Accuracy of thick and thin intraocular lens power formulas using paraxial vergence calculation.

Purpose: To compare the prediction errors of several thick IOL formulas to a thin lens approach using variations of the same paraxial vergence calculation formula.

Setting: Department of Ophthalmology, Randers Regional Hospital, Denmark.

Design: Prospective, non-interventional study.

Methods: We prospectively and consecutively performed optical low coherence reflectometry biometry in 132 eyes with subsequent phacoemulcification and insertion of the same aspherical IOL model. Clinical refraction was performed two months postoperatively. We retrospectively used the same paraxial vergence formula and varied only the methods for calculating the postoperative IOL position, thickness, and curvatures to construct four formulas: Næser I formula based on thick lens calculation using the manufacturer´s cutting card information; Næser II formula based on thick lens calculation using calculated IOL data from open sources; Næser III formula based on thick lens calculation and a fixed IOL thickness of 0.62 mm; Næser IV formula based on thin lens calculation with fixed IOL position 0.31 mm anterior to the postoperative capsule. Each formula was optimized separately, hereby assuring a mean error of zero. The prediction error (PE) was defined as the difference between the measured and predicted spherical equivalent refraction.

Results: Mean absolute error amounted to 0.30 (±0.26) D for all four formulas with no statistically significant difference. PE averaged zero for short, normal, and long eyes.

Conclusions: The additional optical information provided by thick IOL calculations appears insignificant compared to other sources of error, related to the accuracy of IOL manufacturing, natural lens refractive index, and postoperative refraction.