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Abstract The structural characteristics of the cornea facilitate its essential functions, specifically to serve as both a transparent barrier and the predominant refractive element of the eye. The cornea is formed of keratocytes which represent the predominant cellular components of the corneal stroma, collagen that constitutes more than 70% of the dry weight of the cornea. And proteoglycans forms the major component of ECM and located in the spaces among major collagen fibers in the stroma of the cornea. The orientation of successive collagen fibril layers throughout the entire cornea is an important factor determining the mechanical properties of the cornea. The metrics of biomechanical properties of the cornea is evaluated by stress strain relationship which in biological tissues typically exhibit a nonlinear response named Young’s modulus of elasticity. The constitutive representations of the cornea can be useful in predicting the corneal biomechanical response to various types of surgical manipulations or to controlled deformation in order to measure intraocular pressure (IOP). Summary 126 They include viscoelastic and hydration models and finite element models. There are various methods of clinical testing of corneal biomechanical properties these include: The Ocular Response Analyzer (ORA), the Corneal Visualization Scheimpflug Technology (Corvis ST), dynamic corneal surface topography, swept source ocular coherence tomography (ssOCT) combined with an air puff, brillouin optical microscopy, quantitative ultrasonic spectroscopy (QUSi), corneal transient elastography (CTE), radial shearing speckle pattern interferometry (RSSPI), dynamic corneal imaging (DCI) , optical coherence tomography elastography. Collagen cross linking is commonly used to treat keratectasias such as keratoconus, corneal ectasia after refractive surgery, and pellucid marginal degeneration (PMD) and works by halting the progression of the condition. However, the anti-edematous and antimicrobial properties can be helpful in the auxiliary treatment of bullous keratopathy and infectious keratitis. Management of keratoconus, PMD and post-surgery ectasia depends on their severity and the extent of irregular astigmatism. Mild cases are correctable with spectacles and Summary 127 soft toric contact lenses. However, with progressive disease, the cornea becomes more irregular and rigid gas permeable lenses are required. In 15-20% of keratoconic patients, surgery, typically keratoplasty, becomes necessary, as a result of contact lens intolerance, corneal scarring and thinning. None of these interventions, while often successful in terms of visual rehabilitation, treat the underlying causes of kerectasia and its progression. It is only with the advent of corneal collagen cross-linking (CXL) that a hope to slow, stop or even to a limited extent reverse keratoconus is started. Riboflavin has a modest affinity for nucleic acid and its absorption of UV-A leads to the oxidation of guanine bases, thus preventing the replication of the viral and bacterial genome. This effect is synergistic with any direct antimicrobial effect of UV-A irradiation itself and with any damage to microbial cell membranes and DNA caused by oxygen radicals. The antimicrobial efficacy of the combination of riboflavin and UV-A against a range of common bacterial pathogens causing infectious keratitis has now been well demonstrated in vitro. Also the treatment responses of the infectious ulcers indicate that photochemically activated riboflavin could be used for future management of infectious keratitis, allowing for less Summary 128 frequent application of topical antibiotics and fewer patient visits. Use of the method might reduce the frequency of complications associated with corneal infections and increase the healing rate in the treatment of microbial keratitis. Corneal collagen cross-linking has also been suggested as a treatment for corneal oedema. This concept is supported by changes in the hydration behaviour of the porcine cornea after CXL and the observation that stromal compaction follows CXL in a similar experimental model. Corneal collagen cross-linking with riboflavin is a method to increase the biomechanical stability of the cornea by inducing additional cross-links between or within collagen fibers using UV-A light and riboflavin as photomediators. The first patients were treated in 1998 by Prof. Theo Seiler and Prof. Eberhard Spoerl at the University of Dresden, Germany. Currently, the classical corneal collagen cross-linking (CXL) protocol is strictly formulated. Various new methods for epithelial disruptions were introduced to the classical technique. Current treatment protocols utilize UVA energies of 3mW/cm2 and require 30 min of UVA exposure to achieve the desired clinical effect. It has been established that by Summary 129 increasing the UVA fluence while simultaneously reducing the exposure time as accelerated type of CXL. This is then used with LASIK patients as an Xtra CXL procedure. The news in CXL treatment is represented by the possibility of realizing cross-linking keeping the epithelium unaltered. Combination of the riboflavin drops with a tense-active substance, have a more efficient penetration to the cornea. This substance act as a vector for riboflavin, with a double effect: Reaching the cornea and filling the epithelium, contributing so far to its strengthening. Riboflavin administration can also be done transepithelial within the stroma via a femtosecond laser created pocket or after ICRS insertion. Another non-invasive procedure called iontophoresis where small electric current is applied to enhance the penetration of an ionized substance into a tissue without epithelial removal. “Athens protocol” introduced by Kanellopollus where CXL and topography- guided surface ablation is performed in the same session rather than sequentially over time offers a safe and effective approach for normalizing Summary 130 the cornea and enhancing visual function in eyes with ectatic conditions. There are several studies have been conducted in different cornea centers that provide clinical data to support the efficacy and safety of the CXL procedure. Several long-term and short-term complications of CXL have been studied and documented. |