Transcript Slide 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
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
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
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
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)
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
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
Results - Corneal Power Range vs.
Average Corneal Power
Maximum - Minimum
Corneal Power (D)
1.4
1.2
1
R2 = 0.0092
0.8
0.6
0.4
0.2
N = 20
0
41
42
43
44
45
46
47
Average Corneal Power for 5 Methods (D)
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)
Results – Optimized Surgeon Factor
& Absolute Prediction Error
Corneal Power
Measurement
Galilei SimK
Galilei TCP Avg
Galilei TCP Central Avg
IOL Master
Atlas SimK
Optimized
Absolute Prediction
Surgeon Factor
Error (D)* ± SD
1.67
1.50
1.70
1.73
1.79
0.43 ± 0.34
(0.03 – 1.58)
0.45 ± 0.36
(0.04 – 1.61)
0.46 ± 0.36
(0.02 – 1.71)
0.40 ± 0.29
(0.05 – 1.06)
0.51 ± 0.34
(0.05 – 1.38)
* No statistically significant differences b/w absolute prediction errors
% Eyes
Results - Absolute Prediction Error
100
90
80
70
60
50
40
30
20
10
0
Galilei SimK
Galilei TCP Avg
Galilei TCP Ctr Avg
IOL Master
Atlas SimK
PE <= 0.5D PE <= 1.0D PE <= 1.5D
Absolute Error Group
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
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