Assisting femto incisions with nomograms

Treat corneal astigmatism during cataract surgery.

By Eric Donnenfeld, MD & Eric Rosenberg, DO

Recent advancements have allowed cataract surgeons to achieve great visual results quickly, safely and consistently. As a result, patients have increasingly high expectations for both functional visual improvement as well as spectacle independence.^1,2

To provide good surgical outcomes, quality of vision and satisfaction for patients requesting refractive cataract surgery and optimal postoperative uncorrected visual acuity, you must manage patients’ astigmatism. In astigmatism, corneal steepening at a specific meridian causes incoming light rays to be focused at more than one location anterior, posterior, or directly on the retina. This causes visual distortion and decreased visual acuity.

ASTIGMATISM MANAGEMENT

Its impact on cataract patients

When performing cataract surgery, it is important to account for pre-existing or surgically induced astigmatism — residual astigmatism of 0.50D or less may result in glare, symptomatic blur, ghosting and halos.³ Also, a recent study of 4,540 eyes of 2,415 patients showed that corneal astigmatism is present in the majority of patients undergoing cataract surgery, with at least 1.50D measured in 22.2% of study eyes.⁴ Approximately 38% of eyes undergoing cataract surgery have at least 1.00D of pre-existing corneal astigmatism, and 72% of patients have 0.50D or more.⁵

While astigmatism management is important regardless of the IOL that is used, patients implanted with presbyopia-correcting IOLs are significantly more sensitive to even minor refractive errors than those implanted with conventional monofocal IOLs. Unfortunately, the majority of cataract surgeons are uncomfortable performing manual incisional astigmatic incisions. In a survey of 233 surgeons, only 73 routinely performed limbal relaxing incisions (LRIs).⁶ In a separate study performed by the ASCRS Young Physicians Committee, only 48% of residents while in training performed an LRI while in training.⁷

Role of nomograms

Since the conception of the LRI to efficiently minimize astigmatism during surgery, there has been concurrent research into the appropriate incision placement, length, and depth.^8-15 The parameters for such incisions were often guided by various nomograms, which corrected for small amounts of cylinder, and eventually validated by individual surgeon outcomes. One study on the effectiveness of LRIs showed a 60% reduction of cylinder.¹⁵ This compares favorably with the results achieved using toric IOLs given that 79% of patients corrected to less than 1.00 D of cylinder and 59% corrected to less than 0.5 D of cylinder. Overall, this amounts to a mean 58.4% reduction in cylinder.¹⁶

LRI calculation

There are multiple significant predictors with respect to the amount of astigmatic correction, and therefore surgical nomograms can be referenced for the number of incisions, incision length, age, gender or cylinder axis. In addition, the phacoemulsification incision must be factored in preoperatively to determine the final postoperative residual cylinder.

Using the Holladay, Cravy, Koch vector analysis method, the surgeon must determine the proper axis for the astigmatic incision. With the aid of online LRI calculators (Femtosecond Laser nomogram, above), the surgeon can readily employ vector analysis to calculate LRI-incision parameters based on preoperative patient keratometry, preoperative factors and the surgeon’s location and extent of the planned phaco incision, which affects postoperative astigmatism. If you select the Donnenfeld and Nichamin nomogram, a visual map of the axis and lengths of incisions is provided. Then, surgeons can bring a printout of the LRI calculator to the operating room as a guide when marking the cornea and performing LRIs.

Given the many variables that go into planning and performing the LRI incisions, values could be measured incorrectly or subjected to internal standard errors, such as cyclorotation and corneal marking. The incision itself, if performed manually, is only as precise as the surgeon making the incision and the accuracy of the blade. Any and all of these errors can compound and result in suboptimal visual acuity. Therefore creating manual LRIs is as much an art as it a science.

FEMTOSECOND LASER-GUIDE PROCEDURES

A shift from manual

Not surprisingly, LRIs have been subject to the natural progression of any surgical procedure with the aim of reducing risk and improving outcomes. Potential complications of manual incisions include decreased corneal sensation, discomfort, infection, overcorrection, under-correction, perforation of the cornea and induced irregular astigmatism. The LRI procedure is not generally associated with glare or starbursts.

