Estimation of corneal power after myopic laser refractive surgery: comparison of methods against back-calculated corneal power

Abstract

Purpose: To compare methods of corneal power estimation after myopic laser refractive surgery with back-calculated corneal power (K).

Setting: Private practice, Edmonton, Alberta, Canada.

Design: Case series.

Methods: Patients with previous myopic laser surgery followed by cataract extraction were studied. Back-calculated K obtained with the Holladay IOL Consultant was compared with that obtained by the clinical history method (CHM), the modified Maloney method, an adaptation of the Maloney method using individualized Orbscan IIz-derived posterior corneal power values, Orbscan IIz quantitative area topography, and the Gaussian optics formula. A mixed effects linear model was used for analysis.

Results: The mean spherical equivalent (SE) before laser treatment was -6.43 diopters (D) ± 3.52 (SD). The estimated means of all methods except those obtained with the CHM, modified Maloney method, 2.0 mm total axial map, 1.5 mm total mean map, and 1.5 mm total optical map were significantly different from the mean of the back-calculated K. Estimates from the 1.5 mm total mean map were generally 0.06 D higher. The 2.0 mm total axial map, modified Maloney method, 1.5 mm total optical map, and CHM underestimated corneal power by 0.11 D, 0.13 D, 0.22 D, and 0.26 D, respectively. Unit increases in optical zone and pre-laser myopic SE were associated with decreases in corneal power of 1.58 D (P = .047) and 0.55 D (P = .0001), respectively.

Conclusion: The modified Maloney method, 2.0 mm total axial map, 1.5 mm total mean map and 1.5 mm total optical map of the Orbscan IIz may provide estimates closer to the back-calculated K than the CHM.

Financial disclosure: No author has a financial or proprietary interest in any material or method mentioned.