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Ibrahim Seven, Abhijit Sinha Roy, William J Dupps; Adjunctive Collagen Crosslinking of the Residual Stromal Bed in LASIK: Finite Element Analysis of Impact on Refractive Outcome and Surgically Induced Deformation. Invest. Ophthalmol. Vis. Sci. 2014;55(13):2990.
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© ARVO (1962-2015); The Authors (2016-present)
Corneal collagen crosslinking (CXL) of the residual stromal bed (RSB) in LASIK has been described (LASIK Xtra, Avedro Inc., MA) for augmentation of RSB material strength with the potential advantages of reduced ectasia risk and improved postoperative stability. Stiffening of the anterior cornea has demonstrated the potential to produce central corneal flattening, but no prior studies have explored the refractive impact of stromal stiffening below the level of the flap in LASIK. This study compares the refractive outcomes and corneal displacements of conventional LASIK and LASIK with RSB CXL on the same 3D finite element model.
The geometry was generated from Scheimpflug tomography elevation data. Epithelial and stromal flap layers were defined by offsetting the anterior corneal surface by 50 microns and 110 micron uniform thicknesses respectively. A hyperelastic, anisotropic, incompressible and depth dependent material formulation was defined for the stroma using normative experimental values for the material constants. FE analyses were performed using Abaqus 6.11 with 15mmHg intraocular pressure. A myopic LASIK procedure with a -4.25D spherical correction was simulated by using a wavefront optimized ablation profile with an optical zone diameter of 6.5mm and overall treatment diameter of 9mm. CXL was simulated as an increase in stiffness of the central 9mm of the RSB with an effective depth of 200 micron and a stiffening factor of 1.5. Tangential (instantaneous) curvatures of anterior surface points were calculated by a custom Python 2.7 script (Fig 1).
Spherical equivalent refractive error was -4.25D in the preoperative case and was reduced to -0.27D in the LASIK-only simulation and 0.03D in the LASIK with RSB CXL simulation. Surgically induced corneal displacements were significantly lower in the LASIK with CXL group (Fig 2).
LASIK with CXL of the RSB employing a 50% increase in anterior RSB stiffness contributed less than 0.25D of hyperopic effect to the postoperative refractive error and significantly reduced corneal displacements after LASIK. These preliminary simulation-based results are consistent with early clinical reports showing no systematic trend toward overcorrection of myopia in LASIK xTra even in the presence of a 50% increase in RSB stiffness.
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