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Refractive
Surgeries
(LASIK, CustomLASIK, IntraLase,
PRK, and Epi-LASIK)
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Eye
Anatomy and Refractive Errors |
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Recent advances
in refractive surgery are designed to permanently
reduce or eliminate the need for corrective eyewear
to see objects clearly. Today, more than 3 million
corneal refractive procedures are performed worldwide
each year. Presently, more than 100 million patients
wear contact lenses for correction of their refractive
errors.
To better understand refractive surgeries, one should
understand how a perfectly normal eye (20/20) focuses
light and what are different types of refractive
errors? Let us first understand the main components
of eye.
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Anatomy of the Eye |
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Cornea:
It is the transparent front wall of the
eye. Cornea is one of the most critical
components of the eye and is the “window”
of your eye, just like the anterior window
of your camera. Cornea works to refract
light rays from the object viewed so that
they focus on the retina at the back of
your eye. Refractive
errors occur when the cornea
fails to focus light rays precisely on the
retina. Cornea is the tissue which is reshaped
during laser surgery. |
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Iris:
The iris is the colored portion of the eye. It
is similar to the aperture of a camera. The iris
regulates the amount of light entering the eye.
Pupil:
The adjustable opening at the center of the iris
that allows varying amounts of light to enter
the eye. Dilation drops used by your eye doctor
increase the size of the pupil so that the internal
health of your eye can be evaluated.
Lens:
The lens focuses the light entering through the
pupil, directing it to the retina. The curvature
of the lens adjusts to focus at different distances.
Optic
Nerve: The optic nerve
is similar to the cable that connects your video
camera to the VCR. The image seen in the eye is
transported to brain where the image is processed.
The optic nerve head is the portion of the optic
nerve that is seen by an experienced eye care
professional to evaluate for eye diseases such
as glaucoma,
papilledema (increase pressure in the brain),
and optic neuritis.
Retina:
It is a nerve layer at the back of your eye that
senses light and sends images to your brain. The
retina is similar to the film in a camera. This
is where the picture seen by the eye is converted
to electrical signals and then transported to
the brain via the optic nerve. The retina is made
of cells called rods and cones. Rods are primarily
used to aid with night and peripheral vision,
while cones are designed to aid with color
vision and crisp 20/20
vision.
Macula:
The macula is a small area located at the posterior
portion of the retina. It is comprised of cones
and functions in crisp 20/20 vision. Diseases
that affect this area can cause a decrease in
vision. Disease processes such as macular
degeneration, retinal
detachments, diabetes, and hypertension
can potentially cause blindness.
Sclera:
The sclera is the white portion of the eye. It
is comprised of tough connective tissue that gives
the eye its shape.
Conjunctiva:
It is a very thin layer covering the sclera. This
layer carries blood vessels and very importantly
it contributes to the innermost layer of a healthy
tear film. Conditions affecting this layer such
as surgery, infection etc. can affect your tear
film.
Muscles:
Ocular muscle alignment is critical in developing
sharp three-dimensional vision. During a routine
eye exam, an eye care professional
evaluates the integrity of the muscles and the
nerves which supply them. Muscle imbalance can
cause lazy eye
if it is not corrected during childhood.
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Refractive
Errors
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Normal Eye |
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In a perfectly normal eye, the cornea and
lens contribute to sharp focusing of incoming
light rays from distant objects directly
on the retina. The cornea acts as a natural
lens and its curvature determines its power.
Cornea is responsible for 80% and the lens
is responsible for about 20% of the eye’s
refractive power. Nerve fibers in the retina
transmit this image to the brain, where
it is interpreted as vision. This perfect
eye system sees 20/20.
If the light rays do not come to a pinpoint
focus on the retina, the patient has a refractive
error. There are four types of refractive
disorders. These are: |
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Myopia (also
known as near-sightedness) |
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a person can see objects nearby with no
problem, but reading road signs or making
out the writing on a bill board is more
difficult, he may be near-sighted. Myopia
occurs when light rays are focused in front
of the retina, causing distant images to
appear blurry. Far-objects appear blurry
because the light rays are spread apart
instead of focused when they strike the
retina. Myopia is the result of an eyeball
which is too long, or a cornea which is
too curved.
