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Refractive Surgery | Oct 2010

Femtosecond-Assisted Astigmatic Keratotomy

Enabling precise and predictable corneal incisions.

The treatment of moderate to high astigmatism is challenging for the refractive surgeon. Even in experienced hands, 40% of patients who undergo penetrating keratoplasty (PKP) are left with more than 4.00 D of astigmatism.1 Spectacles, contact lenses, excimer laser ablation, toric IOLs, and tissue-relaxing procedures have been used for a long time in the treatment of preexisting and postsurgical corneal astigmatism.2-4 We propose the use of femtosecond-assisted astigmatic keratotomy (AK) for treatment of moderate to high astigmatism.

Because AK is a simple and effective technique to manage astigmatism,5 it is the most commonly used surgical treatment for post-PKP astigmatism.6 The technique is similar to limbal relaxing incisions; the corneal incisions perpendicular to the steep meridian produce a flattening effect along the axis centered over the incision (Figure 1).7 Likewise, the incisions induce steepening at the nonincised meridian 90° away. This coupling effect avoids significant change in the spherical equivalent refraction.7,8

AK can be performed with a freehand technique, but mechanized devices such as the Hanna arcitome (Moria, Antony, France) allow more reproducible results.9 Nevertheless, corneal perforation, wound dehiscence, and the decreased efficacy that result from such events lessen the reliability of these devices.2,7 Surgeons have described use of the femtosecond laser as an alternative to perform AK.10-12 Combining the predictability of the femtosecond laser with the simplicity and effectiveness of AK enhances reproducibility and improves accuracy and safety.10,11

In this article, we demonstrate our surgical technique for femtosecond-assisted AK in a case with high against-the- rule corneal astigmatism after PKP.

Case Report

A 55-year-old man with keratoconus was referred for evaluation of decreased vision in his right eye. He had undergone PKP in his left and right eyes in 1990 and 2004, respectively. Additionally, he had unsuccessful PRK in his right eye several years ago. The preoperative examination revealed 20/400 UCVA in both eyes. BCVA was 20/30 in the right and 20/25 in the left eye. The manifest refraction was -7.75 +11.50 X 15° in the right, and there was -9.25 sphere in the left eye. Slit-lamp examination revealed trace haze in the right eye and microcystic edema due to recent decompensation in the left. Schirmer values and tear break-up time were normal in both eyes.

Fundoscopic examination was normal in both eyes. Corneal topography (TMS-4; Tomey Corp., Nagoya, Japan) revealed marked against-the-rule astigmatism with simulated keratometry readings of 51.78 at 3° and 34.80 at 93° in the right eye (Figure 2A). Topographic astigmatism was 17.00 D. The patient was scheduled for femtosecondassisted AK for the management of high corneal astigmatism in his right eye.

Surgical Technique

Corneal thickness was measured with ultrasonic pachymetry (Sonogage; DGH Technology, Inc., Exton, Pennsylvania) at the 6-mm optical zone along the planned incision sites. A sterile marking pen was used to mark the cornea in the steep axis, and the surgical procedure was performed under topical anesthesia. The position of the suction ring and cone was aligned according to the center of the corneal graft (Figure 3A). During laser treatment, the tear fluid meniscus was kept beyond the graft-host junction.

Sidecut diameter and angle, used to determine the length and position of the relaxing incisions, were determined according to a modified version of our own nomogram for AK in high astigmatism (Table 1). Paired incisions were made in the graft, centered on the steep axis according to corneal topography (Figures 1 and 2). In our case, two paired, symmetrical corneal incisions were positioned at 3° and 183° (80° arc length, from 43° to 323° and from 143° to 223°; Figure 2).

The posterior depth selected by the laser software controls the depth of the corneal incisions from the glass applanation surface. Using the keratoplasty software, incisions were created at 75% depth of the thinnest corneal measurement (420 µm) at the 6-mm zone. We used the IntraLase 30-kHz femtosecond laser (Abbott Medical Optics Inc., Santa Ana California; Figure 3A and 3B). Table 2 summarizes the laser settings.

