الفهرس 
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Abstract
Cross linking (X-linking) of the cornea is a new curative approach to increase the biomechanical and biochemical stability of corneal tissue; biomechanical by increasing the interfibrillar linkages of the collagen and biochemical by increasing the resistance to enzymatic digestion. The aim of (his treatment is to create additional chemical bonds inside the corneal stroma by means of a highly localized photoploymerization while minimizing exposure to the surrounding structures of the eye (Spoerl et al., 1998).
Photo-polymerization usmg UV -light was found to be the most promising technique to achieve cross-links in connective tissue. Photoopolymerization is activated by means of a non-toxic and soluble photo mediator and a wavelength which is absorbed strongly enough to protect deeper layers of the eye (riboflavin-UVA technique). using UVA at 370nm, the photo sensitizer riboflavin, is excited into its triplet state generating so-called reactive oxygen species (ROS), being mainly singlet oxygen and to a much lesser degree super oxide anion radicals. The ROS can react further with various molecules inducing chemical covalent bonds bridging amino groups of collagen fibrils (type II photochemical reaction). This process leads to physical cross-linking of the corneal collagen fibers. The wave length of 370 nm was chosen because of an absorption peak of ribol1avin at this wavelength, while remaining below harmful DNA and retinal radiation levels (Spoerl et al., 2007).
Current indications for corneal cross-linking are corneal ectasia disorders such as keratoconus, pellucid marginal degeneration, and iatrogenic keratectasia after refractive lamellar surgery, and corneal melting that is unresponsive to conventional therapy.
Riboflavin ophthalmic solution is instilled several times [or 15- 20 minutes before irradiation with the proper syringe-dispenser. Then during the treatment with UV -A, Riboflavin solution is administered every 5 minutes [or an overall time of 30 minutes. Riboflavin associated to an UV-/\ rays source is suitable for the parasurgical treatment o[ progressive keratoconus and corneal ectasias. The administration of both Riboflavin and UV -A rays induces a decrease in keratoconus progression, thus preventing corneal transplantation (Spoerl et al., 1998).
The procedure is typically performed under sterile conditions using topical anesthetics, The surgeon debrides the central 7-9 mm of thc corneal epithelium with a blunt spatula Then the corneal penetration o[ riboDavin solution is checked under a slit lamp with blue light. continued administration of riboflavin solution every 3 minutes, The irradiating source is placed 10 cm from the cornea’s center and applied for 30 minutes using an ultraviolet-A double diode. Postoperatively, the eye wears a bandage contact lens to reduce the post-operative pain and to enhance epithelialization. Also the eye receives a topical antibiotic, artificial tears and non-steroidal antiiinflammatory DROPs until corneal re-epithelization is complete, followed by a 6-week course of a topical steroid. The epithelium usually heals in two to three days and the effect is noted within a couple of weeks.
A study was performed to quantify the effects o[ the treatment.
According to the study’s results, maximal cross-linking occurs within the anterior 300 /lm of the cornea the change and improvement in visual acuity was not parallel to changes in topographic variables. This may be due to remodeling of the collagen fibers following C3-riboflavin treatment, which provides clear vision but has little effect on topography and refraction. This contrasts with Intacs treatment, because the topographic improvement is dramatic, but subjective complaints about vision due to halo, glare, irregular astigmatism, or monocular diplopia persist.
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