The Effect of Dilation on Toric Intraocular Lens Aligning and Centring

Purpose: To determine the effect of dilation on toric intraocular lens (IOL) alignment using conjunctival blood vessel or iris features and centration relative to the pupil. Methods: Thirty-six patient’s right eyes were imaged every two minutes after dilation with 0.5% Tropicamide. The orientation of conjunctival blood vessels on either side of the limbus and peripheral horizontal and vertical iris features were measured and compared to the magnitude of the dilation. The centration relative to the limbus and pupil shape were also assessed. Results: The pupil was essentially symmetrical and remained so with dilation, whereas the limbus was horizontally oval, remaining constant over time. Pupil dilation had no effect on the average orientation of the conjunctival blood vessels, horizontal or vertical iris features (F = 2.95, p = 0.069), however, the variability between patients was much greater using iris features (F = 31.233, p < 0.001). The change in pupil size was more strongly correlated to the change in orientation with the horizontal (r =0.13 ± 0.60) and vertical (0.18 ± 0.49) iris features than the conjunctival blood vessels (r = 0.02 ± 0.43; F = 3.149, p = 0.049). The pupil was on average centred superior (0.07 ± 0.09mm) nasal (0.37 ± 0.10mm) compared to the centre of the limbus, moving inferiorly with dilation. Conclusion: Conjunctival blood vessel orientation is a more robust comparator for toric IOL surgical orientation monitoring than iris features as it is less reliant on pupil dilation. Pupil centration changes with dilation and should be considered when centring IOLs.