SPIE '99
Wavefront Coherence Area for Predicting Visual Acuity of Post-PRK and Post-PARK Refractive Surgery Patients


Our paper "Wavefront Coherence Area for Predicting Visual Acuity of Post-PRK and Post-PARK Refractive Surgery Patients" was accepted to SPIE's Photonics West '99 conference. It was shown in Ophthalmic Technologies IX. We presented a poster to go along with the paper.

The Paper

The Poster

Wavefront Coherence Area for Predicting Visual Acuity of Post-PRK and Post-PARK Refractive Surgery Patients
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


The Cornea
  • 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.
Refractive Surgery
  • 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)
Predicted Visual Acuity
  • 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
Coherence Area
  • 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
Simulated & Real PRK
  • 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.
Visual Acuity Indices
  • 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)
  • 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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