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Myopia
is not a disease, nor does it mean that you have
"bad eyes." It simply refers to a variation
in the shape of your eyeball. Myopia usually starts
in childhood and gets progressively worse through
adolescence. It usually stops changing by the
late teens, but it can sometimes continue to get
worse into the mid-twenties. Corrective concave
lenses (“negative” lenses that are
thicker at the edge than in the center) or contact
lenses are prescribed to focus light more precisely
on the retina, where a clear image is formed.
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Hyperopia
or Hypermetropia (also known as far-sightedness) |
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| Hyperopia,
or far-sightedness, is the opposite of myopia.
If a person can see objects at a distance
clearly but has trouble focusing well on
objects close up, he may be farsighted.
It occurs when light rays are focused behind
the retina, causing near images to appear
blurry. Near-objects appear blurry because
the light rays are spread apart instead
of focused when they strike the retina.
Very far-sighted people have difficulty
seeing clearly the distant objects as well.
Hyperopia is the result of an eyeball which
is too short, or a cornea which is less
curved. This creates blurred vision that
is more noticeable for near objects than
distant ones.
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Hyperopia
is not a disease, nor does it mean that you have
“bad eyes.” It simply means that you
have a variation in the shape of your eyeball.
Hyperopia may be present in childhood but does
not usually become apparent until people are in
their late 20’s or 30’s when they
can no longer see up close. When these patients
get into their 40’s or 50’s, they
begin to have difficulty seeing distant objects
as well. Corrective convex lenses (“positive”
lenses that are thicker in the center than at
the edge) or contact lenses are prescribed to
focus light more precisely on the retina, where
a clear image is formed. |
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Astigmatism |
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| Astigmatism
is an overall inability of the eye to focus
clearly at any distance. It occurs when
cornea is shaped more like a football (with
two different curvatures) than a well-rounded
baseball. Images appear blurred or ghost-like
because light rays are refracted unequally.
In extreme cases, images both near and far,
appear blurred. Many people who have myopia
also have some degree of astigmatism. This
creates eye strain. Many people live with
minor astigmatisms that do not require any
type of treatment. Others with more severe
astigmatism can have it corrected by wearing
glasses or contact lenses. |
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Presbyopia |
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Presbyopia
is the inability to focus the eyes at near.
In young individuals, the natural lens of
the eye is soft and pliable. This innate
flexibility permits the natural lens to
change its shape, allowing it to focus on
objects near the eye. As the years pass,
the lens loses its flexibility and can no
longer vary the focus of the eye. This condition
usually becomes noticeable sometime between
40 and 50 years of age. People with myopia
as well as hyperopia have presbyopia after
age 40. |
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Laser
Vision Correction (LVC)
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Laser
vision correction includes those refractive surgeries
that use an Excimer
laser to microscopically reshape
the corneal curvature, (the outer window of the
eye) so that the light rays focus more precisely
on the retina. Following are the most popular LVC
available today: |
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These procedures have either altogether eliminated
or reduced the dependence on glasses or contact
lenses for millions of people around the world.
They are capable of correcting a wide range
of refractive disorders such as near-sightedness
(myopia), far-sightedness
(hyperopia) and astigmatism.
Note that these procedures can improve
vision only if your vision can be improved with
glasses or contact lenses.
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Conventional
LASIK |
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LASIK
is the most commonly performed refractive procedure
today. From 1990 to the present, over 10 million
LASIK procedures have been performed worldwide
establishing refractive surgery as the second
most common ophthalmic surgical procedure, behind
cataract (more than 10 million surgeries per year
worldwide). In 2003, in USA alone, more than 1.2
million LASIK surgeries were performed.
There are
no needles, no stitches, and no pain. Prior to
the procedure, drops are placed in the eye to
anesthetize it (like the ones used to check your
eye pressure for a glaucoma
test) and an instrument called speculum is placed
in the eye to prevent you from blinking.
