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J. Einighammer, T. Oltrup, T. Bende, B. Jean; Comparison of Two Methods to Determine Corneal Asphericity From Topographic Maps. Invest. Ophthalmol. Vis. Sci. 2010;51(13):5658.
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© ARVO (1962-2015); The Authors (2016-present)
To compare two methods and two topography systems to determine asphericity (Q) from corneal measurements, one method based on axial curvature maps and another based on elevation maps.
As a first method, simulated sagittal radius measurement (SRM) after Wilms based on axial curvature maps was used to calculate Q at 20 degrees (this corresponds to approx. 5.4mm diameter for a cornea of 8mm radius of curvature). As a second method, a biconic surface fit approach (BSF) based on corneal elevation maps was used to calculate Q over 6mm diameter. Both methods rely on the same measurements, analyze the asphericities of the steep and flat axes and average those values to derive one Q value for each topography map. Two patient groups were analyzed using two topography systems: 40 cataract and 33 refractive surgery eyes, pre- and postoperatively. The cataract group was measured with C-SCAN (Technomed GmbH, Germany) and OphthaTOP (OphthaSWISS AG, Switzerland), the refractive group only with C-SCAN. For the cataract group the repeatability was investigated based on a subset of 29 eyes that had multiple measurements with both topography units. The "95% limits of agreement" (LoA) and "repeatability coefficients" (RCs) were determined according to Bland and Altman.
See table for detailed results of Q and LoA. No big difference regarding the average values was found between the two methods SRM and BSF on neither system. Both methods and systems show the expected oblateness of the postoperative refractive corneas. The LoA are quite large, showing better agreement for the refractive group than for the cataract group. The RCs were 0.31/0.69 (SRM/BSF) for C-SCAN and 0.36/0.21 for OphthaTOP.
Asphericity determination is critical. The two methods gave nearly identical results for the asphericity, in average. But, looking at individual values, the agreement between methods is somewhat limited. However, an even greater influence than choosing the analysis method goes to the topography systems’ repeatability.
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