INTRODUCTION

For more than a century, limbal and corneal incisions have been used to correct astigmatism during cataract surgery¹. In 1885, Schiotz described performing limbal relaxing incisions (LRI).

In 1939, Sato presented his technique of anterior and posterior radial keratotomy, beginning the era of corneal incisional surgery². A few decades later, arcuate and transverse corneal surgeries for reducing astigmatism were described¹.

Currently, the most used forms of incisional surgery during phacoemulsification are the (1) limbal relaxing incisions (LRI) and (2) arcuate keratotomy (AK). Both techniques can be done using calibrated scalpels or the femtosecond laser (femto).

LIMBAL RELAXING INCISIONS (LRI)

LRIs are relaxing incisions performed on the peripheral corneal (just-limbic) at the most curved meridian, with a usual depth of 600 microns and variable arch length, according to the amount of astigmatism to be corrected (Figure 1).

Due to their limited correction potential, they are generally used to correct low or moderate degrees of astigmatism (up to about 3 diopters (D))^3,4.

Its advantages are high security, simplicity and low cost. Hypercorrections are also rare; there is rapid recovery and visual stabilization (few weeks) and the optical quality of the cornea is preserved^3,4.

As disadvantages we have the limited ability of astigmatic correction, even with large arch length incisions, and the lower predictability when compared to toric lenses, especially in high astigmatism⁵.

Among the factors that have an impact on the outcome of a LRI, we can mention: (1) number of incisions (1 or 2); (2) the length and location of the incisions (the greater and more localized in the vertical meridian (WTR), the greater the correction), (3) distance from the limbus (the more corneal, the greater the correction) (5) corneal diameter, (6) age and (7) depth of the incisions¹.

Nichamin's nomogram is still the most used when performing LRIs³ (Figure 2). This nomogram takes into account the magnitude and direction of the preoperative astigmatism and age of the patient. The phaco incision is considered neutral and the author suggests the making of a clear temporal corneal incision, due to the lower induction of astigmatism.

LRIs are always 600 microns and performed on the curved meridian. Another option when the surgeon prefers to operate elsewhere is to use an online calculator (www.LRIcalculator.com) (Figure 3) that adjusts by vector calculation the influence of the phaco incision on the astigmatism to be corrected. It also allows the use of Donnenfeld's nomogram for LRI as well as Nichamin's.

In the authors' service, Nichamin's nomogram was used, aiming at a slight hypocorrection (0.5 D) in WTR cases and a slight overcorrection in the cases of astigmatism against-the-rule (ATR), thus compensating the astigmatism of the posterior face of the cornea^6-8. Astigmatism is qualitatively assessed through topography and/or corneal tomography and quantitatively (magnitude and orientation) through optical biometry (IOL Master 700, Carl Zeiss, Germany). LRIs are contraindicated in irregular astigmatism (e.g. keratoconus or pellucid marginal degeneration), after previous incisional corneal surgery (e.g. radial keratotomy), and after previous photoablative surgery (e.g. LASIK and PRK). In order to control cyclotorsion, it is essential to mark the curved axis at the slit lamp, with the patient sitting (or using Verion intraoperatively (Figure 4)).

The use of femtosecond laser is possible and should also involve torsional control. In general, when the femtosecond laser is used, the incisions are performed on clear cornea (Figure 5), with an optical zone slightly smaller than in the manual technique, which allows the correction of greater astigmatism, but may result in hypercorrections and complications similar to AK. Intrastromal incisions with the femtosecond laser are also possible, but this is limited to correction of low astigmatism, even with long incisional arches.

Also, among the current developments, we include the intraoperative use of ORA (Alcon, USA) that allows the dynamic refractive evaluation of astigmatism during surgery (Figure 6).

ARCUATE KERATOTOMY (AK)

AK consists of a similar procedure, but performed on clear cornea, with smaller optical zones, ranging from 6 to 8 mm⁹. They are made with diamond scalpels calibrated according to the local corneal thickness (often 95% of the thickness) or with femtosecond laser.

It has the advantage of greater astigmatic correction power, but can cause irregular astigmatism, progressive delayed overcorrection (after several years), increased ocular discomfort and risk of perforation, so they are currently little used during phacoemulsification of virgin eyes.

Its use is reserved for astigmatism following lamellar or penetrating corneal transplantation (Figures 7 and 8). In lamellar cases care must be taken to avoid hypercorrections, which are frequent.

Astigmatism -0.23 (LRI enlarged)