Transcript Refractive Surgery Techniques and Technology 101 Clay Falknor, M.D. Presbyterian Hospital of Dallas August 9, 2005

Refractive Surgery Techniques
and Technology 101
Clay Falknor, M.D.
Presbyterian Hospital of Dallas
August 9, 2005
Case Example
A 26 yo CM fourth-year medical student presents
with best corrected vision of –4.50 OD and –
5.00 OS, now with c/o wanting to get rid of his
contacts, and wanting to see the alarm clock in
the morning so he won’t be late for rounds. He
presents to a cornea fellow at a certain
prestigious medical school for refractive surgery
consultation holding a discount coupon he got
by email.
Basics of Optics
[1]
[1]
Basics of Optics
P = Power
np = nodal point
f,f’ = primary and
secondary focal
points
n,n’ = refractive
indices
[1]
Refraction is measured in diopters (D) = reciprocal of focal
length in meters
• Refractive
media of
eye
• Transparent
media
[2]
Accommodation
• Amplitude of accommodation is the
number of diopters the eye can
accommodate
• Accomplished by the variable optical
power of the crystalline lens
[3]
Vision Correction Basics
• Far point: the point at which an object
must be placed along the visual axis for
light rays to be focused on the retina
when the eye is not accommodating.
• Near point: the point at which an object
will be in focus on the retina when the eye
is fully accommodating. Any object closer
than the near point will not be in focus.
Emmetropia
[4]
• The far point for the emmetropic eye is at infinity.
• Nearer objects brought into focus on retina with
accommodative power of the lens.
Myopia (Near-sightedness)
[4]
• The far point for the myopic eye is between the cornea and infinity
• Refractive myopia: too much refractive power due to steep corneal
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curvature or high lens power
Axial myopia: elongated globe (each 1mm axial elongation = 3D myopia)
Overall, the cornea and lens are too “strong” for the length of the globe.
Distant objects unclear, but near objects within focal point of the eye seen
clearly.
Correct with minus lens
Hyperopia (Far-sightedness)
[4]
• The far point for the hyperopic eye is beyond infinity
• The cornea and lens are too “weak” for the length of the globe.
• Distant objects focused on the retina with accommodation, but
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clear near vision is difficult)
Correct with plus lens
Astigmatism
• Very common, up to 95% of eyes w/ detectable
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astigmatism, and 10-20% >1D.
Can be naturally occurring or surgically induced.
Most often caused by a toric cornea, and less
commonly by astigmatic effects of the lens
Refractive power of the eye different in various
meridians. Light can never be brought into focus
on a single point regardless of distance.
May occur with either myopia or hyperopia.
Regular astigmatism termed “with the rule” when
steepest corneal meridian close to 90°, and
“against the rule” when close to 180°.
When astigmatism regular but not close to 90° or
[1]
180°, termed oblique.
Astigmatism
[1]
Astigmatism
[4]
[1]
• Astigmatism creates two focal lines, one closer to the cornea
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formed by the more powerful corneal meridian, and the second
further from the cornea formed by the less powerful meridian.
The Circle of Least Confusion is the smallest cross-sectional
area between the two focal lines, a circular cross-section of the
conoid of Sturm.
The goal of refractive correction is to place the circle of least
confusion on the retina.
Presbyopia
• As the crystalline lens hardens with age, it is no
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longer able to attain the more spherical form,
leading to decreased accommodation.
Net Effect: A lessening of the accommodative
amplitude and a increase in the near point of the
eye.
Usually has onset in 5th or 6th decade.
Presbyopia is not correctable with laser surgery,
and in fact, the surgery may hasten its noticeable
development
Solution: Reading glasses
More vision correction basics
• A lens with the focal point coincident with
the far point of the eye allows parallel
light rays from infinity to be focused on
the retina
Spectacles and Contact Lenses
• Correct both sphere and cylindrical refractive errors.
• Prescription read as spherical error, then cylindrical
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error, then axis of astigmatism.
Myopia treated with concave lenses with minus power
(divergent) to focus light on the retina.
Hyperopia treated with convex lenses with plus power
(convergent).
Astigmatism corrected with cylindrical lenses.
Ex. of myopic Rx w/ astigmatism: -3.50 + 1.50 x 090
Ex. of hyperopic Rx w/ astigmatism: +4.00 – 2.00 x
180
[1]
Spectacles and Contact Lenses
• Aberrations induced by thick lenses
1. Spherical aberration: light at periphery of lens refracted
more than at center, causing night myopia due to larger
pupil at night.
