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Ibrahim Seven, Ali Vahdati, Cynthia J Roberts, Iben Bach Pedersen, Jesper Hjortdal, William J Dupps; BIOMECHANICAL COMPARISON OF CONTRALATERAL FLAP-BASED AND NO-FLAP FEMTOSECOND LENTICULE EXTRACTION PROCEDURES USING INVERSE FINITE ELEMENT ANALYSIS . Invest. Ophthalmol. Vis. Sci. 2015;56(7 ):1110.
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No-flap lenticule extraction procedure has been introduced in order to conserve anterior stromal collagen fibers (RELEX SMILE Carl Zeiss Meditec, Germany). Prior studies using clinical corneal stiffness measurement devices were limited to differentiate the biomechanical impact of this procedure comparing to flap based procedures. This contralateral study aims at predicting the amount of weakening in collagen fibril elasticity induced by flap-based versus no flap lenticule extraction methods.
Patient specific tomography data were meshed using a custom meshing software consisting epithelium, flap/cap, wound and residual stromal bed. A hyperelastic, anisotropic, incompressible and depth dependent material formulation was utilized for the stroma. Inverse FE analyses were performed using Abaqus 6.11 and Matlab 7.8.0 with 15mmHg intraocular pressure followed by a forward analysis using 30mmHg IOP to better demonstrate the mechanical impact of each procedure. Two cases, one myopic and one myopic astigmatism,who received flap-based and no-flap treatments contra-laterally were simulated with their clinical treatment settings. Percentage weakening in the fiber stiffness within the flap region compared to the no-flap procedure was found by preforming inverse finite element (FE) study.
Flap based procedure demonstrated 68%, 71% surgically induced weakening within flap region for patient1 and patient2 respectively. Lower stress and deformation within the residual stromal bed were noted in the no-flap procedures as opposed to the flap-based procedures. Simulated refractive outcomes closely matched the 6month clinical follow-up topographies.
Based on the results of this study, the no-flap procedure demonstrated higher biomechanical stability following the surgery as opposed to the flap-based procedure when the same preoperative material properties were assigned to each contralateral eye.
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