Incisional Correction of Astigmatism
Several techniques of incisional surgery have been used to correct astigmatism, including tangential (transverse/straight) keratotomy and arcuate (curved) keratotomy (AK), in which incisions are typically placed in the cornea at the 7-mm optical zone; and limbal relaxing incisions (LRIs), which are placed at the limbus. Tangential keratotomy was used in the past in combination with RK to correct myopic astigmatism, but now is seldom used. AK is used to correct post-keratoplasty astigmatism. Along with LRIs, AK is used to correct astigmatism during or after cataract surgery and IOL implantation, as well as after refractive surgery procedures such as LASIK and PRK (Video 3-1). Several femtosecond laser platforms have been approved for incisional keratotomies when used for capsulotomy and phacofragmentation of the crystalline lens.
Femtosecond laser-assisted astigmatic keratotomy. Courtesy of George O. Waring IV, MD.
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When a single meridian is flattened as a result of an astigmatic incision, a compensatory steepening occurs in the meridian 90° away. This phenomenon is known as coupling (Fig 3-3). When the coupling ratio (the amount of flattening in the meridian of the incision divided by the induced steepening in the opposite meridian) is 1.0, the spherical equivalent remains unchanged. When there is a coupling ratio greater than 1.0, a hyperopic shift occurs. The type of incision (arcuate versus tangential) and the length and number of parallel incisions can influence the coupling ratio. Long, straight, and tangential incisions tend to induce a coupling ratio greater than 1.0, unlike short, arcuate incisions. When a correction is less than 2.00 D of astigmatism, the coupling ratio is typically 1.0; however, when a correction is greater than 2.00 D of astigmatism, the ratio tends to be greater than 1.0. In general, LRIs do not change the spherical equivalent.
Excerpted from BCSC 2020-2021 series: Section 13 - Refractive Surgery. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.