C constant: New concept for ray tracing–assisted intraocular lens power calculation

Purpose To evaluate the accuracy of the C constant for ray tracing–assisted intraocular lens (IOL) power calculation. Design Case series. Setting Public university hospital and private clinic. Methods Preoperatively, all intraocular distances were measured using laser biometry. Various IOL designs were studied; powers ranged from −5.0 diopters (D) to +38.0 D. The IOL power calculation was performed with the Olsen formula using the C constant and compared with the Haigis, Hoffer Q, Holladay 1, and the SRK/T formulas on optimized datasets. Outcome measures were the error of the prediction, expressed as the arithmetic error, and the absolute error between the observed refraction and the predicted refraction. Results Two thousand forty-three cases from the 2 centers were studied. No significant differences were found between the Haigis, Hoffer Q, Holladay 1, and SRK/T formulas with the exception of the SRK/T formula, which performed better than the other thin-lens formulas in eyes with an axial length (AL) greater than 27.0 mm (P<.01). Compared with the SRK/T formula, the Olsen formula showed an improvement of 15% and 14% in the mean absolute error and a 39% and 85% reduction in the number of large errors (>1.0 D) for the 2 series, respectively (P<.0001). Contrary to the Olsen formula, all thin-lens formulas showed a significant bias in terms of the AL, keratometry reading, and anterior segment length (P<.0001). Conclusion The C constant is a promising concept for ray tracing–assisted IOL power calculation. Financial Disclosure Dr. Olsen is a shareholder of IOL Innovations Aps, manufacturer of the Phacooptics IOL calculation software. Dr. Hoffmann has no financial or proprietary interest in any material or method mentioned.