After the corneal incisions were placed, a Sinskey hook was used to separate the corneal incisions under the operating microscope. There was significant improvement in the mires of the anterior corneal surface immediately after both incisions were opened (Figure 3C and D). Postoperatively, antibiotic and steroid eye drops were prescribed four times daily for 4 weeks. Three months after surgery, UCVA improved to 20/40 and BCVA to 20/20 in the right eye. Manifest refraction was -4.00 +5.75 X 20°, and corneal topography demonstrated significant improvement in corneal astigmatism (6.00 D) in the right eye (Figure 2B).

Discussion

Surgical planning. Careful preoperative surgical planning is essential with femtosecond-assisted AK. Corneal thickness should be precisely measured with ultrasound pachymetry or anterior segment optical coherence tomography (OCT)—we use the Visante OCT (Carl Zeiss Meditec, Jena, Germany)—to prevent full-thickness corneal incisions. The accuracy of corneal cuts with the femtosecond laser decreases the risk of corneal perforations.10 However, femtosecond lasers cut in a perpendicular plane, which is a challenge in eyes with uneven corneal thickness.13,14 The combination of OCT data and the femtosecond laser will solve this problem and further decrease the risk of corneal perforations in the future.

Creating corneal incisions. Position of the suction ring is crucial when planning the incisions. In post-PKP patients, corneal incisions should be prepared 1 mm inside the graft edge—a difficult task if the graft is decentered. Newer femtosecond laser software includes programming for nonorthogonal incision placement, thus giving the versatility to manage irregular astigmatism.15 The energy settings, layer separation, and spot separation values should be selected according to the manufacturer's suggestions. For instance, all corneal incisions in keratoplasty mode with the IntraLase are cut from posterior to anterior. Depth in contact glass is used to continue the incision into the glass applanation surface. A minus value can be used to stop the incision before reaching the applanation glass.

Keratoplasty software creates both arcuate incisions with a spiral pattern in a single applanation; the laser is capable of creating incisions up to 1,200 µm. We have previously published a femtosecond-assisted AK nomogram for naturally occurring high astigmatism (Table 1).12 In these patients, we prepare corneal incisions at 90% of corneal depth. In post-PKP patients, we use a modified nomogram, because the incision depth should be 75% of the lowest pachymetry reading. We also prepare the incisions with smaller diameters and angular lengths, according to the graft size. However, further refinements of these nomograms and long-term results are needed for femtosecond-assisted AK.

Other considerations. Caution should be used when planning femtosecond-assisted AK in patients with ocular surface disease and severe dry eye. Postoperative refractive results can be evaluated 1 to 3 months after AK.5,16 Several authors have reported overcorrections after femtosecond-assisted AK.14,16 In post-Descemet's stripping endothelial keratoplasty cases, corneal thickness measurements should be made with anterior segment OCT to prevent overcorrection or full-thickness corneal incisions.14

Conclusion

Femtosecond-assisted AK is a simple and effective technique to manage astigmatism. The femtosecond laser enables precise, predictable, and effective corneal incisions for use in AK. Future developments in femtosecond laser software may allow creation of subepithelial stromal incisions, iris registration, and even OCT-guided incisions to further improve results.

Volkan Hurmeric, MD, is a Research Fellow at Bascom Palmer Eye Institute, University of Miami Miller School of Medicine, Florida. Dr. Hurmeric states that he has no financial interest in the products or companies mentioned. He may be reached at tel: +1 312 304 5851; e-mail: hurmeric_v@yahoo.com.

Sonia H. Yoo, MD, is a Professor of Ophthalmology at Bascom Palmer Eye Institute, University of Miami Miller School of Medicine, Florida. Dr. Yoo states that she is a consultant to Carl Zeiss Meditec and Abbott Medical Optics Inc. She may be reached at tel: +1 305 326 6322; fax: +1 305 326 6337; e-mail: syoo@med.miami.edu.

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