Transcript Slide 1

case
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A 56-year-old woman with LASIK for -11 D
Residual refraction: - 0.50D
Preop mean K : 44.5D
Postop mean K: 35.5 D
AL by A-scan: 30.1 mm
13 D IOL implanted base on SRK/T formula
Postop refraction : +4.00 D
IOL power calculations following
keratorefractive surgery should not
be carried out using standard
keratometry combined with any
one of several popular 2-variable
third generation theoretic formulas,
such as SRK/T, Holliday 1 and
Hoffer Q without a special
correction
Sources of errors
• keratometric measurement
• Calculating formula
CAUTION
K-readings
inaccurate
Formulas
Inappropriate
Determining Corneal Power after RK, PRK and LASIK
• The true corneal power following RK, ALK,
PRK and LASIK is difficult to measure by
any form of direct measurement, such as
keratometry, or corneal topography
• Keratometry and topography assume a
normal relationship between the anterior
and posterior corneal curvatures, and
measure the anterior corneal radius.
keratometric measurement errors
• Standard keratometry measures an intermediate
area and extrapolates the central power based on
some very broad assumptions.
• For this reason, keratometry, autokeratometry and
simulated keratometry by topography will typically
over-estimate central corneal power, following
keratorefractive surgery for myopia.
• Failure to keep this important fact in mind will often
result in an unexpected and unpleasant postoperative hyperopic surprise.
Variations within the IOL Power
Calculation Formulas
• Until recently, our attention for the postkeratorefractive eye has mainly been focused
on accurately determining the central corneal
power.
• It is now becoming more widely understood
that a flattened central cornea not only
renders keratometry inaccurate, but also
causes problems with many IOL power
calculation formulas that were previously old
and trusted friends.
Variations within the IOL Power
Calculation Formulas
• Third generation, 2-variable formulas such as
SRK/T, assume that the anterior and posterior
segments of the eye are mostly proportional and
use a combination of axial length, and
keratometric corneal power, to estimate the
postoperative location of the IOL, known as the
effective lens position (ELP).
• If the central corneal power is very low, as we see
following keratorefractive surgery, the formula
assumes that the anterior chamber is shallow.
Errors related to calculating
ELP
formula
A major shortcoming of most 3rd
generation, 2-variable formulas, such as
SRK/T, is that they often assume that the
anterior and posterior segments of the eye
are mostly proportional and use only the
axial length and keratometric corneal
power to estimate the postoperative
location of the IOL, known as the effective
lens position (ELP).
Calculating formula
• Unless a specific correction is made for this situation,
the artifact of centrally flattened Ks following
keratorefractive surgery will have these formulas
assume a falsely shallow post-operative ELP
• The end result is that without a special correction,
following LASIK these formulas will typically
recommend less IOL power than is actually required.
• This is a second, and little recognized, source of
unanticipated post-operative hyperopia following
keratorefractive surgery for myopia.
Importance of ELP
• If a +21.00 D IOL placed within the capsular bag
produces emmetropia, with only a 0.50 mm
posterior displacement, that same lens has an
effective power of +20.00 D. If that same lens is
moved anterior by only 0.50 mm, it then would have
an effective power of +22.00 D.
• With a low central corneal power the formula makes
the assumption that the IOL following cataract
surgery will end up sitting closer to the cornea than
normal and call for less power.
• The flatter the cornea, the bigger a problem this
becomes.
Importance of ELP
• Unless a correction is made for this situation,
the artifact of centrally flattened Ks following
keratorefractive surgery will have these
formulas incorrectly assume a falsely shallow
post-operative ELP.
• The end result is that without a special
correction, 2-variable formulas following LASIK,
PRK and RK will recommend less IOL power
than is actually required.
• This is a second, and little recognized, source of
unanticipated post-operative hyperopia
following keratorefractive surgery for myopia.
