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Jos J Rozema, Gareth D Hastings, Jason D Marsack, Raymond A Applegate; Refractive Correction and Visual Strehl in Keratoconus. Invest. Ophthalmol. Vis. Sci. 2021;62(8):2911.
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© ARVO (1962-2015); The Authors (2016-present)
It is notoriously difficult to determine optimal refractive corrections for keratoconus patients. To investigate the underlying reasons, we compared how visual image quality, in the form of Visual Strehl (VSX), changes over a range of sphero-cylindrical spectacle and scleral lens corrections in eye models with and without keratoconus.
Starting from the previously published SyntEyes models, the ocular biometry sets of 20 healthy and 20 keratoconic eyes were generated, including corneal tomography and all intraocular structures. Using Matlab, these eyes underwent simulated sphero-cylindrical spectacle or scleral lens correction, followed by ray tracing to determine the residual wavefront aberrations, which were used to calculate VSX to assess resulting visual image quality. For each eye, refractive corrections were applied over the entire range of the phoropter, referred to as ‘correction space’; an area of ‘optimal correction’ (i.e. maximized VSX) is called a ‘focus’. To speed up calculations, a smart scanning algorithm was used, consisting of three consecutive scans over increasingly finer grids, considering only those points neighboring a previously calculated point with a VSX above 0.01.
In all 20 healthy eyes the VSX pattern in correction space resembled an hourglass for both the spectacle and scleral lens corrections, with a distinct focus at the narrowest point and a quick drop in VSX away from the focus (Figure). For 18/20 keratoconic eyes the VSX pattern of spectacle corrections resembled a shell. In 9/20 of these cases two foci could be distinguished, separated by a large mean dioptric distance (13.3 ± 4.9D) and a mean orientation difference of 96.4 ± 32.1°. In keratoconic eyes scleral lenses always produced an hourglass pattern with a single focus with a VSX lower than in healthy eyes.
Based on the hourglass pattern found in healthy eyes, it is easily understood how the refracting process automatically funnels practitioners towards the optimal correction. The shell pattern of the spectacle correction in keratoconus, on the other hand, presents a far more complex shape with multiple foci. Hence, depending on the starting point, refracting procedures can often lead away from the optimal correction.
This is a 2021 ARVO Annual Meeting abstract.
log(VSX) in correction space for healthy (top) and keratoconic SyntEye (bottom)
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