Refractive
Surgery Information
How
the Eye Works – The Short Version
Anatomy
of the Eye
Refractive
Errors (nearsightedness, farsightedness, astigmatism)
History
of refractive surgery and methods to treat refractive errors
Laser Vision Correction
How the Eye Works – The Short Version
As
in a camera, the eye works by focusing light rays. Light
entering the eye first passes through the transparent cornea,
which has about 60 percent of the light-focusing ability
of the eye. Light rays are then focused by the lens to make
a clear image on the retina at the back of the eye. The
image formed by the retina is transmitted to the brain as
a series of electro-chemical impulses via the optic nerve.
back to top
Anatomy
of the Eye
To
understand refractive surgery, let's briefly review the
anatomy of the eye.
We'll
call it Eye Anatomy 101.
Think of the eye as a camera. The lids are the lens cap and
the iris is the f-stop or diaphragm which lets in more or
less light through the lens which focuses light on the retina.
The retina is the film in the camera. The retina is connected
by a coaxial type of cable called the optic nerve which
travels to the brain. We'll refer to this camera example
again later.
•
Cornea
The
cornea is the clear dome-shaped front window of the eye,
which provides 60 percent of the light-bending (focusing)
power of the eye. It is the only tissue in the human body
which is transparent to light. In its normal state, the
cornea has no blood vessels. Epithelial cells coat the outside
of the cornea, providing both a smooth and a water-holding
(tear) surface to allow clear vision. The lids not only
are windshield wipers but, by spreading the tears around,
also serve to keep the eye from completely drying out.
•
Sclera and Conjunctiva
This is the muscular outer coat of the eye which maintains
the eye's round shape. It is white and opaque to light.
Covering the sclera and the inside of the eyelids is the
conjunctiva, which is a clear thin membrane containing mucous-producing
cells to keep the ocular structures moist.
• Lens and Ciliary Body
Within the globe of the eye, situated just behind
the cornea, is a usually-clear, semi-solid structure called
the crystalline lens. This lens is enclosed in a thin membrane
(lens capsule) which is itself connected to the muscular
part of the ciliary body by another membrane (zonule). Because
it is the source of nutrient fluid, the ciliary body is
extremely important to the health and function of the eye.
The lens of the eye-composed of the capsule, cortex and
nucleus is held under tension by the zonular membrane and
thus is made thin.
In its relaxed state, the eye is able to discern distant
objects clearly. As an object gets closer, the eye must
change its light-bending power to accommodate the nearness
of the object. How does this automatic focusing occur? The
ciliary muscles are so arranged that when they contract,
the zonule relaxes, allowing the lens to relax and become
thicker in the center. This increased thickness bends the
light more so that nearer objects are seen more clearly.
The anatomical term for this act of focusing is called accommodation.
It is the gradual loss of the property of accommodation
that causes a loss of the ability to see near objects. This
is called presbyopia, which occurs for most people
in their mid-40's. When this crystalline lens turns cloudy
from age or disease, it is called a cataract.
•
Iris: Controlling the light entering the eye
Overlying
the lens is the iris whose purpose is to control the amount
of light entering the eye. It's a structure made of thin
elastic tissue with an opening in the center, surrounded
by a circular muscle called a sphincter. The structure is
the iris; the opening is the pupil. The backside of this
tissue is composed of tissues containing brown pigment which
act to absorb light and prevent its scattering within the
eye.
• Retina: "Film" capturing the image
Coating the back of the eye, covering the sclera,
is a thin layer of nervous tissue-an extension of the optic
nerve-called the retina. The retina is analogous to the
film of a camera. With its 10 layers, the retina is responsible
for gathering light focused upon it by the cornea and lens.
It converts the light to electrical signals that it amplifies
and sends on to the brain for interpretation and construction
of images. The brain actually does the seeing-not
the eye itself.
The
retina lies on a black membrane, which overlies a very leaky
collection of blood vessels called the choroid. The choroid
supplies the retinal sensory cells with nutrients and oxygen
while the retinal arteries and veins, coming from the center
of the optic nerve, supply the rest. The transfer of nutrients
to the sensory cells is done by active transfer across cell
membranes; that's why vision deteriorates when the retina
is detached and no longer in contact with the choroid.