Sugested corrective methods
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Double K" Formula Corrections Formula
Clinical History Method
Contact Lens Method
Corneal Bypass Method
Feiz-Mannis Method
Haigis-L Formula
Holladay IOL Consultant
Latkany Method
Masket Method
Modified Masket Method
Topographic Central Power
Wang Koch Maloney Method
Zeiss IOLMaster
Oculus Pentacam
Post Keratorefractive online Calculators
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Post Hyperopic LASIK
Post Radial Keratotomy
Aramberri "Double K" method,
• Uses preop Ks to calculate ELP and
postop Ks to calculate IOL power
• Suggests IOL Power Adjustment Tables
for SRK/T, Hoffer Q and Holladay l
Formula
Aramberri "Double K" method After MYOPIC Keratorefractive Surgery
IOL Power Adjustment Table for SRK/T Formula.
MYOPIC
AL (mm)
Correction (D)
20
22
24
26
28
30
2
0.7
0.7
0.7
0.6
0.5
0.3
3
1.0
1.0
1.1
1.0
0.8
0.6
4
1.3
1.4
1.4
1.3
1.1
0.8
5
1.7
1.7
1.8
1.6
1.4
1.1
6
2.0
2.0
2.1
2.0
1.7
1.4
7
2.3
2.4
2.5
2.3
2.0
1.7
8
2.6
2.7
2.8
2.6
2.3
2.0
9
2.9
3.0
3.2
3.0
2.7
2.3
10
3.2
3.3
3.5
3.3
3.0
2.6
The numbers in each row of the myopic refractive correction represent the amount in
diopters that must be added to the calculated IOL power when using the SRK/T Formula.
Aramberri "Double K" method After Hyperopic Keratorefractive
Surgery IOL Power Adjustment Table for SRK/T Formula
HYPEROPIC
AL (mm)
Correction (D)
20
22
24
26
28
30
2
0.7
0.7
0.7
0.6
0.4
0.0
3
1.1
1.1
1.1
0.9
0.5
0.0
4
1.4
1.4
1.5
1.2
--
--
5
1.8
1.8
1.9
1.7
--
--
6
2.2
2.2
2.5
--
--
--
The numbers in each row of the HYPEROPIC refractive correction represent the
amount in diopters that must be subtracted from the calculated IOL power when
using the SRK/T Formula If no number appears, it means that the formula is not
applicable for that combination of axial length and refractive correction.
Aramberri "Double K" method After MYOPIC Keratorefractive Surgery
IOL Power Adjustment Table for Hoffer Q Formula
Myopia
AL (mm)
Correction(D)
20
22
24
26
28
30
2
0.4
0.3
0.2
0.2
0.1
0.0
3
0.6
0.5
0.3
0.3
0.2
0.0
4
0.8
0.6
0.5
0.3
0.2
0.0
5
1.0
0.8
0.6
0.4
0.3
0.0
6
1.2
0.9
0.7
0.5
0.4
0.1
7
1.4
1.1
0.8
0.6
0.5
0.1
8
1.6
1.2
1.0
0.7
0.6
0.2
9
1.7
1.3
1.1
0.8
0.7
0.2
10
1.9
1.5
1.2
1.0
0.7
0.3
The numbers in each row of the myopic refractive correction represent the amount in diopters
that must be added to the calculated IOL power when using the Hoffer Q Formula
Aramberri "Double K" method After Hyperopic Keratorefractive
Surgery IOL Power Adjustment Table for Hoffer Q Formula
Hperopia
AL (mm)
20
22
24
26
28
30
2
0.4
0.3
0.2
0.1
0.1
0.0
3
0.7
0.5
0.3
0.2
0.0
0.0
4
0.9
0.6
0.4
0.2
0.0
0.0
5
1.1
0.7
0.4
0.2
0.0
0.0
6
1.3
0.9
0.5
0.2
0.0
0.0
Correction (D)
The numbers in each row of the HYPEROPIC refractive correction represent the
amount in diopters that must be subtracted from the calculated IOL power when
using the Hoffer Q Formula. If no number appears, it means that the formula is not
applicable for that combination of axial length and refractive correction.
