Differences in Simulated Refractive Outcomes of Photorefractive Keratectomy (PRK) and Laser In-Situ Keratomileusis (LASIK) for Myopia in Same-Eye Virtual Trials

Abstract

The use of computational mechanics for assessing the structural and optical consequences of corneal refractive procedures is increasing. In practice, surgeons who elect to perform PRK rather than LASIK must often reduce the programmed refractive treatment magnitude to avoid overcorrection of myopia. Building on a recent clinical validation study of finite element analysis (FEA)-based predictions of LASIK outcomes, this study compares predicted responses in the validated LASIK cases to theoretical PRK treatments for the same refractive error. Simulations in 20 eyes demonstrated that PRK resulted in a mean overcorrection of 0.17 ± 0.10 D relative to LASIK and that the magnitude of overcorrection increased as a function of attempted correction. This difference in correction closely matched (within 0.06 ± 0.03 D) observed differences in PRK and LASIK from a historical nomogram incorporating thousands of cases. The surgically induced corneal strain was higher in LASIK than PRK and resulted in more forward displacement of the central stroma and, consequently, less relative flattening in LASIK. This FE model provides structural confirmation of a mechanism of action for the difference in refractive outcomes of these two keratorefractive techniques, and the results were in agreement with empirical clinical data.

Keywords: cornea; finite element analysis; laser in situ keratomileusis (LASIK); photorefractive keratectomy (PRK); refractive surgery.

Figure 1

A representative mesh of preoperative corneal models for LASIK (left frame) and PRK (right frame). Both models consist of an anterior epithelial layer and underlying stromal layers. Sublayers, defined from anterior to posterior, for LASIK (left) are the epithelium, stromal component of the flap, flap interface wound (bolded boundary), and residual stromal bed. Sublayers for PRK (right) are the epithelium and residual stromal bed (right).

Figure 2

Additional corneal flattening observed in PRK over those observed in LASIK from simulations (black group), and global historical nomogram (red group) plotted as a function of attempted spherical equivalent (SE) refractive error. (D: Diopters).

Figure 3

Post-surgical increase (given as a percentage) in average MPS as a function of SE of the correction for LASIK (blue) and PRK (orange), SE: spherical equivalent (diopters), MPS: Maximum principal strain.

Figure 4

Example of maximum principal strain (MPS) maps for full-thickness vertical cross-sections and en face anterior stromal regions for pre-operative, post-PRK, and post-LASIK simulations, respectively. Strain maxima are indicated by an ‘x’ on the map. Higher MPS values were incurred in LASIK than in PRK in all cases.

Figure 5

Magnitude of the displacements (mm) in three principal directions (x, y, z) for post-PRK (left) and post-LASIK models (right) with identical color scales. Residual stromal bed displacements were greater in LASIK than in PRK.

Figure 6

Axial corneal curvature maps (Diopters) for post-PRK (left) and post-LASIK models (right).