What's The Difference?
Confused about Laser Vision Correction?
You see and hear ads for PRK (Photorefractive Keratectomy), LASIK (Laser in Situ
Keratomileusis), LASEK (Laser Assisted Subepithelial Keratectomy), IntraLase®
, LTK (Laser Thermal Keratoplasty), CK (Conductive Keratoplasty), and Custom LASIK
(i.e., CustomCornea® or CustomVue™). Learn about the different types of
procedures here!
How the Eye Works
How is vision measured? The "20/20", "20/40" etc. that you hear from your eye doctor is
refered to as your "visual acuity." If you have 20/20 vision, you will see what a normal
person sees from 20 feet. If your vision is 20/40 (driver’s license requirement)
you will see at 20 feet what a person with 20/20 will see from 40 feet. At the
20/200, you will see at 20 feet what a normal person will see from 200 feet.
Click below on a visual acuity measurement for a visual simulation:
Glasses prescriptions are measured in diopters, a measurement
of the optical power of the lens. This is usually written with a sphere and
a cylinder. The sphere (round, not shaped like a football) numbers can range
from +15 to over –30. However, most patients are from +6.00 to –6.00.
Cylinder measures the amount of astigmatism and can range up to 6 diopters or
more. Cylinder can be written in a plus or minus cylinder format. Most patients
have less than 3.00 diopters of cylinder. A positive number indicates hyperopia
(farsightedness) and a negative number, myopia (nearsightedness). The larger
the diopters value (positive or negative), the greater the refractive error.
A typical glasses prescription would read:
-3.50 -1.75 x 70o Add 1.75
-2.75 -1.50 x 110o Add 1.75
The top line by convention is the right eye. In this example,
the right eye has -3.50 diopters of sphere and –1.75 diopters of cylinder
at an axis of 70 degrees (the direction of the cylinder). An "Add" is for bifocals
and it is usually needed for people in their 40s and older.
The diopter rather than visual acuity is the precise way of measuring the amount
of correction one needs. Larger numbers require thicker glasses or more corneal
tissue removal for correction of the refractive error.
Click on the tabs below and see changes in refraction:
Options for Presbyopia
Monovision or blended vision - A Solution For Most Presbyopes
Laser vision correction can be used to create Monovision. The refractive error
is corrected in the dominant eye and the other eye is set to see near. This
can also be done in a patient who can see distance well in both eyes. One eye
can be made nearsighted with the excimer laser, LTK or CK. LTK or CK can only
correct hyperopia to make an eye slightly myopic.
Successful adaptation to Monovision occurs in 3 to 4 weeks in about 80% of
patients. The main advantage is not needing reading glasses for seeing near.
The disadvantage is some decrease in depth perception. It has been our experience
that patients in their 40s adapt to Monovision in one to two weeks and patients
in their 60s may take a few months. If a patient does not have significant astigmatism
we have them try Monovision with contact lenses. A few of the patients use glasses
for night driving or for prolonged computer work or reading. Some Monovision
patients use one disposable contact lens in their near eye for good binocular
vision during sports, such as tennis. In rare situations, patient later decided
to have both eyes corrected for distance. The near eye can be ‘enhance”
to allow distance vision.
Dr. Oyakawa had laser vision correction (PRK) with Monovision. He uses prescription
sunglasses for driving, no glasses for night driving, and glasses for prolonged
computer work, reading, and close mechanical work. He is able to read the newspaper
in the morning and function through out the day without glasses. During patient
eye examinations and surgeries the eyepieces of the instruments are adjusted
for binocularity.
PRELEX
Another option for presbyopia is clear lens extraction with a multifocal lens
(Array® IOL) implants in both eyes. This is essentially the same procedure
as a cataract surgery and is termed PRELEX
(Presbyopic Lens Exchange). This procedure is recommended in patients with early
signs of cataracts and who desire refractive surgery with the ability to see
both at distance and near in each eye. This procedure avoids Monovision or blended vision. Not
needing cataract surgery in the future is a major advantage of this procedure.
