New formula for calculating intraocular lens power after laser in situ keratomileusis

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Abstract
Purpose: To assess the validity and accuracy of a proposed formula for keratometry (K) readings after laser in situ keratomileusis (LASIK). Setting: The Eye Center and the Eye Foundation for Research, Riyadh, Saudi Arabia. Method: This studied comprised 34 eyes that had LASIK surgery. Refraction and an automated K-reading (auto-K) were performed preoperatively. Refraction, auto-K, and K-reading assessment by the clinical history method and the proposed formula were performed 4 to 12 weeks postoperatively. The proposed formula is Kpostop = Kpreop – [(Nc – 1) × (Ra-postop – Ra-preop)/(Ra-postop × Ra-preop)], where Kpostop is the K-reading after LASIK, Kpreop is the K-reading before LASIK, Nc is the index of refraction of the cornea (1.376), Ra-postop is the radius of curvature of the anterior corneal surface after LASIK, and Ra-preop is the radius of curvature of the anterior corneal surface before LASIK. Results: Twenty patients (10 men, 10 women) were included in the study. The mean age of the patients was 30.58 years ± 17.68 (SD) (range 18 to 44 years). Preoperatively, the mean spherical equivalent (SE) was –4.99 ± 2.82 diopters (D) (range –1.12 to –15.00 D), the mean Ra was 7.76 ± 0.32 mm (range 7.33 to 8.50 mm), and the mean auto-K reading was 43.45 ± 1.73 D (range 39.62 to 46.00 D). Postoperatively, the mean SE was +0.02 ± 0.63 D (range –2.75 to +1.00 D), the mean Ra was 8.63 ± 0.53 mm (range 7.80 to 9.92 mm), and the mean K-reading assessed by auto-K, clinical history method, and the proposed formula was 39.17 ± 2.35 D (range 34.00 to 43.25 D), 38.79 ± 2.52 D (range 33.1 to 42.78 D), and 38.69 ± 2.51 D (range 33.1 to 43.0 D), respectively. The results obtained by the proposed formula were similar to those obtained by the clinical history method (P = .098). Auto-K readings significantly overestimated the K-values (P<.0001) when compared to the proposed formula and clinical history method. Conclusion: The proposed formula was simple, objective, not dependent on refraction, and as accurate as the clinical history method in determining K-readings after LASIK.