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LASIK Surgery
LASIK is a recently developed operation, which can correct both long
and short 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 steepened centrally.
The laser treatment takes 3 or 4 minutes to complete, after which the
flap of corneal tissue is folded back and a plastic shield protects the
eye.
Photo-refractive Keratectomy (PRK)
PRK was one of the first methods of treatment introduced using the excimer
laser to correct optical defects of the eye. 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 are additional risks associated with the flap.
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 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. At present the laser cannot correct above +10 dioptres
of hypermetropia.
Amblyopia (Lazy eye)
Occasionally in people with severe long sight who are considering having
LASIK it is found that the quality of vision obtained with glasses is
less than 6/6. This is because the visual development of the eye has never
reached its full potential. In such cases the vision following LASIK treatment
will be no better than that which was obtained with glasses before the
treatment.
Presbyopia
As one get older the ability of the lens of the eye to focus on close
objects declines. Around the age of 40, someone who has not previously
needed spectacles finds that they require reading glasses. Similarly someone
in this age group whose hypermetropia is successfully corrected so that
their distance vision is perfect, will still require reading glasses.
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