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Tolerance to astigmatism with a small aperture corneal inlay


Purpose: Small aperture corneal inlays are used to extend depth of focus in presbyopic patients. Visual outcomes depend on residual refractive error. While the impact of defocus was already studied (Tabernero & Artal, JCRS, 2011), we expanded their study to investigate how much uncorrected astigmatism can be tolerated with a small aperture corneal inlay.

Methods: We developed 20 computational eye models using data from corneal topography, ocular axial lengths and wavefront aberrations taken in 20 healthy presbyopic subjects (mean age 59 years, standard deviation 7 years). The axial length was adjusted to render all eyes to be at best focus (emmetropia). A small aperture of 1.6 mm of diameter was placed at the corneal plane. The pupil of the eye was set to 3 mm of diameter. Astigmatism was induced in steps of 0.25 D (axis 90°) by placing a cylindrical lens at 15 mm from the corneal axis and then the eye’s modulation transfer function (for monochromatic light of 550 nm) was obtained for every eye from 0 D up to 3 D of induced astigmatism. The spatial frequency that corresponded to a modulation value of 0.2 (along the orthogonal direction to the axis of the astigmatism) was used as an image quality metric and calculated in every eye with and without the small aperture.

Results: With the small aperture implanted in the cornea, the cut-off frequencies increased significantly for all values of induced astigmatism. On average for the 20 eyes, if 1 D of astigmatism remained uncorrected for a pupil diameter of 3 mm (which corresponds to a 0.2 cut-off frequency of 0.22 decimal visual acuity units) then the equivalent value with the small aperture inlay implanted is obtained at 1.9 D (i.e. an increasing of 0.9 D of tolerance to astigmatism). For a different uncorrected threshold value of astigmatism, for instance to 0.5 D, then the tolerance with the small aperture would double up to 1 D. Since the inlay is actually an annulus (external diameter of 3.8 mm), this tolerance would be lower in those subjects with pupil diameters significantly larger than 4 mm.

Conclusions: The retinal image quality in eyes implanted with a small aperture corneal inlay to extend depth of focus is affected by the remaining uncorrected astigmatism. Although there is an individual variability due to the particular eye’s aberrations, the presence of the small aperture approximately doubles the typical tolerance to astigmatism.

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