Contrast Sensitivity and Depth of Focus of Spherical and Aspheric Intraocular Lenses

1
Contrast Sensitivity and Depth of Focus of
Spherical and Aspheric Intraocular Lenses

• David L Yeh, MD, Li Wang, MD, PhD,
• Douglas D Koch, MD
• Cullen Eye Institute Baylor College of Medicine

2
Disclosures

• Douglas Koch, MD, is a consultant for Alcon
  Laboratories, Inc., and AMO, Inc.
• Laboratory testing was performed at Alcon
  Laboratories, Inc. (Ft. Worth, TX). Data
  acquisition and analysis were performed by the
  listed authors.

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Background

• Physiologic eyes contain higher order
  aberrations, including spherical aberration (SA),
  which may cause sub-optimal image quality.
• Contribution to SA is from cornea (average
  0.28 ?m) and crystalline lens (negative)
  cataract surgery removes the lens component,
  leaving positive SA on average.
• Traditional intraocular lenses (IOLs) are
  spherical with positive SA, further increasing
  the SA of the pseudophakic eye.
• New aspheric lens designs have zero or
  negative SA and may offset the positive SA of the
  cornea, with theoretical benefits in visual
  quality.

4
Background

• However, contrast sensitivity decreases with
  defocus (e.g., moving an object towards the eye
  with a focal point of infinity).
• The ability of a lens to maintain acceptable
  visual quality in the face of defocus is referred
  to as depth of focus (DOF), and may also be an
  important factor in overall quality of vision.
• Studies have suggested that higher order
  aberrations may contribute to improved DOF, and
  reducing or eliminating natural SA may compromise
  this benefit.

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Purpose

• Using an optical test bench, to measure the image
  quality of test objects projected through model
  eyes with spherical and aspheric IOLs
• Image quality measured at best focus and various
  levels of defocus
• The relationship of SA on image quality and DOF
  will be considered

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Methods

• Optical test bench used to project monochromatic
  (550 nm) test image through model eye with
  spatial frequencies of 30 cyc/deg (20/20) and 15
  cyc/deg (20/40)
• Parameters of model eye
• Pupil size 6 mm
• Model cornea ideal with exception of spherical
  aberration (SA, Z40)
• Cornea A SA 0.01 ?m
• Cornea B SA 0.11 ?m
• Cornea C SA 0.16 ?m
• Cornea D SA 0.33 ?m
• IOLs (Alcon Surgical, Fort Worth, TX)
• Spherical AcrySof SN60AT 20.0D SA 0.16 ?m
• Aspheric AcrySof SN60WF (IQ) 20.0D SA -0.2
  ?m

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Methods

• Resultant modulation transfer function (MTF)
  values, a measure of contrast sensitivity, were
  obtained
• Through-focus curves of MTF vs. defocus from -1D
  to 1D were obtained
• Measures of visual quality analyzed
• Maximum MTF (at best focus)
• Depth of Focus defined as the range of defocus
  in which MTF greater than 80 of maximal MTF was
  maintained
• Visual Utility Range defined as the range of
  defocus within -1D to 1D in which MTF greater
  than previously determined absolute contrast
  sensitivity thresholds were maintained (0.06 for
  20/20 object, 0.013 for 20/40 object)
• Visual Utility Area area under Through-focus
  curve in the Visual Utility Range additional
  global measure of visual quality in this range

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Example Through-Focus Curve
MTF Max
80 MTF Max
Depth of Focus
Visual Utility Area (shaded)
Contrast Threshold
Visual Utility Range
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Through-Focus 20/20 Object, Cornea A
Modulation Transfer
Defocus (D)
10
Through-Focus 20/20 Object, Cornea B
Modulation Transfer
Defocus (D)
11
Through-Focus 20/20 Object, Cornea C
Modulation Transfer
Defocus (D)
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Through-Focus 20/20 Object, Cornea D
Modulation Transfer
Defocus (D)
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Through-Focus 20/40 Object, Cornea A
Modulation Transfer
Defocus (D)
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Through-Focus 20/40 Object, Cornea B
Modulation Transfer
Defocus (D)
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Through-Focus 20/40 Object, Cornea C
Modulation Transfer
Defocus (D)
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Through-Focus 20/40 Object, Cornea D
Modulation Transfer
Defocus (D)
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Summary MTF Max(MTF at best focus)

