How FS laser is revolutionizing our astigmatic incisions

Femtosecond guidance is helping us achieve more precise cataract outcomes.

By Eric Donnenfeld, MD, Erfan Nadji, MD, Jamie Sklar

About the Authors
	Eric Donnenfeld, MD, is a clinical professor of ophthalmology at New York University Medical Center, New York. Disclosure: Dr. Donnenfeld disclosed relationships with Alcon, AMO, Bausch + Lomb and LenSx.
	Erfan Nadji, MD, is a fellow with Ophthalmic Consultants of Long island, Rockville Centre, NY.
	Jamie Sklar is a research assistant with Ophthalmic Consultants of Long island.

Modern cataract surgery is one of the most successful and commonly performed surgical procedures worldwide. A series of advancements have allowed surgeons to perform this procedure quickly, safely and with wonderful visual results on a consistent basis. So patients have increasingly high expectations for both functional visual improvement, as well as spectacle independence for distance and often for near as well.^{1, 2}

The single most important aspect of the ability to provide quality of vision, satisfaction and optimal postoperative uncorrected visual acuity for patients requesting refractive cataract surgery is management of their astigmatism. The femtosecond (FS) laser, already a buzz-generating technology in both cataract and refractive surgery, may provide the best means yet of treating astigmatism during cataract surgery.

PREEXISTING ASTIGMATISM

Find it before surgery

In astigmatism, corneal steepening at a specific meridian causes incoming light rays to focus at more than one location anteriorly, posteriorly or directly on the retina. This causes visual distortion and decreased visual acuity. When performing cataract surgery, it is critical that you account for astigmatism that may be preexisting or surgically induced. Residual astigmatism of 0.50 D or even less may result in glare, symptomatic blur, ghosting and haloes.³

As a result, we’ve placed greater emphasis on treating corneal astigmatism at the time of cataract surgery. A recent study of 4,540 eyes of 2,415 patients showed a majority of patients undergoing cataract surgery had corneal astigmatism, with at least 1.50 D measured in 22.2% of study eyes.⁴ Approximately 38% of eyes undergoing cataract surgery have at least 1.00 D of preexisting corneal astigmatism, and 72% of patients have 0.50 D or more.⁵

Calculating the LRI

To prepare for a limbal relaxing incision, you should first determine the axis of corneal steepening where you will make the incision. Then calculate the incision length and depth based on the magnitude of astigmatism to be corrected. If needed, numerous LRI nomograms are available. One study on the effectiveness of LRIs showed a 60% average reduction of cylinder,⁶ with 79% of patients corrected to less than 1.00 D of cylinder and 59% corrected to less than 0.50 D of cylinder. The 60% reduction in cylinder compares favorably with the results achieved using toric IOLs, which result in a mean 58.4% reduction in cylinder.⁷

Many LRI nomograms are adjusted for age and cylinder axis, making them detailed and complex and giving the impression the procedure is precise and unforgiving. In our opinion, however, this simply is not the case. The LRI procedure has numerous chances for error, and the many variables that help determine how the incision is performed can each be measured incorrectly. The incision itself, if performed manually, is only as precise as the surgeon making the incision, and the accuracy of the blade. Any of these errors can compound and result in suboptimal visual acuity.

Potential complications may be the reason why the majority of cataract surgeons choose not to perform manual astigmatic incisions. In a survey of 233 surgeons, only 73 routinely performed LRIs.⁸ Alternative procedures for managing astigmatism exist, including excimer laser photoablation, toric IOL implantation and conductive keratoplasty. These options, however, may not be available to all cataract surgeons.

THE FEMTOSECOND OPTION

Precise and repeatable incisions

Another alternative to manual astigmatic incisions is femtosecond laser-guided astigmatic correction. Femtosecond lasers for ophthalmic surgery have been commercially available since 1999, and more than 4 million procedures have been performed for the creation of LASIK flaps, lamellar and penetrating keratoplasties, and DSEK.^9-14

Ophthalmologists have shifted their attention toward femtosecond laser-guided procedures because they offer a greater degree of precision, accuracy and reproducibility than manual methods.¹⁵ The treatment of astigmatism during cataract surgery with femtosecond laser astigmatic incisions is poised to revolutionize refractive cataract surgery. Similar to the Nd:YAG laser systems, femtosecond laser pulses pass through transparent tissues and can be focused at a pre-determined depth.¹⁶

A major clinical application of the femtosecond laser is creating corneal arcuate incisions. FS allows for precise and repeatable incisions, which are necessary for consistent results not normally achieved through manual methods.¹⁷ In a case study, femtosecond laser arcuate keratotomy was used to correct astigmatism.¹⁸ The authors measured refraction of the patients’ eyes along with corneal thickness. They then evaluated the keratometry parameters and the femtosecond laser created the incisions, the length and position of which were calculated individually, per eye. OCT-controlled corneal pachymetry was performed directly in the area of the intended incisions and then programmed into the laser, which allowed for extremely high levels of precision.

This study showed that with the FS laser, the surgeon could achieve astigmatism correction with a best-corrected visual acuity better than the preoperative values predicted. Furthermore, the depth and location of the incisions were consistent with the surgical plan. Laser technology can create uniform corneal incisions precisely and predictably.

