1. 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

2. 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

3. 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

4. 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.

5. 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

6. Methods Factors evaluated included: Age Gender Pre-refractive surgery:
The previous IOL power was compared to the emmetropic IOL power back-calculated with the post-cataract 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

7. 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 < D ≤ 0.5
0.5 < D ≤ 1
D > 1.0
Clinical History 4
(8.2%) 2
(4.1%) 5
(10.2%) 11
(22.4%) 18
(36.7%)
Modified Maloney Method 6
(10.3%) 3
(5.2%)
(6.9%) 7
(12.1%) 9
(15.5%) 10
(17.2%) 19
(32.8%)
Adjusted EffRP
(5.1%)
(15.4%)
(7.7%)
(23.0%)
(28.2%)
Masket
(4.3%)
(19.6%)
(10.9%) 16
(34.8%)
(8.7%) 8
(17.4%)
Modifed Masket
(6.4%)
(4.2%)
(8.5%) 15
(31.9%)
(12.8%)
(23.4%)

8. Results

9. Results

10. Results
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.

11. 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

12. Results Masket method: 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
Modified Masket method:

13. 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: