Astigmatism and LASIK.

Although laser in situ keratomileusis (LASIK) enjoys a high success rate, postoperative residual or induced astigmatism may limit uncorrected visual acuity and cause starbursts and glare at night. Irregular astigmatism can also cause loss of best-corrected visual acuity, monocular diplopia, and ghosting of images. Astigmatism may be measured by keratometry and refraction, while corneal topographic techniques help to define irregular astigmatism, in particular. Further information may be obtained regarding induced higher-order aberrations with aberrometry. Because astigmatism has both direction and magnitude, its analysis is more complex than that of the spherical component of the treatment. There are multiple approaches to the analysis of surgically induced astigmatism, including vector analysis, conversion to a Cartesian coordinate system, matrix formalism, and linear optics. Both excimer laser and incisional techniques may be used to correct astigmatism after LASIK, but the treatment of irregular astigmatism requires selective zonal ablation techniques or customized corneal ablations, using topographic or wavefront derived data.