Comparison of corneal astigmatism measured with 3 devices Mariko Shirayama, M.D, Li Wang, M.D, PhD, Mitchell P. Weikert, M.D, Douglas D. Koch, M.D. Cullen Eye Institute, Baylor College of Medicine, Houston, TX. Financial Interest Disclosure: Research support--Li Wang;
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Comparison of corneal astigmatism measured with 3 devices
Comparison of corneal astigmatism measured with 3 devices. Mariko Shirayama, M.D, Li Wang, M.D, PhD, Mitchell P. Weikert, M.D, Douglas D. Koch, M.D. Cullen Eye Institute, Baylor College of Medicine, Houston, TX. Financial Interest Disclosure: Research support--Li Wang; - PowerPoint PPT Presentation
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Comparison of corneal astigmatism measured with 3 devices
Mariko Shirayama, M.D, Li Wang, M.D, PhD, Mitchell P. Weikert, M.D, Douglas D. Koch, M.D.
Cullen Eye Institute, Baylor College of Medicine, Houston, TX.Financial Interest Disclosure: Research support--Li Wang;
travel expenses—Mariko Shirayama
Calculating corneal astigmatism
Corneal astigmatism has been traditionally calculated from anterior corneal curvature using 1.3375 as index of refraction
New Scheimpflug images allow calculation of anterior and posterior corneal astigmatism
Purpose
To assess the repeatability and comparability of corneal astigmatism obtained from the
To evaluate the effect of posterior corneal astigmatism on overall corneal astigmatism
Inclusion criteria: No prior intraocular and/or corneal
surgery No trauma, ocular or corneal diseases No contact lens wear
21 eyes of 21 patients included Gender: 6 males and 14 females Age: 34±11.4 yrs (range 18 to 59 yrs)
Subjects
Methods Prospective study
Corneas measured with: 4 techniques using 3 devices Single examiner 3 sets of corneal measurements each Subject repositioned between measurements
Humphrey Atlas Corneal Topographer
The IOLMaster
2) CAAtlas: Difference between steep and flat meridians of simulated keratometry readings from the Atlas
1) CAIOLMaster: Difference between steep and flat meridians of automated keratometry from the IOLMaster
Corneal astigmatism measurements
3) CAGalilei Sim: Difference between steep and flat meridians of simulated keratometry readings from the Galilei over the 1.0- 4.0mm diameter central zone
4) CAGalilei TCP: Difference between the steep and flat meridians of total corneal
power* over the 1.0-4.0-mm diameter central zone
*Total corneal power is calculated by ray-tracing through the anterior and posterior corneal surfaces using Snell’s law.
Galilei Dual Scheimpflug Analyzer
2 categories for astigmatism measurements
Astigmatism estimated from anterior corneal power using 1.3375 CAIOLMaster CAAtlas CAGalilei Sim
Astigmatism estimated from total corneal power
CAGalilei TCP
Data analysisRepeatability
Coefficient of variation (CV) Standard deviation (SD) Intraclass correlation coefficient (ICC) Vector analysis of astigmatism
measurements for each device
Data analysisComparability Comparison of mean corneal astigmatism in
magnitude between devices Paired t-test with Bonferroni correction
Agreement in measurements between devices Bland and Altman method
95% limits of agreement (95% LoA) calculated as mean difference ±
Data analysisEvaluation of effects of posterior corneal astigmatism on total corneal astigmatism Differences in meridian of anterior and posterior corneal astigmatism from the Galilei Ratio in curvatures of anterior and posterior corneal astigmatism Vector difference between CATCP and CASimK
Comparison of astigmatism estimated from Anterior and Total corneas
Results: Repeatability of 3 measurementsMethods CAIOLMaster CAAtlas
CAGalilei
Sim
CAGalilei
TCP
CV 21% 14% 28% 26%
SD 0.14D 0.08D 0.14D 0.13D
ICC 0.931 0.983 0.922 0.918
All techniques provided high repeatability, especially the Atlas
Vector differences between repeated measurements (% of eyes)
Difference between measurements ≤0.25D ≤0.50D ≤0.75D ≤1.00D
CAIOLMaster 71 93 95 100
CAAtlas 76 100 100 100
CAGalilei Sim 45 76 93 98
CAGalilei TCP 36 76 90 98
42 values for each device (21 for measurement 1-2 and 21 for measurement 1-3)
Atlas tended to provide smallest vector differences between repeated measurements
CAIOLMaster CAAtlas CAGalilei Sim CAGalilei TCP
Double-angle plots
Each ring=0.5DOuter ring=2.0D
Mean CA ± SD @ degree
0.46 ± 0.44@87
0.43 ± 0.40@88
0.37 ± 0.38@94
0.17 ± 0.38@77
Double-angle plots for the average astigmatism from each device
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CAGalilei TCP tended to indicate lower astigmatism than other techniques
Mean corneal cylinder measured by each device
There were no significant differences between techniques although the IOLMaster tended to provide larger cylinder