Equivalent Refractive Index of the Human Lens upon Accommodative Response

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Abstract

Purpose.

To experimentally verify the suggestion of Gullstrand (1909), i.e., that the equivalent refractive index of the human lens increases with accommodation.

Methods.

The left eye of five subjects was focused on different accommodation stimuli, while the right eye was imaged with Scheimpflug photography in order to obtain the shape of the lens and cornea during accommodation. The procedure was then repeated, but instead of using the Scheimpflug camera, the accommodative response of the right eye was measured objectively with an aberrometer. The axial length was measured with a Zeiss IOL-master. Combining the results of these measurements made it possible to correct the digital Scheimpflug images for corneal and lenticular refraction, and to simultaneously calculate the equivalent refractive index of the lens for all different accommodative stimuli. Furthermore, a two-compartment model of the lens was constructed, with a nucleus and a cortex.

Results.

In all five subjects there was no significant change in the equivalent refractive index of the lens as a function of accommodation. The mean equivalent refractive index was 1.435 ± 0.008. Furthermore, the accommodative response appeared to be lower than the accommodative stimulus (i.e., accommodative lag). It appeared to be possible to model the optical power of the lens, based on the geometry of cortex and nucleus. Based on a refractive index of 1.406 for the nucleus, the mean refractive index of the cortex was 1.381.

Conclusions.

Gullstrand suggested that there would be an increase in the equivalent refractive index with accommodation; the intra-capsulary mechanism of accommodation. However, we found that the equivalent refractive index of the lens does not change with accommodation when the accommodative lag is taken into account. Furthermore, it appeared to be possible to simulate the accommodative process of a subject with a two-compartment model with constant refractive indices.

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