Shape of the anterior cornea:Comparison of height data from 4 corneal topographers

PURPOSE: To compare the ability of clinical corneal topographers to describe the shape of the anterior cornea for optical modeling.SETTING: University Medical Center Groningen, Groningen, Netherlands.DESIGN: Cross-sectional study.METHODS: The anterior corneal shape of healthy subjects was assessed with 4 topographers (Atlas Placido disk, Galilei dual Scheimpflug, Orbscan scanning slit, Pentacam single Scheimpflug). Exported height data were fit with Zernike polynomials. Mean values with the standard deviation, interdevice variability, and test-retest variability were determined for the defocus Z(2,0), astigmatism Z(2,-2) and Z(2,2), coma Z(3,-1) and Z(3,1), and spherical aberration Z(4,0) coefficients for 5.5 mm and 8.0 mm diameters.RESULTS: At 5.5 mm, the single Scheimpflug topographer showed the smallest coefficient of repeatability; 0.31 mu m for Z(2,0); 0.40 and 0.34 mu m for Z(2,-2) and Z(2,2), respectively; 0.15 and 0.11 mu m for Z(3,-1) and Z(3,1), respectively; and 0.08 mu m for Z(4,0); the other topographers showed up to 10 times larger coefficients of repeatability. The (unsigned) mean differences between the topographers were in the range of 0.20 to 1.21 mu m for Z(2,0); 0.02 to 0.31 mu m and 0.06 to 0.42 mu m for Z(2,-2) and Z(2,2), respectively; 0.03 to 0.18 mu m and 0.03 to 0.35 mu m for Z(3,-1) and Z(3,1), respectively; and 0.00 to 0.14 mu m for Z(4,0). The Placido-disk topographer and single Scheimpflug topographer data corresponded best. Similar trends were found at 8.0 mm.CONCLUSION: Test-retest variability hampered a detailed description of the anterior corneal shape at the level of individual subjects; interdevice variability compromises the exchangeability of the devices.