Excimer Laser Vision
Correction
Laser vision correction with the excimer laser is a two step procedure: first
an inner layer of the cornea is exposed, then the computer controlled excimer
laser reshapes the inner layer(s) to correct refractive errors (myopia, hyperopia
and astigmatism). A different pattern of excimer laser ablation is used
for each type of correction. The terms PRK, LASEK (also called epi-LASIK or
Advanced Surface Ablation), LASIK, and IntraLase® are just different techniques
to expose the inner corneal layer prior to treatment with the excimer laser.
PRK and LASEK are Non-Flap procedures and LASIK and IntraLase® are Flap
procedures.
Flap and Non-Flap Procedures
Click on tabs to view demonstrations:
Comparison of Flap and Non-Flap Procedures
Flap procedures have flap complication and Non-Flap procedures do not have flap
complications but have slower recovery and more pain.
All the methods share the risk of temporary or permanent dry eyes, night vision
problems such as halos, glare, or starbursts, undercorrection
or overcorrection, and loss of best corrected visual acuity (vision can not be
corrected with glasses to the preoperative level with glasses) regression and
other complications.
Flap Procedures: LASIK and
IntraLase®
In the 1950s, Dr. Jose Barraquer developed the microkeratome to remove corneal
tissue to correct refractive errors. The design is based on the “carpenter’s
plane” and subsequent refinements were used in Automated Lamellar Keratoplasty
(ALK) surgery into the early 1990s. Two cuts were made in the cornea. The first
cut made a flap and exposed the inner cornea and the second cut removed a disk
of tissue to correct myopia and the flap was replaced. Unfortunately, it was
not very accurate and very difficult to perform. The first cut was relatively
easy, but making the second slice precisely was difficult. It is similar to
cutting a first cut (flap) in a melon and then making a second slice of precise
thickness (to correct the refractive error). The excimer laser was introduced
in the early 1990s and it was able to remove corneal tissues precisely allowing
improved results with refractive surgery. PRK was the first procedure to use
the excimer laser to correct myopia. However, it was not well accepted by patients
due to its moderate discomfort, delay in visual recovery, and the usual preference
of doing one eye at a time.
European surgeons combined the first step of ALK surgery to create a flap and
the second step of precise removal of corneal tissue with the excimer laser.
This has become the traditional LASIK procedure.
Traditional LASIK is the main refractive surgery performed due to very little
if any post-operative pain, more rapid recovery, less postoperative haze, easier
enhancement procedures, and reduced need for long-term steroids. The main disadvantage
is flap related complications. While complications
have been rare, many have been directly related to how precisely and uniformly
the flap was created. Complications increase in eyes with extreme curvatures
(corneas too steep, K>47 or too flat K< 40) and small eye openings. Epithelial
abrasions increase in patients with anterior basement membrane dystropy (ABMD)
and diabetes.
Just as removal of corneal tissue has evolved from using a mechanical microkeratome
in ALK to an excimer laser in LASIK, flap creation now can be done with a laser.
This is now an All-Laser Solution™. Rather than a surgical blade, the IntraLase™
approach uses the IntraLase® Femtosecond Laser to create a CUSTOMIZE
CORNEAL FLAP. The main advantage is that intra operative flap complications
may be practically eliminated. Additional advantages include the ability
to make a CUSTOMIZE CORNEAL FLAP of precise diameter, uniform thickness,
desired thickness, and position. These capabilities are important in large refractive
errors, large pupils, small corneas, and thin corneas. Flap creation is possible
in eyes too small to fit a microkeratome (Asians and others with small eye openings).
The uniform thickness and vertical edges decreases post-operative flap complications
such as slipped or wrinkled flaps caused by accidental rubbing or trauma the
eye. In addition, the vertical edge is felt to makes it more difficult for epithelial
cells to migrate under the flap.
The main disadvantage of the IntraLase® procedure compared to traditional LASIK is the increase in time required for the total procedure. It takes longer to
create a flap, about a minute versus a few seconds for a microkeratome. Then,
the patient has to be moved to the excimer laser for the correction of the refractive
error. Also, there is more redness of the eye due to the longer period of time
the suction ring is on the eye. However, the IntraLase® procedure uses a lower pressure
on the eye than a microkeratome and this is thought to be safer. Cost is another
disadvantage. The IntraLase® Femtosecond Laser is about the same price
as an excimer laser with a similar royalty fee to the manufacturer.
