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Thin-lens formula comparable to ray-tracing method of IOL power calculations
J Cataract Refract Surg. 2009;35(4):650-662.
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The thin-lens formula proved comparable to the ray-tracing method in IOL power calculations in normal eyes and eyes that underwent refractive surgery.
Preoperative prediction of IOL positioning plays a key role in IOL power calculation, the study authors said.
“Considering that more complicated procedures and more parameters are needed in the ray-tracing method, the thin-lens method is suggested in IOL power calculations,” they said. “As the ray-tracing method can account for [higher-order aberrations] of the human eye, it may play a significant role in evaluating the visual quality of the pseudophakic eye and in designing custom IOLs.”
The retrospective study included 302 normal corneas and 25 corneas that underwent myopic LASIK or PRK. The normal cornea group included 52 cataract cases.
Investigators used the Haigis, Hoffer Q, Norrby and Olsen 2 algorithms to predict IOL position and anterior chamber depth. They used Scheimpflug photography to measure corneal radii and corneal surface elevation.
Data showed that mean absolute prediction errors of the thin-lens formula were 0.64 D for Haigis, 0.57 D for Hoffer Q, 0.59 D for Norrby and 0.61 D for Olsen 2. Mean absolute prediction errors of the normal ray-tracing method were 0.64 D for Haigis, 0.58 D for Hoffer Q, 0.59 D for Norrby and 0.62 D for Olsen 2. There were no statistical differences between the methods.
The commonly used thin lens formulas provide accuracy equal to ray-tracing. This result is surprising given the assumptions inherent in Gaussian, paraxial optics.
It gives us more confidence that we are doing the right thing when we use the double-K method (which is implicit in the Holladay 2 formula) in post-LASIK patients.
– Mark Packer, MD, FACS
Drs. Fine, Hoffman and Packer Ophthalmologists Eugene,
Ore.