The correlation between the Kpost and the difference between Kpost and Kpre, obtained by analyzing the 174 left eyes with a preoperative refraction of -4.87 [+ or -] 2.5 D (range: -10.0 to -0.5 D) and a 6-month postoperative refraction of 0.57 [+ or -] 0.81 D (range: -1.88 to +2.5 D), gave the following formula (Figure 1): y = 0.8197x - 36.907 (where x = Kpost and y = Kpost - Kpre).
The double-K method described by Aramberri requires a knowledge of the Kpre to estimate the effective lens position in the case of cataract surgery in patients who had previously undergone corneal refractive surgery.
This is an easy task if the Kpre have been recorded, but regrettably, this rarely happens.
Theoretically, if the achieved correction is known, it should be possible to calculate Kpre by adding this value to the postoperative ones.
Few methods have been described to calculate the preoperative Kpre [13, 16, 26, 27].
 described a formula which was not designed with the purpose of calculating the Kpre, but tested in the present paper for such a purpose, and have shown that the results are better than those obtained when the pure effective treatment is used.
 described a method to calculate the Kpre, starting from the assumption that it is possible to predict the preoperative anterior corneal power from the postoperative posterior one in patients who had undergone LASIK surgery.