Changes in Equivalent and Gradient Refractive Index of the Crystalline Lens with Accommodation

Abstract

The ocular dimensions and refraction of the eye were measured for accommodation stimulus levels of 0.0, 1.5, 3.5, 5.5, and 8.0 D for 11 subjects aged 18 to 28 years, mean 21.2 +/- 2.62 years using keratometry, autorefraction, A-Scan ultrasonography, and video phakometry techniques. The subjects had refractive errors in the range + 0.50 to -4.25 D, mean -1.88 +/- 1.64 D. With the maximum level of accommodation the anterior chamber depth decreased by 0.23 +/- 0.09 mm, the lens thickness increased by 0.28 +/- 0.09 mm, and no significant differences were recorded in axial length or vitreous chamber depth. The radius of curvature of the anterior surface of the crystalline lens decreased from 11.54 +/- 1.27 to 6.59 +/- 0.97 mm and the posterior surface from -6.67 +/- 0.97 to -5.30 +/- 0.4 mm. We determined the equivalent refractive index to be 1.4277 +/- 0.0011, with no significant differences at different levels of accommodation. When the crystalline lens was modeled as a gradient refractive index (GRIN) structure with elliptical iso-indicial lines, the mean surface refractive index of the lens was 1.3859 +/- 0.0009 for an assumed central refractive index of 1.406. The power of the anterior surface of the lens increased from 4.38 +/- 0.49 to 7.59 +/- 0.34 D, the posterior surface increased from 7.67 +/- 1.28 to 9.32 +/- 0.64 D, and the GRIN power increased from 9.70 +/- 1.31 to 13.74 +/- 0.77 D for the maximum accommodative stimulus of 8.00 D. On the basis of the model used, a substantial part of the increase in power of the crystalline lens with accommodation resulted from the change in refractive index distribution within the lens.