Abstract
BackgroundEvaluating the corneal biomechanical changes using the Ocular Response Analyzer and the Corvis ST in eyes with incision lenticule extraction (SMILE) and laser assisted in situ keratomileusis (LASIK).MethodsThis is a retrospective study that included 50 eyes equally divided into two groups. The first group included eyes that underwent SMILE procedure using VisuMax® 500 kHz laser system (Carl Zeiss Meditec, Jena, Germany) and the second group included eyes that underwent LASIK procedure using the EX500 Allegretto excimer laser platform (Wavelight GmbH, Erlangen, Germany). The Ocular Response Analyzer (ORA) and the Corvis ST (CST) measured the corneal biomechanical changes before and after the procedures.ResultsThe ORA showed significant decrease of corneal hysteresis (CH) and corneal resistance factor (CRF) in both groups postoperatively. The percentage of change of CH and CRF were found to be significantly higher in group II. There was no significant difference in the IOP with the ORA and the CST pre and postoperatively in either group. Using CST, the deformation amplitude and HC peak distances increased significantly in both groups. It was also noted that the mean percentage of change of the deformation amplitude was nearly five times higher in group II than group I.ConclusionBoth LASIK and SMILE substantially decreased the corneal biomechanical properties with greater reduction in the LASIK group.
Highlights
Evaluating the corneal biomechanical changes using the Ocular Response Analyzer and the Corvis ST in eyes with incision lenticule extraction (SMILE) and laser assisted in situ keratomileusis (LASIK)
Corneal hysteresis Corneal resistance factor Intraocular pressure corneal thickness compensated Is the intraocular measurement based on A1 time Is the time from starting until first corneal inward applanation Time from starting until highest corneal concavity is reached Time from starting until second corneal outward applanation Cord length of applanated cornea during A1 Cord length of applanated cornea during A2 Distance between corneal peaks at point of highest concavity (HC) Maximum inward movement of corneal apex at point of HC Radius of curvature of the corneal concavity at point of HC
When comparing the mean percentage of change of corneal biomechanics between both groups, we found significant difference regarding the corneal hysteresis (CH) and corneal resistance factor (CRF) with greater reduction of the corneal biomechanics in the LASIK group
Summary
Evaluating the corneal biomechanical changes using the Ocular Response Analyzer and the Corvis ST in eyes with incision lenticule extraction (SMILE) and laser assisted in situ keratomileusis (LASIK). The cornea acts as a viscoelastic material having elements of both viscosity and elasticity. This means that the stress– strain relationship of the cornea is nonlinear, during both the loading and unloading phases [1]. A 25-millisecond air pulse generated by the machine applies pressure to the cornea to produce an initial applanation followed by progressive inward concavity of the cornea. This initial applanation triggers the cessation of the air puff but because of the inertia of the air puff piston, the air puff continues for fractions of a second
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