May 2004
Volume 45, Issue 13
ARVO Annual Meeting Abstract  |   May 2004
An objective method for measuring epithelial thickness
Author Affiliations & Notes
  • M.E. Arnoldussen
    Research, VISX, Inc, Santa Clara, CA
  • S. Somani
    Research, VISX, Incorporated, Santa Clara, CA
  • Footnotes
    Commercial Relationships  M.E. Arnoldussen, VISX, Incorporated E; S. Somani, VISX, Incorporated E.
  • Footnotes
    Support  none
Investigative Ophthalmology & Visual Science May 2004, Vol.45, 178. doi:
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      M.E. Arnoldussen, S. Somani; An objective method for measuring epithelial thickness . Invest. Ophthalmol. Vis. Sci. 2004;45(13):178.

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

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Abstract: : Purpose: During corneal refractive surgery, the amount of fluorescent light emitted depends upon the layer of tissue being ablated. When removing the epithelium with the laser, the surgeon can visually distinguish when the stroma has been reached by a significant diminishing of fluorescent emmission. The surgeon stops the laser and judges whether the stromal layer has been fully exposed so that the refractive correction can begin. Several groups have suggested monitoring this change of fluorescence to automate epithelium removal, but a practical method has not been developed. The purpose of this investigation is to validate the accuracy of a compact silicon detector that is being developed to objectively monitor laser epithelium removal in situ. Methods: Freshly enucleated porcine eyes with epithelia intact were ablated with a VISX laser. The monitoring device (a silicon photodiode equipped with a bandpass filter that rejects the source radiation as well as room lights) was used to record broad UV signals during ablation. A proprietary optical profilometer captured the shape of the ablation crater, providing a real–time measure of the depth corresponding to each fluorescent signal. Same–size, static–location ablations limited the experiment to well–known ablation characteristics. Results: The level of the fluorescent signal changed from high to low when the ablation transitioned from epithelium to stroma.The slope of the transition varied in steepness as a function of spot size due to the characteristic ablation response of water–dominant tissue. The transition begins when the outer perimeter of the ablation crater first reaches the stroma and is completed when the center of the crater fully penetrates the epithelium. The thickness of the epithelium can be derived from the knee of this curve, which is consistent with published values for central corneal epithelium thickness in pig eyes. Conclusions:A compact sensor was shown to respond consistently in its ability to differentiate ablation of the epithelial layer from ablation of the stroma.The difficulty of relating fluorescent changes during corneal ablation to penetration depth has been solved by using a real–time depth profilometer. For clinical success with a scanning–spot procedure, variation in the signal with respect to the size and location of the ablation spot, geometric capture efficiency of the detector, and biological factos need to be accounted for. Given a suitable model of fluorescence and careful calibration, a compact silicon photodiode can be used to automate epithelium removal.

Keywords: refractive surgery: PRK • cornea: epithelium • depth 

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