LASIK is a 3-step procedure:
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Step 1 |
A
specially trained ophthalmologist
uses an automated instrument called
a microkeratome
(similar to a carpenter's plane)
to create a thin, uniform flap in
the cornea. |
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Step 2 |
The
surgeon then folds the flap back
(like the page of a book) to apply
a computer-guided Excimer laser
treatment to gently remove a thin
layer of tissue from the exposed
corneal surface to achieve a desired
correction. This is a cold laser
and it ablates or vaporizes tissue
away to microscopically reshape
the corneal surface. |
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Step 3
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After
the computerized laser therapy is
applied, the surgeon replaces the
flap, where it naturally re-adheres
in its original position. |
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This
treatment microscopically reshapes the
cornea, resulting in greatly improved
visual
acuity without glasses
or contacts. In just 24 hours following
surgery, a vast majority of people see
20/40 or better. This continues to improve
as the days pass, with the cornea healing
with the assistance of medicated drops.
Many people are able to return to work
in one to two days. Discomfort is generally
limited to some mild burning sensations
or foreign body sensations in the eyes
for a few hours.
Today,
LASIK has become the procedure of choice
because the healing occurs in the interior
of the cornea, and no re-growth of surface
cells is required. The corneal flap protects
the treated area and there is usually
little or no post-op discomfort. Visual
rehabilitation is much quicker and regulation
of the healing process with long term
post-op drops is unnecessary since the
center of the cornea tends to have much
less of a healing reaction than the surface.
There are two new advancements to further
improve the safety and precision of standard
LASIK. These include CustomLASIK and IntraLase. |
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CustomLASIK
/ CustomCorneaTM / WaveFrontTM
LASIK / CustomVueTM WaveScanTM
LASIK
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There are two types of
optical distortions: low order and high
order. Low order distortions include nearsightedness
(myopia), farsightedness
(hyperopia), and astigmatism.
Higher-order aberrations are corneal irregularities,
other than refractive errors, that can cause
decreased contrast sensitivity or night
vision, ghosting ("double" vision),
starbursts, glare, shadows, and halos that
some patients have had after the conventional
LASIK surgery. Conventional LASIK surgery
is limited to treating low-order aberrations.
However, CustomLASIK (CustomVue) uses Wavefront
technology to address both low-order and
higher-order aberrations.
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CustomVue
System, which includes the Wavefront Map
and 3D ActiveTrak™ Excimer laser with
Variable Spot Scanning (VSS™) is used
to measure and treat the optical distortions.
This system allows the surgeon to treat
your optical distortion by performing Wavefront
guided laser treatment according to your
individual customized treatment plan. WaveScan
transmits a safe ray of light into your
eye. The light is then reflected back off
the retina, out through the pupil, and into
the device, where the reflected wave of
light is received and arranged into a unique
pattern that captures your lower- and higher-order
aberrations.
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The resulting refractive map (Wavefront
map) used in CustomLASIK is as individual
as a fingerprint. No two eyes are the
same. Like your DNA or fingerprint, your
vision is unique. This map provides information
on more than just the front surface of
the eye. It shows the variability of how
light is focused in different areas of
the eye. This information is then transferred
to the Excimer laser, enabling the surgeon
to customize the LASIK procedure to your
unique visual requirements. There is no
significant difference in how the actual
LASIK procedure is done, whether Wavefront
or conventional software is used. Like
conventional LASIK, CustomLASIK won't
cure all vision-related problems, so it's
important to discuss its applications
with your surgeon to determine if you
are a good candidate.
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Until now, laser vision correction (LVC)
treatment was based on diagnostic technology
similar to that used for the prescription
eyeglasses or contact lenses. The new CustomLASIK
takes LVC to an entirely new, personalized
level by combining exclusive personalized
diagnostic technology with the Excimer laser.
