Effect of Power and Axis of Posterior Corneal Astigmatism on Toric IOL Positioning
To report the refractive effect of combining both power and axis of posterior corneal astigmatism on positioning of toric IOL.
Total astigmatism in eyes with difference of more than 20 degrees between anterior and posterior corneal astigmatism was calculated using vector analysis. Positioning of the toric IOLs was adjusted according to the combined astigmatism. Simulated astigmatism was plotted with the IOL placed both at the combined axis (anterior and posterior astigmatism) and the axis determined by anterior measurements only.
Toric IOLs were implanted using the combined axis calculation in 9 eyes of 7 patients with difference between anterior and posterior corneal astigmatism greater than 20 degrees. Post-operative residual astigmatism ranged between 0.0 to 0.75 Diopters (mean = 0.3 diopters). Adjusted IOL axis using the combined astigmatic axis shifted the astigmatic outcome towards the with-the-rule (WTR) zone in all cases. Positioning of the IOL axis as recommended by common toric calculators using the anterior astigmatism only would most likely result in significantly higher residual astigmatism.
Modifying toric IOL positioning using posterior axis of corneal astigmatism (and not only power) improves accuracy of astigmatic correction by toric IOLs.