الفهرس 
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Abstract
Surgical correction of refractive errors progressively becomes popular, quick and effective for correction of myopia, hyperopia and astigmatism. Photo refractive keratectomy (PRK) has been introduced effectively and reliably for many years in the treatment of myopia. In the PRK, the laser is applied directly to the anterior corneal stroma without creating a flap, It is one of the developing procedures that have been regarding to avoid flap creation. Recently, small incision lenticule extraction (SMILE) has been constructed as a flapless procedure; it has been reported since 2011, for the treatment of myopia and astigmatism. An intra-stromal lenticule is cut by a femto second laser and manually extracted via a peripheral tunnel incision in the cornea leaving stroma overlying the lenticule intact. Now measuring corneal biomechanics is challenging field in modern ophthalmology, in the past it was not possible to measure corneal biomechanics in vivo with any device, however the mechanical instability is thought to be the initiating event of the disease. In 2005 Garbner et al, introduced a way of In-vivo measurements, Corneal dynamic imaging (CDI), a physical probe indents the cornea with simultaneous video-topography imaging of cornea. This enables the measurement of curvature changes over the entire corneal surface during indentation. Thereafter ultra high speed Scheimpflug camera imaging devices were introduced. Two devices are currently available for characterising the corneal biomechanical properties in a clinical setting; Ocular Response Analyzer (ORA) (Reichert, Buffalo, New York, USA) based on bidirectional applanation tonometry and Corvis -ST (Oculus Optikgeräte GmbH, Wetzlar, Germany) based on corneal deformation estimation using Scheimpflug imaging. Ocular response analyzer was used to assess corneal hysteresis (CH) and corneal resistance factor (CRF) to characterize changes in corneal biomechanics. The other device is Corvis –ST which produces air pulse on the cornea then measure and records the movements using a high-speed Scheimpflug video camera in real time. Corvis biomechanical index (CBI) included different corneal response parameters, as deformation amplitude ratio at 1 and 2 mm, applanation 1 velocity, slandered deviation of deformation amplitude at highest concavity, Ambrosios relational thickness to a horizontal profile and a novel stiffness parameter, It is highly sensitive and specific to differentiate healthy cornea from keratoconic one. The aim of our study is to compare the changes in corneal biomechanical properties before and after PRK and SMILE . Our study included 40 eyes of 20 patients who were randomly divided into two groups: group A included eyes that received PRK group B included eyes that received SMILE. Patients were evaluated preoperatively. This included history taking, slit lamp examination, Fundus examination and intraocular pressure, uncorrected distance visual acuity (UDVA, Corrected distance visual acuity (CDVA) using glasses, cycloplegic refraction. The CORVIS-ST (CST) was used to assess the corneal biomechanics. Patients were evaluated at one month after surgery using CORVIS-ST to assess corneal biomechanics and Pentacam, also after 6 months Patients were evaluated using CORVIS-ST and Pentacam. The two groups of the study included group A was 4 males (20%) and 16 (80%) females with average age26.6 ± 4.62 and group B was 8 males (40%) and 12(60%) females with average age25.9 ± 3.51 there was no statically significant difference between both groups. Difference In group A from pre to 6months post was: insignificant decrease in IOPb (p=0.495) and significant increase in A1velocity (P=045*), A2 velocity showed a statistically significant difference recorded (p=0.001*). Difference of D.A. Ratio from pre to 6 months was (p<0.001) and in integ. Radius was (p<0.001*), ARTH difference was (p<0.001*), SPA1 showed decrease with a statistically insignificant difference from pre to 6 months post-operatively (p=0.064). Corvis biomechanical index (CBI) showed an increase with a statistically significant difference from pre to 6months post-operatively (p<0.001*). Difference In group B from pre to 6months post was: (IOPb) decreased with a statistically significant difference (p=0.157*), A1velocity was increased with a statistically significant difference (P=0.006*). A2 velocity showed a significant difference (P=0.042*). D.A. Ratio (p=0.001*). integ. Radius (p=0.001*) ARTH (p=0.001*) SPA1 showed decrease with a significant difference (p<0.001*). Corvis biomechanical index (CBI) showed an increase with no significant difference from pre to 6 months post-operatively. (p=0.540). According to correlation with the residual CCT; we found that all the parameters were significantly correlated with the residual CCT except the stiffness parameter at the first applanation (SPA1) and intra ocular pressure corrected biomechanically IOPb in both group. This shows that IOP would be an indicator to predict the biomechanical properties of the cornea with independent of the tissue volume. The significant difference between two groups at 6 months after surgery considering IOPb=0.027*. Integ. Radius=0.001*. SPA1=0.003*. This difference makes Femto SMILE better than PRK procedure for more stability of the corneal biomechanics and possibly lowers the incidence of postoperative ectasia.