Effect of Correcting Non-Orthogonal Astigmatism in Corneas with Novel Optical System



Abass, AMFF ORCID: 0000-0002-8622-4632, White, L, Jones, steve, Elsheikh, A and Clamp, J
(2017) Effect of Correcting Non-Orthogonal Astigmatism in Corneas with Novel Optical System. [Poster]

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Abstract

For normal eyes, astigmatism is assumed to have orthogonal axes between its optical power meridians. Irregular (non-orthogonal) astigmatism is defined as having axes with less than 90° between them. The eye condition Keratoconus generally results in non-orthogonal (NO) astigmatism and this cannot be fully corrected by conventional orthogonal optics. Objects viewed by people with significant NO astigmatism can present as multiple images or appear severely ghosted or distorted. All ophthalmic spectacle lenses and toric contact lenses assume astigmatism has orthogonal axes, making it difficult to correct NO astigmatism optically. Additionally, topography machine software imposes orthognal axes on their power map outputs, so it is not possible to easily assess the extent of NO astigmatism present in any individual eye. An investigation was undertaken to attempt to correct NO astigmatism with an appropriate optical system and assess the ffect on viausl acuity. Raw data was taken from scanning topography machines and processed to detect the natural maximum and minimum power meridians of the anterior and posterior cornea. Axial and tangential maps were created as well as power maps achieved through Light Ray Tracing A means of creating spectacle trial lenses with NO power axes was developed and three sets made to use as refraction trial lenses. The axes of each set were orientated at 80°, 70° & 60° respectively and cyl powers -1.00DC to -6.00 DC in 1.00DC steps plus an additional -0.50DC lens. Three subjects were chosen to be refracted by these lenses: two with mild keratoconus and one with longstanding, non strabismic ambylopia. They were refracted with each set of lenses, using a standard LogMAR Chart. The chart was changed randomly in between testing each set. For each subject, the refraction starting point was taken from the orthogonal spectacle prescription. The subjects were refracted with NO cyls from each set of lenses and wre asked to locate the point of optimal acuity by rotating the lens. This was independently checked by the examiner. To ensure that the subject was not experiencing a placebo effect, the lenses were randomly “flipped” during the examination. Unlike conventional cyl lenses, which present the same power meridians whichever way the lens is presented to the eye, NO lenses will present meridians at different axes when flipped which should cause an obvious difference in VA.

Item Type: Poster
Depositing User: Symplectic Admin
Date Deposited: 15 Jun 2017 10:36
Last Modified: 02 Apr 2021 08:14
Open Access URL: https://www.researchgate.net/publication/312489727...
URI: https://livrepository.liverpool.ac.uk/id/eprint/3007996