Measure central corneal thickness to avoid iatrogenic keratectasia.

To the Editor: In their recent article, Durrie and colleagues1 reported results of lift-flap retreatment after LASIK. The authors concluded that this procedure appears to be effective and is associated with minimal complications. They reported that before reoperation, mean spherical equivalent refraction was -2.34 D (range, -0.67 to -5.75 D). The Summit Apex excimer laser (Summit Technology, Waltham, MA) was used in the retreatment procedure with a 6-mm ablation zone. Munnerlyn and colleagues2 defined the relationship between ablation depth, diopters of correction, and ablation zone as ablation depth = square of diameter of ablation X diopters of correction. This equation is known as the Munnerlyn formula. According to this formula, the excimer laser ablates about 12 μm per 1.00 D of myopic correction for a 6-mm ablation zone. When ablation depth is calculated for the amount of myopic correction in the Durrie article, this amount would be 28 μm (range, 8 μm to 69 μm). Since we may induce iatrogenic keratectasia after LASIK, calculation of ablation depth becomes an important issue. Other important issues in LASIK are preoperative corneal topographic analysis, diameter of ablation zone, ablation smoothness, centration of ablation, nomogram for emmetropia, transition zone of ablation, regular-shaped flap, and preoperative and postoperative care. Insufficient attention to detail in these parameters may lead to visual discomfort, most often easily managed. However, in the event of iatrogenic keratectasia, surgery is required. For this reason, measuring central corneal thickness is very important. Koch3 reported that only five cases of iatrogenic keratectasia have been reported in the literature. Both the threshold thickness for posterior stromal tissue to become keratectasic and the etiology of iatrogenic keratectasia are unknown. The major reason in one of these five reported cases of iatrogenic keratectasia seemed to be performing LASIK without measuring central corneal thickness.4 Central corneal thickness was not reported in the Durrie article; an ablation depth of 28 μm (range, 8 μm to 69 μm) may indude keratectasia.