| LASIK Surgery
LASIK is a recently developed operation which can correct both short
and long sightedness and also astigmatism. An excimer laser is used to
sculpt the cornea to a new curvature that will correct the optical defect.
LASIK surgery is carried out with a local anaesthetic (eye drops). A
small metal suction ring is placed on the surface of the eye around the
cornea. This stabilises a cutting device (microkeratome) that shaves 160
microns of tissue from the surface of the cornea to create a very thin
flap. The microkeratome and metal suction ring are then removed from the
eye and the tissue flap is temporarily folded to one side. The exposed
area of cornea is sculpted by an excimer laser and flattened centrally.
The laser treatment takes 1 or 2 minutes to complete, after which the
flap of corneal tissue is folded back and the eye is protected by a plastic
shield.
Photo-refractive Keratectomy (PRK)
PRK was one of the first methods of treatment introduced using the excimer
laser to correct myopia. In PRK treatment, the thin layer of cells (epithelium)
that covers the surface of the cornea is gently scrubbed off by the surgeon
and the laser treatment is applied directly onto the surface of the cornea.
At the end of the operation the normal covering of epithelial cells is
missing from the treated area, and it is necessary for the surrounding
cells to grow back over the central part of the cornea. Following PRK
the eye is very painful for the first day or two. Although the cells will
cover the central area in a few days, it takes several months for the
cell layer to stabilise over the treated area, and there is often a degree
of haze or scarring of the cornea in the treated area.
Advantages of LASIK
LASIK is a modification of PRK in which a tissue flap covers the laser-treated
area. There is no raw area of cornea exposed at the end of the operation,
and so little or no pain after the surgery, and the eye settles down much
more quickly. The optical recovery is faster and the vision generally
stabilises rapidly. It is rare to encounter scarring in the visual axis
after LASIK.
Disadvantages of LASIK
LASIK surgery is technically more complex compared to PRK and requires
additional instrumentation. This makes the procedure more expensive, and
there is the additional risk of flap related complications.
Problems with Refractive Surgery
The amount of corneal tissue that can safely be removed by the laser is
limited by the thickness of the cornea. To minimise the amount of tissue
that needs to be removed by the laser, the optical correction is restricted
to a small central area of the cornea. It can be seen from the diagram
that the optically corrected area is generally smaller than that achieved
by a contact lens. Provided that the treated area is directly over the
pupil then the optical performance of the eye is satisfactory. If the
pupil is very large (e.g. when driving at night) then the light rays may
pass through both optically corrected and uncorrected parts of the cornea
and this can lead to glare and poor vision. If the optical area treated
is not perfectly centred on the pupil the quality of vision may also be
reduced. If the strength of the optical correction achieved by the treatment
is inaccurate, it is generally possible to carry out further treatment
by lifting the flap and applying additional laser to the bed. A small
optical correction requires less laser treatment than a larger correction.
The greater the refractive error that is being corrected the greater the
chance of a less than perfect outcome.
Presbyopia.
As one gets older the ability of the lens of the eye to focus on close
objects declines. Around the age of 40, most people who have not previously
needed spectacles find that they require reading glasses. Similarly someone
in this age group whose myopia is successfully corrected so that their
distance vision is perfect, will then require reading glasses.
Progression of myopia.
In the long term, regardless of whether LASIK surgery is carried out or
not, it is possible that the underlying myopic condition may progress.
Thus even if the myopia is initially fully corrected, over the years,
although the corneal condition may remain stable, the eye may become myopic
again to some extent due to further increase in the length of the eyeball.
Some patients with high myopia are in the long run at an increased risk
of developing other eye problems associated with myopia, such as cataracts,
glaucoma, and retinal degeneration. It is unlikely that LASIK treatment
influences the risk of these conditions occurring.
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