Non-Flap Procedures: PRK and LASEK
The main advantage of a non-flap procedure is that no flap complications are experienced.
Flap complications increase in patients with extreme curvatures (corneas too
steep, K>47 or too flat K< 40), anterior basement membrane dystrophy (ABMD),
and in patients with small eye openings. Additional advantages exist for patients
with corneas too thin in relation to the amount of tissue ablation needed (more
tissue removed for larger corrections and larger ablation zones which may be
needed for larger pupils), mild corneal scars, and previous glaucoma surgery.
The main disadvantages are mild to moderate pain after surgery for three to
four days, slower visual recovery than LASIK or IntraLase®, and the risk
of corneal haze or scaring. Patients may need to use eye drops for months.
PRK
and LASEK are essentially the same procedure except for rolling back the epithelium.
This acts like a second bandage layer. However, this tissue is not viable (like
a scab) and has to be replaced with new epithelium just like PRK. Some refractive
surgeons feel this may actually slow the recover.
LASEK may have slightly less
pain and possibly faster visual recovery than PRK. Advocates of LASEK feel that
it avoids the pain of PRK and the flap complications of LASIK. Due to the slower
recovery of vision, many surgeons do one eye at a time for PRK/LASEK. The surgery
on the second eye is done a week or two later after the vision has recovered
sufficiently in the first eye for the patient to function. This may present
a problem for patients who wear glasses rather than contact lens since the two
eyes may have difficulty working together.
In rare situations (In addition to
the risk factors listed above, anterior scleral buckles, optic nerve disease,
a risky occupation or activity, or stable keratoconus) PRK or LASEK may be the
only choice. Recovery from PRK is now much faster and less painful with less
complication with the newer excimer lasers compared to first generation excimer
lasers. Dr. Oyakawa’s recent upgrade with CustomCornea® PRK recovered
much more quickly and with less pain than his original procedures six years
earlier.
Excimer Laser
Technology
The excimer laser reshapes the cornea to correct the refractive error. The cornea
is “flattened” by removing tissue from the center of the cornea to correct
myopia. The periphery of the cornea is treated in a donut shape to “steepen”
the central cornea to correct hyperopia. Astigmatism is treated with a cylinder-shaped
pattern to remove the “football” shape of the cornea.
The excimer laser was developed in the late 70s by IBM to make extremely precise
microcircuit patterns in computer chips. This is a human hair (about 1/5th to
1/6th the thickness of the human cornea) cut by an excimer laser.
Unfortunately, the human eye is not an inanimate computer
chip or hair and constantly moves making accurate placement of the laser beam
difficult. Accurate tracking is required for precise beam placement.
There are two major classes of excimer lasers used in refractive surgery: broad-beam
lasers and scanning lasers. Scanning lasers are either slit scanning or spot
scanning. The broad beam lasers as the name implies uses a relatively large
beam diameter from 5.5 to 9.0 millimeters. These were the first excimer lasers
and resulted in the shortest procedure time. Ablations are not as smooth as
the scanning lasers. At first, there were complications of central
islands but were later avoided with improvements in software. Slit scanning
lasers use a rotating device with slit holes that can change size during the
ablation. They have a uniform beam and smoother ablation than broad beam lasers.
Spot scanning lasers have the smoothest ablations and can treat small irregularities
in the cornea.
There are many brands of excimer lasers. At Sharper Vision
Centers, we use the LADARVision® 4000 and VISX Star S4. Both have the ability
to perform Custom LASIK.
LADARVision® 4000 LADARVision® 4000 has the most advanced technology of any excimer laser
system. It is the only system that has a closed-loop, laser radar tracker, registration,
and a small-spot Gaussian beam.
Laser Radar Tracker
Click on tabs to learn more about tracking the eye's movement:
Registration
This is the only laser system with registration, a process where the eye can
be marked prior to treatment to correct for cyclotorsion and for proper alignment
for CustomCornea®. Cyclotorsion is important in laser vision correction
because measurement of the eye’s refractive errors and wavefront analysis
are made in a sitting position and laser treatment is done in a supine position.
