March 2016

 

CORNEA

 

Corneal incisions with a femtosecond laser


by Ellen Stodola EyeWorld Staff Writer

 
   
Using OCT

Using OCT can help ensure a high level of accuracy with corneal incisions and can also provide key information for the intended architecture in relation to where each incision is placed on the cornea. Source: Michael Collins, MD

Surgeon weighs in on the value of using a femto laser for incisions

With the femtosecond laser being used in a variety of cataract and other procedures, Michael Collins, MD, Collins Vision, Fort Myers, Fla., spoke about why it might be helpful to use the laser for corneal incisions. He noted, however, that this is one of the lesser used indications of the laser.

Femto incisions

One of the reasons that femto incisions are not used as often, Dr. Collins said, is that some surgeons have issues opening the femto incisions. Because each platform is unique in how it interfaces with the cornea, the level of precision in creating incisions varies among platforms, he said. Surgeons with certain platforms also feel that the femto-made incisions do not seal as well as incisions made with a blade. However, in Dr. Collins experience with the Victus laser (Bausch + Lomb, Bridgewater, N.J.), he finds that the incisions open easily as the platform cuts well, even through arcus. We also know our incisions seal just as well if not better than incisions made with a blade, he said. The Victus laser is designed and optimized for performance for making the capsulotomy and doing lens fragmentation, as well as for making incisions within the corneal plane. For the corneal incisions, the cornea is fully applanated to the laser interface. Femtosecond lasers designed for capsulotomy and lens fragmentation in cataract surgery rely on a fluid interface (such as the Victus) or biogel interface in order to avoid applanation of the cornea and introduction of corneal folds, Dr. Collins said. Although this method is ideal for lenticular work, it does not provide sufficient control of the corneal tissue that is required for the precision of LASIK flaps and other corneal incisions. If an attempt was made to create a LASIK flap on a femtosecond laser utilizing a fluid interface, there would likely be significant epithelial breakthrough with the laser energy. He added that there has to be a certain measure of force against the cornea when placing the laser pulse to ensure each spot is placed in the desired location.

Femtosecond lasers that applanate for corneal work typically utilize the known location of the patient interface as a reference point for which the laser pulses are placed. This is why femtosecond lasers can create thin LASIK flaps with high precision, he said. The laser is programmed to place each pulse for the LASIK bed cut 90100 microns from the known location of the interface. This method is much more precise than relying on the OCT to determine the exact location of the cornea through a fluid-filled interface. Dr. Collins added that the Victus laser relies on a swept-source OCT to guide the lenticular aspects of the cataract procedure but also utilizes the reference of the patient interface to drive the precision of the corneal work.

In the last few years, Dr. Collins said, there has been a lot of discussion about the configuration of the laser and whether or not it was optimized for work at the corneal versus the lenticular plane. The real optimization of the laser is how it interfaces with the eye for each procedure type, he said.

How OCT helps

The swept-source OCT on the Victus runs live throughout docking, treatment planning, and the actual treatment, Dr. Collins said. Having optimized centration while docking is key to avoiding any visual parallax that can interfere with accurate incision placement and architecture, he said. Since the OCT runs live during the docking process, he can utilize the video microscope as well as the OCT to ensure optimized centration. When planning for treatment, live cross-sectional OCT images are shown for each incision, which helps demonstrate the intended architecture in relation to where each incision is placed on the cornea. The OCT scans the location of the patient interface, and each incision is made from the known location of that interface; this yields an extremely high level of accuracy for the corneal incisions, Dr. Collins said. Because I do a Wong incision with my laser, the OCT is critical in order to ensure ideal placement of this extra incision. The Wong incision is cut after the primary incision, as it is situated anterior to the primary incision. Unfortunately, when creating corneal incisions with a femtosecond laser, the gas byproduct will escape into the interface between the patient interface and cornea, Dr. Collins said. This is not normally an issue with routine femtosecond incisions, but when placing one incision on top of another, it can create a problem. However, the live OCT allows visualization of any gas that may interfere and then adjustments can be made to get the optimal incision.

Dr. Collins uses a Sinskey hook with firm pressure to pop open and unzip the anterior plane of the incisions. Another benefit to the live OCT is being able to see how well the wound opened by the laser pulses during the actual creation of the incisions.

Other options

One of the beauties of femto, Dr. Collins said, is there are almost limitless options with regard to the angles and depths of the different planes of the incision. With the newer software, we can place our incisions anywhere we want and have the ability to customize each incision location for each individual cornea, he said. Dr. Collins said that typically 1 primary triplanar, 1 secondary biplanar, and 1 or 2 AK monoplanar incisions are placed, as well as a small monoplanar Wong incision/pocket. Each incision can have shared or independent architecture (from single to triplane) and can be placed or dragged to various locations out to a 12.5 mm diameter. Dr. Collins has recently been working with Wong incisions and said the idea for this came from the observation that femto incisions sealed better with hydration of the incision roof. The next logical step was to place a Wong incision anterior to the primary incision, he said. With minimal hydration, we get an amazing seal. The arcuate incision options are flexible, as the laser is capable of creating single or paired arc incisions. Paired arcs are not required to be of the same arc length, diameter, and do not have to be locked to the same meridian, he said. The arcuate incision angle is also adjustable, and the laser is capable of creating intrastromal arcuate incisions as well. These intrastromal arcuate cuts are sometimes used to facilitate toric IOL alignment or opened and extended postoperatively to treat residual astigmatism in the case of undercorrection, Dr. Collins said.

Surgically induced astigmatism

Femto incisions are more predictable and precise, allowing for more control and precision with astigmatism correction, Dr. Collins said. There is no way to make manual incisions the exact same depth, length, shape, angle, and optical zone, which is possible with a femtosecond laser CRI. Of course there are other variables that come into play with astigmatic correction, but if we can maximally control these variables, we will get more predictability, he said. A big struggle for some femtosecond surgeons, he added, is the ability to place the primary incision as far out on the limbus as they typically would with a keratome incision. The more anterior the incision, the more it can impact the surgically induced astigmatism. Because the cornea is applanated during the corneal incision mode, the laser is able to achieve greater precision and performance when placing cuts close to the limbus, he said.

Editors note: Dr. Collins has financial interests with Bausch + Lomb.

Contact information

Collins
: mjc@collinsvision.com

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