Clinical History Method
The Clinical History Method for corneal power estimation was first
described by Holladay and later by Hoffer as:
Ka = Kp + Rp – Ra
Ka = the estimate of the central corneal power after
surgery.
Kp = the average keratometry power before surgery
Rp = the spherical equivalent before keratorefractive
surgery
Ra = the stable spherical equivalent after surgery
Clinical History Method
Example:
• Ks before LASIK were 45.37 D / 46.00 D
• The refraction before LASIK was -6.00 +0.75 x 135
(Corrected for an estimated vertex distance of 12
mm)
• The stable refraction after LASIK was -0.25 sphere
then...
• Ka = Kp + Rp - Ra
• Ka = (45.69) + (-5.27) - (-0.25) = 40.67 D
Contactlens
Lensmethod
Method
Contact
• Following all forms of ablative
keratorefractive surgery (LASIK, PRK,
etc.) a review of the literature now
suggests that the hard Contact Lens
method may be less accurate than
originally thought.
• For this reason it is no longer
recommended in this clinical setting
Corneal Bypass Method.
• Described by Walter et al in the March, 2006 issue of the Journal
of Cataract and Refractive Surgery.
• The advantage of this method is that it is done without having to
calculate the post-LASIK corneal power.
• IOL power is calculated using the post-LASIK axial length and the
pre-LASIK keratometry
• The target refraction is set for the pre-LASIK spherical equivalent.
This "bypasses" the post-LASIK corneal power.
Example:
If the pre-LASIK keratometry was 45.37 D / 46.00 D,
the spherical equivalent before LASIK was -5.62 D, and
the post-LASIK IOLMaster axial length is 26.32 mm, then
the IOL power for a -5.62 D refractive result would be +17.27 D
Feiz-Mannis Method
• when good historical data is available.
• least likely to result in a post-operative hyperopic surprise.
• IOL power is calculated using the pre-LASIK keratometry, as
though the patient had never undergone keratorefractive surgery.
• No special formula correction that needs to be employed
• Calculated pre-LASIK IOL power is then increased by the amount
of refractive change at the spectacle plane divided by 0.7
• Example:
• If calculated IOL power before LASIK is +9.18 D, and
the change in refractive power at the spectacle plane
is -5.38 D, then:
• IOLpre - (ΔD / 0.7) = IOLpost
• +9.18 D - (-5.38 / 0.7) = +16.87 D
Latkany Method
• A postoperative regression method developed by
Robert Litany, et al.
• Only requires that the surgeon know the refraction
prior to LASIK.
• Using the flattest K and the SRK/T formula
• The calculated IOL power is then adjusted by the
formula:
-(0.47 [Pre-refractive surgery SEq] + 0.85)
• The final IOL power is rounded to the nearest 0.50 D
Masket Method
• A postoperative regression method developed by Samuel Masket
• There appears to be a linear relationship between the spherical
equivalent of the total amount of the stable laser vision correction (LSE)
and the over-estimation of central corneal power by simulated
keratometry.
• This works following both myopic and hyperopic LASIK.
• By this method, the IOL power is calculated using the Holladay 1 formula
for axial lengths greater than 23.0 mm and the Hoffer Q formula for axial
lengths less than 23.0 mm.
• The SRK/T formula is generally not recommended here as the artifact of
very flat Ks may sometimes result in an under-correction.
• The IOL power is calculated based on Ks provided by simulated
keratometry.
• The calculated IOL power is then adjusted by the vertex distance
corrected laser vision correction spherical equivalent determined at four
to six months after the procedure, multiplied by -0.326 with 0.101 added
to this product.
Masket Method
Example
• (LSE x -0.326) + 0.101 = Post-LASIK IOL power adjustment Where...
• LSE = The vertex distance corrected laser vision correction
spherical equivalent.
