LABORATORY SCIENCE Comparison of the effect of LASIK parameters on the percent tissue altered (1-dimensional metric) versus percent volume altered (3-dimensional metric) Damien Gatinel, MD, PhD, Alain Saad, MD, Perry S. Binder, MS, MD Purpose: To determine the theoretical volumes of flap and tissue ablation altered during laser in situ keratomileusis (LASIK) correc- tion of myopic refractive errors. Setting: Rothschild Foundation, Paris, France. Design: Experimental study. Methods: The theoretical volumes of the flap and ablated corneal lenticules for spherical myopic corrections were calculated by mathematical approximations based on a simplified geometric model. These results were then compared for various zone diam- eters, dioptric corrections, and the percentage of the volumes altered (PVA) with the percentage of tissue altered (PTA). Results: The volume of the flap varied linearly with flap thickness and with the square of the flap diameter. The volume of ablated corneal tissue was estimated to be proportional to the magnitude of myopia treatment and to the 4th power of the treatment diameter. For the same depth of ablation, the volume of tissue ablated can vary significantly, depending on the magnitude of the correction and the optical zone diameter. As a result, the PTA calculation is not predictive of the actual PVA. Conclusions: The flap diameters and the laser correction were the most important determinants of the PVA altered during LASIK surgery. New models estimating the volume of the flap and corneal tissue might be necessary to determine their influence on corneal biomechanical stability and each procedure’s outcome. J Cataract Refract Surg 2018; -:-–- Q 2018 ASCRS and ESCRS F or lamellar refractive surgery, it has been assumed that the residual stromal bed (RSB) thickness might be the critical factor in the cornea’s postoperative biomechanics and stability. This concept was developed from empirical data and previous observations and promulgated without formal investigations. The concept of the percentage of tissue altered (PTA) was introduced by Santhiago et al. 1–3 and Santhiago 4 as a screening metric for refractive surgery candidates. The PVA is an estima- tion of the percentage of central corneal tissue modified during the creation of the laser in situ keratomileusis (LASIK) flap and subsequent stromal photoablation. It considers the relationship between preoperative corneal thickness, the tissue altered through excimer laser ablation and flap creation, and the ultimate RSB thickness. In their study introducing the PTA, Santhiago et al. 1 inves- tigated the association between the PTA and the occurrence of ectasia after LASIK in eyes with normal corneal topography. Their study included 30 eyes of 16 patients with bilateral normal preoperative Placido-based corneal topography that developed ectasia after LASIK and 174 normal eyes of 88 consecutive patients that had uneventful LASIK and at least 3 years of postoperative follow-up. In the ectasia group, a PTA of 40% or greater was the most prevalent risk factor (97%). This value was selected from receiver operating characteristic curves, which showed that a cutoff of 40% led to a specificity of 91% and a sensitivity of 87%. In a recent study comprising 593 eyes, 5 we assessed the specificity value of this PTA metric to 78.7%. In their pa- per, Santiago et al. 1 reported that all cases with a PTA value above 47% developed this complication. In our study, 5 19 eyes (3.2%) that had a PTA above 47% did not develop ecta- sia after 2 years (mean 31 months) of follow-up. The term tissue for the T in the acronym PTA suggests that a volume, not a simple thickness of the elements, is used in the calculation. However, the cornea is a Submitted: August 15, 2017 | Final revision submitted: January 18, 2018 | Accepted: April 13, 2018 From Fondation Rothschild (Gatinel, Saad) and Centre d’Expertise et Recherche en Optique Clinique (Gatinel, Saad), Paris, France; Medical Center (Saad), American University of Beirut, Beirut, Lebanon; Gavin Herbert Eye Institute (Binder), Department of Ophthalmology, University of California, Irvine, Irvine, California, USA. Corresponding author: Damien Gatinel, MD, PhD, Fondation Rothschild, 25 rue Manin, Paris, France. Email: gatinel@gmail.com. Q 2018 ASCRS and ESCRS Published by Elsevier Inc. 0886-3350/$ - see frontmatter https://doi.org/10.1016/j.jcrs.2018.04.041 1