As a patient moves from a seated to a supine position (laying down), naturally
occurring cyclotorsion can cause the eyes to rotate from 2 to 12 degrees. Astigmatism
correction is less accurate without compensation for cyclotorsion. Registration
is critical in CustomCornea®. It allows the accurate transfer of the information
captured during wavefront analysis to the LADARVision®4000 by matching the
position of the eye during wavefront analysis and laser treatment.
The flap is created. Then the eye is tracked, registered, and lasered.
Small-Spot Gaussian Beam
In addition to the most advance eye tracker available, the LADARVision®
4000 also has a small spot laser beam with a Gaussian profile that allows for
improved resolution. It produces a smoother surface ablation than a “flat
top “ surface provided by a broad beam laser.
It has a programmable pattern to allow the largest optical
zone to significantly decrease the risk of glare and halos and to allow for
custom treatments. LADARVision® 4000 is the first system approved by the
FDA for wavefront-guide customized ablation (CustomCornea®) that may improve
the quality of vision. It also has a broad range of treatment and is able to
treat myopia and hyperopia with or without astigmatism, and mixed astigmatism.
CustomCornea® is the LADARVision® 4000’s
wavefront guided custom laser vision correction. This is a CUSTOM LASIK procedure.
VISX S4 ACTIVE TRAK™
VISX is the market leader in laser vision correction and was the second laser
approved in the U.S. for treatment of myopia. Initially a broad beam laser,
it has been continuously upgraded and now has a variable spot beam and a video-based
tracker. This is not an active tracker. This is an open-loop system that follows
and shuts off the laser if the eye moves beyond limits set by the doctor. It
cannot measure and respond to eye movements fast enough to actively track the
eye for super-accurate laser spot placement. The VISX Star S4 laser has an excellent
track record and is a good choice for many patients. It is the second laser
to have FDA approval for wavefront guided custom ablation, CustomVue™.
It works on a similar principle to CustomCornea®. However, unlike CustomCornea®,
the pupil is not dilated for the wavefront measurement and treatment.
Dr. Oyakawa uses both the VISX Star and the LADARVision®4000.
He has treated many physicians, staff, and family members (daughter, brother,
in-laws, and cousins) on the VISX laser. Dr. Oyakawa had his eyes treated with
the VISX laser in 1997 and had his distance eye upgraded with CustomCornea®
on the LADARVision®4000.
Custom LASIK - Wavefront Guided Custom Laser Vision Correction
Traditional methods to determine refractive errors only measure sphere, cylinder,
and axis(see refraction). These are considered lower
order aberrations. They are determined in a subjective manner by the ophthalmologist,
optometrist or technician and the patient. The accuracy, as in any subjective
test, depends on the ability of the patient to be able to detect differences
in changes in refraction during the testing and to communicate this to the doctor
or technician. We all know how difficult it sometimes can be to determine which
is better “one or two”. The refraction (sphere, cylinder, and axis)
is programmed into the excimer laser for treatment. This is essentially using
a glasses prescription for your laser correction. They only correct lower order
aberrations.
There are also higher order aberrations that cannot be determined by refraction.
It is believed that these higher order aberrations contribute to glare, halos,
ghosting, night vision problems and a decrease in the quality of vision. Higher
order aberrations are pupil-size dependent and increase with pupil size. Pupils
are larger at night and in dim light conditions. These higher order aberrations
cannot be corrected with glasses, contact lens, or conventional LASIK.
Wavefront analysis started in 1900 by astronomer Johannes Hartman. He devised
a method for measuring the ray aberrations of mirrors and lenses by using a
metal disc with holes to isolate rays of light so they could be traced. Roland
Shack in 1971 modified the Hartman screen to use a lenslet array. This modification
is now called Hartmann-Shack wavefront sensor. This technology is used in adaptive
optics to compensate for atmospheric aberrations caused by turbulence in the
earth’s atmosphere. This technology has now been adapted to refractive
surgery.