• The Masket method for the eye following LASIK would be carried
out as follows:
• If the calculated IOL power is +15.26 D for a -0.25 D result, and ...
• The stable laser vision correction spherical equivalent is -5.38 D,
then ...
• (LSE x -0.326) + 0.101 = Post-LASIK IOL power adjustment (-5.05
D x -0.326) + 0.101 = +1.75 D = IOL power adjustment +1.75
D + 15.26 D = 17.01 D = Final adjusted IOL power
Modified Masket Method:
• The calculated IOL power is multiplied by -0.4385 and 0.0295
added to this product.
Wang - Koch - Maloney Method
Described by Robert Maloney and subsequently modified by Doug Koch and Li Wang.
Requires no historical data and has been reported to have a low variance when used with
either the Holliday 2 formula or a 2-variable formula combined with an Aramberri double K
method correction nomogram published by Koch and Wang.
The central corneal power is estimated by placing the cursor at the exact center of the Axial
Map of the Zeiss Humphrey Atlas topographer. This value is then converted back to the
anterior corneal power by multiplying the Axial Map central topographic corneal power by
376.0/337.5, which is the same as 1.114. An assumed posterior corneal power of 6.1 D is
then subtracted from this product.
(CCP x 1.114) - 6.1 = Post-LASIK adjusted corneal power Where...
CCP = the corneal power with the cursor in the center of the Axial Map of the Zeiss Humphrey
Atlas topographer.
The Modified Maloney method for the eye following LASIK is carried out as follows:
If the Axial Map central corneal power is 40.91 D, then...
(CCP x 1.114) - 6.1 = Post-LASIK adjusted corneal power
(40.91 D x 1.114) 6.1 = 39.47 D
Haigis-L Formula - Post Myopic LASIK
Using an IOLMaster corneal radius measured in mm (r
meas), the Haigis-L algorithm generates a corrected
corneal radius (r corr), which is then used by the
regular Haigis formula to calculate the IOL power
331.5is calculated as follows:
• The corrected radius
r corr = --------------------------------------------------5.1625 x r meas + 82.2603 - 0.35
Example:
• If IOLMaster keratometry in mm following myopic LASIK is
8.82 mm x 090 and 8.63 mm x 180, the calculation of the
corrected corneal radius would be as follows:
• Average corneal radius = (8.82 + 8.63)/2 = 8.73 mm = r
meas
331.5
• r corr = --------------------------------------------------- = 9.0 mm
5.1625 x 8.73 + 82.2603 - 0.35
To calculate “D” from “r” use formula: D = 337.5/r
D = 337.5 / 9.0 = 37.5 D
Haigis and Haigis-L Formula
• Haigis formula does not tie the effective lens
position estimation to the central corneal power.
Actual ELP is estimated by direct measurement of
ACD
• Haigis-L measures corrected central corneal
power based on a regression-derived algorithm.
If this corrected is then entered into the regular
Haigis formula, it will do a good job with the
calculation
• The Haigis-L formula is now available on
IOLMaster software version 4.xx and higher.
Holladay 2 Formula
• IOL power calculations following any form of
keratorefractive surgery can be carried out
using the Holladay 2 formula (contained
within the Holladay IOL Consultant)
• If your office does not have this software
package, a trial version can be downloaded
from the Internet at: Holladay IOL Constant
Pentacam
• Rotating Scheimpflug camera, generating information in
three dimensions.
• Measures both anterior and posterior corneal curvatures
• During the two second measuring sequence, a total of
25,000 data points are generated which includes the
center of the cornea.
• All keratometers and corneal topographers do not see
the corneal center and are forced to extrapolate this
information, which gives the Pentacam this unique
advantage in terms of accuracy.
The Equivalent K Reading (EKR)
• The Equivalent K Reading feature of the Pentacam
uses information from both the anterior and the
posterior cornea to generate a range of central
corneal power values in keratometric diopters.
