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    How to improve IOL power calculations in keratoconus

    Inaccuracy can be minimised by using an adjusted keratometric approach

    One source of error in IOL power calculation is the use of the classical keratometric approach for the characterisation of the corneal optics.1 This approach is based on the assumption of only one corneal surface and a fictitious index of refraction (keratometric index, nk) for obtaining an estimation of the corneal power (Pk).2

    Specifically, the use of the classical value of nk of 1.3375 has been shown to overestimate the corneal power in healthy,3 post-laser refractive surgery4 and keratoconus eyes.5 Some algorithms that have been developed, for different types of IOL design in eyes with corneal problems or previous surgeries, minimise the impact of this keratometric error in IOL power calculations. There are also algorithms for optimising the estimation of the effective lens position (ELP).6,7

    To this date, few studies have been conducted to investigate how to optimise IOL power calculation in keratoconus eyes. It should be considered that the posterior corneal curvature and thickness is abnormal in this type of eyes.

    Park do and colleagues8 found, in patients with posterior keratoconus, that IOL power calculations from conventional keratometry may be inaccurate, and secondary piggyback IOL procedures might be needed after cataract surgery. Thebpatiphat and co-authors9 concluded in a retrospective cases series evaluating 12 keratoconus eyes undergoing cataract surgery that IOL calculation was more predictable in mild keratoconus than in moderate and severe disease.

    It should be considered that the most significant increase in posterior corneal curvature and decrease in central corneal thickness are present in more severe keratoconus cases compared with the rest.10 We have recently conducted a simulation and clinical study to investigate the influence of the error in the calculation of corneal power due to the use of nk on IOL power calculation, as well as the potential benefit of using adjusted keratometric algorithms.

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