Comparison of Corneal Astigmatism Measured with Combined Placido

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Comparison of Corneal Astigmatism Measured with Combined Placido Powered By Docstoc
					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
  IOLMaster
  Humphrey Atlas corneal topographer
  Galilei Dual Scheimpflug Analyzer
To evaluate the effect of posterior corneal
astigmatism on overall corneal astigmatism
    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)
    Methods
   Prospective study

   Corneas measured with:
        4 techniques using 3 devices
        Single examiner
        3 sets of corneal measurements each
        Subject repositioned between measurements
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
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) 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.
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 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 CATCP and CASimK
       Comparison of astigmatism estimated from Anterior and Total
        corneas
Results: Repeatability of 3 measurements

Methods         CAIOLMaster       CAAtlas      CAGalilei     CAGalilei
                                                  Sim              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)
42 values for each device
(21 for measurement 1-2 and 21 for measurement 1-3)

Difference between
                             ≤0.25D        ≤0.50D        ≤0.75D     ≤1.00D
measurements

CAIOLMaster                     71           93               95     100


CAAtlas                         76          100               100    100


CAGalilei Sim                   45           76               93     98


CAGalilei TCP                   36           76               90     98


Atlas tended to provide smallest vector differences between
repeated measurements
Double-angle plots for the average
astigmatism from each device

                    CAIOLMaster                            CAAtlas                 CAGalilei Sim                      CAGalilei
                                                                                                                             TCP

                               45                                45                            45                             45
                          60         30                     60         30                 60         30                 60          30


Double-angle        75                     15         75                     15     75                     15     75                      15


   plots           90                        0   90                            0   90                        0   90                         0


 Each ring=0.5D                                                                                                  105
                   105                            105                              105                                                    165
 Outer ring=2.0D                           165                               165                           165


                         120                               120         150               120         150               120          150
                                     150
                               135                               135                           135                            135




Mean CA ± SD        0.46 ± 0.44                   0.43 ± 0.40                           0.37 ± 0.38              0.17 ± 0.38
  @ degree              @87                           @88                                   @94                      @77


CAGalilei TCP tended to indicate lower astigmatism than other techniques
Mean corneal cylinder measured by
each device

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

CAIOLMaster              0.74          0.41         0.11 - 1.49

CAAtlas                  0.70          0.39         0.20 - 1.48

CAGalilei Sim            0.65          0.33         0.11 - 1.29

CAGalilei TCP            0.59          0.31         0.12 - 1.12

There were no significant differences between techniques although
the IOLMaster tended to provide larger cylinder
                                               Results: Comparability
                                               Agreement in corneal astigmatism measurements
                                               between devices
Difference in corneal astigmatism (D)




                                                                                                      Difference in corneal astigmatism (D)
                                        1.0                                                                                                   1.0                                                                                                   1.0




                                                                                                                                                                                                                Difference in corneal astigmatism
                                        0.6                                                                                                   0.6                                                                                                   0.6


                                        0.2                                                                                                   0.2                                                                                                   0.2

                                                                                   LoA=0.72                                                                                                  LoA=0.71                                                                                               LoA=0.88
                              -0.2 0.0                  0.5       1.0        1.5                                                              -0.2                                                                                                  -0.2 0.0         0.5        1.0        1.5
                                                                                                                                                     0.0        0.5       1.0          1.5

                              -0.6                                                                                                            -0.6                                                                                                  -0.6


                              -1.0                                                                                                            -1.0                                                                                                  -1.0
                                               Mean corneal astigmatism (D) of




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




                                                                                                                                                                                                        Difference in corneal astigmatism (D)
                                                                                              Difference in corneal astigmatism (D)
Difference in corneal astigmatism (D)




                                   1.0                                                                                                        1.0                                                                                                   1.0


                                   0.6                                                                                                        0.6                                                                                                   0.6


                                   0.2                                                                                                        0.2                                                                                                   0.2
                                                                                   LoA=1.23                                                                                                  LoA=1.36                                                                                               LoA=0.74
                                                                                                                                                     0.0        0.5       1.0          1.5                                                                 0.0       0.5         1.0          1.5
                            -0.2 0.0                    0.5       1.0       1.5                                                               -0.2                                                                                              -0.2



                            -0.6                                                                                                              -0.6                                                                                              -0.6



                            -1.0                                                                                                              -1.0                                                                                              -1.0
                                                                                                                                                     Mean corneal astigmatism (D) of                                                                        Mean corneal astigmatism (D) of
                                              Mean corneal astigmatism (D) of
                                                                                                                                                     the CAAtlas and CAGalilei TCP                                                                          the CAGalilei SimK and CAGalilei TCP
                                              the CAIOLMaster and CAGalilei TCP



                                    The ranges of the 95% LoA between CAIOLMaster and CAGalilei TCP, and CAAtlas
                                    and CAGalilei TCP were wider than other pairs
Results:      Mean vector difference between
              CAGalilei SimK and CAGalilei TCP
             Astigmatism estimated from
               anterior vs. total cornea
                                         CAGalilei SimK - CAGalilei TCP
                                         45
                                    60          30

                              75                     15

   Double-angle plots
    Each ring=0.5D          90                        0
                                                           0.21 ± 0.11D @ 86
    Outer ring=2.0D          105                     165

                                   120         150
                                         135



Mean vector difference between anterior and total corneal astigmatism
was 0.21D
      Mean differences in meridians of anterior
      and posterior corneal astigmatism
                                            90
   Mean difference in meridians (degrees)




                                            80


                                            70


                                            60


                                            50


                                            40


                                            30


                                            20


                                            10


                                            0
                                                 Each subject



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
If the meridians of ant and post corneal              If the meridian are 90-degrees apart,
astigmatism are the same,                             posterior corneal astigmatism
posterior corneal astigmatism reduces                 increases anterior corneal astigmatism
anterior corneal astigmatism
                                                         Anterior astigmatism    Posterior astigmatism
   Anterior astigmatism    Posterior astigmatism              0.20D@13                 -0.26@103
        0.59D@87                 -0.35@90




corneal astigmatism                                   corneal astigmatism        corneal astigmatism from
                           corneal astigmatism from
estimated from
                           total cornea: 0.24@83
                                                      estimated from             total cornea: 0.46@13
anterior cornea: 0.53@87                              anterior cornea: 0.18@13
Relationship between anterior and posterior
astigmatism from the Galilei
Ratio in curvatures of posterior/anterior corneal astigmatism


                                                           25th to 75th
                               Median          range
                                                            percentile



  Ratio in curvatures of
posterior/anterior corneal      0.34        0.05 to 1.07   0.14 to 0.43
       astigmatism




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

				
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posted:3/29/2011
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