• Depending on the EKR zone selected, this value can
then be used with the Holladay 2 Formula for IOL
power calculations following keratorefractive
surgery, or with one of the popular 3rd generation,
2-variable formulas combined with an Aramberri
"Double K" method
Pentacam Holladay Report
• This new feature was developed by Oculus together with Dr.
Jack T. Holladay to improve the measurement of the central
corneal power for patients who have undergone prior
keratorefractive surgery.
• For both myopic and even hyperopic LASIK, the 4.5 mm EKR
zone has been shown to have a high correlation with the
central corneal power calculated by the clinical history
method.
• However, depending on the type of ablation employed,
some surgeons are finding a better correlation for myopic
LASIK using a smaller EKR area, such as the 3.0 mm EKR zone
Pentacam Holladay Report
IOLMaster
• Uses partial coherence interferometry to
measure the length of the eye (as opposed to
standard ultrasound measurements).
• It has an accuracy of +/- 0.02 mm or better! This
represents a fivefold increase in measurement
accuracy when compared to standard A-scan
ultrasound (which has a resolution of +/- 0.1 to
0.12 mm).
• Optical AL measurement
• IOLMaster K's measure a smaller 2.5mm zone.
• Incorporated Haigis L formula
Topographic Central Corneal Power Adjustment Method
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Originated by Doug Koch and Li Wang
Based on determining the central power of the cornea using either the Zeiss
Humphrey Atlas topographer or the adjusted effective refractive power (EffRPadj) of
the Holladay Diagnostic Summary of the EyeSys Corneal Analysis System.
The 1, 2, 3 and 4 mm power values of the Numerical View of the Zeiss Humphrey Atlas
topographer are averaged together and used as the central corneal power value (Ccp).
As an alternative, the adjusted effective refractive power (EffRPadj) of the Holladay
Diagnostic Summary of the EyeSys Corneal Analysis System can be used.
This value is then reduced by 19% for every diopter of myopia corrected by LASIK. We
have added the 3 mm and the 4 mm power values from the Numerical View of the
Zeiss Humphrey Atlas topographer to this calculation based on the fact that Holladay
has found that sampling the 4.0 mm central cornea for the post-LASIK eye has
returned very good results with the Oculus Pentacam.
Topographic Central Corneal Power Adjustment
Method
CCP + (ΔD x 0.19) = Post-LASIK adjusted corneal power Where...
CCP = the EffRPadj, or the averaged Zeiss Atlas central corneal
power, and... ΔD = the refractive change after LASIK at the
spectacle plane.
The topographic central corneal power adjustment method for
the eye would be:
If the averaged topographic central corneal power is 41.97 D,
and...
The change in refractive power at the spectacle plane is -5.38 D,
then...
CCP + (ΔD x 0.19) = Post-LASIK adjusted corneal power 41.97
D + (-5.38 x 0.19) = 40.95 D
Hyperopic LASIK and PRK
• Determining Corneal Power after Hyperopic
LASIK.
• For eyes that have undergone hyperopic LASIK,
the central corneal power estimation technique
is much easier than for myopic LASIK.
• This is thought to be due to the fact that the
ablation takes place outside the central cornea.
Hyperopic LASIK and PRK
• An interesting feature of hyperopic LASIK is that following this
procedure, the ratio between the posterior and anterior corneal radii is
increased, much like what is seen following radial keratotomy.
• For this reason, it is possible to use a similar method of central corneal
power estimation. It is intriguing that a myopic incisional technique (RK)
and a hyperopic ablative technique (hyperopic LASIK), would alter the
posterior : anterior corneal radii ratio in much the same way.
• Based on the experience of Drs. Wang, Jackson and Koch, the average
of the 1 mm, 2 mm and 3 mm annular power rings of the Numerical
View of the Zeiss Humphrey Atlas topographer is sufficiently accurate to
serve as a reasonable estimate of central corneal power following
hyperopic LASIK.