2. Coma: A comet-shaped blur when object and image are
off the optical axis.
3. Astigmatism of oblique incidence: When a spherical lens is
tilted, it gains a small astigmatic effect causing curvature
of the field. Helpful since matches curvature of the retina.
4. Chromatic aberration: Shorter wavelengths bent more.
5. Distortion: Greater magnification in periphery. A high plus
lens produces pincushion effects, and high minus produces
barrel distortion.
[1]
Refractive Surgery Techniques
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Radial Keratotomy (RK)
Freeze keratomileusis
Photorefractive Keratectomy (PRK)
Laser Epithelial Keratomileusis (LASEK)
Laser-assisted in-situ Keratomileusis (LASIK)
Others:
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Astigmatic Keratotomy (AK)
Intracorneal ring segments (Intacs)
Phakic Intraocular Lens Implants
Refractive lensectomy
Radial Keratotomy
• First used in U.S in 1978.
• Treats low to mod myopia in outpt
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setting using topical anesthetics.
RK reduces myopia by steepening
the cornea peripherally, which
secondarily flattens the cornea
centrally.
The surgeon makes deep radial
incisions with a diamond blade in a
spoke-like pattern, leaving a clear
optical zone in the center.
Refractive effect determined by the
number, length, and depth of the
incisions, as well as the size of the
spared central optical zone.
The smaller the optical zone, the
greater
the
central
corneal
flattening (reduction in myopia),
but greater risk of side-effects.
[5]
RK
[6]
• 8 incision RK shown above with uniform central optical zone
• American (centrifugal with angled blade), Russian (centripetal with vertical
blade), and combined techniques
• Russian technique allows for deeper incisions and more refractive effect, but
greater risk of entering optical zone.
• Standardized nomograms based on analysis of previous cases determine the
number of incisions and size of optical zone to create a given refractive
effect. Typically 4 incisions for low myopia, 8 for moderate.
RK
[1]
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[6]
Infected RK incision shown above left
16 incision RK w/ hypertrophic scarring & irregular optical zone above right
Advantage: RK patients have excellent uncorrected vision on POD1.
Disadvantage: As with other surgical procedures, incisions leave permanent
changes to the cornea, as contrasted with contact lenses or spectacles. The
cornea may be left weakened.
• Complications include: glare, diurnal fluctuation in refraction, hyperopic
shift, corneal perforation, infection.
RK Evidence
• Prospective Evaluation of Radial Keratotomy (PERK)
– 60% of RK treated eyes were w/in 1D of emmetropia up to
10 years post-op.
– After 10 years, 53% at least 20/20 uncorrected, and 85% at
least 20/40.
– 43% eyes w/ progressive shift toward hyperopia ≥ 1D after
10 years, and worse for eyes w/ optical zone <3mm diam.
– Only 3% pts lost 2 or more lines of best corrected acuity,
and all 20/30 or better best-corrected.
– <1% c/o severe glare or starburst during night.
– 2% with corneal perforation, none req’d suturing.
– Best results for low myopia group (-2.00 to -3.00D)
[1]
Laser technology
• Excimer laser: EXCited dIMER
• AKA “cool laser beam” because little thermal damage to
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adjacent tissues.
193nm wavelength ultraviolet laser with sufficient energy
to disrupt intermolecular bonds within the corneal stromal
tissue (photoablative decomposition).
First excimer lasers FDA approved in 1995, with beam
width 4-5mm, now available less than 100 microns.
Each laser pulse removes a given volume of stroma
Three types of laser application: wide-area ablation,
scanning slit, and flying spot lasers.
[1,4,5]
Laser technology
• In myopia, laser flattens central cornea to decrease its
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focusing power to bring secondary focal point back to
retina.
In hyperopia, the laser removes peripheral corneal tissue
thereby secondarily steepening the central cornea,
increasing the focusing power of the cornea.
Astigmatism treated with elliptical or cylindrical beams
that flatten the steepest corneal meridian.
To minimize glare and halos, optical zone should be
larger than the dilated pupil.
Corneal Topography
• Computer-based videokeratography used to evaluate the corneal curvature.
Most systems use a video camera to detect reflected images of rings
projected onto the cornea, while others use slit beams, which can also
measure the corneal thickness.
• Pre-operative and post-operative topographic maps can be used to generate
a “difference map” to isolate the procedure-induced changes.
• Subtle abnormalities, such as early keratoconus or contact lens-induced
corneal warping can be picked up.
