LASIK, or “laser-assisted in situ keratomileusis,”
is the most commonly performed laser eye surgery to treat myopia, hyperopia and astigmatism.
The goal of the treatment is to reshape the cornea to correct the refractive error of
the eye. The cornea is the transparent dome-shaped
structure in front of the eye. The cornea refracts light and accounts for about two-thirds
of the eye’s total optical power. Altering the curvature of the cornea changes the way
light rays enter the eye. As a result, the light rays can be focused properly onto the
retina for clearer vision. For nearsighted people, the laser is used
to flatten the cornea. For farsighted people, the cornea is made steeper. For patients with
astigmatism, the laser is used to smooth the irregularly-shaped cornea into a more regular
shape. The outer layer of the cornea – the epithelium
– is capable of replacing itself within a few days after being damaged or removed.
The deeper layer of the cornea – the stroma, on the contrary, is a permanent corneal tissue
with very limited regenerative capacity. The stroma, if reshaped by a laser, will remain
that way permanently. In this procedure, a thin, circular “FLAP”
is created in the surface of the cornea to gain access to the permanent corneal tissue.
This can be done with a mechanical cutting tool called a microkeratome, OR, for a blade-free
experience, by a femtosecond laser. An excimer laser is then used to remove some corneal
tissue to reshape the cornea. Excimer laser uses cool ultraviolet light beams to vaporize
microscopic amounts of tissue in a precise manner to accurately reshape the cornea. The
excimer laser is computer-controlled and is programmed based on the patient’s refractive
error. The flap is then laid back in place and is allowed to heal.
LASIK eye surgery is mostly painless and can be completed within minutes. Improved vision
can usually be seen overnight. PRK, or photorefractive keratectomy, was the
first type of laser eye surgery for vision correction and is the predecessor to the popular
LASIK procedure. In PRK, NO flap is created. Rather, the epithelial cells on the eye surface
are simply removed. An excimer laser is then used to reshape the cornea just like it does
in LASIK. The vision correction outcomes of PRK surgery
are comparable to those of LASIK, but the recovery period is longer. This is because
the epithelium is completely removed in PRK and it takes a few days to regenerate. PRK
patients also have more discomfort and haziness of vision in the first few days after the
surgery. Improved vision also takes longer to achieve.
PRK does, however, offer certain advantages. Because PRK does not involve creation of a
flap, which contains both epithelial and deeper stromal tissue, the entire thickness of the
stroma is available for treatment. The treatment range is therefore higher. This is particularly
useful for patients with high levels of myopia or for those whose cornea is too thin for
LASIK. PRK is also free of flap-related complication risks.