Objective method to measure corneal clarity before and after laser in situ keratomileusis

To develop, evaluate, and use an objective method to determine the effect of laser in situ keratomileusis (LASIK) on corneal clarity.

Centre for Myopia Research, The Hong Kong Polytechnic University, and the Hong Kong Laser Eye Center, Hong Kong, China.

Color photographs of corneal sections were taken using a digital camera and converted to 8-bit gray-scale images. The desired area of the photograph was isolated using a preset mask, and a gray-scale or corneal clarity index of the desired area was obtained by averaging the "intensity" indices of individual pixels within the area. The reliability of the clarity index measures was determined by comparing test and retest measures. The sensitivity of the method was quantified by its ability to identify a small (clinically undetectable) decrease in corneal clarity produced by tight-fitting soft contact lenses worn for 30 minutes. Finally, corneal clarity was measured and compared in 24 patients before and 1 day, 1 week, and 1 month after LASIK.

The reliability value was 4.11 corneal clarity units, and the change in corneal clarity due to soft contact lens use was 16.24 units. In the LASIK patients, there were statistically significant decreases in corneal clarity from preoperatively to 1 day and 1 week but not to 1 month.

The method measured changes in corneal clarity that were undetectable clinically and were 4 times greater than 95% of the differences between test and retest measures. The method is therefore reliable and sensitive. Corneal clarity decreased after LASIK and recovered within approximately 1 month.