[1]
Photorefractive Keratectomy
• PRK can effectively treat low to
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mod myopia or hyperopia +/astigmatism.
Performed as outpt with topical
anesthesia.
First, the corneal epithelium in the
area to be ablated is removed to
expose Bowman’s layer and the
underlying corneal stroma
(spatula, laser).
Excimer laser then applied as
directed by the corneal
topography-driven computer
program.
Topical antibiotics, steroids, and
NSAIDs applied, along with a
bandage contact lens (BCTL)
[5]
PRK
• In the post-op period, pt may experience tearing,
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photophobia, blurred vision, and discomfort due to
abrasion of central epithelium.
This can be controlled with topical steroids and NSAIDs.
Pts occ. require systemic analgesia for severe pain
BCTL removed once epithelial defect healed (avg 3-4
days).
Abx continued several more days, and steroids for up to 3
months post-op.
Visual acuity improves once the epithelial defect heals,
but fluctuates for a few months and finally stabilizes at
~3 months.
Glare, halos, and dry eye symptoms common the first
month post-op, usually diminishing/disappearing by 3-6
months.
[4,5]
PRK
[6]
[1]
[5]
[5]
• Left: mild stromal haze at 3 months
• Center: moderate-to-severe stromal haze at 6 months
• Right: light microscope of rabbit cornea showing epithelial hyperplasia in
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ablated region
Bottom: fluorescent microscope in rabbit cornea 1 month post PRK showing
new connective tissue deposition between stained old stroma and
epithelium.
Post-op corneal haze seen in a minority of patients at 3 months, and in none
at 1 year
Initially following PRK, corneal epithelium hyperplastic, modifying refraction.
Deposition of new collagen and GAGs by activated stromal keratocytes,
manifesting as stromal haze or subepithelial scarring.
PRK Evidence
• A 2001 prospective study of 72 cases showed with
significance that a larger ablation area (7mm) with
transitional zones has less pronounced corneal optical
aberration after PRK than with first generation (5mm)
ablation areas with out transitional zones. [7]
• A 1999 multi-center prospective study demonstrated PRK
to correct myopia from -1 to -10D +/- astigmatism showed
refractive stability, excellent UCVA with no significant loss
of BSCVA, and very low levels of corneal haze at one year
post-op. [8]
• A 2004 British 12-year prospective follow-up study of PRK
patients showed that for mild myopia, refractive stability
achieved at 1 year was maintained to 12 years without
hyperopic shift, diurnal fluctuation, or late regression in
the long term. Night halos remained a significant problem
in the subset of pt’s with small ablation zones. [9]
PRK Evidence
• A large 1998 prospective study (3000 cases) in Spain to
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monitor complications of PRK for myopia +/- astigmatism at
two years revealed that only 0.7% of eyes lost 2 or more
Snellen lines for BCVA at one year post-op. Retreatment for
undercorrection was performed in 7% of the low myopia group
and 39% of the high myopia group. There were no cases of
progressive hyperopia. Severe corneal haze was only present
in 0.5% at one year. Only rare occurrences of surgically
induced astigmatism (0.5%), delayed re-epithelialization, or
recurrent corneal erosion. [10]
Results of PRK are comparable to RK for similar magnitudes of
myopia.
– 68% w/in 1D of emmetropia uncorrected (RK 60%)
– 60% with at least 20/20 uncorrected (RK 53%)
– 90% with at least 20/40 uncorrected (RK 85%)
• For low to mod myopia (-1.5 to -3.0D), 80% ≥ 20/20
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uncorrected
Hyperopic shift infrequent in PRK compared to RK.
Keratomileusis
[5]
Laser Sub-Epithelial
Keratomileusis
• LASEK can treat mild to moderate myopia and hyperopia
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+/- astigmatism.
Can be performed as an outpt with topical anesthesia
The corneal epithelium is incompletely incised using a
microkeratome with a 70 micron deep blade.
A hinge is left at the 12 o’clock position.
Dilute alcohol (20%) drops are applied to the exposed
tissue and left for ~30 seconds. The area is then
washed with water and allowed to dry. The excimer
laser is applied as in PRK to the sub-epithelial stroma.
The epithelial flap is repositioned afterward.
[4,5]
LASEK Evidence
• In theory, since the flap is repositioned with the
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epithelium intact, there is less post-op pain, faster visual
recovery, and less incidence of infection.
