Carlos G. Arce, MD

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Qualitative and Quantitative Analysis of Aspheric
Symmetry and Asymmetry on Corneal Surfaces

• Carlos G. Arce, MD

Associate Researcher and Ophthalmologist Ocular
Bioengineering Refractive Surgery Sectors,
Institute of Vision, Department of
Ophthalmology, Paulista School of Medicine,
Federal University of São Paulo, Brazil
Medical Director - R D Consultant, Ziemer Group
AG, Port, Switzerland carlos.arce_at_ziemergroup.com
Speaker, Bausch Lomb do Brasil Territory
Manager for Latin America, Vista Optics Limited,
Widnes, UK Author does not have financial
interest in the commercialization of equipments
mentioned
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Purpose

• To describe a method how aspheric symmetry or
  asymmetry of corneal surfaces may be assessed and
  the patterns found in normal corneas and with
  keratoconus
• Qualitative Galilei best fit toric aspheric
  (BFTA) elevation maps with a custom ANSI style 5
  µm color scale were used to evaluate the aspheric
  symmetry or asymmetry of both corneal surfaces
• Quantitatively 1 Kraneman-Arce index was defined
  
• Quantitatively 2 The coma found with the Galilei
  corneal wave front report was correlated with the
  patterns found using the BFTA elevation maps

Methods
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Concept of Asphericity

• All corneas have symmetric or asymmetric toric
  aspheric surfaces.
• Symmetric aspheric meridians have uniform change
  of curvature from the center to the periphery in
  both hemimeridians
• Symmetric aspheric meridians fits well the BFTA
  referential surface and therefore will have
  elevation values close to zero with points within
  the green range ( 5 µm)
• Asymmetric aspheric meridians have different
  change of curvature from the center to the
  periphery in each hemimeridian
• When curvature has a slower progression rate the
  elevation values are negative and therefore
  points are within the blue range ( -10 µm)
• When curvature has a faster progression rate the
  elevation values are positive and therefore
  points are within the yellow range ( 10 µm)

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Concept of Asphericity

• Kranemann-Arce index Designed to quantify the
  asymmetry of asphericity of a corneal surface
• K-A Index is the total difference between the
  maximum negative BFTA elevation and maximum
  positive BFTA elevation (without considering
  mathematic sign) within central 6-mm-diameter
  data zone

Example (Anterior Surface) Max negative
elevation (in the blue range) -10 µm Max
positive elevation (in the yellow range) 15
µm Kranemann-Arce index 10 15 25
µm Example (PosteriorSurface) Max negative
elevation -28 µm Max positive elevation 30
µm Kranemann-Arce index 28 30 58
µm Symmetric aspheric meridian Both
hemimeridians within the green range in the 120
to 300 meridian (blue line) Asymmetric
asphericity Hemimeridians with blue or yellow in
the 20 to 200 meridian (red line) In this case
both surfaces had congruent symmetry and
asymmetry of asphericity
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Case A Congruent symmetric asphericity of both
surfaces in normal astigmatic cornea Case B
Incongruent symmetric asphericity of anterior
surface and asymmetric asphericity of posterior
surface in a cornea with crossed astigmatic Case
C Congruent asymmetric asphericity of both
surfaces in normal astigmatic cornea with
asymmetry more related with the flatter axis of
astigmatism Case D Congruent asymmetric
asphericity of both surfaces in a cornea with
keratoconus with asymmetry more related with the
steeper axis of astigmatism
BFS (at left) and BFTA (at right)
elevation maps of anterior (top) and
posterior (bottom) corneal surfaces

• Red line
• - Steeper axis of astigmatism
• Asymmetric aspheric meridian
• Blue line
• - Flatter axs of astigmatism
• - Symmetric aspheric meridian

C
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Custom Selection of IOL
Coma 0.91 D _at_ 62.7

• Standard (SA 0.18 µm)
• AcrySof IQ (SA -0.20 µm)
• Tecnis (SA -0.27 µm)
• SofPort (SA 0 µm)
• Rayner (SA 0 µm)

Spherical Aberration 0.29 µm -0.22 D
central rays focus beyond outer rays
central rays focus in front of outer rays

• Galilei measures the total corneal wave front
  from
• both surfaces
• Spherical aberration is linked to contrast
  sensitivity
• Coma is linked to aspheric asymmetry and
• keratoconus progression
• Hypothesis Symmetry or asymmetry of aspheric
  corneal
• surfaces may be related with satisfaction or
  visual symptoms
• and complains after implantation of IOLs with
  symmetric
• aspheric surfaces

All rays are focused at same point
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Conclusions

• Normal corneas and with keratoconus have a
  variety of patterns of BFTA elevation maps.
• Aspheric asymmetry of corneal surfaces is easy
  recognized by using the BFTA elevation maps.
• Aspheric symmetry was represented by a more green
  map and asymmetry was recognized by blue and
  yellow zones usually in the same meridian but
  opposite side.
• Aspheric asymmetry of anterior surface seems to
  correlate with the amount of corneal coma
  especially when asymmetric (irregular)
  astigmatism was present.
• Normal corneas had aspheric symmetry of both
  corneal surfaces, asymmetry of only one of them,
  or asymmetry on both.
• The axis of the aspheric asymmetry may fit the
  flatter axis of astigmatism, the steeper axis of
  astigmatism or none of them.
• The aspheric asymmetry of both surfaces may be
  oriented in the same or in different axis.
• Corneas with keratoconus use to have congruent
  asymmetry of both surfaces at the same axis.
• The relation of these corneal surface shapes and
  visual symptoms after multiphocal IOL implantatio
  is under study