Purpose
To evaluate how the iris obstructs rays at large visual angles.
Setting
Simpson Optics, Arlington, Texas, USA, and Spektrum Eye Clinic, Wroclaw, Poland.
Design
Retrospective case study.
Methods
Anterior segment optical coherence tomography (AS-OCT) (Visante) images were exported for normal, short, and long eyes and eyes with negative dysphotopsia. Iris characteristics were manually identified, including the maximum iris thickness and its distance from the pupil, and the gap between the iris and intraocular lens (IOL). Ray-tracing software was used for optical modeling.
Results
The study comprised AS-OCT images of 63 eyes (21 normal eyes, 20 short eyes, and 19 long eyes, as well as 3 eyes with negative dysphotopsia). Preoperative and postoperative iris dimensions were similar, with a mean maximum thickness of 0.47 mm ± 0.08 (SD) at 0.94 ± 0.19 mm radially from the pupillary margin. The mean pupillary plane distance from the corneal epithelium moved from 3.23 ± 0.50 mm preoperatively to 3.99 ± 0.26 mm postoperatively, and the mean gap between the posterior iris and the IOL was 0.50 ± 0.24 mm, with a systematic increase with axial length for both parameters. The anterior iris surface obstructed light at very large angles and contributed to the limiting visual angle for rays focused by the IOL (although light from lower angles also bypassed the IOL). Models for the eyes with negative dysphotopsia had dark retinal regions between the focused image and peripheral light for small pupils.
Conclusions
Rays at very large angles can be obstructed by the anterior iris surface, which can be modeled to have a triangular profile near the pupillary margin. This limits the visual field for light focused by the IOL.
