Rules in refractive surgery

Rules
in Refractive Surgery
Dr .Hilal Mohamed Hilal
Elnour eye center
Damietta Egypt

‫زهور‬‫التوليدو‬

4 map
Anterior (Axial) sagittal
Corneal thickness map
Anterior elevation map
Posterior elevation map

The most common display is a 4-map

In each map, both
Shape
Should be studied
Parameters

1-Anterior sagittal (axial)map
Normal pattern
1- Symmetric bow tie
pattern
2-Segments S and I are
equal,
3- Their axes are aligned.
.

Normal Parameters.
At 5mm circle inferior
power higher than
superior less than
1.5D on the steep
axis
The superior point may
rarely have a higher
value than the
inferior one; less
than 2.50 D.

Abnormal shapes

Angulations more than 30”

Asymmetric bow tie
More lower steepness
More than 1.5 D

•Asymmetric bow tie
upper steepness
difference more than
2.5D

•Assymetric + angle

•Smile predisposed to
ectasia

•Gunctional predisposed
to ectasia

Vortex predisposed to ectasia
‫دوامه‬

Important risky point in
Anterior sagittal (axial)map
1-K>48D
2-Angulation
3-Astigmatism >6D in either surface
4-Aginest the rule astigmatism
5-Inferior superior asymmetry
6-Difference between the inferior and
superior more than 1.50 D

-Keratometry readings (k1, k2)-
-Radii of curvature (Rh, Rv),
-Mean keratometry mm zone (Km),

Pachymetry data of the
1-Pupil center,
2- apex,
3- thinnest point, and
their locations are
followed by maximum
curvature amount and
location.
.

‫زهور‬‫األوركيده‬

2-3-ANTERIOR AND POSTERIOR
ELEVATION MAPShape.
The normal shape
is the hourglass
Abnormal shapes -----
-Irregular,
-Tongue-like
extensions
-Isolated islands

2-3-ANTERIOR AND POSTERIOR
ELEVATION MAP
Parameters..
The highest plus value
within the central 5-
mm zone;
Normal values are
<12 μm anterior
<15 μm posterior
Abnormal difference
between front and
back elevation more
than 5u

Normal shape
is the
hourglass
Abnormal
shapes include
irregular,
tongue-like
extensions
Abnormal
isolated islands
Abnormal shapes
include irregular,
tongue-like
extensions and
isolated islands

The cone can be localized using the
elevation maps.
The location may be
-central,
-eccentric,
-peripheral

Isolated island in front or
back with normal elevation

‫السوسن‬ ‫زهور‬

4-PACHYMETRY MAP
Pattern
The normal pattern
is concentric
The abnormal patterns
include
1-Horizontaldisplacement,
2-dome-like,
3-globus,
4-and bell shapes

4-PACHYMETRY MAP
Abnormal
-Dome shape.
-Displaced thinnest location

4-PACHYMETRY MAP
Abnormal
Horizontal displacement
of corneal shape

4-PACHYMETRY MAP
The bell shape seen in
pellucid marginal
degeneration

4-PACHYMETRY MAP
Abnormal parameter
1- Abnormal less than 470
μm thickness at the TL
with normal tomography,
2- Abnormal less than 500
μm thickness at the TL
with abnormal
tomography 0 μm.

4-PACHYMETRY MAP
Abnormal parameter
3-Thickness Difference
between bachy apex and
thin. Loc .> 10u Early KC
4-Bachy apex Lower Location
Y more than -500

4-PACHYMETRY MAP
Abnormal parameter
5-Bachy. Thickness between
superior(S) and inferior (I)
points is ≤ 30 μm. At 5mm
circle
6-Abnormal thickness at the
TL between the patient’s
two eyes; more than 30
μm.

Pachy Apex
The computer considers the apex as the origin of the
coordinates, where X and Y are horizontal and Y
vertical meridians respectively.
X
It represents thickness at the apex of the cornea.
Zero is displayed in both squares of pachy apex

Pupil Center
Corneal thickness Location
Important in decentration technique when treating
hyperopia, astigmatism or corneal irregularities.
They are also important to evaluate angle kappa.
Normal x-coordinate ≤ 200 μm (or ≤ 5°).

