Estimation of the effective lens position using a rotating Scheimpflug camera

Purpose: To describe a no-history method of estimating the effective lens position (ELP) for double-K intraocular lens (IOL) power calculation in eyes that had previous refractive surgery. Setting: Departments of Ophthalmology, Taipei Medical University Hospital and Taipei City Hospital, Taipei, Taiwan. Methods: The corneal height (H_m) and anterior chamber diameter (AG_m) in 106 unoperated eyes were measured using a rotating Scheimpflug camera. The theoretical anterior corneal radius (R_rt) was then derived from H_m and AG_m by regression and rearrangement of the Fyodorov equation. The ELP estimate was then calculated from R_rt. The performance of this ELP estimation method in double-K IOL power calculation and the performance of other methods were compared retrospectively in 11 eyes having cataract surgery that had previous refractive surgery. The refractive results 9 to 12 weeks after cataract surgery were selected for data analysis. Results: The new ELP estimation method, combined with the BESSt formula or the Savini et al. method for estimating post refractive-surgery corneal power (K_post) in the double-K SRK/T formula, provided the best IOL power prediction results. The mean arithmetic and absolute IOL prediction errors were -0.05 ± 0.62 diopters (D) and 0.49 ± 0.34 D, respectively, when combined with the BESSt formula and 0.03 ± 0.73 D and 0.60 ± 0.36 D, respectively, when combined with the Savini et al. method. With either combination, all 11 eyes were within ±1.00 D of the refractive prediction error. Conclusion: This ELP estimation method may be helpful for IOL power calculation in post refractive-surgery eyes when historical data are unavailable.