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

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 K_postop = K_preop – [(N_c – 1) × (R_a-postop – R_a-preop)/(R_a-postop × R_a-preop)], where K_postop is the K-reading after LASIK, K_preop is the K-reading before LASIK, N_c is the index of refraction of the cornea (1.376), R_a-postop is the radius of curvature of the anterior corneal surface after LASIK, and R_a-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 R_a 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 R_a 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.

In Brief

The results of a new proposed formula that calculates the K-reading according to the change in the anterior radius of curvature and independently of the assumed index of refraction were similar to the results obtained by the clinical history method.