Pupil diameter
Diameter of pupil in
(photopic, mesopic scotopic).
Adjusting optical zone(OZ)diameter,
1-0.5 mm larger than the scotopic pupil size.
2-0.5 mm then insert the intracorneal ring

Thinnest location (TL):
Thickness at the TL is used in calculations for photorefractive
surgery.
Myopic,
Hyperopic,
Astigmatic, and
Wave front
Topography-guided treatments

5-K readings
Normal maximum not more than 49D
Normal minimum not less than 34D
The normal difference between K max and the steep
less than 1.00 D.

5-K readings
Flat myopia
The rule -1.00 D correction reduces the flat K by 0.75 D.
The final flat sim K should not go below 34.00 D, or
positive spherical aberration will be induced

5-K readings
Steep Hyperopia
The rule each 1.00 D correction increase the K max 1.20 D
The final K max should not exceed 49.00 D; otherwise,
negative spherical aberrations are induced.
.

Thickness Profiles
corneal thickness spatial profile
(CTSP)
The average progression of thickness
starting from the TL to corneal
periphery
percentage thickness increase (PTI).
The percentage of progression of the
thickness
The normal profile is a curved line
plotted in red, following the black
dotted curves,
High average , Fast transition of
thickness between the
Low flat average corneal edema.
A normal profile follows the normative
curves with an average < 1.2 (red ellipse)

Thickness Profiles
Abnormal profiles include:
a. Quick Slope
The red curve leaves its
course before the 6-mm
zone. It is encountered in
forme fruste keratoconus
(FFKC) and ectatic disorders..
The average is usually high (> 1.1)

Thickness Profiles
b. S-shape
The red curve has a shape
of an “S”. It is
encountered in FFKC and
ectatic disorders.
The average is usually high (> 1.1).

Thickness Profiles
c. Flat shape
The red curve takes a
straight course.
It is encountered in
diseased thickened
(oedematous) corneas
such as Fuch’s
dystrophy and cornea
Guttata.
The average is low < 0.8 (red ellipse)

Thickness Profiles
d. Inverted
The red curve follows an
upward course.
It is encountered in some
cases of PMD.
The average is very low (< 0.8) and may
take a minus value

Measure the spherical aberration
Q value
Measurement undertaken
At 6mm diameter at 4
meridian
Sum. Vertical is most
important
(Normal vertical -0.25 -0.52
Aspheric cornea with least
spherical apparition )

Q value (spherical aberration)
-2 advanced keratoconus or after
hyperopia correction+5
-1 moderate keratoconus or after
hyperopia correction+2
-0.25 -0.52 Vertical normal Aspheric
cornea with least spherical
apparition
0 spherical cornea with spherical
apparition
+1 after correction of -5 myopia
+2 after correction of -12 myopia

6-Topographic astigmatism..
Disparity between these Topographic and manifest astigmatism
1-misalignment during capture,
2-irregular astigmatism,
3-tear film disturbance,
4-corneal haze
5-lenticular astigmatism (including subtle cataract).

Disparity between Topographic
astigmatism and manifest astigmatism.
If lenticular astigmatism is present without cataract and
there disparity,.. avoid overcorrection or converting
the orientation of the topographic astigmatism
-/-3x180 corrected as -0.5/-2x180

Disparity between Topographic
astigmatism and manifest astigmatism.
For example,
if the manifest astigmatism is -3.00 X 180º and the
topographic astigmatism is -2.00 X 180º,
correcting the full manifest astigmatism will induce -1.00 X
90º, which the patient may not tolerate despite zero
manifest refraction.
In such a case, one of the recommendations is to correct -2.00
X 180º and adjust the sphere to achieve the same spherical
equivalent (eg, 0.00 -3.00 X 180º corrected to -0.50 -2.00 X
180º.

7-Pupil coordinates.
The horizontal (x) coordinate of the pupil center reflects
angle kappa.
The normal value of the latter is less than 100 μm (<5°).
Angle kappa is important for the decentration technique
used in hyperopic and highly astigmatic photorefractive
correction. A large angle kappa can also explain the
skew seen in some curvature and elevation patterns.

