2020–2021 BCSC Basic and Clinical Science Course™
13 Refractive Surgery
Chapter 3: Incisional Corneal Surgery
Incisional Correction of Astigmatism
Arcuate Keratotomy and Limbal Relaxing Incisions
AK is an incisional surgical procedure in which arcuate incisions of approximately 95% depth are made in the steep meridians of the midperipheral cornea at the 7–9-mm optical zone. LRIs are incisions set at approximately 600 μm depth, or 50 μm less than the thinnest pachymetry measurement at the limbus. They are placed just anterior to the limbus (Fig 3-4). AKs differ from LRIs by their midperipheral location and greater relative corneal depth. However, AKs and LRIs are similar in that both have coupling ratios of 1.0 and therefore correct astigmatism without inducing a substantial hyperopic shift. Increasing the length of an LRI increases the magnitude of the astigmatic correction. For AK, the amount of cylindrical correction is increased by increasing the length or depth of the incision, using multiple incisions, or reducing the optical zone (Table 3-1). Older patient age is associated with increased effect of astigmatic incisions.
Figure 3-3 Coupling effect of astigmatic incisions. A, A limbal relaxing incision has a coupling ratio of 1.0, and the spherical equivalent and average corneal power are not changed. B, A transverse incision has a coupling ratio greater than 1.0, which causes a hyperopic change in refraction by making the average corneal power flatter.
(Illustration by Cyndie C. H. Wooley.)
Figure 3-4 Limbal relaxing incision. A relaxing incision is made at the limbus with the use of a diamond knife. The coupling ratio is typically 1.0 and does not change the spherical equivalent.
(Courtesy of Brian S. Boxer Wachler, MD.)
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.