الفهرس 
Corneal Anatomy and PhysiologyOnly 14 pages are availabe for public view
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Abstract
T
he corneal stroma composed mainly of type I collagen fibrils with types III, V, and VI also found, these are woven into equidistant lamellae. By inducing cross links between neighboring collagen fibers the corneal rigidity improved.
Riboflavin and UV-A cross-linking of corneal collagen is a relatively new method that attempts to increase the biomechanical stability of the cornea by inducing additional cross-links between or within collagen fibers, (increase the formation of intra and interfibrillar covalent bonds by photosensitized oxidation).Riboflavin is used as a photomediator when exposed to UV-A light.
This method results in an increase in corneal tensile strength, with no medium term adverse effects on its normal architecture. Clinically, treated patients display improvement in both visual acuity and keratometric readings.
Since first introduction of Collagen Cross-linking (CXL) into ophthalmology 5 years ago, the use of riboflavin UVA collagen cross-linking has been gaining popularity in the treatment of progressive keratoconus, and current estimates suggest that 1000 procedures are performed each month worldwide
With this growing popularity, other novel applications are being proposed for the use of riboflavin UVA cross-linking in ophthalmology, such as the treatment of postoperative LASIK ectasia, strengthening of recalcitrant corneal ulcerations, halting of progressive axial myopia.
This treatment has also been used to treat infectious corneal ulcers with apparently favorable results and also in cases of bullous keratopathy.
CXL has also been used in combination with other treatments, such as intracorneal ring segment implantation and limited topography-guided photoablation, with some success.
Corneal CXL mediated by riboflavin and UVA appears to be a safe and efficacious procedure in halting the progression of keratoconus and iatrogenic ectasia. CXL reduces the corneal curvature, spherical equivalent refraction and refractive cylinder in eyes with corneal instability and progressive irregular astigmatism due to keratoconus and ectasia. The CXL technique is promising in treating corneal melting conditions or infectious keratitis because cross-linking would strengthen a collagenolytic cornea while UVA irradiation eliminates the infectious agent.
Bullous keratopathy (BK) is a visually-disabling corneal disorder and a state of abnormal corneal epithelial hydration caused by endothelial cell dysfunction. Without sufficient endothelial function, fluid accumulates in the extracellular spaces between collagen fibers and lamellae causing corneal swelling.
As the most important factor in the pathogenesis of bullous keratopathy is endothelial dysfunction leading to chronic corneal edema, stromal scarring, epithelial bullae and recurrent erosions, recently, an antiedematous effect of collagen cross-linking (CXL) using riboflavin and UVA (374 nm) has been suggested by an in vitro experiment that have been conducted in porcine eyes. Corneal cross-linking showed to be a safe procedure and potential therapeutic alternative for the treatment of corneal edema.
Corneal collagen cross-linking influence the swelling behavior of the cornea by its action on the collagen fibers. The compaction of the fibers of the stroma might increase the resistance of the tissue to osmotic and hydrostatic fluid accumulation.
Corneal CXL is being advocated of late as a new tool in the treatment for the temporary reduction in corneal edema in patients with bullous keratopathy. It has been found to improve corneal transparency, corneal thickness, and ocular pain after surgery.
On the other hand, former investigators suggested that corneal cross-linking (CXL) might have beneficial impact against corneal melting from bacterial and fungal enzymes because of the antimicrobial effect of UV-A radiation.
Corneal collagen cross-linking with riboflavin and UVA treatment of infectious keratitis can be an effective method to treat corneal infections resistant to medical therapy. It may help to decrease the need for urgent penetrating keratoplasty.
Management of resistant corneal infections by corneal collagen crosslinking with UVA and riboflavin seems to be promising but it needs further studies with larger number of patients. A longer period of follow up should be taken into the consideration and the response of different microbial infections to CXL should be addressed.