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

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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.

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