the laser is fired at moderate power levels. A poor cleaved surface on the back (connector) end of the fiber usually results in a geometric exit pattern upon beam examination. Most frequent is a “doughnut” shape that has a hot “ring” on the periphery and a cold center. Also, probably not desirable.
For this reason all Katalyst Surgical laser probes are polished, using a decreasing grit size and flat plattens. Further EACH Katalyst probe is tested for power output, spot size and shape, as well as the output pattern to assure no stray rays or hot spots. Yes, this is a more expensive and time consuming process, but you often get what you pay for.
Not discussed yet is the handle and tip. Other than the Steerable™ Probe, nothing externally distinguishes the front tip of the probe other than the tip stiffness of the 25 and 27ga probes. Our probes feature a front tip material that is 50% stiffer than any competitor’s product. This is a result of a development effort that has resulted in Katalyst probes employing a custom-made tip material that no other competitor uses. Internally, the fiber that we choose to use features a Kevlar-like cladding material that remains intact to the tip. If you recall, this is what prevents the fiber from breakage. By choosing this type of fiber, Katalyst believes that it has fewer out-of-the-bag failures than its competitors, who use a cheaper fiber that must be stripped of its Tefzel buffer before polishing or cleaving.
Following your reading of this “Anatomy of a Laser Probe” discussion, I hope that it is clear how many variables go into making a better product. Katalyst’s engineering expertise has produced more varieties of probes than any other in the history of ophthalmology. There are others that know far more about fibers and their production, but no one knows more than Katalyst about applying these amazing fibers to deliver a superior product for ophthalmology.
Katalyst Surgical is always looking to improve the state of the art in surgical products for Ophthalmology. Please contact Gregg Scheller directly with your ideas at email@example.com