A 2004 randomized prospective clinical trial at Travis Air
Force base compared LASEK with PRK in different eyes
in the same patients (n=30) for subj. pain levels, visual
acuity, and corneal healing.
– No statistical advantages in pain levels or in visual acuity.
– There was a statistically significant smaller median epithelial
defect in the LASEK-treated eyes on POD1, but by POD3 the
PRK defects were smaller and by POD7, there were no
detectable defects in either group.
– Overall, no clinical advantage was seen in LASEK over PRK. [11]
LASEK Evidence
• A 2002 non-randomized, retrospective study of 58
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LASEK-treated eyes with myopia +/- astigmatism
resulted in 45% with 20/40 or better UCVA at POD1, and
89% at 1 month, and 97% at 6 months, with 73% with
UCVA 20/20 or better. 7% of eyes had visually
significant corneal haze at 6 months, and no eyes lost 2
or more lines of BSCVA. [12]
Similar results found in a 2002 South Korean study with
6 month follow-up of LASEK treated eyes for low to
moderate myopia (-3.25 to -7.00D). [13]
Laser-assisted in-situ Keratomileusis
• LASIK can treat mild, moderate, and high
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myopia and hyperopia +/- astigmatism.
Can also be performed as an outpt with
topical anesthesia
LASIK is now the most commonly performed
refractive surgery in the world.
A suction ring is applied to the anesthetized
cornea and a microkeratome is used to raise a
corneal flap of ~160microns thickness (2530% of the corneal thickness), hinged at the
12 o’clock position.
The suction is turned off and the flap is lifted
aside, exposing stromal tissue
The excimer laser is applied as with PRK and
LASEK, controlled by the topography-driven
computer software, to reshape the cornea.
The flap is replaced on the stromal bed
without sutures or a BCTL, as the endothelial
pumps create a driving force to keep the flap
in position.
[5]
LASIK
• The use of the suction ring helps hold the cornea steady
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and provides for a uniform cut by the microkeratome.
Flaps can be formed by an automated process involving
a blade guide on the suction ring to guide a turbinedriven microkeratome, producing a very smooth, regular
cut
Patients usually sent home on topical antibiotics,
steroids, and NSAID drops. Pt is usually seen ~POD 1,
and 7, then at 1, 3 and 6 months.
Benefits include little pain, quick recovery of vision, and
potential to treat higher levels of myopia. LASIK
enhancements are also easily performed.
[1]
LASIK Evidence
• A 1996 Saudi Arabian retrospective study looked at the efficacy of LASIK
to correct myopia from -2 to -20D, and showed promising results in the
entire range of refractive error. [14]
• A 2001 cohort study by McDonald et al also showed refractive efficacy
and stability, good UCVA outcomes, no significant loss of BCVA, and
accurate correction of astigmatism in the range of -1 to -11D with up to
-5D of astigmatism. [15]
• At six months, for spherical myopes,
– UCVA 20/20 or better in 61%
– 20/40 or better UCVA in 94%
– 0.6% lost 2 lines of BSCVA, and none > 2 lines
• At six months for astigmatic myopes,
– UCVA 20/20 or better in 52%
– 20/40 or better UCVA in 94%
– 0.9% lost 2 lines of BSCVA, and none > 2 lines
• A refractive stability was achieved between 1 and 3 months in 98% of
spherical and 99% of astigmatic myopes, and 100% between 3-6
months for both groups.
LASIK Evidence
• A 2001 retrospective study by Tabbara et
al showed efficacy in LASIK refractive
correction of hyperopia from +0.5 to
+11.5D at six months follow-up [16]:
– 44% with UCVA of 20/20 or better
– 98% with UCVA of 20/40 or better
LASIK Evidence
• Even though simultaneous bilateral LASIK has been shown to be safe
(Gimbel et al, 1999 [17]), Chiang et al in 1999 showed that the
refractive predictability between a person’s two eyes after LASIK is
correlated, and therefore that using a correction gained from the first
eye to customize the procedure for the second eye has better
outcomes, esp. in mild myopes. [18]
• A prospective 2001 study at Bascom Palmer and a retrospective
study at Univ. of Washington showed that irritation, or “dry eye”
symptoms are due to sensory denervation of the ocular surface
following bilateral LASIK (neurotrophic epitheliopathy), and resolve
by 6 months post-op. [19], [20]
• A 2003 Ohio State retrospective study examined risk factors for
decreased patient satisfaction and showed that most are satisfied
with their vision after LASIK, but that increasing age, flatter preoperative minimum corneal curvature, and surgical enhancement
were significant factors for decreased satisfaction and increased
night vision symptoms. [21]
Comparisons
• PRK vs. LASIK
– 1998 prospective study Hersh et al) [22] showed similar refractive
outcomes, though faster results in LASIK, and undercorrection more
likely in LASIK than PRK
– 2000 control-matched study also showed equal refractive outcomes
between LASIK and PRK up to -9D, but LASIK 2x more likely to cause
halos [23].
