New and Improved Astigmatic Keratotomy

Femto-assisted AK updates an older procedure.

By Ana Paula Canto, MD, and William W. Culbertson, MD

Astigmatic keratotomy (AK), which in recent years has lost popularity to limbal relaxing incisions as a way to reduce or eliminate corneal astigmatism in cataract surgery patients, may be getting a new lease on life thanks to the emergence of femtosecond laser technology.

With today's patients having higher-than-ever expectations due to the general improvement in IOL performance and specifically the availability of so-called “premium” IOLs, the reduction or elimination of astigmatism now plays a major role in achieving outcomes that satisfy patients. Here, we will describe step-by-step a more efficient and precise way to perform AK using a femtosecond laser.

Corneal Astigmatism: A Prevalent Problem

According to a recent study, the degree and prevalence of corneal astigmatism before cataract surgery is greater than 1.00 D in 36% of the patients, and greater than 1.50 D in 17% to 22% of the patients.

Uncorrected astigmatism reduces a patient's near and distance visual acuity, reduces contrast sensitivity (a key contributor to night driving difficulty), hinders reading speed and decreases the subjective clarity rating when viewing a mobile phone or a computer screen. Visual challenges for these tasks reduce the quality of life, and sometimes the independence, of patients with uncorrected astigmatism.

Therefore, optimal management of astigmatism at the time of cataract surgery is an area of extreme importance. Limbal relaxing incisions, AK and toric IOLs are considered effective options.

In the United States, there are currently no toric multifocal or other presbyopia-astigmatism correcting IOL options available, and the common toric IOL has limited cylinder power gradations. That makes LRI and AK even more important, when the goal of cataract surgery is not just removing the cataract, but also offering patients their optimum vision. Thus, we are transforming a procedure that in the past was just intended to deal with a problem into a refractive procedure with real, increased visual benefits.

LRI vs AK

LRIs are corneal incisions placed adjacent to the limbus, just anterior to the vascular arcade. They relax the steep axis of the corneal astigmatism while steepening the flat axis, and they must be as accurate as possible to yield the best result. AK shares the same principles, but the incisions are performed more centrally in the cornea. In both techniques, a coupling phenomenon is seen in which there is flattening of the incised steep meridian, accompanied by steepening of the unincised meridian 90 degrees away. The effectiveness of both LRI and AK has been studied for many years and both have proven to be a practical, simple, cost-effective and convenient way of approaching astigmatism correction.

The success of an AK depends upon several steps. The first is measuring the astigmatism power and axis. There are several methods available, including manual keratometry, optical keratometry using IOLMaster (Carl Zeiss Meditec) or Lenstar (Haag Streit), and corneal topography using placido discs, Scheimpflug camera or a combination of both.

The second step is choosing the surgical plan based on a nomogram that takes into consideration the age of the patient, the axis of the astigmatism, and the magnitude of the astigmatism. Based on the nomogram, an optical zone diameter (mm), the depth (%) and the angular length (degrees) of the incision will be determined. Step three is identifying the accurate thinnest pachymetry at the intended site of incisions (this can be measured with an ultrasound pachymeter or anterior segment optical coherence tomography, using the caliper tool to determine the thickness at the planned incision location) and set the adjustable diamond knife at the appropriate depth.

The next step is marking the appropriate optical zone and the steepest axis of the cornea where the incisions will be placed. The final step is achieving the precise depth and length planned for the corneal incision, and finally, performing a uniform incision.

Manual vs. Femto-assisted AK

Manually performed astigmatic keratotomy can be successful, but greatly depends on the surgeon's technique and planning. Inaccuracies can be caused by a lack of precision and reproducibility of incision depth and length, the presence of skip lesions and potential axis misalignment.

Consistent incisions at a uniform depth of 80% to 90% are essential to achieving the best results. One example of lack of precision due to manual technique is shallow incisions, which create a small gap and have minimal positive effect when less than 60% of desired depth is achieved.

Disadvantages of manual AK include corneal perforation, varied healing responses, limited cylinder correction, under-correction, overcorrection and regression. Additionally, long incisions may distort or denervate the cornea.

Contraindications for AK include patients with ectatic disorders, highly irregular astigmatism, limbal peripheral corneal pathology, extreme dry eye and patients who have had radial keratotomy.

Enter the Femto Method of Performing AK

What does the femtosecond laser offer in astigmatic keratotomy? Quite simply, precision. Femtosecond lasers create a precise incision construction, in the precise optic zone and axis, with the exact length and depth, with no skip lesions, while minimizing the risk of perforation.

