Effect of Cataract Surgery on the Refractive Index of the Cornea Estimated by Optical Pachymetry

To noninvasively estimate the refractive index (RI) of the central cornea along the antero-posterior direction before and after routine phacoemulsification.

Using 2 setups for a standard optical pachymeter, the ratio of observed optical section widths (OSWs) is a function of the RI. Thus, the corneal RI could be estimated using a calibration equating OSW ratios with known RI values. The OSW was measured by 2 observers for 1) normal subjects for estimating interoperator errors and effects of sex and age on the RI and 2) before and after patients underwent routine phacoemulsification.

First, the average interoperator difference (ΔRI) was +0.0005 (SD = ±0.0044, 95% confidence limit, -0.0002 to +0.0012). The root mean square difference between measurements obtained by the observers was 0.0032. There was a significant correlation between the ΔRI and the mean of each pair of measured values (r = -0.172, n = 153, P = 0.003). The mean RI (±SD) was 1.435 (±0.005, n = 82) for females and 1.429 (±0.005, n = 71) for males. There was no significant between-sex difference or association between the RI and age (mean age, ±SD, and range, 44.31, 20.38, and 19-88 years, respectively). Second, the difference (y) between the preoperative (x) and postoperative RI was, y = 0.844x - 1.203 (r = 0.694, n = 31, P ≤ 0.001) according to observer 1 and according to observer 2, y = 0.755x - 1.108 (r = 0.681, n = 31, P ≤ 0.001).

The RI of the human cornea along the antero-posterior axis can be estimated using a modified application of traditional optical pachymetry. The average values for the corneal RI were higher compared with those reported in previous reports. The change in the RI after phacoemulsification could be predicted from the preoperative value.