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H. S. Ginis, D. De Brouwere, I. Pallikaris; Single-Pass Measurement of Corneal Scatter in Rabbit Eyes After Photorefractive Keratectomy. Invest. Ophthalmol. Vis. Sci. 2007;48(13):3511.
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It is known that the postoperative healing response after photorefractive keratectomy (PRK) affects the transparency of the cornea through the development of newly formed structures that lack the homogeneity of the intact cornea. These structures include activated keratocytes, and extracellular structures mainly new collagen that lacks the periodicity seen in normal tissue. The purpose of this work was to develop an optical setup for the measurement of optical transparency of corneal tissue following PRK and to associate the findings with confocal microscope images of the treated corneas.
Six pigmented rabbits underwent bilateral PRK (-4 D at a 6mm OZ). All eyes underwent corneal confocal microscopy (HRTII, Heidelberg Engineering, Germany) preoperatively, as well as at weeks 1, 2 , 3 and 6 postoperatively. Corneal transparency was assessed using three methods: microscopically by statistical analysis of confocal images, macroscopically in terms of a subjective haze grade under slitlamp examination and objectively by means of a purposely developed optical setup. This setup involved a custom camera lens that incorporated excised rabbit corneas enclosed between glass elements as an imaging system. This system was used to image light sources including a point source and a bright semiplane against a dark semiplane to estimate the percentage of scattered light. This single pass imaging technique was employed at the sixth week postoperatively, after the animals were sacrificed.
After objective classification of haze, ten corneas were characterized as practically clear (trace haze or mild haze) and two corneas as moderately hazy. Confocal microscopy revealed increased reflectivity of keratocyte nuclei indicating an increased difference of refractive index compared to the extracellular matrix at all postoperative intervals. A characteristic highly reflective semi-continuous subepithelial layer was present in all eyes. The reflective structures forming this layer had dimensions ranging between 5 and 120 micrometers. The small-angle scatter light measurements revealed that the scattered light distribution is relatively narrow, being approximately1.5 degrees FWHM. The appearance of light sources through post PRK corneas was directly visualized.
Light scattered at the cornea following PRK-induced inflammation and healing response has a narrow forward distribution. The width of this distribution can be explained on the basis of the anomalous diffraction approximation of Mie theory involving particle sizes equal to these observed microscopically.
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