| LASIK | Laser Assisted In-Situ Keratomelusis |
LASIK, or Laser Assisted In-Situ Keratomelusis, is another excimer laser technique to correct refractive errors. In addition to the excimer laser, a second instrument called a microkeratome is needed for performing the LASIK procedure. The microkeratome creates a flap through the middle layer of the cornea called the stroma. NOTE THE DISTINCTION OF LASIK CREATING A STROMAL FLAP NOT AN EPITHELIAL FLAP AS IN LASEK. (Discussed under LASEK) The LASIK procedure is also performed using only a drop of anesthesia. Because of the necessity to use the microkeratome, and the drop's inability to totally anesthetize the eye other than the cornea, some patients report mildly more discomfort during the actual procedure than with PRK. The average cornea thickness is approximately 530 microns depth and the stromal cornea flap thickness is between 130 and 180 microns. One of the problems of LASIK is the inability to predict the precise depth of the flap. Total thickness of the cornea does vary among individuals and is measured in a few seconds by an ultrasonic instrument called a pachymeter using a drop of anesthetic on the eye prior to creating the flap. In order to create the flap, a round suction ring is placed with the central opening around and peripheral to the cornea on the white part of the eye. Suction or vacuum is applied to the eye by the ring. This raises the pressure in the eye to make the eye firm for creating the flap. It is absolutely necessary to raise the pressure in the eye and make the eye firm in order to safely make the cornea flap. During the short time that suction pulls and attaches the ring to the eye, vision may dim or go completely dark because circulation of blood to the optic nerve is interrupted for a brief time. After being sure the eye is firm, the microkeratome is centered upon the ring and moved so that the very thin microkeratome blade creates the cornea stromal flap leaving the flap attached nasally or superiorly. 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 central tissue must be removed with the 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. At a recent major ophthalmology meeting a patient with residual corne thickness of 310 microns following LASIK was reported to develop cornea ectasia (central bulging of the cornea) and instability. One study found a 2.5% incidence of cornea ectasia for patients having residual cornea thickness of 250 microns or less after LASIK. Ectasia results in irregular astigmatism and poor fluctuating vision which may require cornea transplant surgery. Thus, in LASIK, one very important consideration is how much residual central thickness of the cornea will remain after the laser vision correction. This is a VERY important consideration 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). 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.) A presentation at a recent professional meeting presented a patient who developed a displaced and dislocated flap ten years after having LASIK. Some of the more common complications of LASIK are: 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 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.) The fact the flap can be raised even months or years after the laser correction illustrates again one of the significant negatives to LASIK. Flaps can be dislocated or even lost with significant scarring following ocular injury or blow to the eye years following the LASIK procedure.
© Earl W. Nepple, M.D., 2003 - All Rights Reserved |
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.
