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Longitudinal spherical aberration in contact lens correction of aphakia

By Michael J. Collins, Ross Franklin and Brett A. Davis

Abstract

Purpose\ud \ud Correction of aphakia in infants with standard spherical contact lenses results in significant levels of positive longitudinal spherical aberration (LSA). These infants often achieve relatively poor levels of visual acuity. We measured visual acuity for different object vergences in the presence of representative levels of LSA, and make recommendations for LSA management in contact lens corrections for aphakic infants. \ud \ud Methods\ud \ud Levels of LSA induced by rigid and soft lenses on eye were calculated using raytracing for typical aphakia corrections found at the ages 1 month, 1 year and 4 years. We then measured the likely effects of these LSA levels on visual performance. Twelve subjects between the ages of 21 and 28 were recruited and cycloplegia was induced prior to testing. LSA was induced by means of custom-designed aspheric rigid contact lenses positioned close to the eye. Visual acuity was measured with the Freiburg visual acuity test for object vergences from 2 to +2D. An artificial aperture of 4 mm was used to control the level of LSA. Eight LSA levels were tested between -3.67D and +8.14D. \ud \ud Results\ud \ud Peak visual acuity reduced at a rate of approximately 0.7 logMAR per dioptre LSA. For an average aphakia correction for a 1 month old (+35DS) and a 4mm pupil, the LSA induced by a rigid spherical lens on-eye is +4.86D. This would correspond to a drop in acuity of approximately 3 lines (0.33 logMAR). A soft spherical lens of the same power on-eye would induce +2.10D LSA which corresponds to approximately one and a half lines of acuity loss (0.14 logMAR), provided the lens flexed perfectly to the underlying cornea. Visual acuity improved for negative vergences (closer objects) in lenses with positive LSA and for positive vergences (further objects) in lenses with negative LSA, as expected. \ud \ud Conclusions\ud \ud The poor optics due to positive LSA in contact lens corrections for aphakia will potentially contribute to amblyopia. Correction of LSA in contact lenses can be achieved with an aspheric lens surface and correction strategies are presented for unilateral and bilateral aphakia in infants. CR: Non

Topics: 111300 OPTOMETRY AND OPHTHALMOLOGY, 111303 Vision Science, 111302 Optical Technology
Year: 2001
OAI identifier: oai:eprints.qut.edu.au:12193
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