Asphericity of the anterior human cornea with different corneal diameters

PURPOSE: To measure the anterior corneal asphericity (Q) with different corneal diameters. SETTING: Department of Physics (Optometry), University of Minho, Braga, Portugal. METHODS: Thirty-six eyes of 36 patients were evaluated using a videokeratoscope, and the Q-values were recorded. Topographic data were also analyzed using Vol-CT 6.89 software (Sarver & Associates, Inc) to obtain the Q-values with different corneal diameters (3.0 mm, 4.0 mm, 5.0 mm, 6.0 mm, and 7.0 mm). Variable Q models of corneal sagittal height were compared against models assuming constant Q-values obtained with the Medmont E300 videokeratoscope (Medmont Pty. Ltd.) and a standard Q model of -0.26. RESULTS: The peripheral rate of change in corneal Q with different corneal diameters increased as corneal astigmatism increased. As a result, differences in the sagittal height between the constant model and variable model were evident beyond the central 3.0 mm area. There were significant differences between low and high astigmatic corneas in Q-values measured by the Medmont along the flattest meridian (P =.004) and Q-values obtained with Vol-CT software with a 7.0 mm corneal diameter (P =.026). CONCLUSIONS: There were differences in sagittal corneal height calculations considering constant or variable models of Q. Concern arises when surgical interventions depend on corneal Q-values to predict the outcomes. Surgeons should be aware which procedure is behind Q computing by different corneal topographers and that a constant Q-value cannot reflect the actual shape of the cornea as significant departures from the actual sagittal height can arise depending on which Q-value is assumed.