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Cornea

The cornea is the transparent front part of the eye that covers the iris, pupil, and anterior chamber, providing most of an eye's optical power. The cornea has unmyelinated nerve endings sensitive to touch, temperature and chemicals; a touch of the cornea causes an involuntary reflex to close the eyelid. Because transparency is of prime importance the cornea does not have blood vessels; it receives nutrients via diffusion from the tear fluid at the outside and the aqueous humour at the inside and also from neurotrophins supplied by nerve fibers that innervate it.

The cornea is the only part of a human body that has no blood supply; it gets oxygen directly through the air. The cornea is the fastest healing tissue in the human body, thus, most corneal abrasions will heal within 24-36 hours.



LAYERS OF THE CORNEA

Unlike the corneas of cats, dogs, and other carnivores that have four layers, the human cornea has five layers. From anterior to posterior they are:

1. Corneal epithelium: a thin epithelial multicellular layer of fast-growing and easily-regenerated cells, kept moist with tears. Irregularity or edema of the corneal epithelium disrupts the smoothness of the air-tear film interface, the most significant component of the total refractive power of the eye, thereby reducing visual acuity.

2. Bowman's layer (also erroneously known as the anterior limiting membrane, when in fact it is not a membrane but a condensed layer of collagen): a tough layer that protects the corneal stroma, consisting of irregularly-arranged collagen fibers. This layer is absent in carnivores.

3. Corneal stroma (also substantia propria): a thick, transparent middle layer, consisting of regularly-arranged collagen fibers along with sparsely populated keratocytes. The corneal stroma consists of approximately 200 layers of type I collagen fibrils. There are 2 theories of how transparency in the cornea comes about:

Theory 1 The lattice arrangements of the collagen fibrils in the stroma. The light scatter by individual fibrils is cancelled by destructive interference from the scattered light from other individual fibrils.

Theory 2 The spacing of the neighboring collagen fibrils in the stroma must be < 200 nm for there to be transparency.

4. Descemet's membrane (also posterior limiting membrane): a thin cellular 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 and solute transport between the aqueous and corneal stromal compartments. (The term endothelium is a misnomer here. The corneal endothelium is bathed by aqueous humour, not by blood or lymph, and has a very different origin, function, and appearance from vascular endothelia.)



SURGICAL PROCEDURES OF THE CORNEA

Various refractive eye surgery techniques change the shape of the cornea in order to reduce the need for corrective lenses or otherwise improve the refractive state of the eye. In many of the techniques used today, reshaping of the cornea is performed by photoablation using the excimer laser.

If the corneal stroma develops visually significant opacity, irregularity, or edema, a cornea of a deceased donor can be transplanted. Because there are no blood vessels in the cornea, there are also few problems with rejection of the new cornea.

There are also synthetic corneas (keratoprostheses) in development. Most are merely plastic inserts, but there are also composed of biocompatible synthetic materials that encourage tissue in-growth into the synthetic cornea, thereby promoting bio-integration.

Dr. Lee Webster is now performing the latest, advanced procedure in corneal transplantation. This cutting-edge procedure is named Descemets Stripping Endothelial Keraroplasty, or DSEK for short. DSEK has replaced the traditional full thickness corneal transplant, known as Penetrating Keratoplasty, in a number of patients. A countless number of these patients have a corneal disease involving only the most inner layer of the cornea, the DSEK procedure replaces just the abnormal layer, leaving the remainder of the healthy cornea untouched; where the traditional procedure replaced the whole cornea in its entirety. The advantages of DSEK are numerous and significant. One, the procedure is safer, well tolerated, and performed under local anesthesia. Two, the healing time is much faster, as DSEK patients often regain vision in a mere few weeks compared to many months or even a year with the traditional procedure. And last, the DSEK procedure has only a slight effect on the patient’s glasses prescription while the traditional procedure may cause dramatic changes and the induction of tremendous astigmatism.


NON-SURGICAL PROCEDURES OF THE CORNEA

Orthokeratology is a method using specialized hard or rigid gas-permeable contact lenses to transiently reshape the cornea in order to improve the refractive state of the eye or reduce the need for eyeglasses and contact lenses.

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