Keratoconus: it is hard to define, but ...

MLA

Belin, Michael W., and Stephen S. Khachikian. “Keratoconus: It Is Hard to Define, But ..” American Journal of Ophthalmology, vol. 143, no. 3, Mar. 2007, pp. 500–03. EBSCOhost, https://doi.org/10.1016/j.ajo.2006.12.030.

APA

Belin, M. W., & Khachikian, S. S. (2007). Keratoconus: it is hard to define, but .. American Journal of Ophthalmology, 143(3), 500–503. https://doi.org/10.1016/j.ajo.2006.12.030

Chicago

Belin, Michael W, and Stephen S Khachikian. 2007. “Keratoconus: It Is Hard to Define, But ..” American Journal of Ophthalmology 143 (3): 500–503. doi:10.1016/j.ajo.2006.12.030.