The retina is divided into several areas, the most critical
the macular area. The macular area is responsible for daylight
and color vision and contains specialized cells for that
purpose. The very center of the macula-the fovea- is where
best vision occurs. The rest of the retina provides side
vision or field, and does most of the seeing at night.
Refractive
Errors: Don't Take It Personally
As a camera cannot produce a clear image if the incoming
light is not focused on the film, the eye cannot produce
a clear image if the incoming light is not focused precisely
onto the retina. In the normal eye, the image is focused
by the cornea and crystalline lens and falls on the retina.
Why
are some people nearsighted or farsighted or have astigmatism?
Blame it on heredity at least part of the time. The causes
of refractive errors are not really known. Some researchers
feel that reading at an early age may contribute to increasing
nearsightedness. Some even find a link between increasing
nearsightedness and intelligence. If you are nearsighted,
and reading this, you may be more intelligent than average.
However, this does not mean that farsighted individuals
are less intelligent!
• Myopia = Nearsightedness or shortsightedness (the
length of the eyeball, not the inability to plan ahead)
An
eye is nearsighted when the light rays are focused in front
of the retina, rather than on the retina. Nearsighted people
see near objects more easily and better than those objects
farther away. In nearsightedness, light rays from distant
objects are focused not onto the retina but in front of
the retina. Nearsightedness occurs because the cornea and
crystalline lens have excessive focusing power in relationship
to the length of the eye.
In
general, a nearsighted eye is a big eye. If
the cornea and crystalline lens had less combined focusing
power, or if the eye were shorter, the light rays would
focus on the retina. Measurement of the front to back (axial)
length of the eye indicate that 1 mm of added length will
produce 4 diopters of myopia.
Glasses and contact lens change the position of where the
light rays entering the eye focus. In myopia, they decrease
the light rays, convergence so they fall on the retina and
not in front of the retina. Nearsightedness typically begins
in late childhood (age 8 or 9), depending on the ultimate
severity of the myopia. It usually stabilizes by the mid-twenties.
•
Hyperopia = Farsightedness (no, these people are not
gifted with the ability to read crystal balls)
Farsighted
people see far-away objects better than those objects that
are near. Farsightedness results when the cornea or crystalline
lens together have too little focusing power for the length
of the eye. Light rays from a distant object are focused
behind the retina rather than on it.
If
the eye were longer (a farsighted eye is slightly
smaller than normal), or if the cornea's crystalline
lens had more focusing power, the light rays would land
on the retina. That's because of the ability of the crystalline
lens to accommodate or increase in power, especially in
younger people.
Q: How is farsightedness corrected with lenses or contacts?
A: The lenses are convex and move up the focus point
of light rays entering the eye. These light rays now focus
on the retina rather than behind it.
Q: At what age does farsightedness typically occur, and
does the vision ever stop changing?
A: Farsightedness frequently does not cause symptoms
until the fourth or fifth decade of and then progressively
gets worse as the lens of the eye becomes stiffer and unable
to focus.
•
Astigmatism = Blurred Vision
If the focusing power of the eye is uneven, astigmatism
occurs. The cornea should be round and spherical like a
soccer ball; light rays then achieve a point focus.
If
the cornea is shaped more like a football or an egg, with
two radii of curvature, there are two different focal points.
This causes blurred vision. Astigmatism can occur with myopia
or hyperopia, meaning the two focal points can be in front
or behind the retina or even straddle it.
Astigmatism,
when it is regular or symmetric, can be corrected with a
special type of lens that has power in one axis and no power
in the other. These special types of cylindrical (toric)
lenses must be oriented properly to the eye. Contact lenses
correct astigmatism by using the tear film to fill the space
between the contact lens and the cornea.
• Presbyopia (Why are my arms not long enough?)
Presbyopia is the inability of the inner (crystalline) lens
in the eye to focus images. Before the age of 40, when you
want to see an object close to your eye, the inner lens
changes in shape, and focuses the light rays on the retina.