‫الجنه‬ ‫عصفور‬ ‫زهره‬

General guidelines
Thickness Rules
K-reading Rules
Astigmatism Rules
Pupil Center and Angle Kappa Rule

Thickness Rules
Munnerlyn formula
calculates the ablation depth (AD) for myopia and myopic
astigmatism:
AD (μm) = 1/3 × (OZ diameter [mm])2 × (intended correction [D]).
.

Pupil diameter
Scotopic
The range of low light levels below
cone threshold where visual
responses has only rod signals
Mesopic
The range of intermediate
light levels between cone threshold
and rod saturation
Photopic
The range of high light levels above
rod saturation where vision is
mediated by signals from cone
photoreceptors.

Thickness Rules
When Scotopic pupil >5.5mm
Ablation OZ = 6.5 mm
So 1D Ablate 14um.
When Scotopic pupil <5mm
Aplation OZ = 5.5 mm
1D Ablate 10um
In small thickness save tissue
Do Do

Thickness Rules
RSB Rule 1
The RSB should be at least 300um

Thickness Rules
RSB Rule 2
The AD should be at most 20% of the original corneal
thickness at the TL.
.

Thickness Rules
RSB Rule 3
In LASIK, the AD differs
according to OZ diameter
and laser profile.
For easy calculations,
15 μm will be used.
.
.
Aplation OZ = 5.5 mm
1D = 10um
Ablation OZ = 6.5 mm
1D = 14um

Thickness Rules
RSB Rule 4
For safety,
Least ablation depth
Most residual stromal bed .

Thickness Rules
RSB Rule 5
AD In PRK, not to exceed 80-90 to avoid haze,
Corrected about 6 D(X15)
Final residual stoma not less than 400um.
•

Thickness Rules
RSB Rule 6
In LASIK and PRK, use the absolute sum of the spherical
and
cylindrical components
–4 D sph/–3 D = 7x15
.

Thickness Rules
RSB Rule 7
In hyperopic treatment the central ablation is zero, whereas
the maximum AD is peripheral where the cornea is thick.
Correct +4 D by LASIK or PRK in order to minimize
biomechanical response
In general, the preoperative TL should be > 470 μm.

Thickness Rules
RSB Rule 8
For calculations in mixed astigmatism, The equation should
be converted into plus cylinder formula before calculating
the RSB.
+2 D sph/–4 D cyl converted to –2 D sph/+4 D cyl
RSB rules are applied on the –2 D sph

Thickness Rules
RSB Rule 9
In WFGT profiles, the AD differs according to the type and severity
of HOA(s).
Therefore, AD and RSB should be calculated on site.

K-reading Rules
First The recommended amount of correction should
be calculated according to RSB rules
Second then according to K-reading rules.

K-reading Rules
Flat K Rule
Correcting each –1 D reduces the flat K by 0.75 D.
Final flat K according to the amount of myopic ablation
should be > 34 D.

K-reading Rules
K-max Rule
Correcting each +1 D increases K-max by 1.2 D.
K-max according to the amount of hyperopic ablation
should be < 49 D.

K-reading Rules
Correcting each –1 D reduces
the flat K by 0.75 D.
The final flat K > 34 D.
K-max Rule
Correcting each +1 D increases
K-max by 1.2 D.
The final steep K < 49 D.
Flat K Rule

Astigmatism Rules
Myopic Astigmatism Rules
The astigmatic correction flattens
the steep K and brings it to flat K
Thereafter, the spherical correction
flattens all.

Astigmatism Rules
Hyperopic Astigmatism Rules
the astigmatic correction steepens
the flat K and brings it to steepK
steepens all .
.

Astigmatism Rules
Mixed Astigmatism Rules
The astigmatic correction steepens
the flat K and brings it to steep
K
flattens all.

Pupil Center and Angle Kappa Rule
Angle Kappa is the angle
between the visual axis
and the axis that passes
through the pupil
center..
Angle kappa is considered
significant when it is > 5
(x > 200 μm).
•Angle Kappa

Large angle Kapa
When angle kappa is > 100 μm (x > 200 μm), the capture should be
repeated to exclude misalignment.
1-false positives or false negatives such as the skewed hourglass pattern
in elevation maps.
2- When treating hyperopia or ≥ 2 D of astigmatism, optimal resul ts can
be achieved when the center of ablation coincides with the optical
axis of the eye.
This can be achieved by decentering the ablation profile for the amount
of angle kappa; this is called “offset pupil” or“decentration”.
3-Finally, decentered pupil (corectopia) is a case of concern, especially
when PIOL implantation is indicated.