The advanced diagnostic portion of the new
Wavefront technology produces a precise,
detailed analysis of your vision and provides
a personalized LVC plan that addresses your
individual needs. This WaveScan Map is coordinated
with the VISX
STAR S4 ActiveTrak™ Excimer Laser
System to create one of the most advanced
systems available. The 3-D eye tracker with
auto-centration adds a new level of precision,
comfort, and safety.
The benefits of CustomLASIK
(CustomVue) include higher chance of seeing
20/20;
the ability to treat people with larger
pupils; reduced frequency of night vision
problems following treatment; and clearer,
crisper results compared to treatments performed
without CustomLASIK.
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In standard
LASIK, an ophthalmologist uses an oscillating
blade called a microkeratome
to create the flap. Sometimes, microkeratome
is a source of surgical complications during
traditional LASIK. A new laser, called the
IntraLase
replaces the microkeratome. It may make
LASIK even safer. IntraLase creates the
corneal flap and then the surgeon uses the
same Excimer
laser to perform the rest
of the LASIK procedure.
The "all-laser" LASIK approach,
using the IntraLase and Excimer laser, not
only avoids microkeratome-related complications
such as corneal irregularities and scarring,
but may also offer the potential for better
vision than after standard LASIK. Precision
and control in LASIK surgery have everything
to do with accuracy of vision correction,
quality of resulting vision and reproducibility
among differing patients and surgeons. Replacing
the mechanical microkeratome with a computer-guided
laser is a significant advancement.
People previously “disqualified”
based on the corneal thickness may now be
eligible for LASIK with the IntraLase laser.
The thickness
of the cornea is an important
parameter in determining the eligibility for
LASIK. Most people have corneas about 550
microns thick. Most microkeratomes cut flaps
that are about 130-160 microns thick. For
people with thin corneas, |
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LASIK surgeons tend to move cautiously,
balancing the thickness of the cornea against
the amount of vision correction needed.
They may even decide that the procedure
is inadvisable for a particular patient.
The IntraLase laser can make thinner corneal
flaps without compromising the safety of
the procedure. This may allow surgeons to
perform safe LASIK in people with thinner,
steep or flat corneas.
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IntraLaseTM
with CustomLASIK |
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CustomVue
with IntraLase is the very best possible
LASIK surgery today. It is a blade-free;
all-laser individualized laser vision
correction. It uses a computer-controlled
laser rather than a hand held mechanical
devise (microkeratome) to cut the
corneal flap. This minimally invasive
approach offers more safety and precision
than ever imagined. With CustomVue,
surgeons can now identify, measure,
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imperfectionsin an individual’s
eyes 25-times more precisely than
with standard LASIK. CustomVue with
IntraLase provides an unprecedented
level of safety and precision. |
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| PRK |
PRK
is a variation of LASIK, differing in that
the surgeon does not create a corneal
flap. Instead, the surface
skin cells of the cornea (the epithelium)
are gently removed, and then cornea is treated
by the same Excimer
laser. After this painless
procedure, a contact
lens is placed on the eye
and left in place for three to four days,
allowing the epithelium to grow back over
the corneal surface. Although vision recovery
is slightly slower compared with LASIK,
some people are better candidates for PRK
based upon corneal thickness and epithelial
health. |
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LASEK / Epi-LASIK
/ E-LASIK |
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LASEK
(laser epithelial keratomileusis) is a relatively
new procedure that is a modified form of PRK.
LASEK is mostly for people with corneas that are
too thin or too flat for LASIK. It reduces the
chances of complication that occur when the flap
created during LASIK is not of the ideal thickness
or diameter.
LASEK procedure requires the
surgeon to loosen the outer layer of the cornea,
called the epithelium. Unlike PRK, the epithelium
is not removed in LASEK. The surgeon instead makes
an epithelial flap by using a mechanical blade.
Epithelial flap is then folded back so that the
laser can reshape the exposed cornea. After laser
application, the surgeon replaces the epithelial
flap over the corneal bed and places a soft contact
lens on top. Visual recovery after LASEK is generally
faster than in PRK but slower than in LASIK. Currently,
the use of the Excimer laser for LASEK is not
FDA-approved.
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