Today’s Practice Refractive Fundamentals

SEPTEMBER 2011 CATARACT & REFRACTIVE SURGERY TODAY EUROPE 89 N umerous factors make the treatment of corneal astigmatism with laser corneal surgery more challenging than treatment of spherical error alone. Astigmatism can be irregular and complex, and it can be compounded by myopic or hyperopic error. Each scenario requires choice of an appropriate treatment approach. Choosing the wrong treatment can lead to regression of effect, excessive tissue ablation, and other complications. Selection of the best ablation approach can help to minimize some of the difficulty inherent in treating astigmatism, avoid postoperative complications, and ensure happy patients. Regular astigmatism is characterized by the presence of two principal, orthogonal meridians on the cornea: the steepest and the flattest meridians. Even with simple orthogonal astigmatism, however, refractive surgeons must be aware that astigmatism always includes a component of higher-order aberrations (HOAs).1 HOAs on the corneal surface, which are naturally irregular and asymmetric, influence the cylinder power and axis measured preoperatively and must be corrected before the true astigmatism can be identified. The correction of HOAs may reveal the true cylinder power and axis, eliminating the comaand trefoil-related portions of astigmatism (Figure 1). This approach spares precious tissue during the refractive procedure.