Principles of Kerato- Refractive Surgery

Refractive Surgery
PG Corner
Principles of KeratoRefractive Surgery
Abhishek Dave
MD, FMRF, FICO
Abhishek Dave MD, FMRF, FICO Vinay Arora MS, Sridevi Gunda DNB
Prachi Jhala Dave MS, DNB, Umang Mathur MS, FLVPEI
Cornea & Refractive Surgery Services, Dr. Shroff’s Charity Eye Hospital, New Delhi.
R
efractive surgery procedures for the correction of myopia,
hyperopia, presbyopia and astigmatism can broadly be
divided into 2 categories: keratorefractive techniques and
lenticular and scleral based procedures. Keratorefractive
surgeries rely on at least five major methods to reshape
the corneal surface – incisions, lasers, thermal procedures,
corneal implants and non-laser lamellar surgery. Incisional
procedures and non-laser lamellar surgeries have largely
been replaced by laser refractive procedures. Corneal
implants are mainly used in corneal ectatic disorders and
thermal procedures limited to hyperopic corrections. So
we will be limiting our discussion mainly to laser refractive
surgeries.
Microkeratomes: Principles and Settings
The basic components of a microkeratome are: suction
ring, keratome head, blade control and variables. The main
variables are as follows:
1. Suction ring size and flap diameter
2. Flap thickness and hinge location
3. Vacuum setting
4. Blade selection and controls
Suction Ring Size
The size of the suction ring is the primary determinant of the
flap diameter. Preoperative keratometry influences the flap
diameter1. Use of a large diameter suction ring for patients
with steep corneas can result in flaps that exceed the clear
corneal diameter, resulting in damage to limbal vessels and
also increasing the chances of button-hole complications.
So a smaller diameter ring is recommended for significantly
steep corneas. Flatter corneas present less tissue through a
given ring size and produce smaller diameter flap increasing
the risk of free-flaps.
Flap Thickness and Hinge Location
Factors affecting flap thickness are head-plate depth, corneal
thickness, pre-operative refractive error, blade sharpness,
blade length, translational speed and intraocular pressure.
The 2 most important patient factors that decide the head
plate depth are the corneal thickness and pre-operative
refractive error2. IOP can affect flap thickness with lower
IOP resulting in thinner flaps. Another important factor is
the actual flap depth produced by different microkeratomes.
The Flap Thickness Study Group found significant difference
in the flap thickness produced by different microkeratome
systems3. Now with the help of femtosecond lasers we can
create thinner flaps thus giving us more room for ablation
preserving a good residual bed. The femtosecond flaps
varies much less in thickness also.
The two hinge locations used are superior and nasal.
Superior hinge has the advantage of upper lid compression
maintaining flap position and centration. But the same
is associated with greater dry-eye symptoms due to the
corneal hypoesthesia produced by severing of the nasal and
temporal nerve plexus. Nasally placed hinge fairs better in
this regard4. However few studies contradict this.
Residual Posterior Stromal Thickness (RPST)
This is the most important factor to be considered when
choosing the flap thickness. A minimal RPST of 250
µm is essential to prevent post lasik ectasia5. But it is
recommended to preserve a RPST of 280-300 µm. Thus 250
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PG Corner: Principles of Kerato-Refractive Surgery
µm plus the flap thickness are subtracted from the lowest
central pachymetry to determine how much ablation can be
performed. The ablation depth can be approximated using
the Munnerlyn formula: Depth of Ablation = Dioptres of
Correction X (Ablation Diameter)2/3. The decision to create
a thinner flap to treat higher powers should be weighed
against the disadvantages of a thin flap i.e. higher risk of
flap folds, buttonhole formation and increased difficulty in
handling thinner tissue.
Beam Homogeneity
Vacuum Setting
The smaller the beam size, the easier it is to control the
homogeneity of the beam. The trade-offs for a smaller beam
size include increased treatment time, corneal dehydrations
and small amounts of decentration. Beam sizes of ≤ 1mm
are adequate to correct up to Fourth-order Zernike terms.
