September 2016
Volume 57, Issue 12
Open Access
ARVO Annual Meeting Abstract  |   September 2016
Sampling density and configuration of stimulator points in determination of astigmatism, coma and higher order aberrations of the posterior surface of the cornea using Purkinje imaging reflection technology
Author Affiliations & Notes
  • Roland Piper
    Research, Cassini BV, Den Haag, Netherlands
  • Enrique Gambra
    Research, Cassini BV, Den Haag, Netherlands
  • Victor Sicam
    Advanced Development, Cassini BV, The Hague, Netherlands
  • Footnotes
    Commercial Relationships   Roland Piper, Cassini BV (E); Enrique Gambra, Cassini BV (E); Victor Sicam, Cassini BV (E), Cassini BV (P)
  • Footnotes
    Support  None
Investigative Ophthalmology & Visual Science September 2016, Vol.57, 1911. doi:
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      Roland Piper, Enrique Gambra, Victor Sicam; Sampling density and configuration of stimulator points in determination of astigmatism, coma and higher order aberrations of the posterior surface of the cornea using Purkinje imaging reflection technology. Invest. Ophthalmol. Vis. Sci. 2016;57(12):1911.

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      © ARVO (1962-2015); The Authors (2016-present)

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Abstract

Purpose : Research on improving intraocular lens outcomes is shifting focus onto accurate measurement of the posterior corneal surface (Koch DD. Ophthalmology. 2015 Jun;122(6):1070-1.). The Cassini Corneal Shape Analyzer (Cassini BV, The Hague, Netherlands) is at the frontier of anterior and posterior surface measurement technology (Ch. 5 of Refractive Lens Exchange, ed. Wang MX, Slack Inc © 2016). In this paper we investigate the sampling density required for this technique to reconstruct a Zernike model of the posterior surface of the cornea.

Methods : Source and image points are simulated for a test eye with an 8 mm radius spherical anterior surface and a 6.5 mm base radius posterior surface, with corneal aberrations described by Zernike coefficients of up to 6th order. Gaussian noise (25 μm s.d.) is added to the image points to simulate experimental noise. Source points are arranged in 9 configurations of concentric rings (3, 4 and 5 rings containing 7, 14 or 21 points each) providing even angular coverage up to a 6 mm corneal zone diameter (CZD). A novel raytracing algorithm that considers sequential surface interactions is used to calculate a posterior surface height map. This map is used to calculate a Zernike fit of arbitrary order of the posterior surface. Fits of up to 3rd, 4th and 5th order are made. The precision of the resulting shape contribution to astigmatism (2nd and 4th order), coma (3rd and 5th order), trefoil, and quadrafoil (where possible) is compared. The threshold of acceptable corneal height precision is 10 μm at 6 mm CZD, relating to around 0.25 eq. Dioptre.

Results : A minimum of 37 stimulator points covering the 6 mm CZD is found to be sufficient to achieve a precision of better than 10 μm for astigmatism, using an order 4 Zernike fit. Third order coma is found to have a precision of better than 5 μm with this configuration. Attempting a fit of up to order 5 is not sufficiently stable for clinically relevant determination of 5th order coma or higher order aberrations with this level of experimental noise, up to and including 105 points split between five rings.

Conclusions : The results show this algorithm reconstructs up to 4nd order astigmatism and coma to a clinically relevant precision when using more than 37 stimulator points as input.

This is an abstract that was submitted for the 2016 ARVO Annual Meeting, held in Seattle, Wash., May 1-5, 2016.

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