11 Correction of Astigmatism with a Femtosecond Laser

Sperl Philipp, Kraker Hannes, and Günther Grabner

Keywords: astigmatism, femtosecond laser, arcuate astigmatic incisions, small incision lenticule extractions, laser-guided arcuate corneal relaxing incisions

11.1 Introduction

Since the very beginning of ophthalmic surgery, the ultimate goal was to provide best uncorrected visual acuity to patients. Within the last decades, we witnessed a rapid progress in refractive surgical techniques. Consequently, nowadays patients undergoing ophthalmic surgery have high expectations from their physicians. Spherical errors are easier to treat, whereas correction of astigmatism seems to be a bigger challenge to ophthalmic surgeons.

From earlier publications, we have seen that the percentage of naturally occurring corneal astigmatism at a range from 0.25 to 1.5 D in patients undergoing cataract surgery is 64%. The rate of astigmatism higher than 1.5 D is 22%. ¹ Another study evaluated 23,239 eyes and found corneal astigmatism of 0.75 D or higher in 36.05% of the patients. ²

Furthermore, the group of patients after posterior penetrating keratoplasty suffering from postoperative astigmatism is not negligible. Studies showed a high rate of intolerable astigmatism after keratoplasty. ³

Naturally occurring astigmatism and surgical-induced astigmatism have a strong influence on uncorrected visual acuity and therefore sufficient surgical procedures are needed to achieve satisfying visual outcomes.

To manage corneal astigmatism, a wide range of treatments have been invented. We can differentiate between noninvasive methods (i.e., contact lenses and glasses) and surgical procedures. All methods to correct corneal astigmatism have shown significant reduction of astigmatism. Nevertheless, limitations were seen in every method. For example, fitting contact lenses to a patient with high astigmatism can be extremely challenging. Surgical procedures such as manually performed wedge resection, corneal relaxing incisions, or compression sutures have limitations in predictability.

Within the last years, reducing corneal astigmatism using a femtosecond laser has become more popular. There are two surgical principles in femtosecond laser–guided astigmatism correction for different indications. Laser-guided arcuate corneal relaxing incisions earlier performed manually are used to correct high astigmatism, that is, after posterior penetrating keratoplasty. Good clinical results prompted surgeons to use this technique to treat low to moderate astigmatism. Arcuate incisions can be intrastromal or anterior penetrating incisions. Higher precision and better reproducibility are expected when using a computer-guided surgical device. Several authors published promising results. ^{4,​ 5,​ 6} The second and newest principle of femtosecond laser astigmatism correction is the small-incision lenticule extraction (SMILE).

11.2 Principles of Femtosecond Arcuate Astigmatic Incisions

As mentioned earlier, a widely used technique to correct corneal astigmatism is placing arcuate relaxing incisions. The femtosecond laser offers the possibility to perform limbal relaxing, intrastromal, or anterior penetrating incisions. Therefore, symmetric cuts are placed on the steep meridian of the cornea. The surgical aim is to achieve a flattening effect of the steep meridian enhanced by a steepening of the corresponding flat meridian. A coupling ratio of 1 leads to no change in spherical equivalent, whereas a ratio greater than 1 will lead to a hyperopic shift and a ratio of less than 1 will result in a myopic shift. ⁷

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