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 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  IOLMaster  Humphrey Atlas corneal topographer  Galilei Dual Scheimpflug Analyzer 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) CA Atlas :Difference between steep and flat meridians of simulated keratometry readings from the Atlas 1) CA IOLMaster : Difference between steep and flat meridians of automated keratometry from the IOLMaster Corneal astigmatism measurements

3) CA Galilei Sim :Difference between steep and flat meridians of simulated keratometry readings from the Galilei over the mm diameter central zone 4) CA Galilei TCP : Difference between the steep and flat meridians of total corneal power* over the 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  CA IOLMaster  CA Atlas  CA Galilei Sim Astigmatism estimated from total corneal power  CA Galilei TCP

Data analysis Repeatability  Coefficient of variation (CV)  Standard deviation (SD)  Intraclass correlation coefficient (ICC)  Vector analysis of astigmatism measurements for each device

Data analysis Comparability 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 ± 1.96 standard deviation (SD) Interdevice correlation  Pearson correlation coefficient

Data analysis Evaluation 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 CA TCP and CA SimK  Comparison of astigmatism estimated from Anterior and Total corneas

Results: Repeatability of 3 measurements MethodsCA IOLMaster CA Atlas CA Galilei Sim CA Galilei TCP CV 21%14%28%26% SD 0.14D0.08D0.14D0.13D ICC 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 CA IOLMaster CA Atlas CA Galilei Sim CA Galilei TCP 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

CA IOLMaster CA Atlas CA Galilei Sim CA Galilei TCP Double-angle plots Each ring=0.5D Outer ring=2.0D Mean CA ± degree 0.46 ± 0.43 ± 0.37 ± 0.17 ± Double-angle plots for the average astigmatism from each device CA Galilei 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 Mean (D)SD (D)Range (D) CA IOLMaster CA Atlas CA Galilei Sim CA Galilei TCP

Results : Comparability Agreement in corneal astigmatism measurements between devices The ranges of the 95% LoA between CA IOLMaster and CA Galilei TCP, and CA Atlas and CA Galilei TCP were wider than other pairs Difference in corneal astigmatism (D) Mean corneal astigmatism (D) of the CA Atlas and CA IOLMaster LoA=0.72 Mean corneal astigmatism (D) of the CA Galilei SimK and CA IOLMaster Difference in corneal astigmatism (D) LoA=0.71 LoA=0.88 Mean corneal astigmatism (D) of the CA Galilei SimK and CA Atlas Difference in corneal astigmatism (D) LoA=1.23 Mean corneal astigmatism (D) of the CA IOLMaster and CA Galilei TCP Difference in corneal astigmatism (D) LoA=1.36 Difference in corneal astigmatism (D) Mean corneal astigmatism (D) of the CA Atlas and CA Galilei TCP LoA=0.74 Mean corneal astigmatism (D) of the CA Galilei SimK and CA Galilei TCP Difference in corneal astigmatism (D)

Results: Mean vector difference between CA Galilei SimK and CA Galilei TCP Astigmatism estimated from anterior vs. total cornea CA Galilei SimK - CA Galilei TCP Double-angle plots Each ring=0.5D Outer ring=2.0D 0.21 ± Mean vector difference between anterior and total corneal astigmatism was 0.21D

Mean differences in meridians of anterior and posterior corneal astigmatism There were large range of differences in meridians of ant and post corneal astigmatism

Sample cases showing for the effect of posterior corneal astigmatism on total corneal astigmatism Anterior astigmatism Posterior astigmatism corneal astigmatism from total cornea : corneal astigmatism estimated from anterior cornea : If the meridians of ant and post corneal astigmatism are the same, posterior corneal astigmatism reduces anterior corneal astigmatism Anterior astigmatism Posterior astigmatism corneal astigmatism from total cornea : corneal astigmatism estimated from anterior cornea : If the meridian are 90-degrees apart, posterior corneal astigmatism increases anterior corneal astigmatism

Relationship between anterior and posterior astigmatism from the Galilei Ratio in curvatures of posterior/anterior corneal astigmatism Medianrange 25 th to 75 th percentile Ratio in curvatures of posterior/anterior corneal astigmatism to to 0.43 There was wide variation for the ratio of post/ant corneal astigmatism

Conclusion  Astigmatism measurements from all techniques were highly reproducible and correlated  There were no significant differences in mean corneal cylinder between techniques  There was wide range of differences in steep meridian between anterior and posterior corneal astigmatism  Mean vector difference between anterior and total corneal astigmatism was 0.21D

Conclusion  Our result indicates that posterior corneal astigmatism compensates for astigmatism from anterior surface in some cases and augments it in others  Ignoring posterior corneal surface might yield incorrect estimation of total corneal astigmatism