How do corneal relaxing incisions performed manually with a knife compare to those made with a femtosecond? Femtosecond laser bubbles leave many uncut residual collagen fiber connections, and it is necessary to mechanically open the incision to allow the cornea to flatten. Femtosecond-created incisions may also heal differently from blade incisions. The average depth of laser incision may be deeper than the average depth of a blade incision for the same percentage thickness of intended depth, and beveled laser incisions may produce different results than vertical incisions.

Why adopt femtosecond laser technology? Femtosecond laser incisions may improve outcomes, increase patient satisfaction, have a lesser detrimental effect on higher-order aberrations when compared to manual astigmatic keratotomy, and minimize the risk of perforation. It is safe, effective and precise—exactly what surgeons and patients want and need in an ophthalmological procedure.

In the United States, femtosecond-assisted astigmatic keratotomy can be performed using the IntraLase femtosecond laser (Abbott Medical Optics). With this equipment, we use the keratoplasty mode, and when planning the treatment all the cut options are turned off and only the anterior side cut remains on.

The parameters entered in the planning window are: posterior depth in microns (percent depth of corneal thickness at incision site, usually 90% of the thinnest pachymetry in the planned incision site), anterior side-cut diameter in mm (optical zone diameter), anterior side-cut energy in mJ, anterior side cut 1 position (corneal steepest axis), anterior side cut 1 angle (arc length), anterior side cut 2 position (180 degrees from position 1), anterior side cut 2 angle (arc length of incision, in degrees), anterior side cut angle, anterior side cut spot separation, anterior side cut layer separation, depth in contact glass (selected according to manufacturer recommendations).

Femtosecond-assisted AK: The Procedure

Before bringing the patient to the laser room, the steepest corneal meridian is marked at the slit-lamp station. Under the surgical microscope and using anesthetic eye drops, an optical zone marker centered on the pupil is used to mark the optical zone diameter. An axis marker is then used to mark the planned locations of the incisions. The suction ring, followed by the cone applanation, is applied to the eye and centered on the pupil. The treatment screen shows the location of the incisions and the suction ring can be used to rotate the eye to ensure proper axis alignment. The centration of the optical zone can be modified on the screen, if necessary. After the laser treatment, the incisions are opened with a Sinskey hook. Antibiotic and steroid eyedrops are prescribed four times daily for one week. The patient is instructed to avoid rubbing the eye and to use artificial tears frequently.

Femto-assisted Outcomes

In our experience, femtosecond laser-assisted AK offers good outcomes in naturally occurring astigmatism and post-cataract astigmatism. In a recent retrospective study performed in 14 eyes at Bascom Palmer and presented at ASCRS this year (unpublished data), this procedure was shown to be safe and effective. Significant improvement of the uncorrected distance visual acuity, refractive cylinder and keratometric cylinder were shown. However, further studies to formulate a specific nomogram for femtosecond astigmatic keratotomy are needed.

AK can also play a role in correcting astigmatism developed after penetrating keratoplasty. The average magnitude of astigmatism after penetrating keratoplasty ranges from 4 to 6 D, and as many as 8% to 20% of these patients have intolerable astigmatism. The arcuate incisions for penetrating keratoplasty should be placed 0.5 to 1 mm inside the graft/host junction. Generally, the incisions should be no deeper than 60% of corneal thickness at the intended incision position.

An Entire Femto-assisted Procedure

With the new option of femtosecond laser-assisted cataract surgery in which an image-guidance system is available, it is now possible to make a real-time determination of the corneal thickness at the planned position of the arcuate incision. This ensures that the femtosecond laser pulses are delivered precisely to the intended location at the proper depth, creating a uniform incision. Another benefit of this technology is that all the corneal incisions, astigmatic incisions, capsulorhexis and lens softening will be done in a single procedure.

Femtosecond AK also allows room for surgical creativity, such as when performing entirely intrastromal incisions in which the corneal epithelium and Bowman's membrane would remain intact. The advantages of these incisions are that there would be no epithelial ingrowth, minimal loss of corneal sensation and greater predictability. (See examples in Figures 1 and 2.)

Figure 1. (Left) Perfect FS laser-created intrastromal arcuate incisions immediately postop for treatment of 1.0 D of residual corneal cylinder three months after multifocal IOL cataract surgery. (Right). Same eye on first postoperative day with almost no visible evidence of the incisions.

Figure 2. Comparison of preoperative to postop topography showing resolution of 1.0 D of astigmatism.

In conclusion, the advent of the femtosecond laser has created new options in refractive and cataract surgery, and now, with femto-assisted AK, in the correction of corneal astigmatism as part of basic cataract surgery or in an all-femto-assisted cataract surgery procedure. This major advance in cataract and refractive technology—offering more precise measurement, mapping and treatment—has significantly, safely and effectively expanded surgical options and improved the outcomes for patients requiring a wide range of visual correction. OM