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Wavefront Coherence Area for Predicting Visual Acuity of Post-PRK and Post-PARK Refractive Surgery Patients
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Daniel D. Garcia
(ddgarcia@cs.berkeley.edu)
Corina van de Pol
(vandepol@spectacle.berkeley.edu)
Stanley A. Klein
(klein@spectacle.berkeley.edu)
Brian A. Barsky
(barsky@cs.berkeley.edu)
The OPTICAL Research Project
University of California, Berkeley
http://www.cs.berkeley.edu/optical/
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BACKGROUND The Cornea
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- Transparent tissue covering the front of the eye.
- Performs two-thirds of the refraction, or bending, of light in the eye, and focuses light towards the lens and the retina.
- Subtle variations in the shape of the cornea can significantly diminish visual performance.
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BACKGROUND Refractive Surgery
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- Two common laser procedures for correcting vision
- PRK (photorefractive keratectomy)
- PARK (photorefractive astigmatic keratectomy)
- These dramatically change the patient s vision
- Sometimes the patient has decreased Best Spectacle-Corrected Visual Acuity (BSCVA)
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BACKGROUND Predicted Visual Acuity
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- Current corneal topographers provide predicted acuity indices based on corneal smoothness.
- The vision of post-refractive surgery patients (e.g. PRK and PARK) is often not predicted well.
- Visual acuity is not necessarily determined by corneal smoothness but by having some part of the cornea able to focus light coherently onto the fovea.
- Off-centered ablations are poorly predicted
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METHODS Coherence Area
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- For each point on the cornea, we:
- Determine where incoming parallel light focuses
- We start a wavefront of light emanating from there and refract it out of the cornea to a reference plane
- We compare the Optical Path Length (OPL) with that of the original reference ray for that point
- All rays whose OPL are within 0.25 l are coherent
- We search for the maximum coherence area
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RESULTS Simulated & Real PRK
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- The value at every point is the coherence area for the rays going through that point.
- For both of these corneas, the coherence area is quite low; typically the values are between 20-80%
- The poor coherence in the transition zone in both eyes is characteristic of post-PRK corneas
- The circled region in the real cornea is a region of good coherence.
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RESULTS Visual Acuity Indices
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- We studied 62 post-PARK and post-PRK patients
- Our coherence area (CA) index, compared to:
- CVP: Coefficient of Variation of Corneal Power
- SAI & SRI: Surface Asymmetry / Regularity Index
- is better at correlating actual vision metrics
- Small Letter Contrast Tests (SLCT) in (L)ight, (D)ark
- Comparable for High Contrast Visual Acuity (HCVA)
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CONCLUSION
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- Indices provided by current corneal topographers sometimes fail for corneas whose shape differs from simple ellipsoidal models
- This is the case with post-PRK and post-PARK refractive surgery patients.
- Our coherence area representation has many advantages, and promises to be a better predictor of visual acuity than current shape measures.
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