Download Cornea Implants

Cornea Implants
Topics:
– Structure of the cornea
– Implant materials for
the cornea
– History, current and
future research
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The Cornea
The cornea is the
transparent front part of
the eye that covers the
iris, pupil, and anterior
chamber.
Together with the lens, it
refracts light and
provides 80% of the
eye’s optical power
The focus of the cornea
is fixed; the lens provides
fine focus tuning
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Cornea Structure
• The cornea is transparent, thus it has no blood
vessels. It is the only part of the body that has
no blood supply
• It receives its oxygen directly through the air,
and its nutrients via diffusion from the tear fluid
on the outside and aqueous humour on the
inside, and slo from neurotrophins supplied by
the nerve fibers that innervate it.
• It is ~0.6 mm thick at the center, and 11.5 mm
in diameter
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Layers
1. Corneal Epithelium: a thin multicellular,
stratified tissue layer of fast-growing and easilyregenerated cells
2. Bowman’s layer: a tough layer, 8-14 mm thick,
that protects the corneal stroma, consisting of
irregularly-arranged collage fibers.
3. Corneal stroma: a thick, transparent middle
layer made up of ~200 regularly-arranged layers
of collagen fibers along with sparsely populated
keratocytes. 90% of the cornea's thickness is
made up of stroma.
4. Descemet’s membrane: a thin acellular layer
that serves as the modified basement membrane
of the corneal endothelium.
5. Corneal endothelium: a simple squamous or
low cuboidal monolayer of mitochondria-rich cells
responsible for regulating fluid an solute transport
between the aqueous and corneal stromal
compartments
For the meaning of the letters a- consult
Gray’s anatomy, subject #225 1006
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Innervation
• The cornea is one of the most sensitive tissues
of the body, and is densely populated with
sensory nerve fibers
• Nerve enter the cornea through the sclera,
episcleral and conjunctival levels.
• Receptive fields at each nerve ending are very
large and may overlap
• Corneal nerves of the subepithelial layer
converge and terminate near the apex of the
cornea in a spiral pattern
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Maintaining Transparency
• Upon death, or removal from the eye, the cornea
absorbs the aqueous humor, thickens and becomes
hazy.
• Transparency can be restored by placing it in a warm,
well-ventilated chamber at 31 oC, allowing fluid to leave
the cornea
• When placed in reservoirs of glycogen and sugar, the
cornea can remain transparent for at least a day
• The endothelium controls the ejection of fluid from the
cornea
• When the cornea is damaged, e.g. a viral infection, the
collagen used to repair the process is not regularly
arranged, leading to an opaque patch
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Diseases and disorders
Laser eye surgery uses
photoablation to reshape the
cornea
If the corneal stroma
develops opacity,
irregularity or edema, a
cornea of a deceased
donor can be transplanted.
Because there are no blood
vessels in the cornea, there
are few problems with
tissue rejection
Synthetic corneas are
being developed that
encourage tissue ingrowth
and promote biointegration
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Corneal disease is a leading cause of blindness. Examples
of corneal disease include infection such as ocular
herpes and corneal dystrophy. It is estimated that 10
million people worldwide have lost their sight to corneal
disease
Stanford scientists have developed a cornea made of two
interwoven polymer gels, similar to the materials used in
soft contact lenses
One layer is exceptionally strong, and the other can absorb
large amounts of water
This results in a transparent, highly permeable substance
with a water content similar to that of the natural cornea
High water content is needed in order to feed nutrients to
the cells within the eye – otherwise, they die
The artificial cornea is currently being tested in animals
Only one artificial cornea is approved in the U.S. Called
AlphaCor, the failure rate is ~20% per year
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