Case 1
A 27-year-old female
has a stable
refractive error

Case 1Right eye
Case 1 The anterior
elevation map:
Show tong-like
extension.
Normal values
within the
The anterior sagittal
curvature map shows a
SB with an insignificant
SRAX and an I-S difference
The posterior
elevation map
symmetric
hourglass
pattern.
normal values
The pachymetry map
shows normal shape.
There is an insignificant S-
I difference

Case 1
Thickness profiles show normal
slopes with a normal average

QS is OK
K-max—steep K is < 1D
Thickness at the TL is > 500μm
Pachy-Thinnest difference is < 10 μm Y-coordinate of
the TL shows an
insignificant
displacement
Pupil coordinates
indicate a
significant angle
Kappa
K-readings
including D K- <
49 max

Left Eye
Case 1
AB pattern
WTR astigmatism
Anterior
elevation map
skewed
hourglass.
Posterior
elevation map
symmetric
hourglass
pattern.
The pachymetry map shows a
normal shapewith a borderline
S-I difference

Thickness profiles show
normal slopes with
a normal average (0.8).

Left Eye
Case 1
QS is OK
K-max—steep K is <
1D
Thickness at the TL
is > 500 μm
K-readings
including K-
max are < 49
D.
Y-coordinate of
the TL shows
an insignificant
displacement
Pupil coordinates
indicate an
insignificant angle
Kappa.
Pachy—Thinnest
difference in
thickness is < 10
μm

Case 1
Borderline shapes and parameters such as
skewed hourglass,
tongue-like extension
and thickness parameters.
Therefore,
I would recommend PRK rather than LASIK

Case 2
A 35-year-old male has a
stable refractive error
with no other
complaints.
Eye examination is
normal.

The posterior
elevation map:
show moderately
skewed hourglass
pattern.
Nothing seems to be abnormal
in the pachymetry maps
Nothing seems
to be abnormal
in the anterior
elevation maps
Mild tong ex.
The anterior sagittal curvature
map shows a SB with an
insignificant SRAX and a S-I
difference

normal slopes with

QS is OK
Pachy—Thinnest
difference in
thickness is < 10
μm
Thickness
at the TL is
> 500 μm
K-are < 49 D
Y-coordinate
shows no
vertical
displacement
ix. Pupil
coordinates
indicate an
insignificant
angle Kappa
K-max—steep
K is < 1 D

Discussion
Compound myopic astigmatism in both eyes.
Tomography show normal shapes and parameters except for the
skewed hourglass on the posterior elevation map.
This patient is a good candidate for photorefractive surgery with a low
risk score.
Full correction by LASIK is possible as shown by thickness
and K-reading calculations.
In spite of high AD, some surgeons may go for PRK since the RSB is >
400 μm

Case 3
stable refractive error.
His recent glasses and
corresponding VA are
shown in table

Case 3The anterior sagittal
curvature map shows
a AB because of an
insignificant I-S
difference and SRAX.
The posterior
elevation map:
irregular pattern.
normal values
The anterior
elevation map:
almost symmetric
hourglass pattern.
normal values
The pachymetry map
shows normal shape
with an insignificant S-I
difference

show normal slopes
with a normal
average (0.9).
The red curves
deviate after the
6-mm zone (blue
arrows).

Case 3
ACD is > 3.0 mm
K-max—steep K is
< 1D.
K-max are < 49 D
Pachy—Thinnest
difference in thickness
is < 10 μm.
Y-coordinate shows
superior displacement,

Case 3
It is a high refractive error case.
Corneal thickness not good enough to proceed with
photorefractive surgery.
Partial correction may be proceeed.
Anterior chamber parameters and patient’s age are suitable
for phakic IOL

Case 4
His recent glasses
Cyclopligic refraction
Post mydriatic test.
A 35-year-old male has a refractive error, and he has strain
in near tasks.

