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Related Experiment Videos

Corneal ablation profilometry and steep central islands

J K Shimmick1, W B Telfair, C R Munnerlyn

  • 1VISX Incorporated, Santa Clara, California, USA.

Journal of Refractive Surgery (Thorofare, N.J. : 1995)
|May 1, 1997
PubMed
Summary

Large diameter photorefractive keratectomy (PRK) can cause steep central islands due to non-uniform laser ablation. Modifying the PRK ablation algorithm corrects this common refractive surgery side effect.

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Area of Science:

  • Ophthalmology
  • Laser Surgery
  • Corneal Reshaping

Background:

  • Photorefractive keratectomy (PRK) using large diameter, uniform laser beams can lead to central undercorrections, known as
  • steep central islands
  • detected via videokeratography.
  • This phenomenon is a significant challenge in achieving optimal refractive outcomes.
  • Understanding the causes of these aberrations is crucial for improving PRK procedures.

Purpose of the Study:

  • To investigate the factors contributing to central undercorrections in PRK.
  • To develop and validate a method for measuring corneal ablation profiles.
  • To re-engineer the PRK ablation algorithm for improved refractive accuracy.

Main Methods:

Related Experiment Videos

  • A custom optical profilometer was developed and verified to measure corneal ablation profiles.
  • The VISX excimer laser system was used to perform photorefractive keratectomies on porcine and rabbit eyes.
  • Ablation algorithms, diameter, depth, and dioptric correction effects were systematically analyzed.

    • Main Results:

    • The profilometer achieved high accuracy (within 3 microns) in measuring test surfaces.
    • PRK resulted in peripheral over-ablation and central under-ablation, increasing with ablation diameter and correction.
    • Stromal ablation rates varied spatially, independent of ablation depth, influencing ablation profiles.

    Conclusions:

    • Steep central islands in PRK are caused by spatial variations in tissue ablation with uniform laser irradiance.
    • Modifying the laser ablation algorithm can correct for these spatial variances.
    • This algorithmic adjustment leads to photorefractive keratectomies with enhanced sphericity and improved outcomes.