الفهرس 
• Corneal collagen crosslinking with riboflavinOnly 14 pages are availabe for public view
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Abstract
Corneal collagen becomes progressively cross-linked with age, and this can be regarded as a natural physiological ageing process. Increased cross-linking is associated with increasing rigidity, and this explains why the progressive ectasia seen in keratoconus occurs primarily in younger patients, and progression is not generally seen in later years.
Diabetic patients seem to be protected against progressive keratoconus, and this may be related to increased collagen cross-linking associated with hyperglycaemia, the so called ‘Maillard reaction’.
Cross-linking of the cornea can be achieved by a number of different chemical and physical processes, and the effect of UV light is particularly enhanced by the presence of riboflavin. The cornea is saturated with riboflavin, and illuminated with UV at a frequency of 370nm.
Riboflavin acts as a photo-sensitizer, undergoing fluorescent stimulation, and producing free-radicals which enhance the cross-linking process thus increasing corneal strength and integrity.
The osmolarity of corneal stroma is 380-420 mosmol/l, and the standard riboflavin / dextran solution is 400 mosmol/l, so application to the de-epithelialised surface does not lead to swelling of the cornea.
Riboflavin has peaks in its absorption spectrum at 270, 366, and 445nm. At 366nm there is absorption by pigmented tissues, but relative high transmission of DNA, so this frequency is the optimal one for cross-linking the cornea.
In addition to its action as a photo-sensitizer, the riboflavin has an important action as a shield. More than 90% of the UV-light is absorbed in the cornea and in addition the anterior chamber riboflavin reduces the UV intensity to a level that is a factor of 1000 smaller than the official safety level.
The current CXL inclusion criteria require a minimal stromal thickness (without the corneal epithelium) of 400 µm, as a safety margin. However, in many cases of advanced progressive keratectasia, patients still achieve a satisfying visual acuity with contact lenses, and a low minimal stromal thickness is the only parameter prohibiting safe CXL.
To treat these patients, the treatment parameters have been modified, using a hypo_osmolar riboflavin solution to induce stromal swelling and increase the stromal thickness prior to CXL in cases with preoperatively thin corneas. Hypo-osmolar 0.1% riboflavin solution is generated by diluting vitamin B2-riboflavin-5-phosphate 0.5% with physiologic salt solution.
Current indications for corneal cross-linking are corneal ectasia disorders such as keratoconus, pellucid marginal degeneration, iatrogenic keratectasia after refractive lamellar surgery.
Keratoconus is a degenerative, non-inflammatory corneal disorder, characterised by central and para-central stromal thinning and conical ectasia resulting in corneal distortion and impaired vision due to myopia and severe irregular astigmatism.
PMD is a bilateral, non inflammatory, peripheral corneal thinning disorder, it is characterized by a peripheral band of thinning of the inferior cornea. The cornea in and adjacent to the thinned area is ecstatic.
Post Lasik keratectasia is the progressive steepening and thinning of the cornea that reduces uncorrected and often BCVA, it remains one of the most insidious complications after LASIK.
The treatment of these disorders had only been optical rehabilitation with RGP contact lenses, intacs or keratoplasty. None of which can halt the progression of the disease.
Recently, CXL has also been used for the treatment of corneal melting and ulcers, it significantly increases resistance to collagenase, pepsin, and trypsin digestion especially in the anterior half of the cornea, in addition to the increase in biomechanical stability.