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Comparison of Agreement and Accuracy using Binocular Wavefront Optometer with Autorefractor and Phoropter
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Total corneal power estimation: ray tracing method versus gaussian optics formula.

Li Wang1, Ashraf M Mahmoud, Betty Lise Anderson

  • 1Department of Ophthalmology, Baylor College of Medicine, Houston, Texas, USA. liw@bcm.tmc.edu

Investigative Ophthalmology & Visual Science
|November 13, 2010
PubMed
Summary

Total corneal power (TCP) differs from Gaussian equivalent power (GEP) after refractive surgery. The Gaussian formula overestimates power, potentially leading to errors in intraocular lens calculations post-LASIK/PRK.

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Area of Science:

  • Ophthalmology
  • Corneal Biomechanics
  • Optical Engineering

Background:

  • Accurate intraocular lens (IOL) power calculation is crucial for refractive outcomes after cataract surgery.
  • Standard formulas often rely on assumptions that may not hold true for eyes with altered corneal geometry, such as those post-refractive surgery.

Purpose of the Study:

  • To compare total corneal power (TCP) calculated via ray tracing with Gaussian equivalent power (GEP) in normal eyes, post-LASIK/PRK eyes, and theoretical models.
  • To identify discrepancies and their underlying causes in corneal power calculations.

Main Methods:

  • Corneal topographic data from normal, myopic-LASIK/PRK, and hyperopic-LASIK/PRK eyes were used to calculate TCP and GEP.
  • Corneal models were constructed to analyze ray incident angles at the posterior surface and validate differences using optical design software.

Main Results:

  • TCP was generally less than GEP in normal and myopic-LASIK/PRK eyes, with the reverse in some hyperopic-LASIK/PRK eyes.
  • Differences between TCP and GEP in post-refractive surgery eyes correlated with anterior curvature and posterior/anterior radius ratios.
  • Model corneas showed that parallel rays overestimated negative posterior corneal power, increasing the TCP-GEP difference with lower posterior/anterior radius ratios.

Conclusions:

  • The Gaussian formula's paraxial assumptions introduce errors in total corneal power calculation for eyes that have undergone refractive surgery.
  • These inaccuracies may lead to significant errors in intraocular lens power calculations when using the Gaussian formula post-LASIK/PRK.