Rather than relying on a refraction, wavefront-guided laser correction uses
wavefront analysis to determine - in addition to sphere, cylinder, and axis
- higher order aberrations that can affect the quality of vision.
Wavefront Measurement
Traditional Refraction
It measures the entire visual system of the eye. These higher order aberrations
cannot be determined by refraction.
An objective wavefront analysis tool (LADARWaveTM and Wavescan™
) does not rely on patient’s subjective responses. This computerized wavefront-measuring instrument passes flat waves of light through the eye and measures distortions
in the wavefront of the reflected light.
The LADARWave™ or Wavescan™ generates a 3D map of the unique visual distortions of the eye including both lower and higher order aberrations.
Wavefront Scanning and CustomCornea®/CustomVue™
Dr. Oyakawa has had his distance eye upgraded with CustomCornea®. He had a small residual astigmatism and some higher order aberrations from his original PRK with a visual acuity of 20/25 minus with some ghosting. His visual acuity improved to a very sharp 20/15 after CustomCornea® with significant improving in ghosting.
In summary one can select a combination of Flap or Non-Flap procedures, excimer
laser types and wavefront-guided treatments.
At Sharper Vision Centers we offer Flap and Non-Flap procedures and use both the LADARVision® 4000 with or without Custom Cornea® and VISX Star S4
with or without CustomVue™.
Dr. Oyakawa’s preference is IntraLase® with the LADARVision®
4000 with CustomCornea® or VISX Star S4 with CustomVue™ for safety
and results.
Advantages and Disadvantages
All the methods share the risk of temporary or permanent dry eyes, night vision
problems such as halos, glare, or starbursts,
undercorrection or overcorrection, and loss of best corrected visual acuity
(vision can not be corrected with glasses to the preoperative level with glasses)
regression and other complications.
The main advantage of a Non-Flap procedure is no flap complications. Flap complications increase in patients with extreme curvatures (corneas too steep, K>47 or too flat K< 40), anterior basement dystrophy (ABMD), and in patients with small eyelid openings. Additional advantages exist for patients with corneas too thin in relation to the amount of tissue ablation needed (more tissue removed for larger corrections and larger ablation zones which may be needed for larger pupils), mild corneal scars, and previous glaucoma surgery. The main disadvantages are mild to moderate pain after surgery for three to four days, slower visual recovery than LASIK or IntraLase®, and the risk of corneal haze or scaring. Patients may need to use eyes drops for months.
PRK and LASEK are essentially the same procedure except for rolling back the epithelium. The LASEK epithelium acts like a second bandage layer. However, this tissue is not viable (like a scab) and has to be replaced with new epithelium just like PRK. Some refractive surgeons feel this may actually slow the recovery. LASEK may have slightly less pain and possibly faster visual recovery than PRK. Advocates of LASEK feel that it avoids the pain of PRK and the flap complications of LASIK. Due to the slower recovery of vision, many surgeons do one eye at a time for PRK/LASEK. The surgery on the second eye is done a week or two later after the vision has recovered sufficiently in the first eye for the patient to function. This may present a problem for patients who wear glasses rather than contact lens since the two eyes may have difficulty working together. In rare situations (In addition to the risk factors listed above, anterior scleral buckles, optic nerve disease, a risky occupation or activity, or certain patients with keratoconus) PRK or LASEK may be the only choice.
Complications
and Their Treatments
LASIK surgery has a learning curve and complications decrease with experience.
The chance of a serious vision threatening complication is less than 1%. Most
of these are related to the creation of the flap.
Epithelial Abrasions: Can occur and are treated with bandage contact lenses. Epithelial abrasions are more common in patients with diabetes and anterior membrane corneal dystrophies (superficial corneal problems).
Free Cap: Originally, the flap was completely detached
from the cornea and replaced after the laser correction. Now, a small attachment
was left on one side, a hinge, to allow easier replacement and alignment of
the flap. A free cap requires the extra step of replacing and realigning the
flap.
Partial flaps and Buttonhole flaps: These complications are extremely rare. When they occur, it is best to replace the flap and let the cornea heal. In 3 to 6 months, recut the flap.
