A New Method for Calculating Residual Astigmatism Produced by Toric Intraocular Lens Rotation

Aims: To report a new method for calculating residual astigmatism when exact and inexact cylinder power toric intraocular lenses (IOLs) are rotated.

Methodology: Using Excel spreadsheets and sin2θ values, we expressed the cylinder powers of toric IOLs in percentage terms compared to corneal astigmatic powers and calculated values for various corneal astigmatisms and toric IOLs. From these values, we created charts of residual astigmatic powers and axes demonstrating the results of IOLs ranging from 50% to 150% and rotated 0º to 30º.

Results: Comparing residual astigmatic powers, the rank orders for each IOL were as follows: 100%<90%<80% at 5º of rotation, 90%<100%<80% at 10º of rotation, and 90%<80%<75%<100% at 15º of rotation. Thus, lower cylinder power IOLs perform better than higher power IOLs when rotated over 5º. Furthermore, the residual astigmatic powers of 125% IOLs were always higher than those of 100% IOLs; however, the residual astigmatic powers of 75% IOLs became lower than those of 100% IOLs when rotation exceeded 15º.

Conclusion: Our method shows that lower cylinder power IOLs are advantageous in environments where IOL rotation is likely and when inexact IOLs are utilized.