Estimation of LASIK Flap Thickness

We read with great interest the article by Brenner et al.,1 which compares accuracy of visual estimation of LASIK flap thickness versus ultrasound measurements, and there are some concerns we would like to share. First of all, if the percent of tissue altered (PTA)2 is considered, a customized residual stromal bed should have been estimated for each patient, instead of setting the “safe level” at 300 μm. For instance, for a preoperative 600-μm central corneal thickness (CCT) cornea, a 300-μm residual stromal bed means that the PTA is 50%, clearly beyond the safe zone. Second, although it has been considered as a useful measurement to increase the safety of LASIK, ultrasound subtraction pachymetry is not as accurate as it was once considered to be. According to Realini et al.,3 CCT measurements exhibit great variation, so a single CCT assessment is not adequate to characterize the corneal thickness. Therefore, to rely on subtraction pachymetry may be misleading due to the variability associated with this technique. Moreover, the cornea suffers dehydration when exposed to the microscope light, especially after the flap is lifted, as demonstrated by Rosa et al.,4 so measuring the residual stromal bed after flap lifting may overestimate the actual flap thickness. Changes on tissue hydration may also affect the visual estimation. Given these circumstances and all of the sources of error, we believe that neither visual estimation nor ultrasound subtraction pachymetry can be considered as accurate. Instead, optical coherence tomography or ultrasound biomicroscopy measurements in several locations of the flap, performed at least 1 month after surgery (when major flap thickness changes have occurred, as demonstrated by Rocha and Krueger5) may be considered. We propose, for a given flap target thickness, to measure a pool of flaps and see the performance of the particular femtosecond laser used. When the mean flap thickness and the standard deviation are known, the surgeon may assume that the flap obtained is within two standard deviations above the mean flap thickness obtained with that particular device. We consider this a more reasonable approach because it considers the hypothetical worst-case scenario based on statistical data and is not subject to the high variability of intraoperative measurements.