When you are in your forties, the battle is continually
being waged between focusing light by increasing the power
of the inner lens and the gradual stiffening of the lens.
It is a losing battle. Eventually, near objects such as
print become increasingly difficult to see at one usual
reading distance, and you will need to hold these objects
further away from your eye. Reading glasses or bifocal lenses
are now needed to keep the near objects in focus.
back
to top
History
of refractive surgery and methods to treat refractive errors
Refractive
Surgery: Changing the Way Light Focuses inside the Eye
What is it?
Refractive Surgery changes the way light focuses on the
retina. Believe it or not, doctors have been performing
it, by various techniques, since 1898, when Lans discovered
that thermal burns to the cornea could change its curvature.
Glasses, by the way, have been around a very lot longer,
with modern types of spectacles discovered in the 14th century.
Different Procedures?
There are a number of refractive surgical procedures. Before
to laser vision correction, various other techniques have
been used.
• Radial Keratotomy
Laser vision correction should not be confused with the
Surgical procedure called radial keratotomy
(RK) which is also used to correct nearsightedness. RK involves
surgical incisions deep into the cornea. These incisions
cause flattening of the cornea and reduction in nearsightedness.
These incisions can weaken the eye and this weakening can
create problems. Because Dr. Sher believes RK has a number
of risks and disadvantages, he does not, and has not in
the past, performed RK.
LASERS
Ophthalmology was the first branch of medicine to utilize
the laser in clinical practice. In the 60's these early
lasers produced heat energy and worked well for coagulating
blood vessels. They were not suitable for use on the cornea.
Other lasers, utilizing infrared light, were used to cut
membranes within the eye, especially after cataract surgery
(YAG laser capsulotomy).
Excimer Laser:
In the late 1970's, scientists at IBM developed an ultraviolet
laser to etch microchips for the computer industry. The
excimer laser was an ultraviolet laser that had the unique
property of being able to literally strip away atoms and
produce photo-decomposition of tissue.
In 1983, the first ophthalmic use of the excimer laser was
published by Dr. Steven Trokel of New York; it described
the unique properties of this laser to vaporize corneal
tissue without producing heat or scarring. Clinical investigators
conducted extensive research and development with the excimer
during the next 12 years, culminating in 1996 when the first
excimer laser for ophthalmic refractive use was approved
in the United States. Since that time, several companies
have developed excimer lasers for treating refractive errors.
The Machine: Elegant and Precise
The excimer laser produces an invisible beam of ultraviolet
light to remove microscopic amounts of corneal tissue without
causing damage to surrounding cells. This elegant surgical
tool is so precise that it can etch microscopic layers off
a human hair. To achieve most refractive corrections, a
microscopic amount of corneal tissue -- approximately one-third
the thickness of a human hair -- is sculpted from the surface
of the cornea. The laser beam is directed on to the surface
of the eye in a series of 6 to 10 pulses per second. Each
pulse removes a quarter of a micron of tissue (1/100,000th
of an inch). This precision assures total control of the
correction process. Currently, our surgeons at Eye Care
Associates use the VISX STAR™ S4 excimer laser, the
most advanced available in the United States.
•
Laser Vision Correction
Photorefractive Keratectomy (PRK) or Advanced Surface Ablation (ASA)
Photorefractive keratectomy (PRK) was the first laser vision correction procedure utilizing the excimer laser, approved by the FDA in 1995. I
have been performing this procedure since 1989, as I was a participant in the earliest clinical trials of the excimer laser for laser vision
correction. As with other technology, a number of improvements have been made over the last decade. PRK offers excellent visual results and
some studies show slightly superior visual results from advanced PRK in comparison to LASIK. The number of PRK procedures performed in the US
and elsewhere diminished significantly after the introduction of the LASIK procedure in 1996. (More on LASIK below) In the last several years,
due to significant advances in technology and in medications that control healing, surface ablation techniques such as PRK are gaining in
popularity. With over 16 years of experience with this procedure, its long-term safety and stability have been proven. Combined with the use
of Wavefront technology, many surgeons refer to PRK as Advanced Surface Ablation (ASA). In this article, I will refer to PRK as ASA.