• 20/20 image 20/40 image
• SN60AT SN60WF SN60AT SN60WF
• Cornea A TSA 0.17 -0.19 0.17 -0.19
• 0.233 0.153 0.339 0.311
• Cornea B TSA 0.27 -0.09 0.27 -0.09
• 0.188 0.378 0.281 0.645
• Cornea C TSA 0.32 -0.04 0.32 -0.04
• 0.155 0.608 0.227 0.812
• Cornea D TSA 0.49 0.13 0.49 0.13
• 0.096 0.265 0.156 0.400
• MTF Max in yellow TSA total spherical
  aberration (cornea IOL)

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Summary Depth of Focus(gt80 MTF Max)

• 20/20 image 20/40 image
• SN60AT SN60WF SN60AT SN60WF
• Cornea A TSA 0.17 -0.19 0.17 -0.19
• 0.17D 0.26D 0.32D 0.35D
• Cornea B TSA 0.27 -0.09 0.27 -0.09
• 0.21D 0.13D 0.38D 0.21D
• Cornea C TSA 0.32 -0.04 0.32 -0.04
• 0.22D 0.07D 0.38D 0.18D
• Cornea D TSA 0.49 0.13 0.49 0.13
• 0.26D 0.16D 0.41D 0.38D
• Depth of Focus in yellow TSA total spherical
  aberration (cornea IOL)

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Summary Visual Utility Range(MTF gt Contrast
Threshold)

• 20/20 image 20/40 image
• SN60AT SN60WF SN60AT SN60WF
• Cornea A TSA 0.17 -0.19 0.17 -0.19
• 0.40D 0.54D 2.00D 2.00D
• Cornea B TSA 0.27 -0.09 0.27 -0.09
• 0.48D 0.59D 1.78D 1.84D
• Cornea C TSA 0.32 -0.04 0.32 -0.04
• 0.45D 0.42D 1.29D 1.42D
• Cornea D TSA 0.49 0.13 0.49 0.13
• 0.39D 0.47D 1.91D 1.68D
• Visual Utility Range in yellow TSA total
  spherical aberration (cornea IOL)

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Summary Visual Utility Area

• 20/20 image 20/40 image
• SN60AT SN60WF SN60AT SN60WF
• Cornea A TSA 0.17 -0.19 0.17 -0.19
• 0.066 0.063 0.203 0.232
• Cornea B TSA 0.27 -0.09 0.27 -0.09
• 0.066 0.105 0.209 0.290
• Cornea C TSA 0.32 -0.04 0.32 -0.04
• 0.057 0.116 0.154 0.288
• Cornea D TSA 0.49 0.13 0.49 0.13
• 0.035 0.082 0.131 0.250
• Visual Utility Area in yellow TSA total
  spherical aberration (cornea IOL)

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Conclusions

• Reduction in SA correlates with increased image
  quality at best focus in model eyes.
• Depth of focus, as defined by 80 of MTF max,
  generally decreased with reduction in SA.
• In Cornea D, which most closely simulates the SA
  of the average human cornea, the DOF of the 20/20
  test object was less in the aspheric lens model,
  but the 20/40 test object did not differ
  significantly.
• DOF in this study was quite limited in all eyes
  compared to computer-simulated and patient
  studies this may indicate contributions from
  other naturally existing aberrations and
  neuro-mechanisms not studied here.

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Conclusions

• Visual Utility Range, which uses an absolute
  threshold criteria, did not appear to be
  correlated with SA.
• This may be an additional useful measure of
  depth of focus.
• In eye models with aspheric lenses, Visual
  Utility Area was equal to or greater than the
  corresponding eyes with spherical lenses.
• Aspheric IOLs may improve visual quality by
  reducing naturally occuring SA.
• However, it may be desirable to retain some SA to
  achieve an acceptable compromise with DOF.

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Future Directions

• Effect of varying pupil size?
• Results with stimuli of other wavelengths/polychro
  matic light?
• Testing with corneas of broader range of
  spherical aberrations to simulate post-refractive
  surgery eyes
• How would adding other aberrations (e.g.,
  trefoil, coma) to the test cornea affect results?
• How sensitive are these results to
  decentration/tilt of the intraocular lens?
• Correlation of results to clinical testing

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