Astigmatism induced by intraocular lens tilt evaluated via ray tracing

Purpose: To evaluate astigmatism induced by aspheric and toric intraocular lens (IOL) tilt using a ray-tracing model.

Setting: Cullen Eye Institute, Baylor College of Medicine, Houston, Texas, USA.

Design: Experimental study.

Methods: Ray-tracing eye models with aspheric IOLs (16.0 diopters [D], 22.0 D, and 28.0 D) and toric IOLs (16.0 D, 22.0 D, and 28.0 D each with toricities of 1.50 D, 3.75 D, and 6.00 D) were used. The IOLs were tilted from 1 to 10 degrees horizontally around a 90-degree vertical meridian. Toric IOLs were aligned at 90 degrees and 180 degrees to correct with-the-rule (WTR) and against-the-rule (ATR) corneal astigmatism, respectively. Astigmatism at the corneal plane induced by IOL tilt was calculated.

Results: Induced astigmatism increased with increasing IOL tilt and power. Horizontal tilt around a vertical meridian induced ATR astigmatism. For 5 degrees of tilt, induced astigmatism was 0.08 D, 0.11 D, and 0.14 D for 16.0 D, 22.0 D, and 28.0 D aspheric IOLs, respectively. Ten degrees of IOL tilt produced 0.33 D, 0.44 D, and 0.56 D of induced astigmatism for 16.0 D, 22.0 D, and 28.0 D aspheric IOLs, respectively. Tilting toric IOLs aligned at 90 degrees around a vertical meridian increased the magnitude of induced ATR astigmatism. Tilting toric IOLs aligned at 180 degrees decreased the magnitude of induced WTR astigmatism.

Conclusions: Tilting aspheric IOLs horizontally around a vertical meridian induced ATR astigmatism. Tilting toric IOLs aligned at 90 degrees increased ATR astigmatism, resulting in overcorrection. Tilting toric IOLs aligned at 180 degrees decreased WTR astigmatism, producing undercorrection.