SURFACE ABLATION PRK , LASIK , SMILE

PRK , LASIK , SMILE
FOUR STEPS TO START
TOMOGRAPHY , REFRACTIVE
OPTIONS ,MACHINES ,RULES
INDOREDRISHTI.WORDPRESS.COM

DR DINESH MITTAL DR SONALEE MITTAL
DRISHTI EYE HOSP VIJAYNAGAR INDORE

The Science of Refractive
Surgery
• The goal of refractive surgery is to
reduce dependence on contact lenses
or spectacles for use in routine daily
activities. A wide variety of surgical
techniques and technologies are
available, and all require an appropriate
pre surgical evaluation to determine the
best technique and ensure the optimal
outcome for each patient individually .

Eye’s refractive power determined by
1. Power of the cornea
2. Power of the lens
3. Length of the eye
EMMETROPIA
- IMAGE OF
THE OBJECT
BEING
VIEWED IS
FOCUSED
ON RETINA

1. Myopia
- Near-sightedness
- Image is focused
in front of retina
2. Hyperopia
- Far-sightedness
- Image is focused
behind retina

LASIK (Laser-assisted in situ keratomileusis)
• Most commonly
performed refractive
surgery
• Excimer laser ablation
of corneal stroma
beneath a hinged
corneal flap created
with a mechanical
microkeratome /
femtosecond laser

LASIK STEPS
TOPICAL
ANAESTHESIA
SUCTION
RING
MICROKERA
TOME HEAD
EXCIMER LASER FLAPFLAP PUT BACK

REMOVAL OF
EPITHELIUM
SPATULA
⓴%
ALCHOL
ROTARY
BRUSH

COMPLICATION LASIK ASA
ECTASIA ***** *
FLAP
RELATED
***** NO
HAZE * ***
DRY EYE ** *
INFECTIONS * **
INFLAMMATORY *** *
IRREGULAR * *

TOPOGRAPHIC ANALYSIS
Keratometry & Computerized Videokeratography

TOMOGRAPHY
• ANTERIOR CURVATURE
( TOPOGRAPHY )
• ANT. & POST. CORNEAL
SURFACE ELEVATION DATA
• REGIONAL CORNEAL
THICKNESS
• BELIN / AMBROSIO DISPLAY
FOR EARLY DETECTION OF
ECTASIA
PENTACAM GOLD STANDARD GIVE ALL ABOVE DATA

PACHYMETRY
measuring thickness of cornea
ULTRASONIC
PACHYMETRY
Minimum bed
thickness (300µ)

Visually demanding
occupations

 All corneal refractive surgical
procedures reduce corneal
strength to varying degrees
 Excessive corneal
weakening can result in non
progressive irregular
astigmatism or progressive
corneal ectasia

EVOLUTION OF
REFRACTIVE SURGERY

The birth and evolution of
excimer laser refractive surgery
• The concepts of modern refractive surgery witnessed
its breakthrough when Professor Jose I. Barraquer
described in 1949 his coined technique of
keratomeliosis, setting the foundation for all following
innovation in this field. Few years later, the argon-
fluoride excimer laser was developed and was first
tried on an organic tissue by IBM scientists. The
introduction of excimer laser to be used in the human
eye was done by Stephen Trokel as a precise and safe
tool of corneal shaping, these concepts later defined
the refractive techniques widely used now, when
McDonald performed the most commonly used
epithelium removal technique photorefractive
keratectomy (PRK).

The birth and evolution of
excimer laser refractive surgery
• Ioannis Pallikaris introduced the most widely used
and commonly accepted technique of laser in situ
keratomeliosis (LASIK) in 1990 . The American
Academy of Ophthalmologist (AAO) reports stated
that the substantial level II and III evidence proved
that excimer laser refractive surgery whether
LASIK or PRK, is a safe and effective tool of
correcting the full spectrum of refractive errors but
with some limitations in high hyperopic refractive
errors .

HISTORY OF EXCIMER LASERS
• In the mid-1970s, researchers discovered that rare
gas and halogen lasers (e.g. argon fluoride (ArF),
xenon chloride, and krypton bromide) could be
electrically stimulated to form unstable dimers.
The photons of energy released from this unstable
or excited dimer could then be focused with
sophisticated lenses and mirrors to create an
ultraviolet (UV) laser beam with considerable
energy. The excited dimer (shortened to excimer)
laser beam could be shaped to form different
treatment profiles or patterns.

• Trokel and associates showed that the ArF excimer
laser could precisely photoablate enucleated
bovine cornea by cleaving chemical bonds with no
evidence of thermal damage to adjacent tissue.
This ability to impart very high energy to corneal
stroma without significant thermal effects makes
this laser unique among all other ophthalmic
lasers. Serdarevic showed that the ArF excimer
laser could photoablate a wide area of the optically
used portion of the cornea to recontour the cornea
and maintain corneal transparency long term.

• In late 1986, Marshall and associates used the ArF
excimer laser to recontour the anterior corneal
surface for myopia in a technique known as
photorefractive keratectomy (PRK). Then in 1990,
Pallikaris and associates described a technique to
recontour the corneal stroma beneath the surface
by cutting a lamellar corneal stromal flap first and
then photoablating the anterior surface of the
residual stromal bed. This became known as laser-
assisted in situ keratomileusis (LASIK).

• The ArF excimer laser utilizes a 193 nm UV
wavelength that produces greater energy,
approximately 6.4 eV per laser pulse, than the
required 3.5 eV of energy needed to break covalent
carbon–carbon bonds. This 193 nm wavelength has
been shown to be safe, as it is not mutagenic or
cataract forming and creates minimal thermal
effects (0–9°C increase in temperature). Endothelial
toxicity from the ArF excimer laser radiation is also
negligible since absorption of radiation occurs
within 1 μm of the ablated tissue .

BEFORE 1995
• The excimer lasers have evolved, and the effectiveness
of laser keratorefractive surgery has markedly
improved since 1987. At that time, only two excimer
laser systems , Summit and VISX, were available . Most
of the excimer laser systems introduced in the market
between 1988 and 1995 consisted of broad-beam
lasers with no eye-tracking ability. Broad-beam lasers
were the first-generation systems that had large spot
diameters (5.0–7.5 mm) and relied on expanding
apertures. They were advantageous in that they were
fast and effective, but they had the disadvantages of
irregular beam profiles or poor homogeneity that would
sometimes create rough and irregular surfaces, and
they were limited in their inability to create complex
ablations profiles without masking devices.

1995 TO 1999
• Between 1995 and 1999, small scanning spot excimer
lasers with eye-tracking capabilities were introduced.
Scanning-beam lasers with fixed or variable size, small-
diameter slit beams (0.5–2.0 mm) represented second-
and third-generation excimer laser systems. These
included flying spot, scanning slit, and variable spot
scanning laser systems. All of these systems were an
improvement over broad-beam systems, as the laser
beam profiles were more Gaussian (more energy
centrally than over the edges), produced smoother
ablations, and could treat more complex ablation
profiles.

1995 TO 1999
• All of these systems were an improvement over
broad-beam systems, as the laser beam profiles
were more Gaussian (more energy centrally than
over the edges), produced smoother ablations, and
could treat more complex ablation profiles. They
had the disadvantages of longer treatment times
since smaller spot sizes require more pulses and of
needing sophisticated video or laser radar eye-
tracking systems since any misalignment of small
diameter pulses could result in significant
alteration from the intended ablation profile.

1999 TO 2003
• From 1999 to 2003, newer systems added many
upgrades and improvements to make them faster,
more reliable, and more precise. They also allowed
expandable optical zones and better algorithms for
recontouring the cornea and had better tracking
systems and faster treatment times. With this level
of precision and versatility, these fourth-generation
excimer laser systems had become sophisticated
enough to support customized or wavefront-guided
ablations.

1999 TO 2003
• With the advent of wavefront sensors and FDA
approval of the first wavefront-guided customized
excimer laser platform in October 2002, attention
had now focused more on measuring and
correcting optical aberrations beyond sphere and
cylinder. The ultimate goal has been to achieve an
aberration-free correction that is only limited by
the resolution of the human retina, which is
perhaps around 20/8 vision .

Comparison of lasers for
excimer laser vision
correction

Comparison of lasers for excimer
laser vision correction
• There are now a wide variety of excimer laser
platforms that are used for photorefractive and
phototherapeutic keratectomy (PRK/ PTK), laser in
situ keratomileusis (LASIK), laser in situ epithelial
keratomileusis (LASEK), and other associated
procedures. The basic premise of the laser’s
function in each of these surgical procedures is to
remove corneal tissue in a specific pattern to
correct myopia, hyperopia, and astigmatism or, in
non refractive cases, to remove corneal scar
tissue.

FLUENCE VARIATION
• Energy is used to remove corneal tissue during the
photoablative process, and the amount of energy
present in a given laser beam can be variable. This
amount is expressed in terms of laser fluence
(mJ/cm2). Fluence on a standard excimer laser may
vary from 50–500 mJ/cm2, which is too broad a
range to give predictable surgical outcomes. Thus,
most lasers available have the ability to modify
fluence by adding or subtracting additional energy,
usually by means of diluting the ArF gas with
another molecule (often He), or by varying the
voltage across the laser.

laser vision correction ENERGY
• These methods control fluence to a more
acceptable range between 120 and 180 mJ/cm2.
The ablation threshold for cornea stromal tissue is
50 mJ/cm2. Below this level, only photochemical
changes occur. At very high fluence levels, there is
an increase in disruption of surrounding tissue by
thermal energy, as well as the acoustical
shockwave, both of which may produce undesirable
results.

laser vision correction ENERGY
•All excimer lasers should be
maintained and calibrated regularly
for best performance. Beam
homogeneity and laser fluence
should be checked regularly by
surgical staff to ensure optimal
surgical results.

