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

IOLMasterHumphrey Atlas corneal topographerGalilei 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) 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 ±

1.96 standard deviation (SD)

Interdevice correlation Pearson correlation coefficient

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

15

3045

60

135150

165

0 0

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3045

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75

90

105

120135

150

165

75

90

105

120

0

15

3045

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90

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120135

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0

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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

Mean (D) SD (D) Range (D)

CAIOLMaster 0.74 0.41 0.11 - 1.49CAAtlas 0.70 0.39 0.20 - 1.48CAGalilei Sim 0.65 0.33 0.11 - 1.29CAGalilei TCP 0.59 0.31 0.12 - 1.12

Results: ComparabilityAgreement in corneal astigmatism measurements between devices

The ranges of the 95% LoA between CAIOLMaster and CAGalilei TCP, and CAAtlas and CAGalilei TCP were wider than other pairs

Diffe

renc

e in

cor

neal

ast

igm

atism

(D

) Mean corneal astigmatism (D) of the CAAtlas and CAIOLMaster

LoA=0.72

Mean corneal astigmatism (D) of the CAGalilei SimK and CAIOLMaster Di

ffere

nce

in c

orne

al a

stig

mat

ism (D

)

LoA=0.71 LoA=0.88

Mean corneal astigmatism (D) of the CAGalilei SimK and CAAtlas

Diffe

renc

e in

cor

neal

ast

igm

atism

(D

)

LoA=1.23

Mean corneal astigmatism (D) of the CAIOLMaster and CAGalilei TCP

Diffe

renc

e in

cor

neal

ast

igm

atism

(D

)

LoA=1.36

Diffe

renc

e in

cor

neal

ast

igm

atism

(D)

Mean corneal astigmatism (D) of the CAAtlas and CAGalilei TCP

LoA=0.74

Mean corneal astigmatism (D) of the CAGalilei SimK and CAGalilei TCP

Diffe

renc

e in

cor

neal

ast

igm

atism

(D)

Results: Mean vector difference between CAGalilei SimK and CAGalilei TCP

Astigmatism estimated from anterior vs. total cornea

CAGalilei SimK - CAGalilei TCP

Double-angle plotsEach ring=0.5DOuter ring=2.0D

0.21 ± 0.11D @ 860

15

3045

60

75

90

105

120135

150

165

Mean vector difference between anterior and total corneal astigmatismwas 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 astigmatism0.59D@87

Posterior astigmatism-0.35@90

corneal astigmatism from total cornea: 0.24@83

corneal astigmatism estimated fromanterior cornea: 0.53@87

If the meridians of ant and post corneal astigmatism are the same,posterior corneal astigmatism reduces anterior corneal astigmatism

Anterior astigmatism0.20D@13

Posterior astigmatism-0.26@103

corneal astigmatism from total cornea: 0.46@13

corneal astigmatism estimated fromanterior cornea: 0.18@13

If the meridian are 90-degrees apart, posterior corneal astigmatism increases anterior corneal astigmatism

Relationship between anterior and posterior astigmatism from the GalileiRatio in curvatures of posterior/anterior corneal astigmatism

Median range 25th to 75th percentile

Ratio in curvatures of posterior/anterior corneal

astigmatism0.34 0.05 to 1.07 0.14 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