– 1999 El-Maghraby et al showed LASIK significantly lowers postoperative pain and hastened recovery of vision, but did not alter
refractive outcomes [24].
• LASEK vs. LASIK
– LASEK with thinner flap, corneal ectasia less likely
– LASIK needs more complicated equipment with higher risk of
intraoperative flap complications.
– LASEK lowers risk of DLK
– Lost LASEK flap less risky than a lost LASIK flap.
– LASEK can cause stromal haze similar to PRK
– More studies needed
Wavefront-guided LASIK
• Wavefront testing allows for the measurement of not only
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myopia, hyperopia, and regular astigmatism, but also
higher-order aberrations (irregular astigmatism).
A beam of light is shone onto the eye, reflected off the back
of the eye and refracted on its way back out. The light then
enters a micro-lens array to produce a spot image array of
reflected light.
Computer analysis determines the relative displacement of
each spot image. The images are then processed to give
the local slope and character of the wavefront light.
A 2004 Israeli prospective, non-randomized comparative
clinical study showed that WFG LASIK patients have
significantly improved contrast sensitivity compared to the
standard LASIK patients at one month post-op, even though
visual acuities were not different with significance between
the groups. [25]
LASIK Complications
• Potential complications:
– Intra-operative flap complications: A 2000 UCLA retrospective study of
~4000 eyes found a microkeratome complication rate of 0.7%, but a
higher rate with surgeon inexperience (1.3% in surgeons first 1000
eyes). [26]
– Post-operative flap complications
– Flap-bed interface epithelialization: Walker et al in 2000 showed that
epithelial growth at the interface could significantly be reduced by
irrigating the stromal surfaces and using a BCTL for one day. [27]
– Irregular astigmatism
– Infection:
– Diffuse lamellar keratitis (DLK): (AKA Sands of Sahara syndrome)
Wavy inflammatory reaction at LASIK flap interface 1-3 days post-op of
unknown cause. Treatment involved high-dose topical steroids or
lifting the flap to irrigating the interface.
– Progressive corneal ectasia: progressive corneal thinning and
steepening with worsening irreg. astigmatism thought to result from
too thin a stromal bed after LASIK. Most believe stromal bed thickness
should be at least 250 microns.
LASIK Complications
• A 1998 Canadian retrospective study showed
that even with early techniques, there was no
significant loss of BCVA. 1.9% of procedures
involved microkeratome-related complications,
and 1.3% involved complications with the
suctioning device. Only 1.8% involved post-op
complications requiring repositioning of shifted
or wrinkled flaps. [28]
Poor LASIK Candidates
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Thin cornea
Irregular astigmatism
Keratoconus
Anterior basement membrane dystrophy
Herpes keratitis
Unstable refraction
Pregnant or nursing (unstable refraction)
History of dry eyes
[1]
Good LASIK Candidates
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Proper expectation of outcome
>18 years old
Stable refraction for at least 1 year (<1D change)
Sufficient corneal thickness
Good wound healing potential (no immunosuppresing
conditions or medications or autoimmune conditions).
• Mild to moderate refractive error (though high myopes
and hyperopes, as well as higher-order aberrant eyes are
relatively good candidates)
[5]
Other options…
• Astigmatic keratotomy (AK)
• Phakic intraocular lens implants
• Refractive lensectomy
• Intracorneal rings
[6]
[1]
Sources Cited
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1) Vander, James F. and Janice A Gault, Ophthalmology Secrets, 2nd ed. 2002, pp 11-129.
2) Netter, Frank H. Atlas of Human Anatomy, 2nd ed. 2001, pg 82.
3)Rohen, Johannes W., Chihiro Yokocki and Elke Lutjen-Drecoll Color Atlas of Anatomy 4th ed. 1998, pg 129.
4) Weichel, Eric D and Kraig S Bower, “Laser Refractive Surgery,” www.UpToDate.com, 2005.
5) Abad, Juan Carlos, and Dimitri Azar, Yanoff: Ophthalmology, 2nd ed. 2004. Chapter 15.
