الفهرس 
Corneal BiomechanicsOnly 14 pages are availabe for public view
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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).
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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
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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
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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
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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
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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.