In the ASA procedure, the excimer laser is applied on the surface of the cornea, with only a
drop of topical anesthetic needed to numb the eye. In surface ablation, the epithelium, or surface layer of corneal cells, has to be removed in order to reach
the stromal layer of the cornea. There are a number of methods to remove the cells including a gentle brush like instrument, chemicals such as alcohol, the
laser itself or some other instruments. The bottom line is that it does not make much of a difference which epithelial removal technique is used. There are
also two related procedures, Epi-LASIK and LASEK, which should not be confused with LASIK but are variants of ASA. To date, I have not seen any advantage from
the latter two techniques.
The PRK procedure takes less than fifteen minutes, and is essentially painless. You can see
immediately after the surgery, but there is usually a transition period where your vision fluctuates somewhat, and then continues to improve. The VISX S4
laser, the laser used for your procedure, is equipped with a special tracking device that helps to center the laser on the proper position over your eye
and follows your eye even if it moves. At the end of the surgery, a soft contact lens, which works as a bandage, is placed on the eye and several different
eye drops are administered. The corneal surface cells (the epithelium) regrow to resurface the cornea over the next 3 to 7 days. After these cells are back
in place, the soft contact lens is removed and the vision improves very rapidly after that time. There is continued improvement over the next several months.
During the first several days after the procedure, there is some irritation and discomfort
in the operative eye, with some patients describing this as a foreign body sensation or contact lens irritation. This varies greatly from patient to patient.
Several types of eye drops are given to reduce this discomfort. Most patients return to work the next day. After the epithelium heals, the discomfort disappears.
There is a slower healing time and the need for more postoperative visits and eye drops than
with LASIK. Eye drops are commonly used for 1 to 3 months. The vision is blurred after surgery although the uncorrected vision is improved immediately. It
does not improve significantly until after the bandage soft contact lens is removed in 4-7 days. There is a wide variation among patients. The maximal vision
achieved may take 3-6 months. Currently, there is an interval of 1 or 2 weeks between performing PRK on the first and second eye. Complications with PRK (ASA)
include a small chance of inflammation and superficial infection. For more details, the informed consent document is available here.
PRK INFORMED CONSENT AS PDF
LASIK
LASIK utilizes the excimer cool-beam laser to vaporize a small amount of corneal tissue beneath the surface of the cornea to reshape the cornea. LASIK is an acronym for Laser-Assisted in-Situ Keratomileusis, first described about 1993 by Greek eye surgeon Dr. Ionnis Pallikarus. I went to Greece in 1996 to observe and learn about this new procedure from Dr. Pallikarus.
To perform LASIK, the surgeon uses an instrument called a microkeratome to create and fold back a thin layer of cornea. The creation of this flap takes about 30 seconds and the only sensation the patient feels is pressure and an occasional pinching of the eyelid. As with the PRK procedure, advanced computer technology, including Wavefront technology is used to program the laser for your particular prescription.
As with PRK, the laser energy pulses will then painlessly remove the tissue. You'll hear a steady clicking sound as the laser is operating. The higher your prescription, the more time the surgery will take. The surgeon has full control of the laser and can turn it off at any time.
The thin flap of corneal tissue is folded back into its original position where it bonds after several minutes of drying. There are no stitches required. The entire procedure takes 10 or 15 minutes and the only anesthetic required is a numbing anesthetic eye drop. Since only the edge around the thin layer of corneal tissue is needed to heal, the visual recovery is rapid and patients report very little post-operative discomfort or pain. This discomfort usually dissipates in a few hours. The return of vision is rapid, and 80 to 90% of the visual recovery may be present by the next day. Antibiotic and cortisone eye drops are used for about a week. Most patients return to work the next day.
As with any surgical procedure, there are risks and complications. In LASIK, the most serious risks relate to the creation of the thin flap of cornea. Most of these problems can be managed but loss of significant vision is possible.
