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Human Cornea Before and After Refractive Surgery Using a New Device: VCH-1

Purpose: To test and measure the refractive index (RI) and to estimate the equivalent percentage of water content (%WC) of the anterior layers of the human cornea before and after excimer laser photoablation using a novel contact device, the VCH-1. Methods: (1) Repeatability was estimated by sequential measurement of the RI of 4 separate hydrogel contact lenses on 2 occasions, 5 and 10 minutes after insertion. (2) Patients preselected for photorefractive keratectomy, laser assisted sub-epithelial keratomileusis, laser assisted in situ keratomileusis, and laser assisted in situ keratomileusis with epithelial flap were enrolled. RI was measured for (a) corneal epithelium after topical anesthesia, (b) Bowman layer where appropriate after either removing epithelium or lifting the epithelial flap and remeasured immediately after excimer laser photoablation, and (c) midstroma after lifting the flap (created using mechanical microkeratome) and immediately after photoablation. Results: (1) Average repeatability of RI measurement was ±0.001 (range 0.000 to ±0.002). (2) Mean RI (±SD) and estimated mean %WC of (a) epithelium were 1.382 (0.006) and 75% (n = 8, age 28-63 years), (b) Bowman layer 1.380 (0.011) and 77% changing to 1.392 (0.010) and 69% after photoablation (n = 11, age 26-40 years, P = 0.005), and (c) midstroma 1.369 (0.007) and 83% changing to 1.385 (0.006) and 74% after photoablation (n = 30, age 22-56 years, P < 0.0001). RI of corneal epithelium and Bowman layer was significantly greater than that of midstroma before photoablation (P < 0.001). Conclusions: (1) VCH-1 can reliably measure RI to within ±0.001 units. This is equivalent to an error in water content estimation of ±0.6%. (2) RI and %WC of the human cornea are not uniform. Intersubject and intracorneal variations in RI and %WC should be considered because these factors will affect optical performance of the eye and photoablation increases RI, presumably by way of tissue dehydration.