Case 4
The anterior sagittal
curvature map shows
AB but considered
SB because of an
insignificant SRAX
and I-S difference .
almost symmetric
hourglass pattern
Normal value
irregular pattern.
normal values
pachymetry map
shows a horizontal
displacement of the
TL and an insignificant
S-I difference

Case 4
normal slopes with

Y-coordinate of the TL
insignificant vertical
displacement.
Pupil coordinates
indicate an
insignificant angle
Kappa
QS is OK
K-readings including
K-max are < 49 D
K-max—steep K is <
1D
Thickness at the TL is
> 500
Pachy—thinnest is
< 10 μm.

Case 4
4. Discussion:
compound hyperopic
astigmatism.
The thickness rules allow for full
correction,
The K-reading rules do not
5x1.2 =6D +46.7 =52.7
Final K will be higher than 49 D,
which leads to induction of
negative spherical aberration

Case 5
refractive error.
He is complaining of
blurring vision, halos,
ghost images and
headache.
He feels that
his vision is deteriorating

Right
Case 5
Irregular shape. Tongue shape
Tongue shape
abnormal
values
Atypical concentric shape with
an abnormal S-I difference (>
30 μm).

Right Case 5
Right
Thickness profiles show an
S-shape (blue arrows)
with a normal average
(0.9).

Right Case 5
Pupil coordinates indicate
decentered pupil and a
significant angle Kappa.
the AC is shallow
(ACD < 2.1 mm
Y-coordinate of the
TL is normal
< –500 μm.
QS is OK
K-readings are 48 D
Thickness at the TL is >
500 μm. Pachy—thinnest
difference in
thickness is < 10 μm.

Left case 5
Irregular shape or a superior-steep
shape.
tongue-like
extension
Normal values
within the
central 5-mm
tongue-like
extension
Abnormal
values within
the central 5-
mm
Atypical dome-like. The S-I
difference is > 30 μm

Left case 5
Left
Thickness profiles show an S-
shape (bluearrows) with a
normal average 0.8

Left case 5
QS is OK.
K-max are < 48 D
Thickness at the TL is
> 500 μm
Y-coordinate of the TL
is < –500 μm
Pupil coordinates
indicate decentered
pupil and a
significant angle Kappa

Case 5
Discussion
This is a case of FFKC because it is progressive reffracion
change with corneal abnormal tomography not distinct
enough to be classified ectatic disorders.
Conventional photorefractive surgery cannot be done.
Option
1-wavefront-guided or topo-guided PRK with CXL
2-PIOL is also another option with CXL prior to implantation.

Case 6
A 24-year-old female She is complaining of blurring vision,
strain and headache after near tasks with glasses.
Cycloplegic ref.
Recent glasses
Post medriatic ref.

RT Case 6
Irregular horizontal AB
pattern indicating ATR
astigmatism
Not typical
hourglass but
it is normal
symmetry.
normal values
within the central
5-mm circle.
Tongue-like
extension. normal
values within
the central 5-mm
circle
Normal shape in spite of
superior-temporal
displacement in the TL
There is an insignificant S-I
difference

Case 6
normal slopes and

RT Case 6
K-max are < 48 D.
K-max—steep K is < 1 D.
Thickness at the TL is >
500 μm.
Pachy—Thinnest difference
in thickness is < 10 μm.
Y-coordinate of the
TL is < –500 μm.
ACD is > 3.0 mm
Pupil coordinates indicate
decentered pupil and a
significant angle Kappa

Case 6
There is a big difference among
MR, CR and PMT,
Try contact lenses or glasses
depending on PMT correction
for a couple of weeks,
Then recheck the patient for
adding more correction till the
highest tolerable correction is
reached.
Manifest Ref
Cycloplegic Ref
Post medryatic test
Optimal correction

This case can be treated because preoperative
a.Corneal thickness is 550μm.
b. K-readings concept:
1.2x4 =48.8 D so can de treated ..<49D+44

Case 6
Discussion:
This patient is hyperopic .has accommodation spasm,
(Pseudomyopia)
There are some risk factors in corneal tomography The
anterior sagittal map and the posterior elevation map
which means that PRK will be safer than LASIK.
Treating hyperopia by PRK is unfavorable due to the risk of
1-Peripheral haze
2-High rate of regression.

Spasm of accommodation
Pseudomyopia
Intermittent and temporary shift in refraction of the
eye towards myopia, Cause spasm of the ciliary
muscle
Aetiology
1-organic, stimulation of the parasympathetic
2-functional in young adults who have active
accommodation, such as students preparing for an
exam, or a change in occupation.

