Intraocular lens power calculation with the Scheimpflug camera after refractive surgery

For a long time, we have investigated methods to improve intraocular lens (IOL) power calculation after corneal refractive surgery and thus read with interest the article by Saiki et al.¹ Although the results of their method could be considered satisfactory, it would have been better if other corneal power values provided by the same Scheimpflug camera had been entered into the double-K SRK/T formula.² Using “Km,” as the authors have, introduces the well-known keratometric error or index of refraction error.³ The Km is calculated using the keratometric index (1.3375) to simulate standard keratometry. This index, although commonly used, is no longer valid once corneal refractive surgery disrupts the physiological ratio between the anterior and posterior corneal curvatures. The higher the amount of myopic correction, the lower this index should be to avoid erroneous corneal power calculations.⁴

The Scheimpflug camera used by the authors offers at least 2 options to overcome this issue: (1) corneal power calculated by ray tracing (total corneal refractive power) and (2) corneal power calculated by the Gaussian optics formula (true net power). The former and, although to a lesser extent, the latter have been found to accurately track the refractive change induced by myopic excimer laser surgery, while Km significantly underestimates it.^5,6 Hence, we suggest that Saiki et al. reanalyze the data after entering the corneal power calculated by ray tracing.

Furthermore, in the methods section, the authors state they used the mean postoperative posterior corneal power in the 6.0 mm zone to estimate the preoperative corneal power. In most studies, the 3.0 mm zone is evaluated. We wonder why such a large zone was chosen.

Finally, we would like to point out that the Camellin–Calossi method⁷ relies on the surgically induced refractive change, and it is not correct to state that it does not require preoperative data.