LASER ENERGY DELIVERY
SYSTEM
• Most early lasers used a complicated system of
optics to attempt to deliver a large homogeneous
beam to the surface of the eye. This method of
delivery system is termed broad beam. These early
delivery systems provided surgeons with fairly fast
surgery times, but surgical outcomes were subject
to beam profile. Another method is a small spot or
flying spot, which delivers the laser energy in a
smaller spot (typically 0.5–2 mm), which is moved
around the cornea, overlapping in a specific
pattern to give the desired ablation profile.

SYSTEM
• The homogeneity of the scanning laser beam
becomes less important in small-spot lasers than in
broadbeam lasers because each of the spots is
overlapped by a different spot. Specificity in
ablation profile is theoretically increased, but in
early models surgical times also increased
significantly. There was also some difficulty
ensuring proper distribution of the beam profile to
the correct area of the cornea, as each treatment
took longer and eyes are subject to saccadic
movement or rotation during surgery.

SYSTEM
• A hybrid of the two types is called a variable-spot
scanning laser, which allows for a change in the
size of the beam delivered to the cornea as well as
the ability to move the beam around to generate
the ablation profile. Aside from the beam type,
different delivery systems also vary in the rate in
which the laser beam is administered to the
cornea. This is termed the pulse rate, or Hertz rate
(Hz).

SYSTEM
• Typically this will range from 3 to 500 Hz. Low Hz
machines take more time and are undesirable in
small-spot scanning lasers because no adjacent
spots are treated consecutively. At extremely high
frequencies, optics tend to degrade faster, and a
problem particular to large beam profiles is that
thermal effect increases as less time exists
between pulses to allow for heat dissipation.

SYSTEM
• The majority of excimer lasers currently in use
today are using small-spot or variable-spot
scanning. Development of eye-tracking devices
have drastically improved delivery of the beam to
the desired location, as it was noted in early
delivery systems using small-spot technology that
small saccadic eye movements could alter the
desired positioning of the beam. Increase in the
delivery rate also decreased surgical time.

ABLATION PATTERN
• For myopia, it is necessary for the cornea to
receive more energy in the central portion of the
cornea than it does in the periphery to create an
overall flattening effect. This concentration of
energy may be done in several different manners.
The broad-beam lasers use some type of a masking
device to protect the peripheral portion of the
cornea from the laser energy while delivering a
larger amount of energy to the central portion of
the cornea.

ABLATION PATTERN
• The flying-spot lasers such as the
Bausch & Lomb Technolas 217z Zyoptix
laser (Bausch & Lomb, San Dimas, CA)
or the Allegretto Wave (WaveLight ,
Germany) use computer control to
preferentially place more spots in the
center of the cornea than in the
periphery for creation of myopic
corrections.

ABLATION PATTERN
• The smaller beam lasers require a less powerful laser
head as well as fewer optics to homogenize the beam.
The scanning lasers, however, are dependent on an
eye-tracking or coupling system. Because the surgeon
cannot as accurately follow small saccadic eye
movements when the laser is rapidly treating small
areas on the cornea, computerized tracking of these
small movements is more crucial. Also, because the
spot size is much smaller, higher pulse rates are
necessary to complete the ablation in a reasonable
time . Because no one area is treated with two pulses
in a row, any thermal energy created is allowed to
dissipate between pulses.

Refractive Laser Platforms
• The excimer laser used to perform the surgery is
guided by computer. The earlier lasers used a
broad beam with spot size up to 6 mm to perform
the ablation; newer generations use a scanning slit
or a ‘ flying spot’ protocol, that is, a small spot
(about 1 mm) with many repetitions across the
cornea to remove the desired amount of tissue. The
broad-beam type used an aperture of varying size
to control the size of the beam; the protocol
started with a small beam to ablate the centre, and
the beam was enlarged to continue the ablation
into the periphery.

VISX STAR LASER
• The VISX Star S4 IR excimer laser system (VISX
CA) is an advancement to the previous models of
the VISX excimer lasers . Notably improvements
have come from the integration of a wavefront
system and improvements to ensure alignment of
the eye-tracking system, which ensures accurate
distribution of the laser treatment to the cornea.
The Star S4 IR is a variable spot-scanning laser
with spot size ranging from 0.65 – 6.5 mm .

VISX STAR LASER
• Using the WaveScan aberrometer in combination with
the laser, surgeons may treat optical zones out to 7 mm
using wavefront data and create blend zones as large
as 9.5 mm. It also has variable repetition rate (VRR),
which allows for variation in delivery times to specific
and adjacent sites on the cornea, allowing for improved
ejected tissue removal by the vacuum system and
thermal dissipation on adjacent sites. The eye-tracking
system is a camera-based iris registration (IR) system,
which operates on a three-dimensional platform,
allowing for adjustment along the x−and y-axis plus
rotation .

VISX STAR LASER
• The fluence is somewhat in the mid-range of other
excimer lasers at 160 mJ/cm2. The laser operates
at speeds up to 20 Hz. A joystick on the control
panel allows movement of the patient’s bed up,
down, to the left, or to the right . The patient fixes
on a red flashing He–Ne beam that is coaxial with
the excimer laser beam in the standard treatments
and aligned with the patient fixation on WaveScan .

ZEISS MEDITEC MEL-80 EXCIMER
LASER
• Zeiss Meditec (Jena, Germany; Dublin, USA)
manufactures the MEL excimer laser. The MEL-80
excimer laser is a small-spot scanning laser with a
fixed spot size of 0.7 mm. It operates at a pulse rate of
between 10 and 250 Hz. Each pulse duration lasts
between 4 and 6 ns. Average fluence level at the
cornea is kept in check by diluting the ArF mixture with
helium gas. A blend zone can be created out to 10 mm.
A tissue-saving algorithm is used, which results in
lower asphericity and preserves more of the corneal
tissue for use on thinner corneas. The eye tracker is a
high-speed camera-based system combined with an
integrated iris-recognition system operating at 250 Hz,
which does not require the patient to be dilated .

BAUSCH & LOMB TECHNOLAS
EXCIMER LASER
• The Technolas 217z with the Zyoptix wavefront
system (Bausch & Lomb Surgical CA) is a small-
spot scanning system . It uses a small-size spot
with integration of the Zyoptix wavefront software
program for treatment of myopia, astigmatism, and
hyperopia. The laser delivery system runs at 50 Hz,
with pulse duration of 18 ns. The Technolas 217
laser incorporates an eye tracker with a camera-
based iris-recognition system, which runs at 120
Hz. The patient does not need to be dilated to use
the eye-tracking system.

BAUSCH & LOMB TECHNOLAS
EXCIMER LASER
• The Technolas 217z is approved for myopic LASIK
treatments up to −11.00 D with or without
astigmatism less than −3.00 D. For hyperopic
LASIK, treatments between +1.00 and +4.00 D can
be performed with or without astigmatism up to
+2.00 D . The 217z incorporates a wavefront
system and is approved for myopic LASIK up to
−7.00 D with or without astigmatism up to −2.0 D.

500 Hz TECHNOLAS ® TENEO™ 317
Excimer Laser Platform
THIS IS
LATEST MODEL
OF
TECHNOLAS
WITH 500 HZ
FREQUENCY .

NIDEK EC 5000 CX SERIES
LASER
• The Nidek EC 5000 CX series laser (Nidek, Inc.,
Fremont, CA) is utilized with the Nidek NAVEX
Quest system, which is marketed by Nidek as a
complete refractive surgery package. The laser
uses a rotating scanning slit to deliver the laser
energy. It operates at between 5 and 50 Hz with a
pulse duration of 10–25 ns. Fluence averages 360
mJ/cm2. The laser has an eye-tracking system that
operates at 200 Hz and has a torsion control to
correct for intraoperative cyclotorsional movement
of the eye.

WAVELIGHT ALLEGRETTO WAVE
LASER
• WaveLight ( Germany) manufactures the Allegretto
Wave laser, which is approved in the USA for LASIK
treatments of up to −12.00 D of myopia with or without
astigmatism up to −6.00 D, and for hyperopic LASIK
treatments of up to +6.00 D with or without
astigmatism up to +5.00 D. The Allegretto Wave can
also treat mixed astigmatism up to 6.00 D. Approved in
July 2006 by the FDA is the Wave-Q laser, which is
approved for the same parameters of treatment as the
Wave, with the exception of mixed astigmatism. The
laser is comprised of a small-spot scanning laser with a
fixed spot size of 0.95 mm. Beam profile is Gaussian.

WAVELIGHT ALLEGRETTO WAVE
LASER
• The laser operates at a pulse rate of 500 Hz. Fluence
at the level of the cornea is measured at 130–140
mJ/cm2. This system does not use a gas dilution
system to control fluence. Instead, the Wave uses a
nitrogen purged optical rail, which reduces the amount
of ozone produced during firing of the laser. Optical
zones can extend out to 8 mm, with blend zones out to
10 mm. The eye-tracking system is a camera-based iris-
recognition system operating at 200 Hz. Software for
the laser also uses a slightly different ablation
algorithm, which accounts for normal corneal
asphericity, enabling it to minimize spherical aberration
and resultant glare or halo.

WaveLight Allegretto Wave Eye-Q Laser
• The WaveLight® Allegretto Wave® Eye-Q excimer laser uses
a series of technical innovations to optimize laser vision
correction, providing excellent clinical results in Wavefront
Optimized® and Wavefront-Guided. Now the laser is FDA-
approved for topography-guided laser vision correction as
well, incorporating refractive error of the eye and corneal
irregularities into custom laser ablations.
• The system’s high-speed laser and eye tracker yield both
efficiency and safety, leading to enhanced throughput,
reduced environmental exposure and excellent outcomes.
In addition, its proven, state-of-the art technology offers you
and your patients safety and reliability, with a low rate of
complications and retreatments.

