Reduction of spherical aberration: experimental model of photoablation

Several authors have suggested that a compound ablation for myopia may reduce postoperative spherical aberration. This technique consists of treating myopic ablation by increasing the preoperative sphere 25% and applying a hyperopic ablation of 25% of preoperative sphere. Theoretically, this technique allows optical zone increase, and elimination of the peripheral red ring.

Eight eyes of four patients with bilateral moderate myopia received laser epithelial keratomileusus (LASEK) with the Nidek EC-5000 excimer laser. In each patient, one eye was treated with the compound ablation (Group 1), and the fellow eye with the standard cross-cylinder technique (Group 2). Mean preoperative refraction and BSCVA were comparable within the two groups.

At 1 month postoperatively, UCVA was significantly better in Group 2 than in Group 1 eyes. In neither group was clinically important haze seen. Subjectively, all patients rated more satisfactorily the eye that received the standard ablation (Group 2) than the one treated with compound ablation (Group 1). Evaluation of keratorefractive indices derived from corneal topography revealed significantly better eccentricity in Group 2 eyes. Corneal topography showed the presence of a central island in all Group 1 eyes.

The compound ablation hypothesis was not confirmed. Presence of central island in compound-treated eyes induced significant optical aberrations.