Estimation of true corneal power after keratorefractive surgery in eyes requiring cataract surgery: BESSt formula

PURPOSE: To describe a new formula, BESSt, to estimate true corneal power after keratorefractive surgery in eyes requiring cataract surgery. SETTING: Moorfields Eye Hospital, London, United Kingdom. METHODS: The BESSt formula, based on the Gaussian optics formula, was developed using data from 143 eyes that had kerato refractive surgery. The formula takes into account anterior and posterior corneal radii and pachymetry (Pentacam, Oculus) and does not require pre-keratorefractive surgery information. A software program was developed (BESSt Corneal Power Calculator), and corneal power was calculated in 13 eyes that had keratorefractive surgery and required cataract surgery. RESULTS: In the eyes having phacoemulsification, target refractions calculated with the BESSt formula were statistically significantly closer to the postoperative manifest refraction (mean deviation 0.08 diopters [D] +/- 0.62 [SD]) than those calculated with other methods as follows: history technique (-0.07 +/- 1.92 D; P = .05); history technique with clouble-K adjustment (0.13 +/- 2.39 D; P =.05); Holladay 2 with K-values estimated with the contact lens method (-0.76 +/- 1.36 D; P = .03); Holladay 2 with K-values from Atlas topographer (Humphrey) (-0.55 +/- 0.61 D; P <.01). Using the BESSt formula, 46% of eyes were within +/- 0.50 D of the intended refraction and 100% were within +/- 1.00 D. CONCLUSIONS: The BESSt formula was statistically significantly more accurate than the other techniques tested. Thus, it could significantly improve intraocular lens power calculation accuracy after keratorefractive surgery, especially when pre-refractive surgery data are unavailable.