The cornea is the clear front surface of the eye. It lies directly in front of the iris and pupil, and it allows light to enter the eye. Viewed from the front of the eye, the cornea appears slightly wider than it is tall. This is because the sclera (the "white" of the eye) slightly overlaps the top and bottom of the anterior cornea.

Cornea

The horizontal diameter of the cornea typically measures about 12 millimeters (mm), and the vertical diameter is 11 mm, when viewed from the front. But if viewed from behind, the cornea appears circular, with a uniform diameter of approximately 11.7 mm. This makes the cornea about two-thirds the size of a dime.

The center thickness of the average cornea is about 550 microns, or slightly more than half a millimeter. The cornea has five layers. From front to back, these layers are:

  • The corneal epithelium. This outer layer of the cornea is five to seven cells thick and measures about 50 microns — making it slightly less than 10 percent of the thickness of the entire cornea. Epithelial cells are constantly being produced and sloughed off in the tear layer of the surface of the eye. The turnover time for the entire corneal epithelium is about one week.
  • Bowman’s layer. This is a very thin (8 to 14 microns) and dense fibrous sheet of connective tissue that forms the transition between the corneal epithelium and the underlying stroma.
  • The corneal stroma. This middle layer of the cornea is approximately 500 microns thick, or about 90 percent of the thickness of the overall cornea. It is composed of strands of connective tissue called collagen fibrils. These fibrils are uniform in size and are arranged parallel to the cornea surface in 200 to 300 flat bundles called lamellae that extend across the entire cornea. The regular arrangement and uniform spacing of these lamellae is what enables the cornea to be perfectly clear.
  • Descemet’s membrane. This very thin layer separates the stroma from the underlying endothelial layer of the cornea. Descemet’s (pronounced “DESS-eh-mays”) membrane gradually thickens throughout life — it’s about 5 microns thick in children and 15 microns thick in older adults.
  • The corneal endothelium. This is the innermost layer of the cornea. The back of the endothelium is bathed in the clear aqueous humor that fills the space between the cornea and the iris and pupil. The corneal endothelium is only a single layer of cells thick and measures about 5 microns. Most of the endothelial cells are hexagonal (six-sided), but some may have five or seven sides. The regular arrangement of these cells is sometimes called the endothelial mosaic.

Cornea Function

As already mentioned, the clear cornea allows light to enter the eye for vision. But it has another very important function as well — the cornea provides approximately 65 to 75 percent of the focusing power of the eye. The remainder of the focusing power of the eye is provided by the crystalline lens, located directly behind the pupil.

Most refractive errors — nearsightedness, farsightedness and astigmatism — are due to a less-than-optimal curvature or symmetry of the cornea. Presbyopia, on the other hand, is due to an aging change in the crystalline lens.

In addition to allowing light to enter the eye and providing most of the focusing power of the eye, individual parts of the cornea have specialized functions:

Corneal epithelium – The corneal epithelium provides an optimal surface for the tear film to spread across the surface of the eye to keep it moist and healthy and to maintain clear, stable vision.

Bowman’s layer – The dense nature of Bowman’s layer helps prevent corneal scratches from penetrating into the corneal stroma. Corneal abrasions that are limited to the outer epithelial layer generally heal without scarring; but scratches that penetrate Bowman’s layer and the corneal stroma typically leave permanent scars that can affect vision.

Corneal endothelium – The single layer of cells that forms the endothelium maintains the fluid content within the cornea. Damage to the corneal endothelium can cause swelling (edema) that can affect vision and corneal health.

Cornea Problems

A number of conditions can affect the cornea. Among the more common corneal problems are:

Arcus senilis – As people get older, a white ring often develops in the periphery of the cornea. This is called arcus senilis (also called corneal arcus), and it’s the most common aging change in the cornea. Arcus senilis typically is separated from the limbus by an area of clear cornea. The white ring — which is composed of cholesterol and related compounds — can be barely noticeable or very prominent.
In older individuals, corneal arcus typically isn’t related to blood cholesterol levels; but if it occurs in a person under age 40, blood tests should be performed to check for hyperlipidemia (abnormally high concentration of fats or lipids in the blood).

