ARTICLE Wang-Koch formula for optimization of intraocular lens power calculation: Evaluation at a Canadian center Marko Popovic, BHSc, Matthew B. Schlenker, MD, Xavier Campos-M€ oller, MD, Austin Pereira, BMSc, Iqbal Ike K. Ahmed, MD Purpose: To externally validate the Wang-Koch method for optimization of intraocular lens (IOL) formulas. Setting: TLC Laser Eye Centre, Mississauga, Ontario, Canada. Design: Retrospective case series. Methods: Consecutive cataract patients with an axial length (AL) of 25.0 mm or longer were recruited. The predicted post- operative spherical equivalents (SEs) calculated from the Holla- day 1 formula were compared with the 3-week postoperative SEs to yield prediction errors for Wang-Koch adjusted and unadjusted ALs. A mixed linear model was used to compare the proportion of eyes with a prediction error of G0.25 diopter (D) or worse, G0.50 D or worse, and G1.00 D or worse between groups. The secondary outcomes of mean absolute error and median absolute error were also analyzed. A subgroup analysis was performed based on AL subgroups. Results: Two hundred sixty-two eyes were selected for inclusion with a balanced sex distribution, a mean age of 62.49 years G 9.13 (SD), and a preoperative AL of 26.49 G 1.10 mm. Subgroup prediction error comparisons of G0.50 D or worse favored unadjusted eyes with ALs between 25.0 mm and 26.0 mm (n Z 105; P < .001), no difference in eyes with ALs between 26.0 mm and 27.0 mm (n Z 91; P Z .43), adjusted eyes with ALs between 27.0 mm and 28.0 mm (n Z 36; P Z .003), and adjusted eyes with ALs of 28.00 mm or longer (n Z 30; P < .001). Conclusion: The Wang-Koch adjustment should only be applied in eyes with ALs longer than 27.0 mm that have IOL power calculation with the Holladay 1 formula. J Cataract Refract Surg 2018; 44:17–22 Q 2018 ASCRS and ESCRS M odern cataract surgery is an efficacious and safe procedure. Even though refractive outcomes af- ter intraocular lens (IOL) implantation have improved considerably over time, patient expectations for increasingly precise postoperative refractive outcomes continue to increase. Accuracy in the calculation of IOL power is necessary for realizing the desired postoperative refraction. 1 This accu- racy is largely dependent on 3 factors: preoperative biom- etry data, the IOL power calculation formula, and the quality control of the manufacturer’s IOL power. The axial length (AL) derived from preoperative biometry is one important determinant of eventual IOL refractive power. 2 Specifically, a study of ultrasound (US) biometry using the Binkhorst formula 3 found that 54% of all errors in predicted refraction could be attributed to AL measurement errors, whereas only 8% of errors occurred because of corneal power measurement inaccuracies. Thirty-eight percent of errors were caused by flaws in the estimation of postoperative anterior chamber depth (ACD). Resul- tantly, accurate measurement of AL is likely the most mean- ingful method to improve IOL power prediction. 4 In eyes with long ALs, IOL formulas might not consis- tently produce accurate results. In 89 long eyes with ALs greater than 24.5 mm, Hoffer 5 showed that the Holladay 1 formula 6 produced the lowest mean absolute error (MAE) of 0.41 diopter (D) G 0.31 (SD) when compared with the SRK I, 7 SRK II, 8 SRK/T, 9 and Hoffer Q formulas. 5 Nonetheless, hyperopic error can be found after IOL im- plantation in long eyes. Wang et al. 10 have hypothesized Submitted: June 16, 2017 | Final revision submitted: August 27, 2017 | Accepted: September 22, 2017 From the Faculty of Medicine (Popovic, Pereira) and the Department of Ophthalmology and Vision Sciences (Schlenker, Campos-M€ oller, Ahmed), University of Toronto, Toronto, and Prism Eye Institute (Campos-M€ oller, Ahmed) and the Department of Ophthalmology (Ahmed), Trillium Health Partners, Mississauga, Ontario, Canada. Presented in part at the ASCRS Symposium on Cataract, IOL and Refractive Surgery, Los Angeles, California, USA, May 2017 and the annual meeting of the Canadian Ophthalmological Society, Montreal, Quebec, Canada, June 2017. Corresponding author: Iqbal Ike K. Ahmed, MD, Prism Eye Institute, 3200 Erin Mills Parkway, Unit 1, Mississauga, Ontario L5L 1W8, Canada. E-mail: ike.ahmed@ utoronto.ca. Q 2018 ASCRS and ESCRS Published by Elsevier Inc. 0886-3350/$ - see frontmatter https://doi.org/10.1016/j.jcrs.2017.09.035 17