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Eugenia Kao Thomas, Ling Lin, Srividhya Vilupuru, Robert Ang; An objective method to measure astigmatism tolerance with a small-aperture vs. monofocal intraocular lens. Invest. Ophthalmol. Vis. Sci. 2016;57(12):3105.
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© ARVO (1962-2015); The Authors (2016-present)
The principle of small aperture optics may be practically applied to intraocular lenses (IOLs) to reduce astigmatic blur and bridge the gap between monofocal and toric IOLs for correcting low grade astigmatism. It can offer greater flexibility on tolerance limits for toric lenses and provide greater tolerance for residual postoperative astigmatism. Relative tolerance to astigmatism with a small-aperture (SA) IOL versus a monofocal (MO) IOL was assessed by measuring visual acuity with each IOL and various levels of induced astigmatism.
Nine pseudophakic subjects who had an SA IOL in the nondominant eye and a MO IOL in the dominant eye were evaluated for tolerance to astigmatism by defocus testing with up to 2.5 diopters of cylinder (DC) induced in the same axis as that in the manifest refraction ("natural axis”) or with the rule for a spherical manifest refraction.Each eye was corrected to best visual acuity with the manifest refraction. Next, 2.5 DC was added to the refraction and distance visual acuity re-measured using a Snellen letter chart. Cylinder power was reduced in 0.5 diopter steps to zero, and visual acuity was measured at each decrement.
Visual acuities were converted to logMAR values and defocus curves plotted from mean visual acuities at each defocus point. In pairwise comparisons, the greatest mean difference in acuity was seen with 2.5 DC defocus, where visual acuity with the SA IOL was 0.8 lines better than the MO IOL. With 2.0 and 1.5 DC defocus, the SA IOL was 0.7 lines better than the MO IOL. With 1.0 DC defocus or less, the mean difference between the SA and MO IOLs was not clinically significant.Overall, visual acuity of 0.1 logMAR or better was maintained with the SA IOL through 1.5 DC defocus and about 1.0 DC with the MO IOL. Results suggest that astigmatic tolerance may be affected by the axis of induced astigmatic defocus, which varied between eyes.
This experiment demonstrated a useful method for objectively assessing tolerance to astigmatism with different optical devices. Small aperture technology may be beneficial for subjects with residual postoperative astigmatism due to the limits of toric lens availability or postoperative refractive surprise.
This is an abstract that was submitted for the 2016 ARVO Annual Meeting, held in Seattle, Wash., May 1-5, 2016.
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