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refraction is performed. If the refraction remains the same, the corneal curvature is equal
to the base curve of the contact lens. If the over-refraction results in a higher myopic
error, the difference between the over refraction and the original refraction is
algebraically added to the base curve of the contact lens; a more myopic change
decreases the base curve value, while a more hyperopic change increases this value. The
limitations of this method are three-fold: the availability of Plano hard contact lens in a
clinical setting, the difficulty in obtaining an accurate refraction in presence of a cataract,
and more importantly, the difficulty in differentiating a residual myopia from one that has
been induced by the cataract. The usefulness of this method in clinical practice is limited
and it was not used in our study.
An accurate way to calculate the true corneal power after Lasik surgery is the
history-derived method (6); the corrected corneal power (Kc.hd) is obtained by
algebraically adding the myopic correction at the corneal plane to the pre-operative
corneal power. This method requires access to the refractive surgery data (pre-Lasik K
readings and amount of myopia corrected). Care should be taken not to include in the
calculations any myopic shift induced by the cataract. In our study, we used this history-
derived method as a benchmark for comparison.
More recently, a refraction-derived method to correct the corneal power after
refractive surgery has been advocated (9-12). Our study confirmed the validity of this
method where the measured post-Lasik keratometry readings are reduced by 0.23 D for
each diopter of myopia corrected by the refractive surgery. This method is most useful
when the pre-Lasik K readings are not available but the amount of myopia corrected can
be retrieved either from the patient's old glasses or the optometrist's prescriptions.
