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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;

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Comparison of corneal astigmatism measured with 3 devices

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  1. 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

  2. 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

  3. 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

  4. Subjects • 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)

  5. Methods • Prospective study • Corneas measured with: • 4 techniques using 3 devices • Single examiner • 3 sets of corneal measurements each • Subject repositioned between measurements

  6. Corneal astigmatism measurements The IOLMaster 1)CAIOLMaster: Difference between steep and flat meridians of automated keratometry from the IOLMaster Humphrey Atlas Corneal Topographer 2) CAAtlas:Difference between steep and flat meridians of simulated keratometry readings from the Atlas

  7. Galilei Dual Scheimpflug Analyzer 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) CAGalileiTCP: 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.

  8. 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

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

  10. 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

  11. 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 CATCP and CASimK • Comparison of astigmatism estimated from Anterior and Total corneas

  12. Results: Repeatability of 3 measurements All techniques provided high repeatability, especially the Atlas

  13. Vector differences between repeated measurements (% of eyes) 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

  14. Double-angle plots for the average astigmatism from each device 45 45 45 60 60 60 30 30 30 75 75 75 15 15 15 45 60 30 90 0 90 90 0 0 75 15 105 105 105 165 165 165 90 0 120 120 120 150 150 150 135 135 135 105 165 120 150 135 CAGalilei TCP tended to indicate lower astigmatism than other techniques

  15. Mean corneal cylinder measured by each device There were no significant differences between techniques although the IOLMaster tended to provide larger cylinder

  16. Results:Comparability Agreement in corneal astigmatism measurements between devices Difference in corneal astigmatism (D) LoA=0.72 Difference in corneal astigmatism (D) Difference in corneal astigmatism (D) Mean corneal astigmatism (D) of the CAGalilei SimKand CAAtlas Mean corneal astigmatism (D) of the CAAtlas and CAIOLMaster Mean corneal astigmatism (D) of the CAGalilei SimKandCAIOLMaster LoA=0.74 LoA=0.71 LoA=0.88 LoA=1.36 LoA=1.23 Difference in corneal astigmatism (D) Difference in corneal astigmatism (D) Difference in corneal astigmatism (D) Mean corneal astigmatism (D) of theCAIOLMasterand CAGalilei TCP Mean corneal astigmatism (D) of theCAAtlasand CAGalilei TCP Mean corneal astigmatism (D) of theCAGalilei SimKand CAGalilei TCP The ranges of the 95% LoA between CAIOLMaster and CAGalilei TCP, and CAAtlas and CAGalilei TCP were wider than other pairs

  17. Results: Mean vector difference between • CAGalilei SimKand CAGalilei TCP • Astigmatism estimated from • anterior vs. total cornea 45 60 30 75 15 90 0 105 165 120 150 135 Mean vector difference between anterior and total corneal astigmatism was 0.21D

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

  19. Sample cases showing for the effect of posterior corneal astigmatism on total corneal astigmatism If the meridian are 90-degrees apart,posterior corneal astigmatism increases anterior corneal astigmatism If the meridians of ant and post corneal astigmatism are the same, posterior corneal astigmatism reduces anterior corneal astigmatism Anterior astigmatism 0.59D@87 Posterior astigmatism -0.35@90 Anterior astigmatism 0.20D@13 Posterior astigmatism -0.26@103 corneal astigmatism from total cornea: 0.46@13 corneal astigmatism estimated from anterior cornea: 0.18@13 corneal astigmatism estimated from anterior cornea: 0.53@87 corneal astigmatism from total cornea: 0.24@83

  20. Relationship between anterior and posterior astigmatism from the Galilei Ratio in curvatures of posterior/anterior corneal astigmatism There was wide variation for the ratio of post/ant corneal astigmatism

  21. 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

  22. 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

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