Advanced features for precise treatments
•400-Hz eye tracker:
• Actively tracks eye movement and verifies location before releasing pulses
•Integrated cross-line projector:
• Provides exact alignment of the head and eye position
•Gaussian beam profile:
•Ensures a smooth ablation without grooves or ridges, while the tiny 0.68 mm spot size provides a
• precise ablation of corneal tissue and very small transition zones
•
•PerfectPulse Technology:
•Helps ensure safety and precision at high speeds
•
•Cross-line projector:
• Provides precise centration

Advanced features for precise treatments
• Fine laser beam: Tracks pupils between 1.5 and 8
mm, while the automatic centering mode ensures
perfectly centered ablations
• Considers the unique curvature of each eye:
Addresses spherical distortions that may induce
glare and affect night vision
• Integrated, closed-loop energy control: Allows
stable energy flow throughout the treatment,
enhancing precision and safety
• Thermal shot distribution: Helps minimize potential
for thermal buildup6

Tracking and registration
• Accurate eye tracking and registration are required for
optimal results when applying wavefront-guided laser
refractive surgery to the cornea. Because the custom
ablation profile is highly specific, it must be applied to
the cornea only after achieving precise alignment to
the area that was mapped. To optimize this, technology
is needed that
• (1) can identify landmarks in order to link measured
wavefront to corresponding area on cornea and
• (2) can track them as the eye continually moves
throughout the treatment

• A well-centered treatment ultimately requires good
fixation by the patient. However, natural eye
movements and improper fixation cannot be
avoided during refractive surgery. Active eye
tracking systems were introduced to compensate
for these movements in order to decrease the
incidence of decentered ablations. With the
emergence of wavefront-guided corneal ablation,
one more potentially important step, registration,
has been introduced.

• Registration is the process that links the measured
wavefront to the treatment ablation profile on the
laser platform, thereby facilitating accurate
positioning of the ablation pattern on the cornea.
Registration can occur initially at the beginning of
surgery but ideally would be an active part of the
tracking process to compensate for any eye
rotation and pupil size changes that might occur
during laser ablation.

Tracking and registration EYE
MOVEMENTS
• The major eye movements that occur
physiologically during fixation for refractive
surgery are slow drifts, microsaccades, and
tremors . Moving the eye to a new fixation location
is a voluntary movement called a saccade. The
average velocity of saccades is about 200°/s with
peaks up to 500°/s and amplitudes up to 15°. The
majority of eye movements that occur during
refractive surgery are caused by relatively slow
drifts in eye position

Patients’ satisfaction after
refractive surgery
• Patients’ satisfaction after refractive surgery,
wavefront guided or not, is primarily dependent on
the successful treatment of lower order
aberrations of the sphere and cylinder of the eye.
LASIK has been successful in the correction of
mild to moderate myopic astigmatism, but with
limited reports on the efficacy, predictability and
safety of it in higher myopic astigmatism in the
terms of astigmatic correction of HOA, with
limitation of retreatments needed

Ablation centration
• Ablation centration is a major issue in the excimer laser
development, the decentration of ablation can lead to under
correction and irregular astigmatism, which is most
important in hyperopic patients , who tend to have a larger
angle kappa values . There are four main methods of
centration in laser refractive surgery that has been
suggested in literature; center of the pupil, coaxially
sighted corneal light reflex (CSCLR), corneal vertex normal
and between the pupillary and visual axis . Many reports
had demonstrated that pupil-centered and vertex centered
treatments provide similar visual and optical outcomes.
However, in eyes showing large temporal pupil decentration,
pupil-centered ablation seemed to produce a lower amount
of coma and consequently, a reduced loss of BCVA
compared with vertex-centered patients

Sixth generation excimer lasers
• This generation of excimer laser platforms can be
defined as an excimer laser delivery system that
targets the goal of minimally invasive laser
refractive surgery by reducing the amount of time
and tissue ablated with a faster laser system,
delivering more laser spots per second .

• with a faster treatment time, through the ability of
ablating more corneal tissue in a given time . The 6th
generation lasers speed varies from 400 to 1050 Hz,
being 400 Hz in Wavelight Eye-Q up to 1050 Hz in
Schwind Amaris. On average, a 500 Hz platform will
reduce the time needed per diopter ablation in a 6.5
mm optical zone from 7–10 seconds using older
generation laser platforms to an effective 4 seconds .
Another feature to reduce treatment time is the
advanced fluence level adjustment system, in which a
mix of high and low fluence levels are used. High
fluence level will perform 80% of corneal ablation,
while low fluence will be used for fine correction,
improving resolution, with remarkable precision in high
refractive errors

• Conclusion In summary, the latest generation of
excimer laser platforms had introduced a large number
of features as faster laser, smaller spot size, a high
speed tracker, pupil monitoring and online pachymetry,
all of which provided superior treatment with
significant improvement of induced post operative HOA
and control of thermal damage. This technology is still
facing major limitations in terms of high hyperopic,
presbyopic treatments, along with difficulties in laser
centration along with the limitation of the customized
treatments, generated by the biomechanical patterns
of wound healing .

MEL 80 VS MEL 90
• MEL 80, has fast eye tracker, which allow to treat a
wider range of patients with 3.00 to -12.00 D of
refractive errors. Other key developments with the
MEL 80 included its high ablation rate and
individual treatment planning with the optional
CRS-Master (Carl Zeiss Meditec). It also had a wide
range of applications like conventional and
femtosecond LASIK (Femto-LASIK), PRESBYOND
Laser Blended Vision, topography-guided
treatments, PRK, LASEK, and phototherapeutic
keratectomy

MEL 90
• The MEL 90 has an ablation speed of up to 1.3
seconds per 1.00 D. the FLEXIQUENCE switch
function allows to switch between 250- and 500-
Hz frequencies and perform new procedures. 500-
Hz frequency significantly reduces treatment time
and lowers the risks associated with longer
treatment times .

Equipment for Corneal
Flap Creation

Equipment for Corneal Flap
Creation Microkeratomes
• A suction ring is applied to the surface of the eye
and the microkeratome head unit is mounted on the
suction ring. The surgeon must select both an
appropriate size suction ring and the depth at
which the blade cuts the flap. For a steep cornea
( K > 45 D), a smaller ring is used.

K READINGS effect on flap properties

• Precise gears drive the applanation head across
the cornea, flattening it so that the oscillating
blade mounted within the unit can cut a flap. The
ideal flap is of the desired thickness, of uniform
profile and free from abrasions, buttonholes and
free caps. Very high intraocular pressures (>60
mmHg) may be generated during these steps. The
head motor is reversed to back the blade out from
under the flap, and suction unit / microkeratome
assembly is removed from the globe.

• The flap is then lifted and the exposed stromal bed
is ablated. The microkeratome motor is set so that
a hinged flap is created; excessive travel in early
models resulted in free caps which had to be
replaced accurately to avoid inducing irregular
astigmatism. Extremes of corneal curvature also
risk complications in flap creation :
• steep corneas with K > 48 D are at increased risk
of buttonholed flaps,
• and flat corneas ( K < 40 D) are at increased risk of
a free cap.

• Care must be taken to apply the suction ring
precisely so that the resultant flap is centred on
the desired spot; if this is misaligned, the ablation
profile still can be centred, but there is a risk of the
overall ablation diameter being beyond the edge of
the bed, resulting in halos and ghosting. If the
surgeon is not happy with the location or quality of
the flap, it is important to abort the procedure,
allow time for the flap to heal and reschedule the
patient for an alternative procedure, usually a
surface laser treatment after stabilization
( 3–6 months).

Creation
• Care must also be taken to ensure that the suction
ring is not applied over conjunctiva. This could lead
to pseudosuction, chemosis and unstable fixation
of the microkeratome. There is then a high risk of
suction break or blade movement during flap
creation.

Creation
• A range of mechanical microkeratomes are
available, each with their own characteristics. The
way in which the blade is propelled across the
cornea varies slightly between models. Some are
driven along a straight track forwards and
backwards (Amadeus, NIDEK MK-2000, Moria One
Use-Plus), some rotate about a pivot (Hansatome,
Moria M2) and some act as a pendulum and
describe a slightly concave trajectory, applanating
the central cornea more than the periphery (e.g.
Carriazo-Pendular, Schwind).

Bausch & Lomb Hansatome
and Zyoptix XP
• The Hansatome pivots around a post to create a
superior hinged flap. Flaps tend to be slightly
thinner than the blade rating, especially for thin
corneas. An adapter is used to set up the assembly
for right or left eyes. The successor to the
Hansatome, the Zyoptix XP, offers a number of
improvements, particularly in the low variability in
flap thickness. The orientation of the head
assembly can be changed from right eye to left eye
simply by moving a switch rather than having to
reassemble the unit .

Creation Bausch & Lomb Hansatome
and Zyoptix XP
• The Zyoptix XP is also felt to offer more reliable
suction, better centration and smoother flap beds
than the Hansatome (the previous gold standard).
Most importantly, the actual flap thickness is much
closer to the thickness labelled on the blade,
meaning that surgeons can be more confident
about the safety of the flaps they are creating. In
addition, the gears are completely covered so that
tissue cannot be trapped or jam the mechanism .

Creation Moria M2
• The Moria M2 is relatively lightweight and compact
and enables the surgeon to control flap thickness,
flap diameter and hinge length. A detailed
nomogram is available to guide choice of
equipment for each procedure. The option to use a
smaller hinge length means that the stromal bed
can be exposed over a greater area for ablation of
small eyes or hyperopic corrections; however, a
shorter hinge may be more vulnerable to tearing
and misalignment of the flap when repositioned.