• However, depending on the amount of hyperopic laser vision
correction, a correction to the average of the 1 mm, 2mm, and 3mm
annular power ring value must be made
Hyperopic LASIK (D
LASIK-Induced
Refractive Correction (D)
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
5.5
6.0
Adjustment for Hyperopic
LASIK (D)
Atlas AnnCP
-0.30
-0.20
-0.11
-0.01
0.08
0.18
0.27
0.37
0.46
0.56
0.65
0.75
Hyperopic LASIK and PRK
• As an alternative, Drs. Wang, Jackson and Koch have shown that the
effective refractive power (EffRP) of the EyeSys Corneal Analysis
System also works well.¹ Notice that this technique is very similar to
what is currently recommended for estimating the central corneal
power following radial keratotomy. As well, for this method, a small
correction needs to be made to the effective refractive power (EffRP).
• Remember that some form of correction is still required for IOL power
calculations following hyperopic LASIK in order to avoid a refractive
surprise.
• This can be done by checking the "Previous RK, LASIK, etc." box on the
Holladay 2 formula (contained within the Hollday IOL Consultant), or
by using a hyperopic "double K method" IOL power correction table
based on the work of Aramberri for any of the three popular 3rd
generation, 2-variable formulas.
• outcomes employing the Holladay 2 formula.
Radial Keratotomy
• Unlike ablative forms of myopic keratorefractive
surgery (LASIK and PRK) in which the ratio
between the posterior : anterior corneal radii is
decreased, for eyes that have previously
undergone radial keratotomy, the ratio between
the posterior : anterior corneal radii is increased.
• This allows for a direct estimation of the central
corneal power using elevation data of the central
4.0 mm, if carried out in a certain way.
Radial Keratotomy
• For eyes with prior radial keratotomy,
averaging the 1 mm, 2 mm, 3 mm and 4
mm annular power rings of the Numerical
View of the 995, 994, and 993 Zeiss Atlas
topographer (right) will typically give a
useful estimate of central corneal power.
• For the Zeiss Atlas 9000 topographer, you
would take average of the 1 mm, 2 mm, 3
mm and 4 mm ring (not zone) values.
• If the Zeiss Atlas topographer is not
available, then the adjusted effective
refractive power (EffRPadj) from the
Holladay Diagnostic Summary of the EyeSys
Corneal Analysis System can be used.
Zeiss Atlas Topographer
Annular Ring Power from the
Numerical View feature. Use
the average of the 1 mm, 2
mm, 3 mm and 4 mm annular
power values.
Post Keratorefractive online
Calculators
7/4/07 in Bologna
Î HOFFER®—SAVINI Ê LASIK IOL POWER TOOL

Calculating IOL Power in Eyes with Previous Corneal Refractive Surgery
THIS SPREADSHEET WILL PRODUCE ERRONEOUS INFORMATION UNLESS THE LETTERS NA ARE TYPED
INTO EACH CELL WHERE YOU DO NOT HAVE THE REQUESTED DATA.
Patient Name
Date
James A**k*ki
7/12/2006
DATA
MYP: Low est K HYP: Highest K 
Enter DATA [or NA] Needed For Calculations
*IOLs Calculated By SRK/T® (>24.5 mm)
Pre-LASIK SE Rx =
-6.00
Axial Length
27.55
Clinical History Method
36.60
PO LASIK SE Rx =
-0.37
IOL A-con
118.50
Contact Lens Method
40.65
Rx Δ (RCc) 0mm =
-5.23
Rx Δ (RCs) 12m m =
-5.63
Pre-LASIK K =
41.83
PO MK/TK =
39.00
PO Topog EffRP
38.75
Contact Lens Method
Flattest Man PO K
38.50
CL Base Curve
38.00
PO Topog Central -K
38.00
Hard CL Power
IOLMaster PO K
38.37
IOLMaster IR
1.3375
Pentacam =
37.30
Must Use Hard PMMA
Ant Radius r =
8.68
Aphakic Refrx Methods
Post Radius r =
6.75
C Thickness =
489
O
D P
A
TA
PO
ST
IOL and POWER USED
UCVA
BCVA
WHITE cells are for Data Entry
GRAY cells are for IOL Power Entry
Calculation Results in Diopters
For K Reading
For IOL Power
Eye For Surgery
OD
©2007
Jay Patel, MD
SURGEON
The IOLs must be calculated using other
software and added in the 3 Cells
BELOW .
