The topography of the peripheral cornea (E-Abstract)

Purpose Few studies have investigated the peripheral cornea outside of the central 6mm. We examined the topography of the peripheral cornea in a large group of young adult subjects and report normative data for this population. Methods Corneal topography data were acquired for 100 young adult subjects using a method that allows the combination of central and peripheral maps to produce one large, extended corneal topography map. This computer-based method involves matching the common topographical features in the overlapping maps. Results The average apical radius Ro was found to be 7.78 ± 0.2 mm and the asphericity value (Q) was -0.18 ± 0.09 for the central 6mm corneal diameter. Both Ro and Q were found to change significantly with increasing corneal diameter (p <0.001). For a 10 mm corneal diameter, Ro was found to be on average 7.72 ± 0.2 mm and Q -0.36 ± 0.09. The corneal height data was also averaged along the steepest and flattest corneal meridian for each subject. The average Ro was found to be 7.68 ± 0.26 mm and Q -0.32 ± 0.1 along the steepest meridian and Ro 7.81 ± 0.24 mm and Q -0.30 ± 0.1 along the flattest meridian for a 9mm corneal diameter. The average Ro and Q values were found to differ significantly as a function of corneal meridian (both p <0.01) indicating that the steepest corneal meridian flattens in the periphery at a slightly more rapid rate. The RMS fit errors for the conic section (Q) increase markedly for larger corneal diameters. A better fit to the data was attained for the bigger corneal diameters by using higher order polynomial fits (9th order). The average corneal power vectors “M” “J0” and “J45” were all found to reduce significantly in magnitude with increasing corneal diameter (p <0.01), indicating a significant flattening and reduction in toricity for the peripheral cornea. Conclusions The peripheral cornea is poorly estimated with a simple conic section. The cornea flattens and becomes slightly less astigmatic in the periphery.