The Influence of Anterior Chamber Depth on the Calculation of Piggyback Intraocular Lenses

Objective: Demonstrate the variability of the spherical and toric ratios between the refraction at the intraocular lens (IOL) plane and the refraction at the spectacle plane when the sphere and the cylinder power of toric piggyback IOLs are calculated. Also, a comparison of the results of piggyback IOL power calculation with variable ratios, to the IOL power calculation performed by a major manufacturer online calculator using a fixed ratio.

Design: Experimental study with theoretical eye models.

Setting: Private practice, Buenos Aires Argentina.

Methods: Schematic eyes with different values of pseudophakic anterior chamber depth (ACD), refractive cylinder and sphere were used to calculate the exact toric and spherical power of the IOL needed to achieve emmetropia.

Results: The toric ratio could range between 0.88 and 1.6, while the spherical ratio between 1.01 and 1.61. Therefore, using fixed ratios can lead to errors in IOL power calculation up to 1 diopter. The most likely errors are under correction in eyes with deep ACD and low amounts of sphere and astigmatism and overcorrections in eyes with shallow ACD and high preoperative sphere and/or cylinder.

Conclusion: Manufacturers that calculate the power of piggyback IOLs with a fixed ratio of toricity and sphere should develop calculators that change this ratio according to the preoperatively measured variables. An excel file with the calculator used is provided.