Flap incorrectly positioned-not centered or of incorrect diameter: Usually not a problem unless a large ablation zone is required. This may require replacement of the flap and returning in 3 to 6 months to recut the flap.
Flap too thick or too thin: A flap that is too thick may result in termination of the surgery for an eye needing a large amount of ablation. There is a recommended amount of corneal tissue that needs to remain after the flap and laser treatment. Leaving less than this amount may lead to long-term corneal problems such as ectasia (abnormal bulging). It may not be possible to recut the flap in the future. Replacement of the flap usually maintains vision.
Ectasia may be treated with gas permeable contact lenses; if severe a corneal transplant surgery may be required. A flap too thin may be more difficult to smooth out and may result in striae (wrinkles) that can reduce vision.
Corneal perforation: This is the most serious complication and occurred with early microkeratomes.
Bleeding from peripheral corneal vessels: This is usually treated with drops that constrict the blood vessels. The bleeding stops when the flap is replaced.
Laser complications: Decentered
ablations and central islands were seen with earlier broad beam lasers. These
complications have been largely prevented with advances in excimer laser technology.
Postoperative (During the Healing Period)
Flap Related
with Microkeratomes
Diffuse lamellar keratitis (DLK): Is a rare condition and it usually occurs one to three days after surgery. Patients usually report blurred vision, pain and sensitivity to light. Some patients may not have any symptoms. It is treated with steroid drops and rarely with irrigation under the flap. This condition can be treated without significant visual loss when it is detected and treated early. It is important to attend all scheduled postoperative visits
Flap wrinkles: They usually do not affect the patient’s vision. If they reduce the acuity or quality of the vision, the flap can be lifted and wrinkles removed.
Flap slippage: Repositioning of the flap treats this. It usually occurs on the same day or night of surgery by rubbing your eye. It is important to use the protective eye shield after surgery for a few days.
Epithelial ingrowth: The eye doctor detects this. A small amount of epithelial ingrowth is seen in a few percent of microkeratome cases. It should be removed if it progresses and affects vision, or creates other problems. It is important to attend all scheduled postoperative visits.
Infection: Extremely rare. It is important to use the antibiotic drops as directed and not to get unclean water or other substances into your eyes during the healing period. Avoid the ocean, pools, hot tubs, etc. If infection is left untreated, it can result in permanent loss of vision. It is important to attend all scheduled postoperative visits and report any problems to your surgeon.
Non-Flap Related
Dry eyes: Your eyes may feel scratchy and burning.
This tends to return to normal after a few months. It is important to use artificial
non-preserved tears frequently. If severe, punctual plugs can be used to temporally
plug the tear drainage site to keep your natural tears in the eye.
Glare and haloes are seen especially at night around
light. They usually decrease over one to two months. They are more common in
patients with large pupils and large refractive errors. They were more common
with first generation lasers using a small optical zone.
Long-Term Complications After the Healing Period Overcorrection, undercorrection, and regression: The desired correction was not achieved. This is due to many factors and each person’s eye heals differently. The surgical plan is based on an average response to treatment. Some patients have an increased or decreased healing response. In addition to the state of corneal hydration, the humidity and other unknown factors contribute to the response to excimer laser surgery. These conditions can usually be enhanced (retreatment by lifting up the previously created flap and treating with the excimer laser) when stable. Enhancements are done three to six months after surgery when the eye is stable. It may take longer in eyes with larger corrections to become stable.
Persistent glare, haloes, starbursts, or ghosting: Very uncommon with larger ablation zones. These may be corrected by wavefront-guided custom ablation such as CustomCornea®. Eye drops that constrict the pupil may be of benefit.
Dry eyes: Most cases of dry eyes resolve in a few months. For the few who have persistent dry eyes, temporary or permanent punctal plugs and or drops may offer relief.
Loss of best-corrected visual acuity: Some patients may not see as well with glasses after surgery as before surgery with glasses. Enhancement may help some of these patients.
Aggravation of muscle imbalance: This can occur in patients who have had eye muscle surgery or have an imbalance corrected by glasses.
Caution about glaucoma: After LASIK surgery your cornea
will be thinner and the intraocular pressure will measure falsely low.