Recent developments show femtosecond laser technology’s extreme promise and have shifted the movement from manual LRI and astigmatic keratotomy procedures to femtosecond laser-guided refractive procedures. The treatment of astigmatism during cataract surgery with femtosecond laser astigmatic incisions offers a greater degree of precision and accuracy than manual methods. It improves the risk profile for the possible complications mentioned previously.¹⁷

Energy usage

Femtosecond lasers are photodisruptive lasers, in contrast to photoablation and photocoagulation lasers, with extraordinarily short pulse durations of less than 800 femtoseconds. Because of this extremely short pulse, the femtosecond laser cuts tissue with considerably less energy than traditional lasers. Per-pulse energies can be reduced approximately 1,000‐fold, from around 1^‐10 millijoules for nanosecond lasers aluminum garnet [Nd:YAG]) to 1^‐10 microjoules for femtosecond lasers. Both the short 1053-nm wavelength, which is not absorbed by optically clear tissues at low power densities, and the reductions in per-pulse energy result in substantially reduced collateral tissue damage, shifting concomitant damage from a few spherical millimeters with the nanosecond lasers to just a few microns with the femtosecond lasers.¹⁸

The accuracy and limited collateral effects of the femtosecond laser’s pulses have been established during the myriad LASIK flap creations and lamellar and penetrating keratoplasties. The laser for cataract surgery is approved for four clinical indications: primary incisions, arcuate incisions, lens fragmentation and the capsulotomy. The use of the femtosecond laser to create arcuate incisions is a major clinical improvement over manual incisions. The femtosecond arcuate incisions are more precise due to more accurate arc length, depth, angular position and optical zone. The femtosecond laser allows for precise and repeatable incisions, which are necessary for consistent results not normally achieved through manual methods.¹⁷

Laser technology can create arcuate incisions with digital precision and predictably. The corneal tissue does not absorb the laser wavelength. This allows for a higher margin of safety because a sizable distance is kept from Descemet’s membrane, preventing perforation.

In addition, arcuate incisions with the femtosecond laser are reproducible with continuous curvature that cannot be achieved with manual keratomes. Femtosecond laser incisions provide superior reproducibility and reduced variability compared with conventional manual incisions.¹⁹

Put into practice

In an early study, using 8-mm arcuate incisions and a 33% reduction of the Donnenfeld nomogram a 70% reduction in astigmatism was achieved.²⁰ Additionally, femtosecond laser incisions may be placed intrastromally in the sub-Bowman’s layer of the cornea, which improves healing by sparing corneal epithelial damage. This is a major area of interest, and nomogram development is currently under way to optimize this method and eliminate the need for corneal wound manipulation on the surface.

Performing femtosecond laser arcuate incisions requires the parameters of length, position, depth and distance from the visual axis where the incisions will be created. We use a 33% reduction of the Donnenfeld nomogram and use the LRI calculator (www.lricalculator.com) to determine the length and axis at which the incision should be placed (Figure 1). The depth of our incisions is 85% of the corneal pachymetry in the area of the incision. We have set our distance from the visual axis at 8 mm. This information is all downloaded onto the femtosecond laser. Then, we begin the surgical procedure by docking the laser onto the cornea. An overlay of the incisions is then visible on the surgical screen (Figure 2). OCT imaging of the cornea in the area of the arcuate incision is then visualized, and the depth is confirmed (Figure 3). We first perform the capsulotomy, followed by the lens disruption, and then create our corneal incisions. After treatment, we bring the patient to the operating microscope and open the incisions with a Sinskey hook. Then, we use OCT to confirm the postoperative depth of the incisions (Figure 4). Some surgeons prefer to open one or both of the incisions postoperatively under the guidance of the postsurgical refraction. By using low energy, the incisions do not have significant effect until they are opened.

Figure 1.

Figure 2.

Figure 3.

Figure 4.

Conclusion

Femtosecond laser-assisted arcuate incisions have brought digital accuracy and precision to astigmatism control and cataract surgery. These refractive incisions are computer-controlled and do not rely on the surgeon’s skill or experience. The femtosecond laser system provides faster, safer, easier, customizable, adjustable and fully repeatable astigmatic incisions. Intrastromal ablations cannot be performed with manual incisions and offer advantages in patient comfort and safety. Femtosecond laser arcuate astigmatic incisions will enable the majority of ophthalmologists to enter the field of refractive cataract surgery. OM

REFERENCES

About the Authors
	Eric Donnenfeld, MD, is clinical professor of ophthalmology at New York University and a partner at Ophthalmic Consultants of Long Island.
	Eric Rosenberg, DO, is currently finishing his second year of surgical residency at St. Joseph’s Regional Medical Center in Paterson, N.J.
	Disclosures: Dr. Donnenfeld is a consultant to Abbott Medical Optics, Bausch + Lomb, LenSx and WaveTec.