While LASIK is associated with a number of intraoperative and postoperative complications it's important to note that, according to most studies, the incidence of severe, vision-threatening complications is much less than 1%. Other less serious complications can occur in up to 5% of people who receive LASIK. Almost all LASIK complications, including the serious ones, can be treated and usually resolve within several months of surgery. Rarely, LASIK can cause permanent problems.
In January 2002, the American Academy of Ophthalmology — The Eye M.D. Association — reviewed the scientific literature and found LASIK safe and effective for correcting low-to-moderate nearsightedness and astigmatism, but less predictable for moderate-to-high nearsightedness.
The assessment found that serious complications that result in permanent visual loss occur rarely with LASIK, but side effects such as dry eyes, nighttime starbursts and reduced contrast sensitivity occur more frequently. A complete discussion of the benefits, risks, complications and alternatives are part of the initial consultation and subsequent visits. The US Food and Drug Administration (FDA) website is a good source of information:
http://www.fda.gov/cdrh/LASIK/what.htm
A copy of the booklet I use as a patient information booklet and informed consent document is viewable here:
Which Procedure Do I Favor?
Due to advances in PRK (ASA) in the last several years, I now favor this procedure for almost all of my laser vision correction patients. Despite the additional mild discomfort and longer time to obtain best vision, I feel ASA is the procedure of choice for many of my patients.
There are a number of advantages to surface ablation. These include:
-
Additional safety and minimal serious complications. As there is no corneal flap to be created, flap complications are not an issue.
-
Slightly better visual outcomes after healing than LASIK, although the visual acuity with both procedures is excellent. Studies presented at the recent meeting of the American Society for Cataract and Refractive Surgery (ASCRS) in April 2005 also indicated that the results of Wavefront guided ASA are the same, or may even be slightly better than Wavefront guided LASIK
-
Lack of induction of dry eye symptoms, seen in some patient with LASIK. The dry eye in LASIK is the result of cutting of the corneal nerves. It may last 6-12 months but usually improves sooner. PRK does not cause a dry eye.
-
With over 15 years of follow up, the long term safety and stability of surface ablation have been proven.
-
There is no cutting of the cornea and no significant weakening of the cornea. Corneal thickness issues are not as important. There are less serious consequences from accidental injury to the eye. This makes it the procedure of choice for athletes, military, police and others with some risk of eye injury.
-
Meets occupational requirements for most military and law enforcement agencies.
-
Does not interfere with the treatment of any eye disease that may develop in the future including cataract, glaucoma and macular degeneration.
-
Makes optimal use of Wavefront technology and custom ablation techniques. The reasons for this relate to the extremely precise and sensitive nature of the Wavefront sensing measurements that are done preoperatively. Studies have shown that even the most precise and non-traumatic flap creation during LASIK may induce minor aberrations. ASA, with only the corneal surface cells removed, probably induces minimal or no aberrations.
Disadvantages of ASA
-
Slower best visual recovery (several weeks) and interval of 1-2 weeks between surgeries for each eye.
-
Slightly higher risk of infectious and non-infectious superficial corneal infection, however these tend to be on the surface of the cornea. If promptly treated, the visual outcomes will usually be excellent.
-
Postoperative discomfort, usually mild, lasts until the corneal epithelium heals in 3-7 days.
-
The need for several more postoperative visits. They are brief checkups to assess healing.
Glasses and Contacts: Out of Your Life?
Our goal is to reduce dependence on optical devices. Many of our patients function without glasses or contacts after a lifetime of being dependent on them. The reality is that some people still will need glasses or contact lenses for some tasks. A person may need them for clearer vision while driving or at night, or if past the mid-forties, may need them for reading. The eyeglass correction is usually much less than before the surgery.
For more information, please see FAQ and other links.
What Do Our Patients Think?
The overwhelming majority of our patients are happy with the decision to undergo laser vision correction. A small percent are not happy due to failure to meet their goals and expectations or a complication.
For the first-hand reactions to a number of our patients who have undergone refractive surgery, press here.
back
to top