There are three main types of beam delivery systems:
The suction system should be sufficient to raise the IOP to
at least 65 mm Hg. Lower pressure can produce thinner
flaps and higher pressure increase risk of chemosis and
optic nerve head injury. Newer microkeratome systems
automatically stop oscillations and forward progress if there
is loss of suction.
Femtosecond Laser
Femtosecond laser consists of a solid state Nd: glass laser
source generating pulses at rates of 3-5 kHz with each
pulse of 500 femtosecond duration and 3-5 µm size. Laser
delivery occurs via a scanning spot ablating 0.3 µm/pulse
with an accuracy of approximately 1 µm6. The vacuum
system raises the IOP to around 35 mm Hg. The depth
accuracy achieved is around 10 µm. The first pulses are
delivered along the hinge. A pocket is created to receive
the gases. The edges of flap are prepared at last with a
variable angle chosen by operator (60-90 degree) ensuring
maximal stability. FS laser has the potential advantages of
reduced flap complication rate, increased consistency of
flap thickness and diameter, reduction in suction pressure
required and flexibility of hinge creation.
This is a measure of the consistency of energy distribution
applied over the ablation area. Poor beam homogeneity can
lead to irregular ablations. This becomes very important for
customized ablations. Beam homogeneity can be verified
by test ablations performed on appropriate substrate
material provided by the manufacturer.
Beam size, Beam Delivery Systems & Ablation Profiles
1. Full-beam Systems: They were used in first generation
lasers. They offer more rapid treatment and are less
sensitive to decentration. However, they are more
difficult to homogenize, yield less regular ablation
surfaces and customized treatment is impossible.
2. Scanning Slit Delivery: A diaphragm is placed between
the eye and a full beam, creating a rectangular beam
with a smaller width, improving homogeneity.
3. Flying Spot: In this case beam is small and circular.
Beam direction is controlled by pivoting mirrors. Large
number of pulses, at a very high frequency is used,
each pulse removing only a very small area of tissue.
They have the advantage of providing asymmetrical
ablation profiles.
Excimer Laser: Principles & Settings
The principles for ablation in myopia, hyperopia and
astigmatism have been depicted in (Figure 1, Figure 2) and
(Figure 3) respectively.
Principles
Laser Room Environment
The excimer laser utilizes a 193-nm UV wavelength
produced by the dissociation of an excited dimer, consisting
of Argon and Fluoride. The direct breakage of chemical
bonds by a high energy laser is called photochemical
ablation. Pulses produced are of approximately 10-nsec
duration with laser repetition rates between 50 and 400
Hz. Molecular fragments of approximately 10-20 carbon
atoms are ejected in a plume lasting 3-15 µsec7.
The laser room environment is an important variable which
should be controlled. The room should be cool for optimal
laser performance. The temperature varies from laser to
laser, but a temperature of 18-24°C is optimal. Ambient
level humidity between 40-50 % should be maintained.
Over-corrections are more likely in dry conditions and vice
versa8.
Laser Fluence
Laser fluence is described as the amount of energy applied
per unit area with each pulse (mJ/cm2). The minimum
fluence necessary for proper photoablation of the cornea is
approximately 50-60 mJ/cm2. It should be checked before
every ablation, which is usually performed automatically
in most laser delivery systems. The two factors affecting
fluence are gas pressure and voltage. Low fluence means
that the gas concentration should be raised to avoid under
corrections.
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Other Considerations in Lasik
Pupil Size
Laser should be planned in such a way that the Functional
Optic Zone (FOZ) should be more than the mesopic pupil
size. FOZ is 25% less than the ablation diameter. Larger
pupil diameter requires a larger ablation zone otherwise it
can give rise to significant glare post-operatively. However,
modern LASIK has negated the role of the low light pupil in
predicting adverse visual outcomes after LASIK outside of
the early postoperative period9.
Refractive Surgery
Figure 1: Myopic Ablation Profile.
Figure 3: Astigmatic Ablation Profile.
Optical Zone (OZ)
According to the Munnerlyn’s formula, the depth of ablation
is directly proportional to the square of the diameter of the
OZ. For example, reducing the OZ from 6mm to 5mm,
induces a reduction of nearly 30% of the maximal depth of
ablation. This can allow full correction of high myopic errors
in patients with relatively thin corneas. The disadvantages
of an excessively small OZ include poor optical quality and
increased risk of regression10. For an ablation zone of 6.0
mm, a conventional myopic correction would be expected
to remove approximately 12 μm/D of correction.
