Intraocular lens calculations after refractive surgery

Purpose

To evaluate the effect of refractive surgery on intraocular lens (IOL) power calculation, compare methods of IOL power calculation after refractive surgery, evaluate the effect of pre-refractive surgery refractive error on IOL deviation, review the literature on determining IOL power after refractive surgery, and introduce a formula for IOL calculation for use after refractive surgery for myopia.

Setting

Laser & Corneal Surgery Associates and Center for Ocular Tear Film Disorders, New York, New York, USA.

Methods

This retrospective noncomparative case series comprised 21 patients who had uneventful cataract extraction and IOL implantation after previous uneventful myopic refractive surgery. Six methods of IOL calculation were used: clinical history (IOL_HisK), clinical history at the spectacle plane (IOL_HisKs), vertex (IOL_vertex), back-calculated (IOL_BC), calculation based on average keratometry (IOL_avgK), and calculation based on flattest keratometry (IOL_flatK). Each method result was compared to an “exact” IOL (IOL_exact) that would have resulted in emmetropia and then compared to the pre-refractive surgery manifest refraction using linear regression. The paired t test was used to determine statistical significance.

Results

The IOL_HisKs was the most accurate method for IOL calculations, with a mean deviation from emmetropia of −0.56 diopter ±1.59 (D), followed by the IOL_BC (+1.06 ± 1.51 D), IOL_vertex (+1.51 ± 1.95 D), IOL_flatK (−1.72 ± 2.19 D), IOL_HisK (−1.76 ± 1.76 D), and IOL_avgK (−2.32 ± 2.36 D). There was no statistical difference between IOL_HisKs and IOL_exact in myopic eyes. The power of IOL_flatK would be inaccurate by −(0.47x+0.85), where x is the pre-refractive surgery myopic SE (SEQ_m). Thus, without adjusting IOL_flatK, most patients would be left hyperopic. However, when IOL_flatK is adjusted with this formula, it would not be statistically different from IOL_exact.

Conclusions

For IOL power selection in previously myopic patients, a predictive formula to calculate IOL power based only on the pre-refractive surgery SEQ_m and current flattest keratometry readings was not statistically different from IOL_exact. The IOL_HisKs, which was also not statistically different from IOL_exact, requires pre-refractive surgery keratometry readings that are often not available to the cataract surgeon.