الفهرس 
IntroductionOnly 14 pages are availabe for public view
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Abstract
The increased prevalence of myopia worldwide led to a subsequent increase in the performance of corneal refractive surgeries. Small incision lenticule extraction (SMILE) is a new paradigm for refractive surgery and considered to be the most recent development in femto-second laser–based techniques showing encouraging results in the treatment of myopia and myopia with mild to moderate astigmatic error with a high degree of safety, stability, predictability, and efficacy. Importantly, corneal biomechanics must be considered when planning to perform any kind of corneal refractive surgery, as the cornea will be altered structurally and inevitably altered biomechanically. Also, there is a risk of postoperative corneal ectasia after any refractive surgery. The emergence of non-invasive methods of studying corneal biomechanical changes after these procedures are useful in comparing different methods of refractive surgery. The Corvis ST (Oculus, Wetzlar, Germany) is a novel non-contact tonometer that allows investigation of the dynamic corneal response (DCR) to an air impulse.The CorVis ST uses a high-speed Scheimpflug camera to gathers 4330 frames per second within a 100 ms period, The camera could cover up to 8.5 mm of a cornea and provide excellent image resolution (640 × 480 pixels). therefore recording dynamic deformation of the cornea which allows the evaluation of corneal biomechanical properties. Also, it`s used to calculate the IOP value as well as CCT which then will be displayed on the built-in control panel in ultraslow motion. CorVis ST obtains corneal biomechanical parameters in 3 main stages. During the first stage the air puff is applied, and the cornea is subsequently flattened in the center. This corresponds to the first applanation. In the second stage the pressure continues up to the peak pressure at which the highest corneal concavity is achieved. Finally in the third stage the air puff pressure decreases, and the cornea will be returning to the baseline state passing through a second applanation just before it recovers its original shape. Those parameters as shown in Vinciguerra screening report include: DA ratio, Integ. Radius, ARTh, SP-A1 and CBI. These parameters can be used as an early indicator for evaluation of the effects of refractive treatments on the cornea. Although a considerable amount of the literature have discussed and compared the corneal biomechanical response following SMILE with other flap-based laser refractive procedures, fewer studies have targeted the impact of the corneal cap thickness on the postoperative biomechanical strength. The anterior one-third of the corneal stroma consists of an interwoven arrangement of collagen fibers, while the collagen fibers in the posterior two-thirds of the corneal stroma are arranged in distinct lamellae with a predominant vertical and horizontal arrangement. The collagen arrangement seems to be responsible for the depth-dependent exponential decrease in the corneal tensile strength, with the 40% anterior stroma being the strongest region, and the 60% posterior stroma being at least 50% weaker. In SMILE, a cap thickness of 100 to 130 μm is commonly used for myopic correction. Consequently, the stromal lenticule is usually extracted from within the stronger part of the cornea. The lamellar cut during cap creation may cause only a minor reduction in the corneal biomechanical strength, but the shape of the removed lenticule still requires that several collagen fibers are damaged during the procedure. Lenticle creation in deeper stromal layers is associated with a greater lenticule thickness required to provide a comparable refractive outcome resulting in a thinner residual stromal bed postoperatively. Also, lenticule removal in the deeper layers requires a deeper incision for lenticule extraction, which damages the corneal lamellae. On the other hand, removal of the lenticule in the deeper layers (thicker cap) may better preserve the corneal biomechanical strength than removal in the superficial layer. Furthermore, a thicker cap may also have other possible advantages, such as less disruption of the corneal nerve fibers integrity and better options for postoperative enhancements in the cap. Hence, the benefits of lenticule removal in the deeper corneal layers may counteract the disadvantage of a deeper incision in terms of the corneal biomechanical strength after SMILE. Our study included 40 eyes with moderate myopia with or without myopic astigmatism. Eyes will be randomly distributed into two groups each including 20 eyes. All eyes are treated by SMILE (group A with 100 μm cap thickness while group B with 120 μm cap thickness). All participants are evaluated preoperatively and 3 months postoperatively. This included history taking, UDVA, CDVA, cycloplegic refraction, manifest refraction, comprehensive slit lamp examination, fundus examination, evaluation of corneal topography by using Scheimpflug corneal tomography (Pentacam; Oculus gmbh, Wetzlar, Germany) and corneal biomechanics by using Corvis ST non-contact tonometer (Oculus, Wetzlar, Germany, Type 7200). Regarding the epidemiological-demographic characteristics of the studied groups there was no statistically significant difference between the two groups regarding age (P= 0.211) with the mean age of the participants was 25.100 ± 5.52 (Range= 19 – 34) in group I while the mean age of the participants was 23.00 ± 4.90 (Range= 19 – 32) in group II. There was only one male (10%) and 9 females (90%) in group I while there was 2 males (20%) and 8 females (80%) in group II with no statistically significant difference between the two groups regarding the gender (P= 0.376). Also, regarding the preoperative CDVA and postoperative UDVA, CDVA, manifest refraction SE, all the corneal tomographic parameters including CCT, Thinnest location, and Km there was no statistically significant differences between the two groups. Moreover, the preoperative corneal biomechanical parameters including DA ratio, ARTh, SP-A1 and CBI showed non-significant differences between the two groups with exception of the Integ. Radius parameter that showed a statistically significant difference (P= 0.020) between the two groups with the mean values of preoperative Integ. Radius was 7.08 ± 0.82 (Range= 5.8 – 8.5) in group I while the mean value was 6.53 ± 0.58 (Range= 5.6 – 7.6) in group II. Postoperatively only the DA ratio showed a statistically significant difference (P= 0.033) in the postoperative DA ratio values in group II (Mean= 5.42 ± 0.45 / Range= 4.7 – 6) as compared to group I (Mean= 5.66 ± 0.22 / Range= 5.3 – 6). We also found a negative statistical correlation between the postoperative CBI and both the preoperative and CCT in both groups.