Estimation of the Effective Lens Position Using the Pentacam 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.