Determinants of Posterior Corneal Biometric Measurements in a Multi-Ethnic Asian Population Marcus Ang 1,2 , Wesley Chong 1 , Huiqi Huang 2 , Tien Yin Wong 1,2,3,4 , Ming-Guang He 5 , Tin Aung 1,2,3 , Jodhbir S. Mehta 1,2,4 * 1 Singapore National Eye Centre, Singapore, Singapore, 2 Singapore Eye Research Institute, Singapore, Singapore, 3 Department of Ophthalmology, National University of Singapore, Singapore, Singapore, 4 Office of Clinical Sciences, Duke- National University of Singapore Graduate Medical School, Singapore, Singapore, 5 State Key Laboratory of Ophthalmology, Zhongshan Ophthalmic Center, Guangzhou, China Abstract Purpose: To describe the corneal and anterior segment determinants of posterior corneal arc length (PCAL) and posterior corneal curvature (PCC). Methods: Cross-sectional, population-based study of 1069 subjects (1069 eyes) aged 40–80 years, from three major Asian ethnic groups. All underwent anterior segment optical coherence tomography imaging and analysis with Zhongshan Angle Assessment Program. Our main outcome measures were determinants of PCAL and PCC using adjusted, multivariate linear regression analysis, adjusted for confounders to obtain the estimated marginal means (EMM) with standard error (SE). Results: The overall mean (6 SD) of PCC was: 6.5160.39 mm; and PCAL was: 12.5260.59 mm. Malays had a relatively longer PCAL (EMM = 12.74 mm, SE = 0.04 mm) than Chinese (EMM = 12.48 mm, SE = 0.03 mm, P,0.001), and Indians (EMM = 12.42 mm, SE = 0.03 mm, P,0.001). Anterior segment parameters had weak-moderate correlations with PCAL, which included: anterior chamber depth (ACD) (r = 0.55, P,0.001), PCC (r = 0.27, P,0.001), anterior corneal curvature (ACC) (r = 0.14, P,0.001) and central corneal thickness (CCT) (r = 20.07, P = 0.023). In multivariate analysis, anterior segment parameters explained only 37.6% of the variance of PCAL, with ACD being the most important determinant (partial R 2 = 0.300; P,0.001). The determinants of PCC included ACC, PCAL and CCT (explaining 72.1% variation of PCC), with ACC being the most important determinant (partial R 2 = 0.683; P,0.001). Conclusion: There was moderate correlation of PCAL with ACD, but anterior segment parameters accounted for only a small proportion of the variation in PCAL. The significant differences in PCAL and PCC amongst different Asian ethnic groups suggests that there is a need to consider this factor when planning for anterior segment surgeries such as endothelial keratoplasty. Citation: Ang M, Chong W, Huang H, Wong TY, He M-G, et al. (2014) Determinants of Posterior Corneal Biometric Measurements in a Multi-Ethnic Asian Population. PLoS ONE 9(7): e101483. doi:10.1371/journal.pone.0101483 Editor: Rajiv R. Mohan, University of Missouri-Columbia, United States of America Received January 13, 2014; Accepted June 6, 2014; Published July 9, 2014 Copyright: ß 2014 Ang et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Funding: National Research Foundation Translational and Clinical Research Programme Grant (TCR:R621/42/2008). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing Interests: The authors have declared that no competing interests exist. * Email: jodmehta@gmail.com Introduction Accurate measurement of the posterior corneal surface is important in the planning and execution of endothelial kerato- plasty, which is increasingly becoming the surgery of choice in endothelial disease.[1] One way to more precisely image the cornea is with the use of anterior segment optical coherence tomography (AS-OCT, Visante; Carl Zeiss Meditec, Dublin, CA).[2,3] However, measurements of anterior segment parame- ters from AS-OCT images are often inaccurate due to subjective placement of calipers or measurement tools.[4] The Zhongshan Angle Assessment Program (ZAAP, Guangzhou, China) has been shown to reliably derive these parameters from AS-OCT images,[5,6,7,8] with high inter- and intra-observer agree- ment.[5,6] The ZAAP software works by extracting the gray-scale images from the AS-OCT images and uses image processing and algorithms to define the anatomical landmarks.[5] This enables users to obtain rapid and objective analysis of anterior segment images for surgical planning and diagnosis of various ocular diseases (ZAAP research software available upon request; none of the authors have any commercial interest).[2,3,9] The ZAAP analysis obtains standard parameters such as central corneal thickness (CCT), anterior chamber depth (ACD), anterior corneal curvatures (ACC) using image-processing algorithms. [5] Parameters derived from ZAAP include the posterior cornea curvature (PCC) and posterior corneal arc length (PCAL).[6] PCC is defined as the radius of curvature of the posterior surface of the cornea,[6] while PCAL is defined as the arc distance of the posterior corneal border between scleral spurs,[7] both reliably derived by ZAAP analysis from AS-OCT images in both normal and corneas with bullous keratopathy. [7] The clinical application selection for endothelial keratoplasties.[7] The PCC and power of PLOS ONE | www.plosone.org 1 July 2014 | Volume 9 | Issue 7 | e101483 of PCAL has been demonstrated in optimum donor graft size