Creation Moria M2
• It also allows the hinge to be placed anywhere
around the cornea according to the surgeon’s
choice. Flaps tend to be slightly thicker than stated
by the blade. The M2 is also available as a single-
use system.

Creation Nidek MK-2000
• The Nidek MK-2000 is a one-piece microkeratome,
in that the suction ring is integral to the
microkeratome unit. It therefore does not require
assembly on the eye, and the suction time can be
reduced. The unit is slightly smaller than others
which facilitates placement in eyes with small
palpebral apertures. It also tends to cut a flap
thinner than stated on the blade. It was designed
for creating a nasal hinge.

Creation Schwind Carriazo-Pendular
• The Carriazo-Pendular has a curved blade and moves
across the cornea in a pendular fashion. The cornea is
compressed more in the centre than at the periphery,
the theory being that this creates a more uniform flap.
Flap thickness was highly predictable, even for thin
flaps, with standard deviation of 10–12 mm, and flaps
tend to be slightly thinner than stated. There is free
choice for the location of the hinge. Four suction ring
sizes are available and flaps of 9 or 10.5 mm diameter
can be created. The hinge can be placed at any
location. Flaps can be cut with a thickness of 90–170
mm (20 mm steps).

Femtosecond Lasers
• Femtosecond lasers operate in the near-infrared
spectrum (1,053 nm). At low power density, the beam
generated is not absorbed by optically clear media. It
can however be focussed to a high power density
which is absorbed by the tissue, creating a cavitation
sphere within clear media such as the cornea. The
power generated is a function of the energy of a pulse
and its duration; since power is energy per unit time,
the shorter the pulse, the higher is the resultant power
of each pulse. The ultra-short pulses delivered by
femtosecond lasers (1 fs = 10 −15 s) are focussed to a
spot which is small enough that the fluence
(energy/area) is sufficient for plasma formation .

Femtosecond Lasers
• The supersonic expansion of the resultant hot
plasma creates a tiny sphere followed by a
shockwave and air bubble generation. The
combination of these effects disrupts tissue and
gives the femtosecond laser its ability to cleave
tissues. The precision of this laser derives from the
short pulse duration which generates cavitation
spheres many orders of magnitude smaller than
those resulting from the Nd-YAG laser ( » 100 m m).

Femtosecond Lasers
• Current generation lasers use spots of 1 mm,
placed by computer-controlled mirrors with an
accuracy of approximately 1 mm. The surgeon
must program the controller software to create the
desired flap diameter and depth and can also
decide on the angle at which the side of the flap is
formed. The laser requires that the cornea is
contacted by a ‘docking plate’ for accurate
placement of the laser spots; different models have
different ways of achieving this.

Femtosecond Lasers IntraLase
FS60 and iFS (AMO )
• This was the first femtosecond laser to be used for
the creation of LASIK flaps, with the first patient
being treated in 2000. The IntraLase FS60 systems
are considered 4th generation lasers and have a 60
kHz performance. The current top model, the fifth
generation model iFS, operates at 150 kHz and
typical energies of 0.8–1.6 m J . A LASIK flap of 9
mm is created in 8–10 s. Spots can be placed with
an accuracy of around 1 m m. The laser unit has a
docking system with a planar surface so that the
cornea is flattened for flap creation .

Femtosecond Lasers
VisuMax (Zeiss )
• The laser pulse rate of the VisuMax system is 500
kHz with low energy (0.3 m J). The typical spot
distance is 3–6 m m. A docking system with a
concave plate is used so that a curved corneal
surface is maintained during flap creation. This
represents a more anatomically normal situation
and induces less stress within the cornea. This
also allows lower suction pressure during flap
creation and consequently a lower rise in IOP.

Femtosecond Lasers
VisuMax (Zeiss )
• The laser beam is automatically centred on the
corneal vertex, increasing the accuracy of the
treatment. The flap interface is created as a spiral
from the periphery towards the centre, allowing the
patient to maintain fixation throughout flap
creation (compared to raster pattern or centripetal
spiral patterns). The unit is very large and requires
a large room but is less sensitive to temperature
and humidity than other femtosecond laser models.

Femtosecond Lasers
Femto LDV (Ziemer )
• The LDV system currently offers the highest
frequency (1,000 KHz) and therefore offers the
lowest pulse energy (in the nJ range). The laser
spots are overlapped so that tissue bridges should
become smaller and the flap should be easier to
lift. The use of more spots which are smaller, lower
energy and more tightly applied may result in
smooth stromal beds. The company claims an
absence of both inflammation and opaque bubble
layer with this model.

Femtosecond Lasers
Femto LDV (Ziemer )
• This model has a focal plane which is fixed during
the cut so the geometry available to the surgeon is
limited to uniplanar flaps (similar to a conventional
microkeratome). The unit is smaller than other
femtosecond laser and delivers the laser via a
flexible arm assembly which fits under the excimer
laser and can be used in a manner analogous to a
microkeratome. The advantage of this is that the
patient does not need to be moved from one laser
to the other.

Corneal Imaging for
Keratorefractive
Surgery

Corneal Optics
• The air–tear-film interface provides the majority of the
optical power of the eye. Although a normal tear film
has minimal deleterious effect, an abnormal tear film
can have a dramatic impact on vision. For example,
either excess tear film (eg, epiphora) or altered tear
film (eg, dry eye or blepharitis) can decrease visual
quality.
• The optical power of the eye derives primarily from the
anterior corneal curvature, which produces about two-
thirds of the eye’s refractive power, approximately
+48.00 diopters (D). The overall corneal power is less
(approximately +42.00 D) as a result of the negative
power (approximately –6.00 D) of the posterior corneal
surface.

Corneal Optics
• Standard keratometers and Placido-based corneal
topography instruments measure the anterior corneal
radius of curvature and estimate total corneal power
from these front-surface measurements. These
instruments extrapolate the central corneal power (K)
by measuring the rate of change in curvature from the
paracentral 4-mm zone; this factor takes on crucial
importance in the determination of IOL power after
keratorefractive surgery . The normal cornea flattens
from the center to the periphery by up to 4.00 D (this
progressive flattening toward the peripheral cornea is
referred to as a prolate shape) and is flatter nasally
than temporally.

Corneal Optics
• Almost all keratorefractive surgical procedures
change the refractive state of the eye by altering
corneal curvature. The tolerances involved in
altering corneal dimensions are relatively small.
For instance, changing the refractive status of the
eye by 2.00 D may require altering the cornea’s
thickness by less than 30 μm. Thus, achieving
predictable results is sometimes problematic
because minuscule changes in the shape of the
cornea may produce large changes in refraction.

Corneal Imaging
• Corneal shape, curvature, and thickness profiles can
be generated from a variety of technologies such as
Placido disk–based systems and elevation-based
systems (including scanning-slit systems and
Scheimpflug imaging). Each technology conveys
different information about corneal curvature, anatomy,
and biomechanical function. In addition, computerized
topographic and tomographic systems may display
other data: pupil size and location, indices estimating
regular and irregular astigmatism, estimates of the
probability of having keratoconus, simulated
keratometry, and corneal asphericity. Other topography
systems may integrate wavefront aberrometry data
with topographic data.

Corneal Imaging
• The degree of asphericity of the cornea can be
quantified by determining the Q value, with Q = 0 for
spherical corneas, Q < 0 for prolate corneas (relatively
flatter periphery), and Q > 0 for oblate corneas
(relatively steeper periphery). A normal cornea is
prolate, with an asphericity Q value of –0.26. Prolate
corneas minimize spherical aberrations by virtue of
their relatively flat peripheral curve. Conversely, oblate
corneal contours, in which the peripheral cornea is
steeper than the center, increase the probability of
having induced spherical aberrations. After
conventional refractive surgery for myopia, with the
resulting flattening of the corneal center, corneal
asphericity increases in the oblate direction, which
may cause degradation of the optics of the eye.

DECISION
PRK
NO RISK DO PRK OR LASIK

SA includes four types:
• a. Photorefractive Keratectomy (PRK): The epithelium is
debrided and removed either mechanically or by 20%
alcohol .
• b. Laser Subepithelial Keratomileusis (LASEK): The
epithelium is loosened by 20% alcohol and moved aside, but
ultimately preserved, then re-positioned after ablation .
• c. Epipolis LASIK (Epi-LASIK): The epithelium is moved
mechanically by a mechanical microkeratome (MMK), but
ultimately preserved, then re-positioned after ablation .
• d. Transepithelial Photorefractive Keratectomy (TE-PRK):
The epithelium is not removed but ablated with a specific
profile designed to remove the epithelium and
simultaneously correct the refractive error.

Surgical Techniques in
Surface Ablation (SA)

Surgical Techniques in Surface
Ablation
• 1. Pre-op: Patients are often administered 5 mg of
diazepam on the day of or the night before the
procedure to reduce anxiety. Females are
recommended to avoid eye makeup for at least one
weak preoperatively since the particles of the
makeup substances may contaminate and may act
as toxic agents .

Ablation
• 2. Anesthesia, Sterilization and Draping: After the
eye has been anesthetized with either topical
proparacaine or tetracaine, the periocular skin is
prepped with 10% povidone-iodine (Betadine) and
4% povidone-iodine drops are applied to the ocular
surface for two minutes and then irrigated. Patients
then receive additional topical anesthetic and
antibiotic drops, the eyelids are draped and a lid
speculum is placed to optimize corneal exposure.
The other eye is occluded. Topical anesthesia is
obtained with proparacaine 0.5% or tetracaine
0.5% drops.

Ablation
• 3. Alignment and Registration: Registration is a
technique in which a fixed landmark is used at the
time of aberrometry or tomography and treatment
to apply the ablation to the correct area of the
cornea; it does not rely on the pupil for laser
centration. The step of registration should be done
before removing epithelium.

