Refractive Surgery Bonus Feature

SEPTEMBER 2010 CATARACT & REFRACTIVE SURGERY TODAY EUROPE 25 A stigmatism is an optical defect that may lead to shadowing, loss of contrast sensitivity, distortion, blurred vision, and ghosting. There are two types of astigmatism, regular and irregular. Irregular astigmatism is often caused by a corneal scar or scattering in the crystalline lens. It is estimated that astigmatism of more than -0.50 D is present in 44.4% of the population.1 Astigmatism may be idiopathic or secondary to surgical procedures, including cataract extraction and penetrating keratoplasty (PKP). Determined by magnitude and orientation, astigmatism is more difficult to manage compared with myopia and hyperopia. Surgical treatment of astigmatism was first attempted with astigmatic keratotomy, in which incisions in the peripheral cornea are made in an attempt to make its shape more spherical. The use of incisional keratotomy has declined in recent years as laser vision correction has gained popularity due to its superior predictability in most cases. The excimer laser corrects astigmatism by removing anterior corneal stromal tissue to alter the curvature of the cornea. Regular astigmatism is mainly generated by excessive corneal toricity. Corneal toricity can be suppressed either by flattening the steepest meridian to match the curvature of the initially flatter meridian or by steepening the flattest meridian to match the curvature of the initially steeper meridian. Simple myopic and hyperopic astigmatic treatments rely on the use of negative and positive cylinder modes, respectively. Compound and mixed astigmatism are treated by the combination of negative and/or positive cylindrical and spherical modes. The key to refractive laser correction of myopic and hyperopic astigmatism is the use of an elliptical pattern of ablation applied along the central part of the flat meridian, leading to flattening of the steep axis. Because the treatment zone is elliptical, the effective optical zone is smaller than the treated area. Numerous surgical techniques and ablation profiles to correct astigmatism have been described. Although many studies have reported good results for the correction of astigmatism with PRK and LASIK,2-16 the ablation profiles usually cause a hyperopic shift because of the coupling effect in the flattest corneal meridian. Coupling describes the phenomenon by which, for every 1.00 D of cylinder that is corrected, 0.25 D of myopia is treated. Assessing the coupling effect is difficult because it seems to be dependent on many parameters, such as epithelial remodeling, the LASIK flap, differences among excimer lasers, and preoperative corneal curvature.