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Viswanathan Ramasubramanian, Norberto Lopez-Gil, Pete S Kollbaum, Arthur Bradley; Axial Intraocular Lens Position: The Principal Determinant of Far Temporal Field Vignetting in Pseudophakic Eyes. Invest. Ophthalmol. Vis. Sci. 2019;60(9):6464.
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© ARVO (1962-2015); The Authors (2016-present)
The axial separation of iris and IOL in a pseudophakic eye results in a proportion of light rays from far-temporal field to miss the IOL due to vignetting. The vignetted rays fail to be imaged on the far-nasal retina causing reduced retinal illuminance, which is seen as a dark shadow or negative dysphotopsia. The goal was to study the impact of pupil size, IOL position, IOL tilt and decentration on vignetting of far-temporal rays.
A four surface pseudophakic eye model with no lens capsule and zero IOL edge thickness was constructed in Zemax OpticStudio. Fifty rays from a 6 mm diameter ray bundle from 00 to 850 in 10 steps in the temporal visual field were traced. Pupil size, IOL position, IOL tilt and decentration were manipulated independently. Proportion of image rays vignetted was calculated using an inbuilt Zemax function.
For a 6 mm pupil, percent vignetted rays increased systematically with increase in axial IOL separation for all field angles. For 850 field ray, percent vignetted rays ranged from 10.6% to 100% for axial IOL positions 0.00 mm to 1.75 mm. For a 0.70 mm axial IOL separation, there was partial vignetting of rays beyond 200 for physiological pupil sizes (2 mm to 8 mm). For a 1.40 mm axial IOL separation, a pupil size dependent vignetting was observed. For large pupils (8 mm), partial vignetting was observed for all field angles beyond 200, gradually increasing to about 92% at 850. For small pupils (< 3mm), vignetting was observed beyond 600, steeply increasing to 100% at 850. IOL decentration (± 1 mm laterally) and IOL tilt (up to 80 towards temporal iris plane), caused partial vignetting for all field angles for a 0.70 mm axial IOL separation. For a 1.40 mm axial IOL separation, IOL decentration of ≥ 0.80 mm toward the temporal sulcus and IOL tilt of ≥ 40 towards temporal iris plane caused 100% vignetting at 850. IOL tilt towards the nasal iris plane and IOl decentration towards the nasal sulcus allowed more image rays and resulted in less vignetting.
Axial IOL position was the dominant parameter to cause vignetting of rays from the far-temporal field. An IOL that is further away from the iris, with tilt and decentration towards the temporal sulcus will likely have an additive effect to cause increased vignetting. Conversely, IOL tilt and decentration towards the nasal sulcus might neutralize effect of the axial IOL separation.
This abstract was presented at the 2019 ARVO Annual Meeting, held in Vancouver, Canada, April 28 - May 2, 2019.
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