Figure 2: Hyperopic Ablation Profile.
angle kappa can be measured using a synoptophore or
major amblyoscope. Most excimer laser platforms centre
keratorefractive procedures over pupillary axis. However,
in cases with a large angle kappa (hyperopes), there is
a greater chance of decentred ablation because of the
increased distance between pupillary axis and visual
axis. Decentred ablation can lead to a number of visual
complaints including glare, distortion, reduced visual
acuity, and diplopia. Thus, accurate centration of the
ablation zone during laser vision correction is essential
for optimal outcome. Excimer laser systems allow for the
movement of the ablation centre away from the pupil
centre with an offset by entering either X and Y Cartesian
or R and Ɵ polar values. Large angle kappa should also be
considered before using a wavefront-guided treatment as
wavefront is currently calculated centered on pupil. The
relatively small degree of angle kappa in myopic eyes and
the larger optical zone in myopic ablation compared to
hyperopic ablation makes myopic ablation less sensitive to
decentration11.
Custom Ablation
The linking of the laser device to various instruments
allows for correction of irregular (eccentric) ablations and
high-order aberrations. Few of the customized ablation
procedures include:
Angle Kappa in Refractive Surgery
Wavefront-optimized ablation profile: In addition to treating
lower-order aberrations, wavefront-optimized treatments
attempt to maintain the preoperative prolate shape of the
cornea by removing more stromal tissue peripherally than
in conventional treatments and adjusts for postoperative
biomechanical and biologic effects.
The exact definition of angle kappa is the angular distance
between visual and pupillary axis. Clinically, the angle
kappa was redefined as the angular distance between the
line of sight (line connecting the pupillary centre and the
fixation point) and the pupillary axis (Figure 4). The exact
Wavefront-guided ablation profile: Wavefront-guided
treatment is customized ablation that attempts to reduce
the optical aberrations and the total wavefront error of
patient’s eye to a reference ideal. The input data are more
complex and time consuming to measure as it is derived
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PG Corner: Principles of Kerato-Refractive Surgery
Q-factor adjusted ablation profile (F-CAT): This approach
is based on refractive error and corneal topography
measurements, and the resultant laser ablation profile
corrects the refractive error, while trying to produce a
more aspheric corneal shape. The target corneal contour
for reducing spherical aberrations is calculated as having a
Q-factor of −0.40. The major advantages of this technique
compared to wavefront-guided treatments are that it is
less time consuming and produces a clinically equivalent
reduction in certain postoperative optical aberrations to
wavefront-guided ablations. Its disadvantages are that
it ablates more tissue centrally than wavefront-guided
treatments and does not correct all higher-order aberrations
as it predominantly corrects spherical aberrations, similar
to wavefront-optimized ablations.
Relex/ Smile Refractive Procedures
These are keratolenticular refractive procedures in which
femtosecond laser can be used to carve out a lenticule
within the corneal stroma. The lenticule can then be
extracted from within the corneal stroma, either by creating
and lifting a hinged flap or by extricating it using a small
incision in the cornea. These techniques of femtosecond
lenticule extraction are known as femtosecond lenticule
extraction (FLEx) and small-incision lenticule extraction
(SMILE), respectively. These techniques have the advantage
of all-in-one femtosecond laser procedure. They represent
novel integrated surgical techniques to perform corneal
laser surgery in a single step needing only one laser to
perform laser refractive surgery and have various clinical,
practical, and economic advantages over the more
traditional two-laser solution12.
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Figure 4: Angle kappa.
6.
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Topography-guided ablation profile (T-CAT): Topography
- guided ablations are an alternative treatment option
for patients with irregularities of the corneal surface like
keratoconus. A major limitation of this technology at this
time is that it does not correct refractive spherical error;
therefore, a two-stage keratorefractive procedure has been
proposed where a topography-guided treatment is followed
by a wavefront-guided treatment.
9.
58 l DOS Times - Vol. 20, No. 7 January, 2015
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