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Femtosecond astigmatic keratotomy before LASIK can be beneficial
Some advantages to this approach include customized arc length pairings, incisional angulation.
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Corneal astigmatism compromises visual quality and needs to be addressed, especially when the patient elects refractive surgery to correct refractive error and wishes to gain freedom from glasses and contact lenses.
Although most corneal astigmatism correction is usually within the realm of LASIK, high levels of corneal astigmatism may fall outside the levels of CustomVue WaveScan-guided ablation. In such cases, surgically reducing the corneal astigmatism can then allow for subsequent use of custom IntraLase LASIK following refractive stability of the treated cornea. Pre-LASIK astigmatic keratotomy in these cases is more precise and reproducible with the application of laser technology as compared with manual corneal incisions. Yet another area beyond refractive surgery in which femtosecond astigmatic keratoplasty is applicable is in selected cases of post-keratoplasty corneal astigmatism management. In cases with a history of contact lens failure, anisometropia of greater than 3 D and/or significant corneal astigmatism, femtosecond astigmatic keratotomy can be beneficial in optimizing vision and improving quality of life.
In this column, Dr. Fox describes his technique of combining femtosecond astigmatic keratotomy with a subsequent LASIK procedure.
Thomas “TJ” John, MD
OSN Surgical Maneuvers Editor
Martin L. Fox
Early in my years as a refractive surgeon performing incisional corneal refractive surgery, I learned that I could achieve the maximal effect of corneal incisions when they were made at 90° to the corneal plane. With the advent of femtosecond laser technology, surgeons now have the ability to create precise incisions anywhere from 30° to 150° relative to the corneal plane. This has allowed for a new means of titrating the effect because angulated femtosecond laser incisions are greater in length, provide an increased corneal healing interface and thus have less effect.
When I first started performing femtosecond astigmatic keratotomy (AK) in virgin eyes with astigmatism, I used maximal settings consisting of an 8-mm optical zone, paired 80° arc length incisions on the steep corneal meridian, a consistent depth of 75% of the thinnest pachymetry readings in the optical zone of treatment and an incision angle of 90°. This approach corrected astigmatism ranging from 2.3 D to 3.5 D (average of 2.8 D), but there were instances of poor predictability and occasional overcorrection.
The iFS advanced femtosecond laser (Abbott Medical Optics) has taken my initial AK procedure to new levels of predictability and safety due to the incisional options it affords. There are four parameters we consider for each patient: optical zone, depth of the incision, arc length of the incisions (either single or paired) and angle of incision (30° to 120°). I have developed a femtosecond AK nomogram using these parameters to allow for greater precision and predictability (Table).
Source: Fox ML
I have also adapted femtosecond AK as a treatment for patients with past manual keratoplasty that resulted in unacceptable levels of postoperative astigmatism. In such iFS cases, the nomogram is adjusted to include an optical zone set at 1 mm inside of the graft-host interface, with maximal settings consisting of 80° to 90° arc length at 75% depth and 90° incision angulation. The addition of two to three 10-0 nylon interrupted compression sutures on the flat corneal meridian can result in outcomes correcting 6 D to 7 D of astigmatism correction. Sutures are typically removed 2 to 3 months postoperatively.
In a post-penetrating keratoplasty Oculus Pentacam study, we were able to accurately determine the pattern of astigmatism, which in many cases is markedly irregular. I have found it advantageous to make customized arc length pairings and incisional angulation to better address such individual findings. This ability to modify and customize my approach allows me to treat even post-radial keratotomy patients. Simply tilting the angle of incision modifies the effect. Our results confirm that incisions placed at a greater angulation relative to the corneal plane tend to produce less effect because they are of a longer length, and as such, they have a longer healing interface. And, as a general principle, we have documented that steep preoperative corneas tend to demonstrate greater response to femtosecond AK
Case study
A 38-year-old man presented for an iLASIK evaluation. WaveScan (AMO) evaluation on the left eye revealed a lower-order aberration of –1.16 –4.52 × 179 and a manifest refraction of –2.00 –4.25 × 180 gave a best corrected visual acuity of 20/25. Pentacam tomography confirmed a pattern of with-the-rule corneal astigmatism (Figure 1). This level of astigmatism is outside the range for CustomVue WaveScan-guided excimer ablation. In order to provide the patient with the inherent benefits of CustomVue excimer ablation, the patient was advised to consider femtosecond AK treatment to be followed by custom IntraLase LASIK (AMO) after refractive stability was achieved in 4 to 6 weeks.
Figure 1.entacam tomography confirms a pattern of with-the-rule corneal astigmatism.
Figure 2. Pentacam tomography confirms reduction of corneal astigmatism.
Images: Fox ML
Technique
After the steep corneal meridian was marked in an examination room, the patient was transported to the treatment room. Here, an 8-mm optical zone centered on the pupil was marked under microscopic guidance. Ultrasonic pachymetry was used to locate the thinnest part of the cornea in the 8-mm optical zone and steep meridian. Making use of the anterior side-cut component of the IntraLase enabled keratoplasty software, paired side-cut incisions were planned at the steep axis of 90° and 270° with arc length of 85°, depth of 75% of the thinnest pachymetry reading and side-cut angulation of 105°. A small ink mark was placed on the visual axis to ensure centration. The patient was then docked with the IntraLase patient interface, and with femtosecond AK, the incisions were produced in 10 to 12 seconds. Once the incisions were completed, the patient was repositioned under the microscope, where AK incisions were completely opened with a Sinskey hook. The incisions were irrigated with balanced salt solution, and a bandage lens was placed with postoperative drops.
It has been my practice to use an AK approach, which penetrates the corneal epithelium, because it is more predictable and enhancement procedures are easy to perform, if necessary.
Six weeks postoperatively, the patient’s WaveScan evaluation showed lower-order aberrations measuring –2.48 –2.24 × 002. The reduction of corneal astigmatism was confirmed on Pentacam (Figure 2), and the patient went on to uneventful LASIK surgery, making use of an 8.7-mm IntraLase flap at a depth of 110 µm. Uncorrected visual acuity 6 months postoperatively was 20/25.
See video of the technique at http://video.healio.com/video/Femtosecond-laser-astigamatic-k;Ophthalmology.