Ablation 4. Epi-Off
• In the evolution of PRK, several methods were
developed to debride the epithelium. Some techniques
involve complete removal and disposal of the
epithelium; other methods offer the option of
preserving and repositioning the epithelium following
stromal excimer laser ablation. The original PRK
techniques utilize mechanical epithelial removal with a
sharp blade, blunt spatula, or a rotating corneal brush.
Some surgeons prefer using alcohol 20% to loosen the
intact epithelium. Central cornea is marked with an 8.0
mm OZ marker delineating the area for epithelial
removal and the epithelial removal method of choice is
begun.

Ablation 4. Epi-Off
• In case of alcohol method, a solution of 20%
alcohol is applied for 20 to 30 seconds in a “well”
created by pressing an 8 or 9 mm diameter OZ
marker onto the corneal surface, to restrict the
alcohol to area to be de-epithelialized . After the
desired exposure time, the alcohol is removed from
the “well” by absorption into a microsurgical spear
sponge and ocular surface is copiously irrigated
with balanced salt solution (BSS). A special brush
is used to debride loosened epithelium and bare
stromal bed .

Ablation 4. Epi-Off
• LASEK was developed in an attempt to preserve
the epithelium. Alcohol is applied in the same
manner described with PRK. Epithelial trephine and
spatula are used sequentially to score and roll up
the epithelium, which remains attached at the
hinge . After photoablation is performed, the
epithelium is replaced .

Ablation 4. Epi-Off
• The Epi-LASIK procedure does not use alcohol,
which is toxic to the epithelium. Instead, a MMK
with a modified dull blade and a thin applanation
plate is used to cleave the epithelium. In the
absence of alcohol, Epi-LASIK may preserve more-
viable epithelial cells. This could improve epithelial
flap adherence, reduce postoperative discomfort,
and improve visual outcomes compared to LASEK.

Ablation 4. Epi-Off
• In both LASEK and Epi-LASIK, around 20% of the
epithelial flaps are sloughed off or become necrotic
postoperatively and the purported benefit of the
epithelium is lost. Both LASEK and Epi-LASIK are
similar to PRK in that the epithelium is removed
and the photoablation is performed directly on
Bowman’s membrane and the anterior stroma. The
visual results are comparable in all three
procedures and Epi-LASIK and LASEK have not
been proven to be beneficial over PRK in
decreasing corneal haze or significantly reducing
postoperative discomfort.

Ablation 4. Epi-Off
• After the epithelium has been removed with any of
the three techniques, a surgical cellulose sponge
lightly moistened with an artificial tear lubricant
(such as carboxy methylcellulose) is lightly
brushed over the surface of the cornea to remove
residual epithelium and to smooth the surface. The
OZ must be free of epithelial cells, debris and
excess fluid before ablation. The epithelium should
be removed quickly and consistently in order to
prevent dehydration of the stroma, which increases
the rate of excimer laser ablation resulting in an
overcorrection.

Ablation 5. Centring and Applying
the Ablation Beam
• Each excimer laser machine has its own pattern of lights for
centring the ablation beam (cross, meniscuses, spot lights,
etc.). The ablation beam should be coaxial with the fixating
blinking target, at which the patient should be asked to
fixate . It is mandatory to be sure that the eye tracker has
recognised the pupil and it is active; otherwise, decentred
ablation will result leading to postoperative decentred zone,
which is the main cause of postoperative induced coma.
Tracking systems, although effective, do not reduce the
importance of keeping the reticule centred on the patient’s
entrance pupil. If the patient is unable to maintain fixation,
the illumination of the operating microscope should be
reduced. If decentration occurs and the ablation does not
stop automatically, the surgeon should immediately stop the
treatment until adequate re-fixation is achieved.

Surgical Techniques in Surface Ablation 5.
Centring and Applying the Ablation Beam
• It is still important for the surgeon to monitor for
excessive eye movement, which can result in
decentration despite the tracking device. Once the
patient confirms that the fixation light of the excimer
laser is still visible and that he or she is looking
directly at it, ablation begins. During ablation, the
surgeon must monitor the patient to ensure that
fixation is maintained throughout the treatment.
Neither tracking nor registration is a substitute for
accurate patient fixation. It is important to initiate
stromal ablation promptly, before excessive stromal
dehydration takes place.

Surgical Techniques in Surface Ablation 5.
Centring and Applying the Ablation Beam
• The change in illumination and in patient position
(i.e., lying down) can cause pupil centroid shift. In
most patients, the pupil moves nasally and
superiorly when it is constricted. It is therefore
important to use the registration technique in
cases sensitive to pupil position .

Ablation 6. Addressing the Eye:
• After photoablation, many surgeons irrigate ablated
stromal surface with chilled BSS and /or apply iced
BSS on surgical spears to control postoperative
pain and possibly reduce incidence of corneal
haze. Drops of antibiotic & corticosteroid are
instilled, followed by placement of a bandage
contact lens (BCL). If Epi-LASIK or LASEK are
performed, surgeon first places epithelial sheet
back into position with an irrigating canula filled
with BSS before applying drops and the BCL.

Post SA Management
• 1. Sedation: During the first 24 to 48 hours, patients
experience a variable amount of pain, which may need
to be treated with an oral narcotic, NSAID , analgesics
(e.g. gabapentin and pregabalin) .
• 2. Medication: As long as the BCL is in place, patients
are prescribed antibiotics and corticosteroids, 4 times
daily and preservative-free artificial tears.
Corticosteroids can be postponed till complete
epithelialisation and BCL removal. Patients should be
followed closely until the epithelium is completely
healed, which usually occurs within 3 to 4 days. At this
point BCL and antibiotics are discontinued .

Post SA Management
• Topical corticosteroids modulate postoperative wound
healing, reduce anterior stromal haze and reduce
regression of the refractive effect. The strength of steroid
and the duration of use remain controversial. Many
surgeons still advocate a tapering dose of topical
corticosteroid drops (4 times a day for 1 month, 2 times a
day for 1 month, once a day for 1 month) and then stop
them. Other surgeons feel that patients with low refractive
errors (<–4 D), shallow ablation depth (AD) (<70 μm), or
those receiving Mitomycin C (MMC) are at lesser risk to
develop postoperative corneal haze and may require a
shorter course of corticosteroid . Duration of occupational
sun exposure has an impact on haze formation; it is
therefore advised to use sun glasses after SA for a long
period of time .

Adjunctive Intraoperative
Mitomycin C (MMC) in SA
• Because corneal haze is a major complication associated
with SA, a soaked pledged of MMC, usually 0.02% or 0.2
mg/ml, can be placed on the ablated surface for 12 seconds
to 2 minutes at the end of the laser treatment . MMC is an
alkylating agent that inhibits DNA synthesis. It is used to
reduce the chance of corneal subepithelial haze in eyes at
high risk for this complication, including higher corrections
corresponding to ablations of more than 80 μm . Some
surgeons use MMC in patients with a history of keloid
formation or in those subjected to prolonged sunlight
exposure because ultraviolet radiation causes
postoperative corneal haze. Recent studies indicate that
shorter application times are equally effective in primary
SA, hence, the trend to 12-second applications, while 1- to
2-minute MMC applications are reserved for high-risk cases.

• Irrigation of the corneal surface with copious
amounts of BSS (30 ml) to remove the excess MMC
and minimize the toxicity is crucial. To prevent
damage to limbal stem cells, the surgeon should
avoid exposing the limbus or conjunctiva to MMC.
The use of adjunctive topical MMC to prevent
postoperative corneal haze in refractive surgery is
an off-label use of the medication.

• Using MMC may cause overcorrection of myopia. Some
surgeons recommended reducing the spherical myopic
component by 10–15% depending on patients age; the
older the patient the higher the percentage to reduce.
On the other hand, in case of myopic astigmatism, the
plus cylinder equation is used; e.g. a 25 y/o patient has
–4D sph –2D cyl @ 120°, it should be converted into –6 D
sph +2 D cyl @ 30°, then the –6 D sph may be reduced by
10%; therefore, the patient will be corrected for –5.4 D
sph +2 D cyl @ 30°. Another example, a 55 y/o patient
with –2 D sph –1 D cyl @ 180°, it should be converted
into –3 D sph +1 D cyl @ 90°, then the –3 D sph will be
reduced by 15%; therefore, the patient will be
corrected for –2.55 D sph +1 D cyl @ 90°.

Surgical Techniques in LASIK
• 1 Pre-op and anesthesia, sterilization and draping
are same as in SA.
• 2. Alignment and Registration: As mentioned in SA,
registration is a technique in which a fixed
landmark is used at the time of aberrometry or
tomography and treatment to apply the ablation to
the correct area of the cornea; it does not rely on
the pupil for laser centration. The step of
registration should be done before flap creation.

Surgical Techniques in LASIK 3.
Decentration (Off-set Pupil):
• In case of high angle kappa, > 1D astigmatism or in
hyperopia, it is recommended to decenter the ablation
beam in order to shift the center of ablation to be
coaxial with the visual axis. In Placido-based
topographers, angle kappa is measured and offset pupil
(or decentration) can be performed either manually by
input of x and y values of angle kappa or automatically
by wavefront-guided profiles. In Scheimpflug-based
topographers, angle kappa is not measured and offset
pupil (or decentration) can be performed either
manually by input of half x and y values of pupil center
coordinates or automatically by the wavefront-guided
profiles.

• In both methods, great care of the location of the flap should be taken;
decentration holds the risk of applying part of the photoablation beam
out of treatment bed leading to irregular treatment and hence,
introducing high order aberrations (mainly coma). Therefore, the flap
should also be decentered to compensate for the decentered
photoablation beam. Decentring the flap is done in following manner :
• a. Perform beam decentration by input of decentered values in the
software of the excimer machine (offset pupil).
• b. Press the pedal test to see the new location of the center of the
ablation beam.
• c. While keep pressing the pedal test, put a mark on the cornea at the
place of the beam.
• d. Use this mark as the center of the suction ring either with MMK or
femtosecond.
• e. Create the “decentered” flap.

