How Lasers Reshape the Cornea
Lasers used in vision
correction produce precise pulses of UV light energy, which can
be delivered accurately onto the cornea. This
energy weakens the bonds between atoms in the cornea so that
microscopic amounts of tissue are removed in a process called
cool
ablation. Since little or no heat is produced, no damage is done
to adjacent tissue allowing the computer-guided laser to generate
a new corneal profile as it sweeps back and forth across the
eye. Myopic eyes are given a flatter
profile and hyperopic eyes
a steeper
profile. Astigmatic eyes
have their profile made more uniform. There are four types of
laser treatment: Epithelial Laser in situ Keratomileusis (EPI-LASIK),
Photorefractive Keratectomy (PRK) Laser Epithelial Keratomileusis (LASEK)
and Laser In Situ Keratomileusis (LASIK).
In PRK the surface layer of cells (the epithelium) is removed
from the cornea and the treatment is applied to the exposed
Bowman's Layer of the cornea, which is removed by the laser.
In
LASEK the surface layer of cells (the epithelium) is displaced
and the treatment is applied to the Bowman’s layer of the
cornea, which is removed by the laser. The displaced epithelial
flap is then put back into place.
In LASIK an instrument called
a microkeratome is used first to form a thin flap of corneal
tissue which includes the epithelium
and Bowman's Layer on which it rests (Figure 1). The laser is
applied to the exposed corneal tissue (Figure 2) then the flap
is carefully
repositioned (Figure 3).
The cornea has amazing bonding properties
and almost immediately the flap adheres to the underlying tissue
and begins to heal. The preservation of the epithelium and Bowman’s
Layer gives LASIK its advantage over
PRK and LASEK and there
is a faster and more comfortable recovery of vision.
EPI LASIK is a new procedure that combines some
of the advantages of Lasek and Lasik. With Epi-Lasik the surface
epithelium is lifted
from the underlying corneal tissue using a high precision separator.
There is no need to use dilute alcohol to "soften" the
epithelium, as is done with LASEK. The epithelium
is preserved in a more healthy and viable condition. Compared with LASEK visual
recovery is therefore usually quicker and with less discomfort.
Yet there is no need to cut the cornea as is done with LASIK.
Click here to see how
the procedures compare
FIGURE 1
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FIGURE 2
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FIGURE 3
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