Residual Astigmatism With Toric Versus Light-Adjustable Intraocular Lenses: A Power-Vector Model of the Effect of Alignment Uncertainty.

PURPOSE: To identify when postoperative cylinder adjustment with a light-adjustable lens (LAL) is expected to reduce residual astigmatism more than a toric intraocular lens (IOL), whose effect depends on axis placement and rotational stability.

METHODS

Power-vector analysis was used to derive a closed-form expected squared residual cylinder for a non-toric IOL, toric IOL and LAL. The toric model included a cylinder-dependent axis-error term and a cylinder-independent RMS floor; the LAL model included a cylinder-independent floor up to a platform-specific deliverable ceiling. Parameters were informed by published biometric, toric-alignment and LAL data. Closed forms were checked by Monte Carlo simulation, with sensitivity analyses across residual floors and axis-error distributions.

RESULTS

The toric RMS residual comprised a floor plus a misalignment term that increased with corneal cylinder and combined axis-error variance. The LAL residual remained near its floor within its deliverable range. With floors of 0.30 and 0.29 D for toric and LAL, respectively, the LAL advantage was essentially the toric misalignment penalty. At 2.00 D of cylinder, RMS reduction was 0.17, 0.77 and 1.06 D at alignment standard deviations of 5°, 15° and 20°, respectively. Below about 2 D on a stable platform, the gain was generally smaller than 0.50 D; with high cylinder and high combined alignment uncertainty, it could exceed a 0.50-D cylinder step. The comparison was robust across plausible floors and the non-Gaussian error distributions tested. Above the LAL ceiling, LAL monotherapy leaves residual astigmatism from a cylinder beyond the deliverable range.

CONCLUSION

Postoperative adjustability has the most astigmatic value when cylinder and alignment uncertainty are both high. For routine eyes, lens choice is more likely to be governed by cost, treatment burden and target refraction than by residual cylinder accuracy alone.