Influence of axial length and corneal power on the astigmatic power of toric intraocular lenses.

PURPOSE: To assess the influence of the anterior chamber depth (ACD), predicted by corneal power (keratometry [K]), and axial length (AL) on the astigmatic power of toric intraocular lenses (IOLs).

SETTING

Private practice, Bologna, Italy.

DESIGN

Noninterventional theoretical study.

METHODS

Meridional analysis (based on the Hoffer Q formula) was applied to an eye model with K ranging from 38.0 to 48.0 diopters and AL ranging from 20 to 30 mm. For each diopter (D) of K and each millimeter of AL, the ratio between the toricity at the IOL plane and the toricity at the corneal plane was computed. Several simulations with the Acrysof toric IOL were performed.

RESULTS

The ratio between toricity at the IOL plane and toricity at the corneal plane depended on the predicted ACD. The highest value (1.86) was found with the steepest K (48.0 D) and longest AL (30.0 mm) and the lowest value (1.29), with the flattest K (38.0 D) and shortest AL (20.0 mm). When applied to the toric IOL, the results suggest that a low ratio (≤1.41) led to overcorrection of astigmatism relative to the value reported by the manufacturer's online calculator. In contrast, a high ratio (>1.60) led to undercorrection of the astigmatism.

CONCLUSION

The ACD influenced the ratio between the cylinder power in the IOL plane and the cylinder power in the corneal plane and should be accounted for when selecting a toric IOL in an eye whose K and AL are far from the average values.