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Medics
Index Member
مركز
دعسان
الاستشاري
للعيون
الدكتور
: سامي عطا
يوسف
دعسان
استشاري
طب وجراحة
العيون
استشاري
طب وجراحة
القرنية
استشاري
تصحيح
البصر
بالليزر-الليزك
THE ANATOMY AND PHYSIOLOGY
OF EYE
In order to understand the mechanism of refractive surgery
and its complications, it is necessary to be familiar with the
anatomy and physiology of the ocular system, with a particular
emphasis on the cornea as it is the sole part of the eye
operated upon by refractive surgeons.
The human eye is similar to a camera where a system of
lenses focuses a picture onto a light-sensitive film.
The eyeball itself is essentially an opaque globe,
encapsulated with a tough, protective white sheath, the
sclera, and filled with a gel-like fluid called the vitreous.
In the front of the eyeball, the sclera gives way to a
transparent dome known as the cornea.
The cornea is similar to the crystal of a wristwatch and
vaults over the anterior chamber of the eye, much as the
crystal vaults over the watch-face. The cornea is a clear
membrane which has the dual purpose of protecting the eye and
focusing light as it enters the eye. It is maintained in a
perfectly transparent state with a constant curvature. After
light passes through the cornea, it then passes through the
pupil, an opening in the iris (the colored part of the eye).
Once through the pupil, light then passes through the
crystalline lens, which along with the cornea is responsible
for the focusing of the eye.
Light then passes though the gel-like vitreous and falls
upon the retina, the light-sensitive tissue at the back of the
eyeball (functioning much like the film in a camera) that
converts the light into electrical signals. The retina
contains cells, called rods and cones, which serve the task of
detecting the intensity and the frequency of the incoming
light. The rods and cones then send nerve impulses through the
optic nerve to the brain, where translation of the impulses
into vision takes place.
THE CORNEA
The cornea is approximately 500 microns thick (.5
millimeter) and is responsible for 80% of the focusing
(refracting) of the light entering the eye. The cornea
consists of five cell layers, which moving from the front of
the eye inward are the epithelium, Bowman’s Membrane, Stroma,
Descemets Membrane, and Endothelium.
The Epithelium is the outermost layer of the cornea and is
the eyes first barrier to infectious organisms.
Riding on the very outside of the corneal epithelium is a
very thin film of water and other chemicals (salt,
antibiotics, etc.) Known as the tear film.
The tear film lubricates the cornea and keeps it moist. A
problem with the tear film causes the sensation of dry eye.
Coursing through the epithelium are a tremendous number of
nerve cells with bare ends. If these nerve endings become
exposed to the air by the slightest defect in the epithelial
surface, a corneal abrasion is created which is usually
accompanied by exquisite pain.
REFRACTIVE ERRORS
In order for our eyes to be able to see, light rays must
be bent or “refracted” so they can precisely focus upon
the retina. A refractive error means that the optics of the
eye do not refract the light properly, so that the image
formed on the retina is blurred. While refractive errors are
called eye disorders, they are not diseases. There are three
primary refractive errors which are addressed by refractive
surgery: myopia, hyperopia, and astigmatism.
Myopia (nearsightedness) is where the distance vision is
blurred at all times while near vision is often excellent
within a certain range. In the myopic eye, the image
ultimately comes to a focus at a point in front of, rather
than directly on, the retina. Myopia is due to an excessively
long eyeball and/ or a cornea that is too steeply curved,
creating an excessive amount of focusing. It is this corneal
curvature which is altered in the various refractive surgery
techniques to be discussed.
Conversely, hyperopia, (farsightedness) is the refractive
condition where near objects may appear blurred while distant
objects typically appear clear.
The third refractive error deal with through refractive
surgery is astigmatism.
Refractive errors are measured in Diopters. Myopia is
measured in terms of minus “-“diopters, hyperopia in
“+” diopters.
REFRACTIVE SURGERY
MODALITIES
The intent of refractive surgery is to change the natural
curvature of the cornea in order to alter the eyes focusing
power… to make a myopic cornea flatter or a hyperopic cornea
steeper.
In the early 80s, eye surgeons became aware of the Excimer
laser, then being used in the computer chip industry. While
most surgical laser beams affect tissue by producing heat, the
Excimer laser uses a charged mixture of argon and fluorine
gases to produce a cool beam of ultraviolet light. The beam
breaks the molecular bonds between cells and vaporizes tissue,
one microscopic layer at a time. The Excimer laser was
formally approved for use in PRK in 1995, although many eye
surgeons were flying their patients to
Mexico
or
Canada
prior to that to circumvent the FDA prohibition.
In PRK, the Excimer laser is used to reshape the cornea in
an effort to effect a change in the refractive characteristics
of the eye and thereby correct or lessen myopia, hyperopia,
and/or astigmatism. Before the laser is applied, the
epithelial layer of the cornea is removed by either mechanical
means (simply scraped away) or chemical (application of
alcohol solution). The laser is then used to photoablate
(vaporize) several microns of tissue from the central and mid
cornea. Usually from 3% to 15% of the central corneal tissue
is utilized for corneal reshaping for myopic corrections from
-1.00 to -7.00.
The epithelium usually regrows over the treated area
within several days. To reduce the amount of myopia in the
eye, the cornea is flattened by removing more tissue from the
center of the cornea than from the midzone cornea. The
resultant central corneal flattening moves the focus point
farther back toward its desired spot on the retina. To reduce
hyperopia, more tissue is removed from the midzone cornea,
thereby steepening the central cornea.
The LASIK procedure is similar to PRK (photorefractive
keratectomy) but does not treat or alter the very front
surface of the cornea (epithelium). In the LASIK procedure, a
liquid anesthetic is dropped into the patients eye, numbing it
for surgery. The surgeon then props the eyelids open and marks
the cornea with water soluble ink to guide in the later
repositioning of the flap. A suction ring is placed on the eye
to secure the eye and maintain pressure within the eye while
the cornea is drawn outward.
Simultaneously, a microkeratome (similar to a carpenters
plane, but automated) is placed in the track of the suction
ring . The blade of the microkeratome then moves across the
cornea, creating a flap of corneal tissue some 20-25% of the
total corneal thickness. This layer down into the corneal
stroma) is not cut away completely, but remains attached at
one side and is then opened like a door on a hinge to reveal
the stroma beneath.
Once the upper corneal flap has been folded back, the
excimer laser is then employed to ablate (vaporize) the amount
of underlying corneal tissue necessary to reshape the corneal
curvature to the desired degree. To correct myopia, the laser
trims the corneas center, making it flatter. For hyperopia, a
doughnut-shaped ring of tissue is removed. The laser is
programmed to ablate the necessary amount with a modified
version of the patients glasses or contact lens prescription.
The corneal flab is then repositioned to its original position
on the stromal bed where it adheres over the next several
months. As in the other procedures , the eye is then treated
with antibiotics, covered with a shield, and the patient is
sent home to recover.
Dr. Sami Ata
Dassan
Medics
Index Member
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