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Eye Anatomy – The Cornea

The eye’s cornea is the clear front surface and is part of the eyeball wall. It connects to the white part (the sclera) at a junction known as the limbus; and connects to the iris (colored part) at a point called the angle. This is the same angle referred to in the term Acute Angle Glaucoma. The iris blends into the sclera through a mesh-like structure called the trabecular meshwork. See our Eye Anatomy Overview for more on the trabecular meshwork.

The cornea and sclera are made of similar tissue, except that the cornea is transparent and has no blood vessels. It must be completely clear to allow light into the eye unobstructed.

If you wear contact lenses, the cornea is where you are placing them. LASIK and its alternate treatments such as PRK are performed on the cornea, modifying its curvature to change the way it bends incoming light.

Aqueous Humor

Behind the cornea is the eye’s anterior chamber filled with fluid called aqueous humor (or just “aqueous”). This is how the cornea is nourished, since it lacks blood vessels. The aqueous bathes both the cornea and the lens, removing waste products as well as providing nourishment.

Three Main Corneal Layers

  1. Epithelium – the top protective layer. It is five cells thick (50 microns – one micron is one-thousandth of a millimeter). Epithelial cells are discarded and replaced continuously, which helps the eye to heal itself from any trauma or damage. The epithelium can regenerate itself entirely in about one week.
  2. Tears keep the epithelium moist and wash out foreign bodies like dust or eyelashes. There are nerve endings in the epithelium, which is why it is painful when something scrapes or hits the eye.

  3. Stroma – the layer that gives the cornea strength and keeps its curved shape. The stroma makes up about 90 percent of corneal thickness. It has several hundred sheets of collagen fibers, each just one to two microns thick. Because the fibers are equally spaced and regular, they maintain corneal transparency. They are embedded in a substance that holds water.
  4. Stromal cells do not register pain. They also do not replace themselves, which is why laser vision correction is done on this layer – the changes will be permanent.

  5. Endothelium – the layer that maintains equal water distribution in the cornea. If too much water builds up, the cornea starts to lose its transparency and impaired vision will result. The endothelium is just one cell thick and these cells pump out excess water. They are tightly packed and waterproof and form the cornea’s inner lining.

Endothelial cells cannot regenerate themselves. As we age, we slowly lose them at the rate of about ten percent each decade. Remaining cells enlarge to fill the gaps.

Corneal Thickness and the LASIK Flap

Laser vision correction works by removing very small pieces of corneal tissue to change corneal curvature. This in turn changes how the cornea bends (refracts) light that enters the eye. For more on how the eye focuses light, please see our page on The Lens.

The first step in a LASIK surgery is creation of a corneal flap. The purpose of the flap is to give the LASIK laser access to the stroma. Typically, the flap is the entire thickness of the epithelium plus a thin section of the stroma. Laser vision correction is then done on the stroma and the flap is replaced. It heals by itself, with the epithelium generating cells to reconnect it to the rest of the corneal surface.

To be a good LASIK candidate, you need to have a minimum thickness of cornea. There must be enough thickness to allow for the flap and the tissue removal, and still enough remaining to maintain the eye’s shape. The cornea can vary in thickness between about 500 and 600 microns, averaging about 555 microns.

In the early days of LASIK, it was sometimes performed on a person whose corneas were too thin. That can cause the complication of ectasia, where the cornea bulges forward, not having the strength to resist the outward pressure of the eye’s internal fluids. Ectasia gives blurred vision.

If you are not a good LASIK candidate, you may qualify for one of the alternative LASIK surgeries. PRK does not create a flap. Epi-LASIK and LASEK create thinner flaps using different implements.

For more information about the cornea, please visit our Cornea Transplant page.

Find out more about Corneal Topography.

Eye Anatomy Overview

Please visit our Ophthalmology Directory if you would like the name of an ophthalmologist in your area.

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