4. Flap Creation
• Creating the flap can be performed using either a
MMK or a femtosecond laser. It is recommended to
create a flap with a diameter of 0.5 to 1.0 mm
larger than the total ablation zone. Usually, the
total ablation zone diameter is 6 to 9.5 mm in
myopic treatment and 8.5 to 10 mm in hyperopic
treatment.

4. Flap Creation
• a. Marking the Cornea .
Make asymmetric sterile
ink marks in the corneal
periphery, away from the
intended flap hinge, just
before placement of the
suction ring. These marks
can aid in alignment of
the flap at the end of
surgery and in proper
orientation in the rare
event of a free cap.

4. Flap Creation
• b. Creating the flap. – The MMK suction ring has two
functions: adhering to the globe, providing a stable
platform for the MMK cutting head; and raising the lOP
to a high level, which stabilizes the cornea.
• – The dimensions of the suction ring determine the
diameter of the flap and the size of the stabilizing
hinge. The thicker the vertical dimension of the suction
ring and the smaller the diameter of the ring opening,
the less the cornea will protrude and hence a smaller-
diameter flap will be produced .
• – The suction ring is connected to a vacuum pump,
which is typically controlled by an on-off-foot pedal.

4. Flap Creation
• – The MMK cutting head has several key components. The highly
sharpened disposable cutting blade is discarded after each
patient, either after a single eye or after bilateral treatment.
• – The applanation head, or plate, flattens the cornea in advance
of the cutting blade. The length of the blade that extends beyond
the applanation plate and the clearance between the blade and
the applanation surface are the principal determinants of flap
thickness.
• – The motor, either electrical or gas-driven turbine, oscillates the
blade rapidly, typically between 6000 and 15,000 cycles per
minute. The same motor or a second motor is used to
mechanically advance the cutting head, which is attached to the
suction ring, across the cornea; although in some models the
surgeon manually controls the advance of the cutting head (this
carries potential personal errors).

4. Flap Creation
• – Smaller and thinner flap size and longer hinge
cord length are more important than hinge location
in sparing the nerves and reducing the incidence
and severity of dry eye. Regardless of hinge type,
patients generally recover corneal sensation to
preoperative levels within 6–12 months after
surgery.
• – The ring should be positioned either symmetrical
to the limbus, or centered with the ink mark that
has been put in case of decentration.

4. Flap Creation
• – Once the ring is properly positioned, suction is
activated . The lOP should be assessed at this point
because low lOP can result in a poor-quality, thin, or
incomplete flap. It is essential to have both excellent
exposure of the eye, allowing free movement of the
MMK and proper suction ring fixation. Inadequate
suction may result from blockage of the suction ports
from eyelashes under the suction ring or from
redundant or scarred conjunctiva. To avoid the
possibility of pseudo suction (occlusion of the suction
port with conjunctiva but not sclera), the surgeon can
confirm the presence of true suction by observing that
the eye moves when the suction ring is gently moved,
the pupil is mildly dilated and the patient can no longer
see the fixation light or can see it blurred.

4. Flap Creation
• – Prior to the flap cut, the surface of the cornea is
moistened with proparacaine with glycerin or with
preservative-free artificial tears. BSS should be
avoided at this point because mineral deposits may
develop within the MMK and interfere with its
proper function. The surgeon places the MMK on
the suction ring and checks that its path is free of
obstacles such as the eyelid speculum, drape, or
overhanging eyelid .

4. Flap Creation

4. Flap Creation
• – The MMK is then activated, passed over the
cornea until halted by the hinge-creating stopper
and then reversed off the cornea. It is important to
maintain a steady translation speed to avoid
creating irregularities in the stromal bed.
• – Before surgery, the MMK and vacuum unit are
assembled, carefully inspected and tested to
ensure proper function. The importance of
meticulously maintaining the MMK and carefully
following the manufacturer’s recommendations
cannot be overemphasized.

4. Flap Creation
• – In addition, the surgeon should be aware that,
regardless of the label describing the flap
thickness of a specific device, the actual flap
thickness varies with the type of MMK, quality of
the blade, patient age, preoperative corneal
thickness, preoperative keratometry, preoperative
astigmatism, corneal diameter, IOP, first eye vs.
second eye and translation speed of the MMK pass.

b. Femtosecond laser.
• – A femtosecond laser also creates flaps by performing a
lamellar dissection within the stroma. Each laser pulse
creates a discrete area of photo disruption of the collagen.
The greater the number of laser spots and the more the
spots overlap, the more easily the tissue will separate when
lifted.
• – The femtosecond laser allows adjustments for several
variables involved in making the flap, including flap
thickness, flap diameter, hinge location, hinge angle, bed
energy and spot separation.
• – Although the goal is to try to minimize the total energy
used in flap creation, a certain level of power is necessary
to ensure complete photo disruption and greater overlap of
spots allows for easier flap lifting.

• – With the computer programmed for flap diameter,
depth and hinge location and size, thousands of
adjacent pulses are scanned across the cornea in a
controlled pattern that results in a flap.
• – With a femtosecond laser, the side cut of the
corneal flap can be modified in a manner that may
reduce the incidence of epithelial ingrowth.

• – The use of a femtosecond laser generally takes
more time than a MMK because it requires several
extra steps. First, the suction ring is centred over
the pupil (symmetrical with the limbus) or centered
with the mark of decentration in case of it. Suction
is applied. Proper centration of the suction ring is
critical and is performed under a separate
microscope, either the microscope from the
adjacent excimer laser or an auxiliary microscope
in the laser suite.

• The docking procedure is initiated under the
femtosecond laser’s microscope, while the
patient’s chin and forehead are kept level and the
suction ring is kept parallel to the eye. The
applanation lens is then centred over the suction
ring and lowered into place using the joystick and
the suction ring is unclipped to complete the
attachment to the docking device. Complete
applanation of the cornea must be achieved, or an
incomplete flap or incomplete side cut may occur.

• The vacuum is then released, the suction ring is
removed and the patient is positioned under the
excimer laser.
• – A spatula with a semi sharp edge identifies and
scores the flap edge near the hinge .
• – A blunt instrument is then passed across the flap
along the base of the hinge , and the flap is lifted
by sweeping inferiorly and separating the flap
interface, dissecting one-third of the flap at a time
and thus reducing the risk of tearing .

• 5. Centring and Applying the Ablation Beam: This
step is similar to that for SA added that the flap
must be lifted and reflected and the stromal bed
should be uniformly dry prior to treatment .

Surgical Techniques in LASIK6.
Addressing the Eye
• After the ablation is completed, the flap is replaced
onto the stromal bed. The interface is irrigated
until all interface debris are eliminated (which is
better seen with oblique rather than coaxial
illumination) . The surface of the flap is gently
stroked with a smooth instrument, such as an
irrigation canula or a moistened microsurgical
spear sponge, from the hinge, or centre, to the
periphery to ensure that wrinkles are eliminated
and that flap settles back into its original position,
as indicated by realignment of the corneal .

Surgical Techniques in LASIK6.
Addressing the Eye
• The peripheral gutters should be symmetric and even. The
physiologic dehydration of the stroma by the endothelial
pump will begin to secure the flap in position within several
minutes. If a significant epithelial defect or a large loose
sheet of epithelium is present, a BCL should be placed.
Once the flap is adherent, the eyelid speculum is removed
carefully so as not to disturb the flap. Most surgeons place
varying combinations of antibiotic and corticosteroid drops
on the eye at the conclusion of the procedure. The flap is
usually rechecked at the slit lamp before the patient leaves
to make sure it has remained in proper alignment. A clear
shield or protective goggles are often placed to guard
against accidental trauma that could displace the flap.
Patients are instructed not to rub or squeeze their eyes.

Operative steps of LASIK
(preflight checklist)
• 1 Calibrate and program the excimer laser.
• 2 Assemble and test the microkeratome.
• 3 Prepare the operative cart with the instruments
and supplies necessary to perform the procedure.
• 4 Prepare the patient with the proper sedation.
• 5 Instill topical anesthesia, antibiotics, and
nonsteroidal drops into the operative eye(s).

Operative steps of LASIK
(preflight checklist)
• 6 Position the patient on the laser table.
• 7 Clean the eyelashes and fornices.
• 8 Drape the eye, being sure to isolate the eyelashes
and meibomian glands.
• 9 Place a locking eyelid speculum to obtain
adequate exposure.
• 10 Center the eye in the operative field by adjusting
the microscope and/or head position.

Operative steps of LASIK (preflight
checklist)
• 11 Place the alignment markings.
• 12 Apply the pneumatic suction ring, being sure that
the ring is seated securely around the limbus, and
activate vacuum pressure.
• 13 Check for adequate intraocular pressure with a
Barraquer tonometer or pneumotonometer.
• 14 Lubricate the surface of the eye with balanced salt
solution (BSS) and insert the microkeratome onto the
suction ring track and advance it to the starting
position.
• 15 Check the operative field for obstacles in the track
of the microkeratome.

checklist)
• 16 Press the forward pedal until the hinge stop is
reached.
• 17 Reverse the microkeratome and remove the
microkeratome head alone or in combination with the
suction ring.
• 18 Dry the fornices of excess fluid with a microsurgical
sponge.
• 19 Lift the corneal flap with a spatula or blunt forceps.
• 20 Use a microsurgical sponge to remove excess fluid
from the bed to obtain uniform hydration.

checklist)
• 21 Ablate the stromal bed with the programmed
refraction in the laser.
• 22 Place BSS on the stromal bed and reapproximate
the flap with a spatula or forceps.
• 23 Irrigate beneath the flap to remove debris and float
the flap into position.
• 24 Dry the keratectomy gutter with a moistened
surgical sponge.
• 25 Check corneal alignment markings and symmetry of
the keratectomy gutter space to assure correct
positioning of the flap.

checklist)
• 26 Wipe the corneal flap with a moistened microsurgical sponge to
smooth any wrinkles.
• 27 Pressure may be applied centrally with a flap compressor. Wait 2–3
minutes for flap adhesion.
• 28 Place a viscous lubricant on the eye and carefully remove the eyelid
speculum without touching the cornea. Also remove the eyelid drapes.
• 29 Instill an antibiotic and steroid or antibiotic–steroid combination.
• 30 Have the patient blink while under the microscope and recheck flap
alignment.
• 31 Recheck the flap alignment again 10–20 minutes after the procedure
to assure correct flap alignment at the slit lamp.
• 32 Place protective shield(s) over the eye(s) and discharge the patient.

