Laser Assisted In-situ Keratomelusis

In LASIK a flap is created by making a cut through the middle layer (called the “stroma”) of the cornea usually between 110 and 180 microns deep. This cut is created either by a mechanical device with a cutting blade called a “microkeratome” or by using a laser called the Femtolaser.

LASIK is also performed using a drop of anesthetic: Some patients report mild discomfort if a microkeratome is used. Because of this and because of increased safety and accuracy, using the Fentolaser has become the preferred method to create the cornea stromal flap. A small section at the superior or nasal side of the flap is left uncut and attached to the cornea stroma. The flap is then folded back at the stromal attachment to expose the cornea stromal cut surface. The excimer laser (not the Femtolaser used to create the flap) is then used to remove and reshape the exposed cornea stromal surface for the degree of vision correction desired. The flap is folded back and repositioned over the treated stromal surface making sure it is positioned correctly as the flap usually adheres because of osmotic forces after a few minutes.

It is important to note the final thickness of the cornea available for laser vision correction is the total thickness of the cornea minus the thickness of the flap created by the microkeratome. Even though the flap is replaced, it does not add sufficiently to the strength of the cornea to be included in the total thickness of the cornea to determine the maximum possible amount of laser vision correction. The higher the degree of nearsightedness, the more tissue must be removed with the laser.

Because the laser must remove more tissue for people with high degrees of nearsightedness, the laser surgeon will usually attempt to make the flap thinner for patients who are very nearsighted. This leaves more tissue available to be ablated by the excimer laser.

The exact residual thickness which must be left after the laser treatment, for visual stability and safety, varies among individuals. Most ophthalmologist will not use LASIK if the calculated residual thickness of the cornea will be less than 250 microns. One study reports a 2.5% incidence of cornea ectasia (central bulging) with cornea instability if residual thickness is 250 microns or less. The excessive thinning may cause irregular astigmatism with poor and fluctuating vision, and require a cornea transplant.Other complications of LASIK than the risk of ectasia are generally also related to the stromal flap. (Note again, the important distinction that these complications occur in LASIK because this is a stromal flap, not an epithelial flap as in LASEK.)

Some of the more common complications of LASIK are:

• Button holing of the central flap (an unintended hole or tear is created in the flap by the microkeratome)
• A displaced flap, in which either at the time of replacement after treatment with the laser, or from accidental mechanical trauma, even years after the original surgery, the flap is not lined up with the margins of the original incision on the cornea.
• Creation of an actual free stromal flap or cap in which the hinged attachment of the flap is severed or torn off with the possibility of complete loss of the stromal flap. (Again either at the time of the LASIK surgery or from mechanical trauma up to years later.)
• Inflammation in the incision of stromal layer after the stromal flap is replaced. This may be caused by an infection or simply inflammation of unknown cause (called DLK or disseminated lamellar keratitis). Either infection in the incision or DLK may lead to melting of the cornea and even loss of the eye.
• Dry eye problems usually lasting only a few months but which may be permanent. Deeper nerves in the stromal layer of the cornea (which carry the stimulus for tearing) are severed by the microkeratome.
• Irregular astigmatism from various problems with the flap such as “striae” or folds in the flap resulting in poor vision which may be very difficult or impossible to correct.
• Epithelial ingrowth—the surface cells of the cornea grow and migrate into the incision between the underside of the stromal flap and the stromal bed of the cornea after the flap is replaced. This may lead to melting of the cornea tissue and even loss of the flap with severe scarring of the cornea. Lifting the flap and removal by scraping these cells off may be necessary multiple times with all the risks of tearing the flap, infection, etc. each time.
• Possibly more risk of retina detachment. (A risk which is already increased for nearsighted eyes and may be related to the suction exerted on the eye by the microkeratome.)

Thus, in LASIK, the central thickness of the cornea which will remain after laser vision correction is an important factor for nearsighted eyes. It is not a limiting concern for farsightedness. This is because in farsightedness, the outer part of the cornea is much thicker than the center part of the cornea and the outer part of the cornea is treated more than the center when treating a farsighted eye (In contrast to nearsightedness).

Despite these potential and real complications, LASIK is the most popular refractive procedure in the United States because of the relatively rapid recovery and the relative comfort the first few days after the procedure is performed. Most patients do not have significant post LASIK discomfort. However, even with LASIK it is important to understand that vision usually requires time, days to weeks or even several months, to stabilize and before the visual result can be assessed. As many as 10-40% of LASIK patients may require laser retreatment (called enhancement) to achieve acceptable uncorrected vision. If further treatment is required, the flap usually can be elevated even years later, and more treatment applied with the laser rather than creating another flap. However, there is a much higher risk of epithelial ingrowth occurring than than for a first primary treatment. (Discussed in possible complications above.)

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