الفهرس 
IntroductionOnly 14 pages are availabe for public view
Abstract
The femtosecond laser (FSL) has revolutionized corneal and refractive surgery. It enables the creation of precise flaps with uniform thickness across the flap. The incidence of flap complications such as button-hole, epithelial abrasion, incomplete flap, free cap, and irregular cuts are substantially reduced with FSLs. However, there are some complications specific to FSLs such as OBL.
Our study aimed to study risk factors responsible for OBL formation in FSL-assisted LASIK. This retrospective study was conducted on 200 eyes of 101 patients who were eligible for FSL-assisted LASIK using FS200 laser machine in the period from January 2015 to January 2017. One hundred eyes had OBL and 100 eyes did not develop OBL.
The median age of the study group was 30 years ranged from 20 to 40 years old. Sixty-four patients of the studied sample were females and 37 patients of them were males.
Careful ocular examination including BCVA, IOP, anterior segment examination and posterior segment examination using indirect ophthalmoscopy and slit lamp biomicroscopy were done. Then all patients were examined by Pentacam for corneal tomography.
The presence or absence of OBL and data of the study were recorded and reviewed to study different risk factors.
For preoperative assessment, data including SE, corneal astigmatism, K1, K2 and CCT were reviewed.
The median of SE was -3.25 diopters (range from -10.75 to +3.25; IQR: +2.75). Regarding astigmatism the median of astigmatism was -1.05 diopters (range from -0.1 to -4.9; IQR: 1.08). Regarding corneal keratometry the mean flat keratometry (k1) was 42.95 diopters ± 1.54 SD (range: 38.8 - 46.9) while the mean steep keratometry (k2) was 44.22 diopters ±1.69 SD (range: 39.5 - 49.2).
Regarding preoperative CCT in relation to OBL formation among the studied 200 eyes, The median CCT was 545 µm (range: 484 – 600; IQR: 46). There was statistically significant difference between both groups (P-value = 0.03) being thicker in OBL group. The corneal pachymetry was more than 545 µm in 57 eyes of OBL group versus 42 eyes in non-OBL group.
For intraoperative assessment, data including flap diameters, flap thickness, corneal canal length offset, flap hinge angle and laser parameters were reviewed.
The mean corneal flap diameter was 8.76 mm ± 0.07 SD ranged from 8.60 to 9.0 mm, the mean flap thickness was 117.28 µm ± 5.22 SD ranged from 100 to 200 µm, the mean canal length offset was 0.54 mm ± 0.28 SD ranged from 0.10 to 1.00 mm and the mean of flap hinge angle was 49.96 ± 0.24 SD ranged from 49 to 51°.
Regarding laser parameters, bed cut spot separations and line separations ranged from 7.0 to 8.0 µm while spot separations and line separations in side cut ranged from 4.5 to 5.0 µm and pulse energy ranged from 0.67 to 0.84 µj in bed cut and side cut.
There were a statistically significant difference between both groups regarding preoperative CCT, corneal canal length offset and steep keratometry being higher in OBL formation group. Regarding regression analysis, CCT and corneal astigmatism were statistically significant independent predictor of OBL formation.