Comparison of Ray-tracing Method and Thin-lens Formula in Intraocular Lens Power Calculations

Purpose: To compare the accuracy of the thin-lens and ray-tracing methods in intraocular lens (IOL) power calculations in normal eyes and eyes after corneal refractive surgery.

Setting: International Vision Correction Research Centre, University of Heidelberg, Heidelberg, Germany.

Methods: Pseudophakic eye models were constructed using Zemax optical software, importing corneal radii (normal ray tracing) and corneal surface elevation data (individual ray tracing) measured by Pentacam Scheimpflug photography. Algorithms to predict IOL position (effective lens position [ELP]) or postoperative anterior chamber depth [ACD(post)]) (Haigis, Hoffer Q, Norrby, Olsen 2) were used in the thin-lens and ray-tracing methods. Intraocular lens power was calculated in 25 eyes after corneal refractive surgery using normal and double-K modified thin-lens and ray-tracing methods.

Results: Back-calculation of ELP and ACD(post) were well correlated. Using algorithms of Haigis, Hoffer Q, Norrby, and Olsen 2 to predict IOL position, mean absolute prediction errors (MAEs) of the thin-lens formula were 0.64 diopters (D) +/- 0.52 (SD), 0.57 +/- 0.46 D, 0.59 +/- 0.42 D, and 0.61 +/- 0.47 D, respectively; MAEs of normal ray-tracing method were 0.64 +/- 0.50 D, 0.58 +/- 0.44 D, 0.59 +/- 0.41 D, and 0.62 +/- 0.45 D, respectively; MAEs of individual ray-tracing method were 0.66 +/- 0.52 D, 0.59 +/- 0.45 D, 0.59 +/- 0.43 D, and 0.62 +/- 0.50 D, respectively. No statistical differences were found between the thin-lens and ray-tracing methods.

Conclusion: Theoretical thin-lens formulas were as accurate as the ray-tracing method in IOL power calculations in normal eyes and eyes after refractive surgery.