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Thomas W Raasch; Optical effects of an intracorneal annulus as a treatment for presbyopia. Invest. Ophthalmol. Vis. Sci. 2015;56(7 ):2987.
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A potential treatment for presbyopia is the use of an intracorneal annulus, exploiting the pinhole effect to expand the depth of field. The purpose of this study is to examine the effects of an intracorneal annulus on the retinal image and retinal illumination over a range of pupil sizes and visual field angles.
Analytical and numerical methods are used to determine the effects on foveal image quality and on retinal illumination across the visual field. Retinal image characteristics are calculated using analytical formulae. Retinal illumination is calculated using ray tracing techniques.
An intracorneal annulus will expand the depth of field by reducing the entrance pupil diameter, thereby improving the clarity of the foveal image for near vision in a presbyopic eye. The opaque band of the annulus occludes at least of portion of the entrance pupil of the eye over a wide range of visual field locations, reducing retinal illumination in an annular ring. Effects depend upon the dimensions and centration of the annulus, anterior chamber depth, and pupil size. In an eye with an extremely small pupil, the reduction in retinal illumination has the potential to produce a relative scotoma. In an eye with a very large pupil and/or a decentered annulus, the retinal image will be formed by ray bundles traversing two paths, i.e. through the center of the annulus and around the outside of the annulus, which may degrade the quality of the retinal image.
Used as a treatment for presbyopia, a well-centered intracorneal annulus produces a pinhole effect, expanding the depth of field. When implanted unilaterally, this can be expected to provide a near-emmetropic presbyope a greater degree of spectacle independence by improving the quality of focus at near. This desired effect may be accompanied by undesired consequences if the annulus is decentered, if the entrance pupil of the eye is unusually small, or if it is larger than the outside diameter of the annulus.
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