الفهرس 
Anatomy of Cornea and PathogenesisOnly 14 pages are availabe for public view
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Abstract
Keratoconus is a degenerative, non-inflammatory
disorder of the cornea, characterized by central and para-central
thinning and subsequent ectasia. This distortion of the corneal
shape, results in irregular astigmatism with associated reduction
in vision. It typically presents in adolescence and progresses in
a variable manner.
Keratoconus is a little-understood disease with an
uncertain cause, and its progression following diagnosis is
unpredictable. The mechanism of this extreme thinning appears
to be related to increased proteinase activity, along with
decreased proteinase inhibitors. Many biochemical and
histopathological changes occur in the cornea in case of
keratoconus which include epithelial changes as oedema of the
basal cells especially at the base of the cone, swelling and
fibrillar degeneration of Bowman’s layer, stromal thinning,
rupture of Descemet’s membrane and flattening of endothelial
cells.
Several methods of measuring the corneal shape have
been used. Corneal topography represents a significant advance
in the measurement of corneal curvature over keratometry.
Topography provides both a qualitative
evaluation of corneal curvature. New technologies have met
the demand for increased precision in evaluation of complex
corneal shapes. These include Placido disk imaging, three
dimensional topography, PAR (The posterior apical radius
imaging device), slit-scanning topography, Scheimpflug
imaging, ultrasound, and interferometric systems.
The management of keratoconus varies depending on the
state of progression of the disease. In very early cases,
spectacles may provide adequate visual correction, but because
spectacles do not conform to the unusual shape of the cornea
and the resultant induced irregular astigmatism, contact lenses
provide better correction. Contact lenses are the mainstay of
therapy in this disorder and represent the treatment of choice in
90% of patients. The type of contact lens used varies depending
on the stage of keratoconus. As the disease progresses rigid
(hard) contact lenses become the mainstay of treatment. In the
majority of eyes such lenses provide good visual rehabilitation.
Unfortunately, they are not the solution in all cases.
Discomfort and patient preference may limit the use of rigid
contact lens wear and in advanced cases fitting may be
problematic. Severe ectasia and central corneal scarring in
advanced keratoconus can significantly limit the amount of
visual rehabilitation achieved by rigid lenses.
For these reasons, between 10-25% of patients with
Keratoconus progress to a point where surgical intervention is
required. Surgical options include: Intra-corneal ring segment
insert (Intacs &Ferrara Rings), Riboflavin / Ultraviolet-A
corneal cross linkage (CR3), Lenticular (lens) refractive surgery
including refractive lens exchange with toric intraocular lenses
& Corneal transplantation (or grafting) including penetrating
keratoplasty and Lamellar keratoplasty.
The development of intra-corneal ring segments has
provided a surgical alternative to corneal transplantation in
some eyes with keratoconus. This technology has been
available for over 10 years and was initially developed for the
correction of low degrees of myopia, up to -3.0 diopters, with
some success. More recently, these ring segments have been
used to reduce the irregularity of the cornea and flatten the apex
of the cone in mild and moderate cases of keratoconus with
some reported success.
Intacs are inserted into the posterior stroma. The circular
intra-lamellar pockets for the rings are created either using a
specially designed vacuum lamellar dissector or with the
femtosecond laser. There is some debate as to whether one or
two intact segments should be inserted. Intacs are the treatment
of choice in the contact lens intolerant eye, with keratometry
less than 53 diopters and no central corneal scarring and should
be attempted before considering DALK (Deep anterior lameller
keratoplasty).
It must be stressed, that intra-corneal ring technology
does not offer a cure for the condition but can very often
produce a marked improvement in unaided and best corrected
visual acuity and allow eyes to be corrected with spectacles
and/or soft rather than rigid lenses.
The Ferrara Ring (FR) implant is mostly for keratoconic
patients of any age with an evolving condition and intolerance
to contact lenses or with sharp distortions in the cornea shape,
which usually occur after transplants. The main purpose of FR
is to reshape the cornea and thus reduce astigmatism, to
reinforce and stabilize the cornea and possibly delay or prevent
KC from progressing and to improve vision acuity.
Corneal collagen cross-linkage (CR3) using Riboflavin/
ultraviolet–A light is a new therapeutic modality which may be
the first available treatment to halt and stabilise the keratoconic
process. Its aims are to increase the biomechanical stability of
the corneal stroma, in terms of its tensile strength and its
resistance to enzymatic digestion, by inducing and increasing
cross-linkages between the stromal collagen fibres. The ability
to halt disease progression at the earliest stages of the condition,
when full visual rehabilitation can still be achieved with
spectacles and soft contact lenses, offers great hope for future
generations suffering with this not infrequent and often visually
devastating condition.
DALK negates the risk of endothelial rejection, improves
postoperative biomechanical corneal stability& should reduce
the risk of postoperative complications associated with
intraocular surgeries. Many techniques are used to separate the
stroma from Descemet’s membrane in lamellar keratoplasty
including corneal injection with air, saline to turn the stroma
opaque and so can be easily differentiated from Descemet’s
membrane.
Further refinements in operative techniques, together
with improvements in technologies, such as the implementation
of femtosecond lasers and mechanical microkeratomes for
DALK, will allow refinement of lamellar techniques and
improve the ease of performing these procedures for both
surgeons and patients alike.
Corneal transplant (penetrating keratoplasty) is the best
and most successful surgical option for keratoconus patients
who cannot tolerate contact lenses or are not adequately
visually rehabilitated by them. Central scarring may preclude
good vision from contact lenses, even when they are tolerated.
A patient with keratoconus has an approximately 10–20%
chance over his/her lifetime of needing a corneal transplant.
The outcomes of PK for keratoconus are generally very
good. Long-term studies have documented five and 10-year
graft survival rates of over 90% in primary transplants.
Increased rates of endothelial cell loss have been reported up to
17 years following PK, casting doubt on long-term graft
survival.
Conductive keratoplasty which entails applying
radiofrequency energy to the corneal stroma may address
significant astigmatism in eyes that cannot accept Intacs due to
their corneal thickness or extreme K readings.
LASIK has been used to treat myopic astigmatism in
patients with keratoconus. The initial visual results appeared
promising, but longer follow up revealed regression of the
refractive outcome in some cases .Excessive thinning of the
stromal bed together with the action of the intraocular pressure
may cause a progressive keratectasia manifesting months after
the LASIK.
While many surgical interventions for keratoconus are
still at an evolving stage, the use of techniques which are
additive (such as Intacs), mechanically and chemically stabilize
the cornea (riboflavin / UVA corneal collagen cross linkage),
assumed to be neutral (conductive keratoplasty) or that replace
the diseased corneal tissue (keratoplasty techniques) must be
deemed preferable to procedures that remove tissue (excimer
laser) and mechanically de-stabilize the cornea (astigmatic and
radial keratotomies), particularly in a condition where the
cornea is known to be thin, ectatic and mechanically unstable.
Combination of different modalities may be done as CR3
with Intacs, LASIK after PKP or and toric phakic intraocular
lenses for correction of residual myopia and astigmatism
following Intacs or PKP.
Thus it is important to emphasize that the development of
new surgical treatment modalities and the refinement of older
techniques with the introduction of new technologies offer the
promise of improved visual outcomes for future generations
with this condition.