Enter
Powers
Emmetropic
IOL c Pre-K
10.88
Target IOL
c PO-K
14.92
TARGET IOL
c PO Flat-K
15.49
Maloney* Method
36.84
TARGET IOL "Single-K"
Koch/Maloney Method
37.07
Using PO K
14.92
Hamed-Wang-Koch
37.92
Feiz-Mannis Formula
18.92
c "Low/High" K
17.58
Haigis Method
36.50
Feiz-Mannis Method
18.04
BESSt© Formula
38.67
Latkany Method Flat K
17.46
Savini-Barboni-Zanini
38.47
**Latkany Method [K]
17.47
0.00
Ronje Method
37.09
Masket Method
16.73
Refrx (SE) c CL
-0.25
Shammas No History
37.66
Refrx (SE) Bare
-3.00
Speicher (Seitz) Method
38.67
Low est or
Highest K
36.50
19.18
Savini IR Method
38.42
PO K
39.00
15.68
Camellin IR Method
37.62
AVG K
37.46
17.85
Jarade IR Method
38.09
Your Choice
of K
NA
NA
Ferrara IR Method
33.35
Wake Forest Method
34.82
38.62
37.30
IANCHULEV
IN OR
8.75
IR METHODS
-0.50
Target PO Rx =
MACKOOL PO
9.25
18.0
SA60AT
Rosa Method
20/25+2
20/20-1
PO ACD
Jarade Formula
5.00
Pentacam =
USING
Single K
Aramberri DOUBLE-K (Target)
18.96
APHAKIC REFRX
Ianchulev Method
17.73
Mackool Method
16.29
Myopes Only
WARNING!!
Ver 1.0
Transient hyperopia following cataract surgery and prior radial
keratotomy
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Patients with previous 8-incision radial keratotomy will commonly show variable amounts of
transient hyperopia in the immediate post-operative period following cataract surgery. This is felt
to be due to stromal edema around the radial incisions, producing a temporary enhancement of
central corneal flattening. While this central corneal flattening is usually transient, it can be as
much as +4.00 D, and is further accentuated by greater than eight incisions, an optical zone of less
than 2.0 mm, or incisions that extend all the way to the limbus.
If a patient exhibits any of the above, significant unanticipated hyperopia may be seen in the
immediate post-operative period, which should gradually resolve after eight to twelve weeks.
Sometimes, due to a lack of corneal stability, the post-operative refraction can continue to slowly
shift myopic over a several month period. We have seen several patients with myopic shifts as
large a -5.00 D over a 12-week period.
If the final post-operative refractive objective remains elusive, plans for an IOL exchange, or a
piggyback IOL, should not be made until at least two months have passed and two consecutive
refractions, two weeks apart (at the same time of the day), are stable (the "rule of twos.").
Also, if more than six months passes before cataract surgery is required for the fellow eye, the
corneal measurements should be repeated due to the fact that additional corneal flattening
frequently occurs over time following radial keratotomy. For this reason, IOL power calculations
are usually targeted for between -0.75 D and -1.00 D and are designed to make the operative eye
more myopic than usual, so that five to ten years from surgery, the post-cataract surgery
refractive error does not drift into hyperopia. This also helps to avoid hyperopic refractive results,
which are quite common, in spite of every precaution being taken.
Of all the various forms of keratorefractive surgery, we have had the best overall accuracy
following radial keratotomy using the above technique.
Conclusions