6)Friedman, Neil J., Roberto Pineda II, and
Peter Kaiser, The Massachusetts Eye and Ear Infirmary
Illustrated Manuel of Ophthalmology, 1998.
7)Endl, MJ, et al “Effect of larger ablation zone and transition zone on corneal optical aberrations after
photorefractive keratectomy” Arch Ophthalmol 2001, Aug; 119(8):1159-64.
8)McDonald, MB, et al “Photorefractive keratectomy for low-to-moderate myopia and astigmatism with a
small-beam, tracker-directed excimer laser.” Ophthalmology 1999, Aug; 106(8):1481-8.
9)Rajan, MS, et al “A long-term study of photorefractive keratectomy; 12-year follow-up.” Ophthalmology
2004, Oct; 111(10):1813-24.
10)Alio, JL, et al “Complications of photorefractive keratectomy for myopia: two year follow-up of 3000
cases” Cataract Refract Surg 1998, May; 24(5):619-26.
11)Pirouzian, A, et al “A randomized prospective clinical trial comparing laser subepithelial Keratomileusis and
photorefractive keratectomy” Arch Ophthalmol 2004, Jan; 122(1):11-6.
12)Rouweyha, RM, et al “Laser epithelial keratomileusis for myopia with the autonomous laser” J Refract Surg
2002, May-Jun; 18(3):217-24.
13)Lee, JB, et al “Laser subepithelial keratomileusis for low to moderate myopia: 6-month follow-up” J
Ophthalmol 2002, May-Jun; 46(3)299-304.
14)Zadok, D, et al “Hyperopic laser in situ keratomileusis with the Nidek EC-5000 excimer laser”
Ophthalmology 2000, Jun; 107(6):1132-7.
15)McDonald, MB, et al “Laser in situ keratmileusis for myopia up to -11 diopters with up to -5 diopters of
astigmatism with the summit autonomous LADARVision excimer laser system” Ophthalmology 2001, Feb;
108(2):309-16.
Sources Cited Continued
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16)Tabbara, KF, et al “Laser in situ keratomileusis for the correction of hyperopia from +0.50 to +11.50
diopters wit the Keracor 117C laser” J Refract Surg 2001, Mar-Apr; 17(2):123-8.
17)Gimbel, HV, et al “Simultaneous bilateral laser in situ keratomileusis: safety and efficacy” Ophthalmology
1999, Aug; 106(8):1461-7.
18)Chiang, PK, et al “Comparing predictability between eyes after bilateral laser in situ keratomileusis: a
theoretical analysis of simultaneous versus sequential procedures” Ophthalmology 1999, Sep; 106(9):168491.
19)Battat, L, et al “Effects of laser in situ keratomileusis on tear production, clearance, and the ocular
surface” Ophthalmology 2001, Jul; 108(7):1230-5.
20)Wilson, SE “Laser in situ keratomileusis-induced (presumed) neurotrophic epitheliopathy” Ophthalmology
2001, Jun; 108(6):1082-7.
21)Bailey, MD, et al “Patient satisfaction and visual symptoms after laser in situ keratomileusis”
Ophthalmology 2003, Jul: 110(7):1371-8.
22)Hersh, PS, et al “Photorefractive keratectomy versus laser in situ keratomileusis for moderate to high
myopia. A randomized prospective study” Ophthalmology 1998, Aug; 105(8):1512-22.
23)Pop, M, et al”Photorefractive keratectomy versus laser in situ keratomileusis: a control-matched study”
Ophthalmology 2000, Feb; 107(2):251-7.
24)El-Maghraby, A, et al “Randomized bilateral comparison of excimer laser in situ keratomileusis and
photorefractive keratectomy for 2.50 to 8.00 diopters of myopia” Ophthalmology 1999, Mar; 106(3):447-57.
25)Kaiserman, I, et al “Contrast sensitivity after wave front-guided LASIK” Ophthalmology 2004, Mar;
111(3):454-7.
26)Tham, VM, et al “Microkeratome complications of laser in situ keratomileusis” Ophthalmology 2000, May;
107(5):920-4.
27)Walker, MB, et al “Incidence and prevention of epithelial growth within the interface after laser in situ
keratomileusis” Cornea 2000, Mar; 19(2):170-3.
28)Gimbel, HV, et al “Incidence and management of intraoperative and early postoperative complications in
1000 consecutive laser in situ keratomileusis cases” Ophthalmology 1998, Oct; 105(10):1839-47.