Post LA Management
• Many surgeons instruct their patients to use topical
antibiotics and corticosteroids postoperatively for
3–7 days. With femtosecond laser procedures,
some surgeons prescribe more frequent
applications of corticosteroid eye drops or a longer
period of use. In addition, it is very important for
the surface of the flap to be kept well lubricated in
the early postoperative period. Patients are advised
to keep their eyes closed for at least 2 hours
immediately after the procedure.

Post LA Management
• Patients may be told to use the protective shield for
1 week when they shower or sleep and to avoid
swimming and hot tubs for 2 weeks. Patients are
examined 1 day after surgery to ensure that the
flap has remained in proper alignment and that
there is no evidence of infection or excessive
inflammation. In the absence of complications, the
next examinations are typically scheduled at 1
week, 1 month, 3 months, 6 months and 12 months
, postoperatively .

SMILE
• The ReLEx small incision lenticule extraction (SMILE)
technique of refractive corneal surgery is a relatively
new procedure and there is a lot of excitement with
quite a few Ophthalmologists actively adapting this
procedure. The introduction of the VisuMax
femtosecond laser led to development of the ReLEx
SMILE technique. In 2006, VisuMax femtosecond laser
was used to create the flap as well as a refractive
lenticule, for manual removal, all in a single step,
popularized as FLEx. Small incision lenticule
extraction, a variant of the ReLEx technique, has been
proposed as an alternative to the conventional LASIK
procedure whereby the femtosecond laser creates a
lenticule within the corneal stroma, which can be
extracted via a small side-cut incision (2-4mm).

SMILE
• From the patients perspective SMILE is very
attractive as it is a painless, short and minimally
invasive procedure. There is almost no special
post-operative precautions or care and patients can
go back to their normal activities including sports
the very next day. Various studies have shown that
quality of vision, dry eye status and refractive
stability is very good after 3 months. Patients
would perceive it as an easier and safer procedure.

SMILE
• corneal biomechanics may lead to reduced incidence of
corneal ectasia and the ability to treat higher refractive
errors safely. The SMILE procedure is still in its infancy and
has to be refined, but many comparative studies with
wavefront optimized Femto LASIK show that the refractive
results and safety may be comparable or better. There is
still a lot of development that has to take place with the
lenticule extraction technique. Software for treating
hyperopia and mixed astigmatism is being developed and
early results from clinical trial sites indicate good success.
Centration in the SMILE treatment is dependent on patient
fixation and in future hardware for confirming good
centration would improve outcomes. Also automatic
cyclotorsion compensation for astigmatism has to be
introduced .

SMILE
• The ability to treat irregular or aberrated corneas
with a link to a topographer or aberrometry is still
a long way off. The lenticule extraction technique
has shown very good stability over time when
compare to LASIK or PRK especially for higher
refractive errors with very low enhancement rates.
Currently the options for enhancement are either
LASIK by converting the cap into a flap using a
special circle software or surface ablation. There is
a possibility of doing re-SMILE for enhancement .

SMILE
• SMILE is a surgeon based procedure with a learning
curve which may be steeper than LASIK or PRK.
Optimizing energy levels, learning to dock and centre
the eye are important and can be mastered initially by
using the VISUMAX laser to create flaps. Identifying
the correct tissue planes and easy dissection with
minimal tissue distortion improves immediate post-
operative recovery. The learning curve with SMILE can
be reduced by observation and proper training. There
can be complications like suction loss, retained
lenticule, lenticule tear which are unique to this
procedure and also difficult dissection due to sub-
optimal fluence levels which have to be handled
properly to ensure smooth outcomes.

SMILE
• Postoperative dryness and aberrations and
reduction in contrast sensitivity, the accepted
drawbacks of any corneal refractive surgery, seem
clinically less significant with SMILE.

Complications
• The most frequent complication observed in any
refractive procedure is the lack in achieving
accurate refraction outcome. As a rule, accuracy
decreases with the amount of refractive error.
Photoablative procedures tend to be the most
accurate ones for low ametropias. Photorefractive
keratectomy (PRK) and laser-assisted in situ
keratomileusis (LASIK) deal with different variables
that may affect predictability: corneal wound
healing and stromal bed elasticity, respectively .

Complications
• Although results minimally favor LASIK, we may
expect that in any photoablative procedure, 60–
70% of eyes will achieve 20/20 uncorrected visual
acuity and will be within +/–0.50 D after surgery. If
we analyze only low myopias (under 6.00 D), 70–
80% will achieve 20/20 uncorrected visual acuity .

LASIK: Intraoperative
(Flap) Complications

LASIK Intraoperative
Complications
• The intraoperative complication rate of 0.7–6.6 % is
most commonly microkeratome related, either
mechanical or femtosecond laser.

LASIK: Intraoperative (Flap)
Complications
• A thin, irregular, or buttonhole flap is a significant
complication of lamellar surgery that typically calls for
aborting case. Thin, irregular or buttonhole flaps can
occur with all keratomes, including the new
femtosecond devices. The cause of a thin, irregular, or
buttonhole flap is often unclear and can be
multifactorial. Causes of a thin, irregular, or buttonhole
flap may include low pressure, poor corneal lubrication,
poor blade quality, preexisting corneal pathology, or a
keratome malfunction. Most thin, irregular, or
buttonhole flap cases can be redone with either LASIK
or PRK and do have a good prognosis. Remember, the
key when faced with a poor flap typically is not to
ablate.

Thin, Irregular,
Buttonhole Flaps

Thin, Irregular, Buttonhole Flaps
• These poor-quality flaps are a significant concern
with lamellar surgery, for example, incidence of
buttonhole flaps using a mechanical
microkeratome ranges between 0.3 and 2.6% of
general LASIK procedures . The incidence with the
femtosecond laser seems lower.

• Keratectomies can be incomplete, decentered, or
uneven. An incomplete keratectomy is usually
caused by a suction break. It is critical to have
good suction for the duration of the keratome pass.
If the keratome stops before the pass is complete,
then there might not be room to place the ablation.
The keratectomy can be extended by hand but will
not be of the same quality of the microkeratome
section. A bad or damaged blade can cause a
grossly irregular keratectomy.

• poor-quality flaps can be caused by one or more of
the following factors in mechanical keratectomies:
• Loss of suction during the transverse cut
• Patient cornea steeper than 46 D prior to surgery
Low or reduction in patient intraocular pressure
• Poor lubrication of corneal surface or keratome
malfunction
• Excess tissue being compressed beyond
applanation by the keratome foot plate, causing
buckling of cornea .

HOW TO MAKE A FLAP
• The cornea should be wet for the pass, but a little dry for
the applanation. Always take a moment to inspect the eye
before the placement of the suction ring. There should be
no chemosis, and the pupil should be centered between the
speculum. If chemosis is present, then the fluid should be
milk out with curved tiers down beneath the lid speculum. A
speculum that provides maximum exposure with reasonable
patient comfort is desirable. The pupil should be
constricted only with the light from the microscope. Again,
the case should be set up for maximum exposure; good
suction must be present, good centration, a slow, controlled
pass of the keratome, and a sharp, accurate blade . The
keratome must be checked for smooth operation, a perfect
blade, and a good suction sterile field and overall clean
technique must be used .

FEMTOSECOND LASER
• The femtosecond laser offers a unique advantage
to the prevention of complications from poor-
quality flaps. Quite often, a poor-quality flap can be
actually detected during the creation of the flap
with a femtosecond laser. This is because the flap
takes longer to create (20 s vs. 6 s), and the flap is
visible at all times during the procedure. With
experience, a thin flap or buttonhole flap can be
seen in its creation and the procedure stopped.

more prone to experiencing
flap quality complications:
• History of collagen vascular disease
• Patient cornea steeper than 46 D prior to surgery
• Conjunctival scarring after prior ocular surgery
• Previous incisional keratotomy
• Prior ocular, specifically cornea injury
• History of keratoconus
• Previous scleral buckling surgery
• Patient with unusually thick epithelial layer (>90
μm)

• Clinical concerns when dealing with poor-quality
flaps include the potential for epithelial cells to
infiltrate the stroma, causing epithelial ingrowth in
the central axis. This may result in corneal
scarring in the visual field, affecting visual acuity.
In addition, invasive epithelial ingrowth can lead to
more severe complications, such as stromal melt.

• If a keratectomy has an irregular surface, then
there is an important safety feature of lamellar
surgery that should be well known by now. No
matter how irregular the surface of the bed might
be, there is a perfect match in the underside of the
fl ap. Th erefore, if the flap is simply replaced the
patient will return to the preoperative refraction
and best corrected vision by the next morning. The
femtosecond laser is even friendlier in this regard,
in that the flap is held in place by the microtissue
bridges of uncut stroma .