Pseudomyopia
Symptom
1-Intermittent blurring of distance vision after prolonged
periods of near work,
2-Asthenopia.
Signs
1-The vision may clear temporarily using concave (minus)
lenses.
2- Use cycloplegic refraction using a strong cycloplegic like
atropine or homatropine eye drops

Pseudomyopia
Treatment
Easily cured if early recognized and
treated, otherwise the relaxation of spasm of
accommodation could become much more difficult
.

Pseudomyopia
Treatment
Organic treatment of the causes may include systemic or
ocular medications, brain stem injury, or active ocular
inflammation such as uveitis.
Functional
1-Modification of working conditions,
2- An updated refraction,
3-Reduction of a myopic prescription to lower myopic
prescription,
4-ocular exercises.

Pseudomyopia
If refraction is not properly done, there is a danger
that pseudomyopia
In these cases prescription of minus glasses could
induce more spasm and make the condition worse.
Over-correction can produce asthenopia i.e. eyestrain,
pain in and around the eyes, headaches, migraines
Pain may radiate to upper neck (occipital) and down
the neck to the shoulders

‫زهور‬‫الكاميليا‬

CASE 1: MYOPIC ASTIGMATISM
A 21-year-old male came
with stable refractive
error

Corneal thickness map no
displacement of the
thinnest location
The difference between the
lower and upper points of
the central 4 mm circle is
<30μ
The sagittal curvature is
symmetrical bow tie, no skew
or angulation between the
lower and upper axes of the
pattern, the difference
between the upper and
lower points of the 4 mm
circle is less than 1.5 dpt.
The elevation front presents regular shape with
no isolated island or tongue-like extension,
normal values within the central 4 mm circle .
The elevation back presents regular shape with no
isolated island or tongue-like extension, normal
values within the central 4 mm circle

The Topometric map (Q value) both the
average vertical value and the lower value are
normal< -0.55).)

The keratoconus
indices curve
lines are within
the normal range,
no deviation
before the 6 mm
circle.
The average is
normal(0.8<1.1).
The indices of
irregularity are
displayed with
white, which is
normal.

‫الجورى‬ ‫زهور‬

CASE 3: HYPEROPIA
Slightly irregular anterior sagittal
curvature map
Tongue-like
pattern on
both
elevation
maps
Significant
displacement of
the thinnest
location on the
corneal
thickness map
Corneal thickness at
the thinnest location
is below 500 μ (496 μ),
and has abnormal coordinates with pachy
apex especially on “y” axis (>-1000 μ).

CASE 3: HYPEROPIA
The Topometric map
Both the average vertical
value and the lower
value are normal
<-0.55

CASE 3: HYPEROPIA
The keratoconus indices
the curve lines are within the
normal range, they
deviate before the 6 mm circle
(red arrows).
The average is abnormal (1.2).
One of the irregularity
indices is displayed in red.

‫الوتس‬ ‫زهره‬

CASE 7: KERATOCONUS
The patient is 21-year-old
male complaining of
progressive reduction of
visual acuity in
both eyes.
His family history raised
the suspicion of
keratoconus

CASE 7: KERATOCONUS
The corneal
thickness map
has a cone-like
a very big superior-inferior
difference
the thinnest location
displacement
The sagittal curvature front map
has AB/IS pattern, but
there is no skew in axes
There is a big
cone on both
elevation maps
There is a big
cone on both
elevation maps

Pach-thinest >10
Abnormal K
Ycoordination
> -0.5
Thiniet 440

CASE 7: KERATOCONUS
Abnormal profiles
(Quick Slope )

CASE 7: KERATOCONUS
The cornea is very prolate as
shown on the topometric
map with more than 0.3
difference between
vertical and horizontal
averages.
+ve keratoconus indices,
keratoconus level three

‫الخشخاش‬ ‫زهره‬

CASE 8: KERATOCONUS POSSIBLE
AB/IS and the vortex
pattern, although there
is no skew in the
very central major
axes.
The posterior
elevation map is
irregular in spite
of its normal
values

CASE 8: KERATOCONUS POSSIBLE
There are two indices
in the keratoconus indices
page with abnormal
values,.

Rules in refractive surgery

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