1. Posterior Corneal Power in IOL Calculations using the Galilei Dual Scheimpflug Analyzer
MP Weikert, M Shirayama, L Wang, DD Koch
American Society of Cataract & Refractive Surgery 2008 Annual Symposium
April 5-8, 2008

2. Goals
Determine the accuracy of IOL calculations with anterior & posterior corneal power measured by the Galilei Dual Scheimpflug Analyzer
Compare IOL calculation accuracy w/ the Galilei to similar measurements with the Humphrey Atlas & IOL Master

3. Background
The measurement of corneal refractive power is essential to the accurate calculation of IOL power in cataract surgery
Current topographers utilize single technologies to calculate corneal power, such as Placido rings, Scheimpflug imaging, or slit beam scanning
Placido devices produce accurate measures of corneal curvature but their measurements are confined to the anterior surface
Scheimpflug and slit scanning devices can measure both the anterior and posterior corneal surfaces, but rely on elevation data that may lack sensitivity in the central cornea*
*Roberts C. Corneal Topography in Refractive Surgery, 2nd Edition.
Dmitri Azar (ed.). Stanford, CT: Appleton & Lange

4. Background Galilei Dual Scheimpflug Analyzer (DSA): 2 Technologies -
Placido ring imaging
Scheimpflug imaging
May provide increased accuracy by combining -
Anterior curvature data of Placido imaging with
Anterior & posterior elevation data measured with a dual-camera Scheimpflug system

5. Methods Retrospective study
Consecutive cataract surgeries at single center
Preoperative corneal power by 5 methods:
IOL Master – average of steep & flat axis
Humphrey Atlas – average of steep & flat SimK
Galilei DSA –
Average of steep & flat SimK
Average of steep & flat meridians for “total cornea power” (measured by ray tracing thru anterior & posterior surfaces)
Average of “total corneal power” (TCP) over central zone with 4-mm diameter (measured by ray tracing also)

6. Methods Axial length measured w/ IOL Master
IOL calculations performed with Holladay 1 formula
Single model of IOL used (Alcon Acrysof SN60WF)
Post-operative manifest refraction measured at 3 to 4 weeks following surgery
Post-op MR spherical equivalent (SE) compared to predicted refractive target for implanted IOL

7. Methods
Surgeon factor (SF) optimized for each method of measuring corneal power
Outcome measures:
Average corneal power for each method
Absolute error b/w predicted target and post-op MRSE
Statistical analysis:
SPSS software
Analysis of variance

8. Results - Corneal Power Range vs. Average Corneal Power
In comparing the corneal powers measured w/ each device/method, no correlation was found with the average corneal power (i.e. the devices did not agree more at flatter or steeper curvatures)

9. Results – Optimized Surgeon Factor & Absolute Prediction Error
Corneal Power Measurement
Optimized Surgeon Factor
Absolute Prediction Error (D)* ± SD
Galilei SimK
1.67
0.43 ± 0.34
(0.03 – 1.58)
Galilei TCP Avg
1.50
0.45 ± 0.36
(0.04 – 1.61)
Galilei TCP Central Avg
1.70
0.46 ± 0.36
(0.02 – 1.71)
IOL Master
1.73
0.40 ± 0.29
(0.05 – 1.06)
Atlas SimK
1.79
0.51 ± 0.34
(0.05 – 1.38)
* No statistically significant differences b/w absolute prediction errors

10. Results - Absolute Prediction Error

11. Discussion
Agreement between devices w.r.t. corneal power was independent of average corneal power
All methods for measuring corneal power produced similar results for IOL calculations:
Absolute error ranged from 0.40 to 0.51 D
Std. dev. ranged from 0.29 to 0.36 D
Galilei SimK’s had the largest percentage of eyes w/ PE’s ≤ 0.5 D
All corneal power methods had 90% of eyes w/ PE’s ≤ 1.0 D

12. Conclusions
IOL calculations using corneal power measured with the Galilei DSA had accuracies comparable to the IOL Master
Total corneal power (TCP) determined by ray tracing through the anterior and posterior cornea produced accurate IOL calculations
This may have significant benefit in post-LASIK or PRK eyes where the relationship between the anterior and posterior cornea is altered
More eyes are needed to increase the study power