• . These “tags” hold the flap in place so that once the
diagnosis is made, the flap is securely attached and can be
allowed to wait until a retreatment is advisable. The
advantage is that the epithelium and Bowman’s is cut last
with a femtosecond laser so the procedure may be aborted,
leaving epithelium and Bowman’s intact. Problems are
created when the bed is altered with an attempted ablation
such that the flap no longer matches. This is important to
remember with incomplete resections also. When in doubt,
put the fl ap back and do not ablate. One of the more
pleasant features of lamellar surgery is that the eye can be
back to the exact preoperative state the next day, and then
reoperated on in the next few weeks or months depending
on the situation. If an incomplete resection is present and
there is room for the ablation, then one can proceed.

Take-Home Pearls
• Identify patients at risk for flap complications.
• Carefully set up and review your microkeratome,
laser, and surgical protocol.
• Be aware of these complications and suspect them
in any uncertain situation.
• Do not ablate a poor quality bed

Incomplete LASIK Flap
• Incomplete flaps occur due to premature stopping
of the microkeratome head before reaching the
intended hinge location. Visual aberrations are
more likely to occur when the created hinge results
in scarring in proximity to the visual axis .

Incomplete LASIK Flap
• Unlike other flap-related complications such as
buttonholes, thin flaps, and free caps, which result
mainly from a combination of anatomical and
mechanical factors, incomplete flap occurrence is
highly related to the surgeon’s own experience (i.e.,
experienced LASIK surgeons have had a much
lower incidence of incomplete flaps than have
novice surgeons) .

• The most common cause of incomplete flaps is
jamming of the microkeratome due to either
electrical failure or mechanical obstacles. Lashes,
drapes, loose epithelium, and precipitated salt from
irrigating solution have been recognized as
possible impediments to smooth microkeratome
head progress . Less common causes may include
suction loss during the microkeratome pass and
gear-advancement mechanism jams .

IMMEDIATE MEASURES
• In cases of incomplete flap where there is sufficient
surface area in the stromal bed for laser ablation, using
a reasonable ablation zone, the procedure can be
completed as usual. In other cases with a small
stromal bed with inadequate room for ablation,
management is best accomplished by immediate
careful repositioning of the partial flap and postponing
the procedure. Topical antibiotic and steroid drops
should be started immediately. Patients must be
followed up regularly until the refractive error is
stabilized, and then a second intervention may be
performed.

Immediate Measures
• Applying excimer laser treatment to an inadequate
stromal bed with a short or incomplete flap is
contraindicated. Serious visual and refractive
complications may take place, such as irregular
astigmatism leading to loss of BCVA, monocular
diplopia, and distorted vision, especially at night .

Delayed Management
• There are two main options for the second
intervention in cases of incomplete flaps. First, the
microkeratome pass may be repeated after a
period. Second, surface ablation may be performed
over the abnormal flap by utilizing either
photorefractive keratectomy (PRK) or laser
subepithelial keratomileusis (LASEK).

Corneal Haze after
Refractive Surgery

Corneal Haze after Refractive
Surgery

Rules and Guidelines in
Refractive Surgery

Rules and Guidelines in Refractive
Surgery THICKNESS RULES
• Munnerlyn formula for myopic and myopic
astigmatism states that AD (μm) = 1/3 x (OZ
diameter [mm])2 x (intended correction [D]). For
instance, correcting –4 D for an OZ = 6.5 mm
indicates an AD = 1/3 x (6.5)2 x 4 = 56 μm.
• This formula is very helpful in calculating the
amount of AD for different OZs; i.e. when the
scotopic pupil is small, a small OZ can be chosen
to save tissue. For example, if the scotopic pupil
size is 5 mm, an OZ of 5.5 mm can be chosen and
the corresponding AD is almost 10 μm per 1 D of
correction.

• RSB Rule 1
• In LA, thickness of the RSB should be at least 55%
of the original corneal thickness at the thinnest
location AND to be at least 250 μm (preferably 270
μm). Example 1: an original corneal thickness of
500 μm means a RSB of 500 x 55 = 275 μm, which
is >270 μm.
• Example 2: an original corneal thickness of 480 μm
means a RSB of 480 x 55 = 264 μm, which is not
favourable.

• RSB Rule 2
• In LA, at most 20% of the original corneal thickness at
the thinnest location can be ablated.
means a recommended AD of at most 500 x 20 = 100
μm. In case of 100 μm flap, the RSB will be: 500
(thickness) – 100 (flap) – 100 (AD) = 300 μm.
• Example: 4: an original corneal thickness of 600 μm
means an AD of at most 600 x 20 = 120 μm. In case of
100 μm flap, the RSB will be: 600 (thickness) – 100
(flap) – 120 (AD) = 380 μm.

• RSB Rule 3
• In LA, the AD differs according to OZ diameter and
profile. In general, correcting –1 D sph ablates an
average of 14 μm and 16–17 μm for 6 mm and 6.5 mm
OZ, respectively. For easy calculations, 15 μm will be
used.
with –5 D sph and 100 μm flap means a RSB of: 500 – (5
x 15) – 100 = 325 μm.
with –8 D sph and 100 μm flap means a RSB of: 600 – (8
x 15) – 100 = 380 μm.

• RSB Rule 4
• Use the most conservative rule from rules 1, 2 and 3.
• Example 7: an eye with an original corneal thickness of 500
μm and –6 D sph refractive error:
• 1. RSB rule 1: RSB = 500 x 55% = 275 μm; therefore, the
recommended AD for a 100 μm flap is 500 – 100 – 275 = 125
μm
• 2. RSB rule 2: AD = 500 x 20% = 100 μm; therefore, the RSB
for a 100 μm flap = 500 – 100 – 100 = 300 μm
• 3. RSB rule 3: AD = 6 x 15μm = 90 μm; therefore, the RSB for
a 100 μm flap = 500 – 100 – 90 = 310 μm
• 4. To be conservative, ablate 90μm and leave 310 μm RSB
(rule 3).

• RSB Rule 5
• In SA, it is recommended not to exceed 80–90 μm
of AD in order to avoid haze; therefore, in case of
6.5 mm OZ, about 6 D can be corrected. On the
other hand, a minimum of 400 μm of final RSB
including the epithelium should be left. For
example, an eye with an original corneal thickness
of 490 μm can be ablated for 80–90 μm, while an
eye with an original corneal thickness of 470 μm
can be ablated for 70 μm. The AD is thereafter
divided by 15 μm to calculate the recommended
refractive correction.

• RSB Rule 6
• In PRT, use the absolute sum of the refractive error in calculating
the RSB.
• Example 11: the amount of AD for a refractive error of –4 D sph/–3
D cyl @120 is (4 + 3) x 15 = 105 μm.
• RSB Rule 7
• In hyperopic treatment ( pure hyperopia or hyperopic
astigmatism), the central ablation is zero, whereas the maximum
AD is peripheral where the cornea is thick. Therefore, the
previous rules cannot be applied. However, the trend now a days
is to correct no more than + 4 D by PRT in order to minimize
biomechanical responses which may impact the results.
• In general, the preoperative thinnest location should be > 470 μm.

• RSB Rule 8
• For calculations in mixed astigmatism, the equation
should be converted into plus cylinder formula.
• Example 12: in a refractive error of +2 D sph/–4 D
cyl, the plus cylinder equation should be used: –2 D
sph/+4 D cyl. Thereafter, the RSB rules are applied
for –2 D sph.

Surgery THICKNESS RULES LASIK
• RSB 1: Keep at least 270 μm of RSB
• • RSB 2: Ablate at most 20% of original corneal
thickness at thinnest location
• • RSB 3: Actual AD differs according to the
diameter of the OZ. In average, 15 μm per -1 D for
an OZ = 6.5 mm
• • RSB 4: Follow the most conservative rule among
rules 1, 2 and 3

Surgery THICKNESS RULES SA
• • RSB 5: Ablate at most 80–90 μm and keep at least 400
μm of RSB including epithelium
• In PRT:
• • RSB 6: Use the absolute sum of the refractive error in
calculating RSB
• In hyperopic treatment:
• • RSB 7: The preoperative thinnest location should be ≥
470 μm and it is recommended not to go beyond +4 D of
correction
• In mixed astigmatism:
• • RSB 8: convert the equation to use the plus cylinder
formula .

CONCLUSION
• PRK AND BOTH LASIK WILL STAY AS GOLD
STANDARAD AND SMILE IS CATCHING UP WITH
THESE TECHNOLOGIES BUT IT IS VERY
EXPENSIVE TO BOTH THE SURGEON AND PATIENT
AND THAT TOO WITH NO PROVEN BETTER RESULT
. SAME THING SOME SURGEONS ADVOCATE
FLACS FEMTO ASSISTED CATARACT SURGERY TO
BE BETTER THAN PHACO BUT WE KNOW THAT IS
NOT THE CASE . BETTER TO KEEP IT SIMPLE AND
TIME TESTED . PHACO FOR CATARACT AND PRK
AND LASIK FOR REFRACTIVE SURGERY ARE THE
BEST AND GOLD STANDARD .

Excimer laser
• After nearly three decades of innovation in excimer
laser, today we are presented with a state of the art
generation targeting minimally invasive refractive
surgery with high speed laser, faster trackers, pupil
monitoring systems and better customization profiles.
These systems are capable of delivering better
treatments with less induced postoperative high order
aberrations. Still, current technology is facing major
challenges in the correction of high hyperopic errors
and in presbyopic treatments, with upgrades in
ablation centration and thermal control needed, which
will ensure better biomechanical results, as a step
closer to perfection in refractive surgery.

THANK
YOU
DR DINESH
DR SONALEE

SURFACE ABLATION PRK , LASIK , SMILE

SURFACE ABLATION PRK , LASIK , SMILE

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