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Factors Potentially Affecting the
Accuracy of Methods to Calculate
Effective Refractive Power After
Keratorefractive Surgery
Helga P Sandoval, MD, MSCR, Amy S Nowacki, MS,
Luis E Fernández de Castro,MD, Kerry D Solomon, MD
Magill Research Center, Storm Eye Institute, Medical University of South Carolina.
Disclosure: HP Sandoval-D: Alcon, Allergan, AMO; AS Nowacki-None; LE Fernández de
Castro-D: Alcon, Allergan, AMO; KD Solomon–A,C,D: Alcon, Allergan, AMO, Advanced
Medical Research, B&L, InSite Vision, Eyemaginations
None of the authors has any financial interest in any product mentioned herein
The problem
• Making accurate measurements of corneal
power after corneal refractive surgery.
• Keratometry - assumes spherocylinder
• To date, no reliable means to measure
corneal power exists with modern diagnostic
technology
• Rely on fudge factor estimates of current
formulas
Methods to calculate IOL post-refractive
surgery (RS)
• Calculation of effective corneal power:

Pre-RS data needed:
• Clinical history
• Adjusted keratometry
• Adjusted effective refractive power

No pre-RS data needed:
• Modified Maloney Method
• Hard contact lens over-refraction (HCLOR)
• Corneal bypass method
• IOL power adjustment methods: (need pre-RS data):



Feiz Mannis method
Masket method
Modified Masket method
Purpose
• To determine the accuracy and agreement of
different methods used to calculate the
effective corneal power and intraocular lens
(IOL) power in patients post-keratorefractive
surgery
• To determine potential factors associated
with the accuracy of these methods.
Methods
• Eyes that underwent myopic LASIK pre-cataract
surgery were included.
• The IOL power was calculated with the Holladay 2
formula and the effective refractive corneal power
obtained from 3 methods:



Clinical history
Adjusted effective refractive power (adjEffRP)
Modified Maloney method
• IOL power was also calculated with IOL power
adjustment methods:


Masket
Modified Masket
Methods
• The previous IOL power was
compared to the emmetropic IOL
power back-calculated with the postcataract manifest refraction and
visual acuity using the secondary
piggyback IOL option from the
Holladay IOL consultant software.
The IOL obtained with the Holladay
R formula was added/subtracted to
the implanted IOL to determine the
emmetropic IOL power.
• Factors evaluated included:



Age
Gender
Pre-refractive surgery:
• Refraction
• Keratometry




Axial length,
Anterior chamber depth
Lens thickness
White to white
Results
Proportion of patients with varying levels of IOL calculation error by method
D > - 1.0
-1.0 ≤ D < -0.5
-0.5 ≤ D < 0
0 < D ≤ 0.5
0.5 < D ≤ 1
D > 1.0
Clinical
History
4
(8.2%)
2
(4.1%)
5
(10.2%)
5
(10.2%)
11
(22.4%)
4
(8.2%)
18
(36.7%)
Modified
Maloney
Method
6
(10.3%)
3
(5.2%)
4
(6.9%)
7
(12.1%)
9
(15.5%)
10
(17.2%)
19
(32.8%)
Adjusted
EffRP
2
(5.1%)
4
(10.3%)
6
(15.4%)
3
(7.7%)
9
(23.0%)
4
(10.3%)
11
(28.2%)
Masket
2
(4.3%)
9
(19.6%)
5
(10.9%)
16
(34.8%)
4
(8.7%)
2
(4.3%)
8
(17.4%)
Modifed
Masket
3
(6.4%)
2
(4.2%)
4
(8.5%)
15
(31.9%)
6
(12.8%)
6
(12.8%)
11
(23.4%)
0D
Results
Results
Results
Weighted
Kappa
H0: kappa = 0
Clinical
History
Modified
Maloney
Method
Adjusted
EffRP
Masket
Modified
Masket
Clinical
History
1.0000
< 0.001
0.1400
0.3025
0.0633
0.7133
0.1599
0.0942
0.1287
0.2135
1.0000
< 0.001
0.1179
0.4142
-0.0969
0.3690
0.1368
0.2331
1.0000
< 0.001
0.3503
<0.001
0.2465
0.0315
1.0000
< 0.001
0.3484
0.0019
Modified
Maloney
Method
Adjusted
EffRP
Masket
Modified
Masket
1.0000
< 0.001
If the two response variables are viewed as two independent ratings of the n subjects, the kappa coefficient
equals +1 when there is complete agreement of the raters. When the observed agreement exceeds chance
agreement, kappa is positive, with its magnitude reflecting the strength of agreement. Although this is
unusual in practice, kappa is negative when the observed agreement is less than chance agreement.
The agreement observed among the Adj EffRP, Masket and Modified Masket
methods is significantly more than expected by chance.
Simple Linear Regression
• Historical method:

PreLASIK spherical equivalent: the higher the
refractive error higher chance to overestimate IOL
power
• Modified Maloney method:

None of the factors were associated
• Effective Refractive Power:

PreLASIK K: Steepest K associated with
overestimation of IOL power
Results
• Masket method:



Age: Older age associated with underestimation of IOL
power
PreLASIK K: Steepest K associated with overestimation of
IOL power
PreLASIK spherical equivalent: higher refractive error
associated with overestimation of IOL power
• Modified Masket method:



Age: Older age associated with underestimation of IOL
power
PreLASIK K: Steepest K associated with overestimation of
IOL power
PreLASIK spherical equivalent: higher refractive error
associated with overestimation of IOL power
Conclusions
• Eff RP, Masket method and Modified Masket method
most accurate methods in these series
• Factors associated with overestimation or
underestimation varied according to method
• Small sample size
• Further multivariate analysis required
Reference
1. Determining corneal power following keratorefractive surgery. Available at:
http://doctor-hill.com/iol-main/keratorefractive.htm