LASIK Candidacy – Corneal Issues

The eye’s cornea is one of the body’s most sensitive areas. It is the clear front part of the eye which covers the iris and pupil, and behind them the lens and the clear fluid (aqueous fluid) that bathes it. Being curved and transparent, it is a type of lens and refracts (bends) light entering the eye. The crystalline lens refracts light further so that in a 20/20 eye, images focus clearly at the back of the eye, on the retina.

While the lens can change its curvature to bend light at differing angles, the cornea has a fixed curvature. LASIK works by modifying the corneal curvature, making it flatter or steeper, according to whether you are nearsighted (myopic) or farsighted (hyperopic). If you are astigmatic LASIK makes the cornea rounder to correct the somewhat oval shape of astigmatism.

Corneal Structure

There are no blood vessels in the cornea, as it must remain completely transparent to allow light into the eye. It receives nutrients from the aqueous fluid behind it and oxygen from the air. There are many tiny nerves with receptive endings which may overlap.

The cornea has five main layers, of which LASIK is concerned with two.

The corneal surface, the epithelium, which is thin and continually renewing its own cells. Tears keep it moist.
The middle layer, the stroma, which makes up about 90 percent of corneal thickness.

LASIK requires a corneal flap to be made and folded back to give the laser access to the stroma. The flap thickness includes the epithelium and part of the stroma. The LASIK laser modifies corneal curvature by removing a tiny amount of tissue according to your treatment plan. Then the flap is replaced and will heal by itself.

Corneal Thickness

To be a good LASIK candidate you need an average or better corneal thickness. There needs to be enough to allow for the flap, then the tissue removal, plus enough left in place to maintain the eyeball’s shape. Fluid inside the eye pushes against the eyeball periphery, which includes the cornea. A good LASIK candidate has a central corneal thickness of 0.5 to 0.6 millimeters or more. Pachymetry is a common method of measuring corneal thickness.

If LASIK were to be performed on a person with thinner corneas than that, there would be a risk of ectasia – forward bulging of the cornea which blurs vision.

Corneal Scarring

Anything on the cornea that interferes with incoming light will affect vision. A good LASIK candidate has a smooth, clear cornea. Scarring can be caused by keratitis (inflammation of the cornea) when it involves the cornea’s deeper levels. Eye injuries or trauma can also leave corneal scars. Any pre-existing scars would interfere with flap creation and laser corneal reshaping.

Dry Eyes

Many people have occasional dry eyes, perhaps after being out in the wind or after prolonged computer use. But if you have chronically dry eyes, known as Dry Eye Syndrome, you would not be a good LASIK candidate. Dry eyes are a temporary side effect of LASIK. Typically the dryness resolves by itself within two or three months and in the meantime moisturizing eyedrops are used.

Sometimes the dryness does not resolve, but becomes chronic. If you already had chronically dry eyes, this problem would now be compounded.

Severe Myopia

If you are myopic with a prescription of -6.0 diopters or higher, you would not be a good LASIK candidate. A myopic eye has a more steeply curved cornea than average. If you have severe myopia, the curvature would be too steep for safety in flap creation.

The implement used to cut a LASIK flap is called a microkeratome and it is a very precise, oscillating blade that your eye surgeon controls. It works very well for corneal shapes within a certain range, but could cut too deep a flap if the cornea is too steep.

It is not possible for most people to determine their own LASIK candidacy. A thorough eye examination with all appropriate tests is required. If you are interested in moving forward with laser vision correction, please contact a highly-qualified LASIK surgeon in your area.