Corneal abrasion – A scratched cornea can be very painful and can lead to an eye infection.

Chalazion – A chalazion is a fibrous growth that starts on the outer sclera of the eye but can encroach upon the cornea, causing irritation, vision problems and disfigurement of the front of the eye.

Dry eyes – Though the cause of dry eyes typically begins in the tears gland and eyelids, it can lead to damage of the corneal epithelium, which causes eye discomfort and vision disturbances.

Corneal ulcer – A corneal ulcer is a serious abscess-like infection of the cornea that can lead to significant pain, scarring and vision loss. 

Corneal dystrophy – A dystrophy is a weakening or degeneration of a tissue. The most common corneal dystrophy — called Fuch’s dystrophy — affects the corneal endothelium, causing corneal swelling, foggy vision, light sensitivity and other problems.

Acanthamoeba keratitis – This is a very serious and painful corneal infection that can cause significant pain and vision loss. 

Fungal keratitis – This is another dangerous corneal infection that (like Acanthamoeba keratitis) tends to affect contact lens wearers more often than people who wear glasses.

Keratoconus – This is a thinning and deformation of the cornea that causes vision problems that can’t be corrected with regular eyeglasses or contact lenses. In some cases, vision problems from keratoconus can be corrected with scleral contact lenses or hybrid contacts. But in severe cases, a cornea transplant may be required.

Cornea transplant

A cornea transplant replaces diseased or scarred corneal tissue with healthy tissue from an organ donor. There are two main types of cornea transplants: Traditional, full thickness cornea transplant (also known as penetrating keratoplasty, or PK) and back layer cornea transplant (also known as endothelial keratoplasty, or EK).

A graft replaces central corneal tissue, damaged due to disease or eye injury, with healthy corneal tissue donated from a local eye bank. An unhealthy cornea affects your vision by scattering or distorting light and causing glare and blurred vision. A cornea transplant may be necessary to restore your functional vision.

Corneal eye disease is the fourth most common cause of blindness (after cataracts, glaucoma and age-related macular degeneration) and affects more than 10 million people worldwide.

When Do You Need A Cornea Transplant?

A healthy, clear cornea is essential for good vision. If your cornea is damaged due to eye disease or eye injury, it can become swollen, scarred or severely misshapen and distort your vision.

A corneal transplant might be required in cases of conditions such as trichiasis, where eyelashes turn inward and rub against the surface of the eye, causing scarring and vision loss.
A cornea transplant may be necessary if eyeglasses or contact lenses can’t restore your functional vision, or if painful swelling can’t be relieved by medications or special contact lenses.
Certain conditions can affect the clarity of your cornea and put you at greater risk of corneal failure. These include:

  • Scarring from infections, such as or.
  • Scarring from trichiasis, when eyelashes grow inwardly, toward the eye, and rub against the cornea.
  • Hereditary conditions
  • Eye diseases
  • Thinning of the cornea and irregular corneal shape (such as with keratoconus).
  • Rare complications
  • Chemical burns of the cornea or damage from an accident
  • Excessive swelling (edema) of the cornea.
  • Graft rejection following a previous corneal transplant.
  • Corneal failure

Are You A Cornea Transplant Candidate?

A cornea transplant is performed to improve the function of the cornea and improve vision. If pain is caused by a significantly diseased or damaged cornea, a cornea transplant may relieve that symptom.
With these factors in mind, you also should consider several important questions before you decide to undergo a corneal transplant:

  • Does your functional vision impede your job performance or your ability to carry out daily activities?
  • Can your vision be corrected with special or other less invasive measures?
  • How will the cost of cornea transplant surgery affect your financial situation if your does not cover everything from pre-screening to post-operative consultations?
  • Are you able to take enough time off from work or school (up to six months to a year in some cases) to recover properly?

All of these questions, in conjunction with a thorough screening and consultation with your eye doctor, must be carefully considered before you make the final decision to have a corneal transplant.

Before The Procedure

Once you and your eye doctor decide a cornea transplant is the best option for you, your name is placed on a list at a local eye bank. You may need to wait a few days to weeks for suitable tissue from a donor eye to become available for a corneal transplant.

