The Excimer Laser

By Maria Clara Arbelaez, MD; and Thomas Magnago, Dipl-Ing Due to recent advances in refractive surgery, the complexities of correcting irregular astigmatism can now receive closer attention than ever before. Today, subjective evaluation of the patient’s refractive deficit may not produce an accurate depiction of his preexisting astigmatism. To minimize poor postoperative results, the golden rule of refracting a patient is to select the measurement with the lowest cylindrical value. In cases in which multiple measurements of the same cylindrical value can be used, we should choose the one with the axis closest to the 180o of with-the-rule astigmatism. In cases where these rules do not help, we should select the one that shows the lowest higher-order aberrations. Although we have strategies to select the most accurate preoperative measurements, we must devise proper treatment strategies that translate these into the best astigmatism correction. We believe that excimer lasers are the best technology to reach this goal; they can compensate actively for all kinds of cyclotorsional errors and customize the treatment zone. Newer ablation algorithms for correction of spherocylindrical values are typically aspheric, compensating for biomechanical influences and thereby preserving existing higher-order aberrations, retaining the patient’s habitual visual impressions and avoiding the need for neural adaptation. These sophisticated algorithms also take into account the geometric shape of the anterior cornea using keratometry (K) readings of the two individual axes. These readings allow calculation of energy loss due to reflection (Fresnel loss) and efficiency loss due to the geometric change of the spot shape.1,2 The higher the cylindrical refraction, the more significant the benefit of these calculations. If we do not consider the individual loss in both axes, a 5.00 D correction of astigmatism will result in approximately 0.30 D of residual irregularity, which will not create a perfect cylindrical shape. Not considering the loss of energy would result in approximately 0.60 D of irregularity.