Safer surgery, reduced variability

Femtosecond lasers allow for more efficacious and safer surgical procedures. The corneal tissue does not absorb the laser wavelength. The photodisruption dissipates within 100 μm of the target, millimeters from the cornea, which allows for a higher margin of safety because a sizeable distance is kept from Descemet’s membrane, preventing perforation. Only one known case report described the inadvertent perforation of the cornea with a femtosecond laser.¹⁹

Moreover, the surgeon can program the laser to create an ideal wound shape for enhanced sealing and healing of the incisions with reproducible induction of astigmatism that manual keratomes cannot achieve. FS laser incisions provide superior reproducibility and reduced variability compared with conventional manual incisions. In a study of initial results with the LenSx (Alcon Surgical, Forth Worth, Texas) femtosecond laser, using 9-mm arcuate incisions and a 33% reduction of the Donnenfeld nomogram, researchers achieved a 70% reduction in astigmatism.²⁰

OUR METHOD

Determine incision parameters

When performing femtosecond laser arcuate incisions, you must first determine the incision length, depth, position and distance from the visual axis. We use a 33% reduction of the Donnenfeld nomogram in conjunction with the LRI calculator (www.lricalculator.com) to determine the length and axis at which we place the incision (Figure 1). We preset the depth of our incisions to 85% of the corneal pachymetry in the area of the incision. We set our distance from the visual axis at 9 mm, and download this information onto the femtosecond laser.

The surgical procedure begins by docking the laser onto the cornea. An overlay of the incisions is then visible on the surgical screen. A built-in safety measure prevents the intersection of the clear corneal and side-port incisions with the astigmatic incision. Then we perform OCT imaging of the cornea in the area of the arcuate incision and confirm the depth (Figure 2).

Figure 1. The Donnenfeld and Nichamin Nomogram is available on the Internet at www.LRIcalculator.com.

Figure 2. OCT imaging of the cornea in the area of the arcuate incision provides precise pachymetry.

We perform the capsulotomy first, then lens disruption, and finally the corneal incisions. Following the conclusion of the femtosecond laser treatment, the patient is brought to the operating microscope and we open the incisions with a Sinskey hook. We may then perform intraoperative aberrometry (ORA, WaveTec Vision, Aliso Viejo, Calif.) to titrate the incision opening. The incisions are symmetric and standardized at 9 mm from the visual axis (Figure 3). If needed, you can open the arcuate incisions postoperatively in the office at the slit lamp using forceps or a Sinskey hook and topical anesthetic.

Figure 3. Overlay of the arcuate incisions.

Refining FS incisions

Femtosecond incisions are no longer an art form but a science with reproducible arc length, depth and angulation. The angulation may play a role in reducing wound gape. That problem can occur with all corneal incisions, but it is much more common following incisions to reduce astigmatism following a penetrating astigmatism. However, while the results have improved over manual LRIs, femtosecond incisions continue to result in an unpredictable response. This is due to the inherent variable response to corneal incisions, which is based on patient age, pachymetry, corneal diameter and curvature, IOP and biomechanics.

A major advantage of femtosecond arcuate incisions is that the refractive incisions may now be made prior to cataract surgery and then modified intraoperatively or postoperatively, or both. In addition, they do not have a full refractive effect until they are opened. We have begun to modify the energy, spot size and separation of the spots to further refine the titration of our results. With higher energy and small spot size separation, we achieve the majority of the effect without opening the incisions.

With lower energy settings and greater spot size separation, the incisions have minimal effect until they are opened. This is the case with femtosecond LASIK flaps that, when left in place, cause no refractive change and essentially disappear until they are opened. A good analogy is that the FS laser incisions are similar to a sheet of postage stamps bound together by serrations. Until the serrations are manually torn apart, the stamps remain in a fixed location.

Role of intraoperative aberrometry

To further refine our results, we have been performing intraoperative aberrometry to titrate our results in the operating room. Intraoperative aberrometry is a type of wavefront analysis in which aberrations in the wavefront are converted into the current refractive value. We remove the cataract, place the IOL and open one of the femtosecond incisions with a Sinskey hook. The IOP is then raised to approximately 25 mm Hg.

Next, we perform intraoperative aberrometry, which provides a highly accurate reading of the existing astigmatism. We may then open the second femtosecond incision partially or completely, based on the intraoperative aberrometry reading, which, if needed, we can take again. If you do not have access to intraoperative aberrometry, you can examine patients with topography and refraction, performed the day of surgery, or even weeks or months afterward. If needed, you can easily open the remainder of the incision, completely or partially, in the desired axis in the office to increase the effect of the incision and adjust the residual astigmatic refractive error.

UNIQUE ADVANTAGES OF FS

Titrating LRI

In addition to the ability to titrate the LRI with a femtosecond incision, this technology offers a second advantage that you cannot achieve with a manual diamond knife incision: The ability to perform intrastromal sub-Bowman’s layer ablations. These incisions are less powerful than anterior penetrating incisions and are performed closer to the visual axis. Intrastromal incisions do not cut through Bowman’s layer, requiring fewer stromal lamellae incisions, which should provide greater corneal stromal integrity,²² preserve epithelial integrity,^23,24 reduce pain and provide a faster recovery.

No need for antibiotics

There is no need for postoperative antibiotics as the surface of the eye has not been violated and late wound drift is less likelihood, although long-term results are not available. Further clinical investigation and nomogram development are under way to optimize this method, which would eliminate the need for corneal wound manipulation on the surface.

A novel technique

The creation of femtosecond laser-assisted arcuate incisions is a novel technique that utilizes the precision of image-guided laser technology. Refractive incisions are now computer-controlled and do not rely on surgeon skill or experience. The use of a femtosecond laser system will provide faster, safer, easier, customizable, adjustable and fully repeatable astigmatic incisions. In addition, intrastromal ablations may increase the safety and accuracy of astigmatism management.

Removing the inconsistencies in the astigmatic procedure will improve our understanding and accuracy of astigmatic incisions and should provide improved refractive results and patient satisfaction. This technology offers immense potential but will require development with different nomograms and surgeons as well as expanded clinical experience. OM

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