Before a donor cornea is released for use in transplant surgery, it is checked for clarity and screened for the presence of any diseases such as hepatitis and AIDS, Only corneas that meet these stringent guidelines are used in corneal transplant surgery to ensure the health and safety of the graft recipient.

During The Cornea Transplant

Your eye surgeon will first administer either local or general anesthesia, depending on your health, age, eye injury or disease, and whether or not you prefer to be asleep during the procedure. If local anesthesia is used, an injection is made into the skin around your eye to relax the muscles that control blinking and eye movements, and eye drops are used to numb your eye. You will be awake during the procedure and most people don’t report any discomfort. After the anesthesia has taken effect, an instrument called a lid speculum is used to keep your eyelids open. Your surgeon then measures the affected corneal area to determine the size of the donor tissue needed.corneal transplant.

Before a donor cornea is released for use in transplant surgery, it is checked for clarity and screened for the presence of any diseases such as hepatitis and AIDS, Only corneas that meet these stringent guidelines are used in corneal transplant surgery to ensure the health and safety of the graft recipient.

During The Cornea Transplant

During traditional corneal transplant surgery, or penetrating keratoplasty (PK), a circular button-shaped, full-thickness section of tissue is removed from the diseased or injured cornea using either a surgical cutting instrument called a trephine or a femtosecond laser
femtosecond which creates bursts of laser energy at an extremely fast rate measured in terms of a unit known as a femtosecond (one quadrillionth of a second). These ultra fast energy pulses precisely target and break apart tissue or other substances at a molecular level, without damaging adjacent areas.

A matching “button” from the donor tissue is then positioned and sutured into place. The sutures (stitches) remain in place typically for a year or more after surgery. Lastly, a plastic shield is placed over your eye to protect it during healing.

Penetrating keratoplasty surgery generally takes one to two hours and most procedures are performed on an outpatient basis, meaning you can go home a short while after the surgery (though you will need someone to drive you home).

Endothelial keratoplasty –  In the past decade, a newer version of corneal transplant surgery called endothelial keratoplasty (EK) has been introduced for certain corneal conditions. Endothelial keratoplasty selectively replaces only the innermost layer of the cornea (endothelium) and leaves the overlying healthy corneal tissue intact. The endothelium controls the fluid balance in the cornea, and if it becomes damaged or diseased, this can cause significant corneal swelling and loss of vision.

In EK, the surgeon makes a tiny incision and places a thin disc of donor tissue containing a healthy endothelial cell layer on the back surface of your cornea. An air bubble is used to position the new endothelial layer into place. The small incision is self-sealing and typically no sutures are required.

Who Can Be A Corneal Tissue Donor?

How old is too old to donate eye tissue for a corneal transplant?
A decade ago, it was common for surgeons to reject corneas from people over 65 years old. But the Cornea Donor Study has concluded that corneas from people aged 34-71 are likely to remain healthy for most recipients after 10 years, with a success rate of 75 percent.

In the United States, about three-quarters of corneas come from this age range, with one-third from people aged 61-70.

Since corneas from people under age 34 do perform better in corneal transplants, it has been suggested that younger people should receive these younger corneas.

Recovering From A Cornea Transplant

Total cornea transplant recovery time can be up to a year or longer. Initially, your vision will be blurry for the first few months — and in some cases may be worse than it was before — while your eye gets used to its new cornea.

As your vision improves, you gradually will be able to return to your normal daily activities. For the first several weeks, heavy exercise and lifting are prohibited. However, you should be able to return to work within a week after surgery, depending on your job and how quickly your vision improves.
Steroid eye drops will be prescribed for several months to help your body accept the new corneal graft, as well as other medications to help control infection, discomfort and swelling. You should keep your eye protected at all times by wearing a shield or a pair of eyeglasses so that nothing inadvertently bumps or enters your eye.

If stitches were used in your surgery, they usually are removed three to 17 months post-surgery, depending on the health of your eye and the rate of healing. Adjustments can be made to the sutures surrounding the new corneal tissue to help reduce the amount of astigmatism resulting from an irregular eye surface. As with any type of surgery, always follow the instructions of your eye surgeon to help minimize corneal transplant complications and expedite healing.

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