الفهرس 
ANATOMY OF THE CORNEAOnly 14 pages are availabe for public view
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Abstract
K
eratoconus is a progressive non-inflammatory, degenerative disease that affects the integrity of the collagen matrix within the corneal stroma. Its hallmark is the formation of a localized cone-shaped ectasia, it is accompanied by thinning of the stroma in the area of the cone. This might cause irregular astigmatism and a steeper corneal curvature . An imbalance between degradative enzymes such as; lysosomal enzymes, cathepsins and matrix metalloproteinase-2 (MMP-2) and their inhibitors; Alpha-1 proteinase inhibitor, alpha-2 macroglobulin and tissue inhibitor of metalloproteinase-1 and 3 is now understood to be the reason behind the development of Keratoconus .
Due to the great variability among patients with keratoconus, it is vital to grade this disease in order to provide some general guidance for the clinician regarding the level of progression and the treatment options that can be offered. There are various classifications depending on which principal factors are considered. Traditionally, the most widely used classification is the Amsler-Krumeich (1946) scale. This scale is based primarily on keratometric criteria but also includes other factors, such as refraction and pachymetry .
Pentacam is a multipurpose, fast and non-contact device used to investigate the anterior segment of the eye. It combines a rotating Scheimpflug camera with a static camera to acquire multiple photographs of the anterior segment to generate the Scheimpflug images in three dimensions . When more than one of the following criteria is found, any of the above mentioned patterns is considered as frank KC, Forme Fruste KC, early stage KC, or at least a case of suspicion according to the severity and amount of parameters.
Until the last decade of the 20th century, the only surgical option for patients with keratoconus was keratoplastic procedures. However, nowadays there are numerous procedures that tend to stop progression of the disease and improve the patient’s visual function, which include corneal collagen crosslinking, Intra-corneal stromal implants and different modalities of keratoplastic procedures, including penetrating keratoplasty (PK) and deep lamellar keratoplasty (DALK).
Corneal Cross-linking (CXL), the term used to describe the formation of chemical bridges following a chemical reaction between proteins and other molecules. It was first performed by a group at the Dresden Technical University where they used Vitamin B2 (Riboflavin) and ultraviolet A to increase the formation of crossed links between the collagen fibers in the corneal stroma, producing a rigid effect capable of halting the progression of the ectasia .
This will generate various effects in the cornea, such as increased stiffness, changes in biomechanical and bioelastic behavior of the tissue and different visual, refractive, topographic and aberrometric changes in the patient. Two techniques are described for this procedure the (Epi-off) technique; involving mechanical removal of the corneal epithelium and the less invasive (Epi-on); with intact corneal epithelium, CXL is indicated in patients under 35 years of age, with visual acuity better than LogMAR 0.1 , pachymetry must be over 400 microns and keratometry readings less than 58 D. Pregnancy and breastfeeding ,Cornea with central opacity and serious dry eye syndrome are contraindications for the CXL procedure.
In our study, the mean age was 24.08 years old ± 5.93. 62.1% of patients were males. Mean preoperative spherical equivalent was - 5.64 ± 5.43 diopters, postoperative mean spherical equivalent was -4.16 D ± 4.87, with no statistically significant improvement in the refraction of the patients (P=0.454). Mean pre-operative cylinder was -3.64 D ± 2.33, while postoperatively the mean cylinder was -3.67D ± 2.27. We found statistically significant decrease in both Flat K reading (K1) and K mean (P= 0.01), with insignificant decrease in all the other values. The posterior keratometric values showed insignificant decrease in all measurements. Highly significant changes in Pachymeteric measurements as well as maximal K reading (Kmax) (P=0.001). Elevation indices showed almost no changes in both anterior and posterior corneal surfaces. Changes in the Pentacam keratoconus indices was only significant in ISV, Center Keratoconus-Index (CKI), IVA were statistically significant; showing a decrease in all values.