The cornea is the outermost layer of the eye. It's the clear, dome-shaped surface that covers the front of the eye. The cornea helps shield the rest of the eye from germs, dust and other harmful matter. The cornea functions like a window that controls and focuses the entry of light into the eye. It contributes between 65-75% of the eye's total focusing power.

The cornea is made up of five layers:

  • Epithelium- the outermost, protective layer
  • Bowman membrane- the strong second protective layer
  • Stroma- the thickest layer of the cornea. Made up of water, collagen fibers and other connective tissue
  • Decemet membrane- a thin, strong inner layer that is also protective
  • Endothelium- the innermost layer made up of cells that pump excess water out of the cornea

Diseases and disorders:

  • Corneal abrasion
  • Corneal dystrophy
  • Corneal erosion
  • Corneal laceration
  • Corneal ulcer
  • Fuchs' dystrophy

Corneal Transplants

A corneal transplant is needed if vision cannot be corrected satisfactorily with eyeglasses or contact lenses, or if pain cannot be relieved by medications or special contact lenses. There are many conditions that can affect the clarity of the entire cornea. For instance, trauma or injury to the cornea (causing scarring), Fuchs' dystrophy (causes corneal failure), and Keratoconus causes a steep curing of the cornea. Sometimes cataract surgery can cause corneal failure. A corneal transplant is done using a human donor cornea. Before a cornea is released for transplant, tests are done for viruses and other potentially infectious diseases. The cornea is also checked for clarity.

During a traditional full corneal transplant surgery (penetrating keratoplasty), a circular portion is removed from the center of the diseased cornea. A matching circular area is removed from the center of a healthy, clear donor cornea. It is then placed into position and sutured into place.

An endothelial keratoplasty (EK) involves just the abnormal inner lining of the cornea being removed. A thin disc of donor tissue containing the healthy endothelial cell layer is placed on the back surface of the cornea. An air bubble pushes the endothelial cell layer into place until it heals in an appropriate position.

With a lamellar corneal transplant procedure, the superficial layers of the cornea are removed and replaced with donor tissue. Sutures are used to secure the new tissue into place. Corneal transplants have proven to be very successful in patients with poor vision, or whose corneas have been significantly damaged because of corneal dystrophies.

© eyeSmart® Eye health information from the American Academy of Ophthalmology. The Eye M.D. Association.

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