The equivalent refractive index of the crystalline lens in childhood

Abstract

Despite the importance of crystalline lens power in ocular development, schematic refractive index values used to calculate lens power have been validated for children. We measured refractive error and ocular component dimensions in 519 schoolchildren, calculating lens power using phakometrically measured lens radii and three different refractive index profiles: (1) Gullstrand-Emsley schematic indices [Gullstrand-Emsley lens power (GELP)]; (2) a 10-shell gradient index model [gradient index lens power (GILP)]; and (3) the equivalent refractive index (IND) needed to bring calculated and measured refractive error into agreement [calculated lens power (CLP)]. GELP was significantly lower than either GILP or CLP, indicating the Gullstrand-Emsley refractive index of 1.416 is too low for use in children. Variation in IND cannot be explained by measurement error alone. GILP and CLP also differed as a function of lens shape, with GILP greater than CLP at steeper external curvatures and less than CLP at flatter external curvatures. Variation in equatorial gradient index profile as a function of lens shape is proposed as an explanation for this bias. Equivalent index appears to be a useful tool for encompassing individual variation in lens gradient profiles as well as for assessing the relative role of lens surface curvature and refractive index changes during lens power development in childhood.